ALBERT

Description: Similarity estimators for irregular and age uncertain time series Climate of the Past Discussions, 9, 5299-5346, 2013 Author(s): K. Rehfeld and J. Kurths Paleoclimate time series are often irregularly sampled and age uncertain, which is an important technical challenge to overcome for successful reconstruction of past climate variability and dynamics. Visual comparison and interpolation-based linear correlation approaches have been used to infer dependencies from such proxy time series. While the first is subjective, not measurable and not suitable for the comparison of many datasets at a time, the latter introduces interpolation bias, and both face difficulties if the underlying dependencies are nonlinear. In this paper we investigate similarity estimators that could be suitable for the quantitative investigation of dependencies in irregular and age uncertain time series. We compare the Gaussian-kernel based cross correlation (gXCF, Rehfeld et al., 2011) and mutual information (gMI, Rehfeld et al., 2013) against their interpolation-based counterparts and the new event synchronization function (ESF). We test the efficiency of the methods in estimating coupling strength and coupling lag numerically, using ensembles of synthetic stalagmites with short, autocorrelated, linear and nonlinearly coupled proxy time series, and in the application to real stalagmite time series. In the linear test case coupling strength increases are identified consistently for all estimators, while in the nonlinear test case the correlation-based approaches fail. The lag at which the time series are coupled is identified correctly as the maximum of the similarity functions in around 60–55% (in the linear case) to 53–42% (for the nonlinear processes) of the cases when the dating of the synthetic stalagmite is perfectly precise. If the age uncertainty increases beyond 5% of the time series length, however, the true coupling lag is not identified more often than the others for which the similarity function was estimated. Age uncertainty contributes up to half of the uncertainty in the similarity estimation process. Time series irregularity contributes less, particularly for the adapted Gaussian-kernel based estimators and the event synchronization function. The introduced link strength concept summarizes the hypothesis test results and balances the individual strengths of the estimators: while gXCF is particularly suitable for short and irregular time series, gMI and the ESF can identify nonlinear dependencies. ESF could, in particular, be suitable to study extreme event dynamics in paleoclimate records. Programs to analyze paleoclimatic time series for significant dependencies are included in a freely available software toolbox.

Description: Impact of precession on the climate, vegetation and fire activity in southern Africa during MIS4 Climate of the Past Discussions, 9, 5391-5438, 2013 Author(s): M.-N. Woillez, G. Levavasseur, A.-L. Daniau, M. Kageyama, D. H. Urrego, and M.-F. Sánchez-Goñi The relationships between climate, vegetation and fires are a major subject of investigation in the context of climate change. In southern Africa, fire is known to play a crucial role in the existence of grasslands and Mediterranean-like biomes. Microcharcoal-based reconstructions of past fire activity in that region have shown a tight correlation between grass-fueled fires and the precessional cycle, with maximum fire activity during maxima of the climatic precession index. These changes have been interpreted as the result of changes in fuel load in response to precipitation changes in eastern southern Africa. Here we use the general circulation model IPSL_CM5A and the dynamical vegetation model LPJ-LMfire to investigate the response of climate, vegetation and fire activity to precession changes in southern Africa during Marine Isotopic Stage 4. We perform two climatic simulations, for a maximum and minimum of the precession index, and use a statistical downscaling method to increase the spatial resolution of the IPSL_CM5A outputs over southern Africa and perform high-resolution simulations of the vegetation and fire activity. Our results show an anti-correlation between the North and South African monsoons in response to precession changes. A decrease of the precession climatic index leads to a precipitation decrease in the summer rainfall area of southern Africa. The drying of climate leads to a decrease of vegetation cover and fire activity. Our results are in qualitative agreement with data and confirm that fire activity in southern Africa is strongly dependent on the vegetation type.

Description: Ensemble meteorological reconstruction using circulation analogues of 1781–1785 Climate of the Past Discussions, 9, 5157-5182, 2013 Author(s): P. Yiou, M. Boichu, R. Vautard, M. Vrac, S. Jourdain, E. Garnier, F. Fluteau, and L. Menut This paper uses a method of atmospheric flow analogues to reconstruct an ensemble of atmospheric variables (namely sea-level pressure, surface temperature and wind speed) between 1781 and 1785. The properties of this ensemble are investigated and tested against observations of temperature. The goal of the paper is to assess whether the atmospheric circulation during the Laki volcanic eruption (in 1783) and the subsequent winter were similar to the conditions that prevailed in the winter 2009/2010 and during spring 2010. We find that the three months following the Laki eruption in June 1783 barely have analogues in 2010. The cold winter of 1783/1784 yields circulation analogues in 2009/2010. Our analysis suggests that it is unlikely that the Laki eruption was responsible for the cold winter of 1783/1784, of the relatively short memory of the atmospheric circulation.

Description: Seasonal changes in glacial polynya activity inferred from Weddell Sea varves Climate of the Past Discussions, 9, 5123-5156, 2013 Author(s): D. Sprenk, M. E. Weber, G. Kuhn, V. Wennrich, T. Hartmann, and K. Seelos The Weddell Sea and the associated Filchner-Rønne Ice Shelf constitute key regions for global bottom-water production today. However, little is known about bottom-water production under different climate and ice-sheet conditions. Therefore, we studied core PS1795, which consists primarily of fine-grained siliciclastic varves that were deposited on contourite ridges in the southeastern Weddell Sea during the Last Glacial Maximum (LGM). We conducted high-resolution X-ray fluorescence (XRF) analysis and grain-size measurements with the RADIUS tool (Seelos and Sirocko, 2005) using thin sections to characterize the two seasonal components of the varves at sub-mm resolution to distinguish the seasonal components of the varves. Bright layers contain coarser grains that can mainly be identified as quartz in the medium to coarse silt grain size. They also contain higher amounts of Si, Zr, Ca, and Sr, as well as more ice-rafted debris (IRD). Dark layers, on the other hand, contain finer particles such as mica and clay minerals from the chlorite and illite groups. In addition, chemical elements, Fe, Ti, Rb, and K are elevated as well. Based on these findings as well as on previous analyses on neighbouring cores, we propose a model of glacially enhanced thermohaline convection in front of a grounded ice sheet that is supported by seasonally variable coastal polynya activity. Accordingly, katabatic (i.e. offshore blowing) winds removed sea ice from the ice edge, leading to coastal polynya formation. We suggest that glacial processes were similar to today with stronger katabatic winds and enhanced coastal polynya activity during the winter season. If this is correct, silty layers are likely glacial winter deposits, when brine rejection was increased, leading to enhanced bottom water formation and increased sediment transport. Vice versa, finer-grained clayey layers were then deposited during summer, when coastal polynya activity was likely reduced.

Description: Is blue intensity ready to replace maximum latewood density as a strong temperature proxy? A tree-ring case study on Scots pine from northern Sweden Climate of the Past Discussions, 9, 5227-5261, 2013 Author(s): J. A. Björklund, B. E. Gunnarson, K. Seftigen, J. Esper, and H. W. Linderholm At high latitudes, where low temperatures mainly limit tree-growth, measurements of wood density (e.g. Maximum Latewood Density, MXD) using the X-Ray methodology provide a temperature proxy that is superior to that of TRW. Density measurements are however costly and time consuming and have lead to experimentation with optical flatbed scanners to produce Maximum Blue Intensity (BI max ). BI max is an excellent proxy for density on annual scale but very limited in skill on centennial scale. Discolouration between samples is limiting BI max where specific brightnesses can have different densities. To overcome this, the new un-exploited parameter Δ blue intensity (ΔBI) was constructed by using the brightness in the earlywood (BI EW ) as background, (BI max − BI EW = ΔBI). This parameter was tested on X-Ray material (MXD − earlywood density = ΔMXD) and showed great potential both as a quality control and as a booster of climate signals. Unfortunately since the relationship between grey scale and density is not linear, and between-sample brightness can differ tremendously for similar densities, ΔBI cannot fully match ΔMXD in skill as climate proxy on centennial scale. For ΔBI to stand alone, the range of brightness/density offset must be reduced. Further studies are needed to evaluate this possibility, and solutions might include heavier sample treatment (reflux with chemicals) or image-data treatment (digitally manipulating base-line levels of brightness).

Description: Halocline water modification and along slope advection at the Laptev Sea continental margin Ocean Science Discussions, 10, 1581-1617, 2013 Author(s): D. Bauch, S. Torres-Valdes, I. Polyakov, A. Novikhin, I. Dmitrenko, J. McKay, and A. Mix A general pattern in water mass distribution and potential shelf-basin exchanges is revealed at the Laptev Sea continental slope based on hydrochemical and stable oxygen isotope data from summers 2005–2009. Despite considerable interannual variations, a frontal system can be inferred between shelf, continental slope and central Eurasian Basin waters in the upper 100 m of the water column along the continental slope. Net sea-ice melt is consistently found at the continental slope; however the sea-ice meltwater signal is independent from the local retreat of the ice cover and appears to be advected from upwind locations. In addition to the along-slope frontal system at the continental shelf break a strong gradient is identified on the Laptev Sea shelf between 122 and 126° E with an eastward increase of riverine and sea-ice related brine water contents. These waters cross the shelf break at ~ 140° E and feed the Low Salinity Halocline Water (LSHW, salinity S 〈 33) in the upper 50 m of the water column. Extremely high silicate concentrations in Laptev Sea bottom waters may lead to speculation on a link to the local silicate maximum found within the salinity range of ~ 33 to 34.5, typical for the Lower Halocline Water (LHW) at the continental slope. But brine signatures and nutrient ratios from the central Laptev Sea differ from those at the continental slope. Thus a significant contribution of Laptev Sea bottom waters to the LHW at the continental slope can be excluded. The silicate maximum within the LHW at the continental slope may be formed locally or at the outer Laptev Sea shelf. Similar to the advection of the sea-ice melt signal along the Laptev Sea continental slope the nutrient signal at 50–70 m water depth within the LHW might also be fed by advection parallel to the slope. Thus, our analyses suggest that advective processes from upwind locations play a significant role in the halocline formation in the northern Laptev Sea.

Description: Migrating subtropical front and Agulhas Return Current affect the southwestern Indian Ocean during the late Quaternary Climate of the Past Discussions, 9, 5521-5551, 2013 Author(s): D. K. Naik, R. Saraswat, N. Khare, A. C. Pandey, and R. Nigam The position of sub-tropical front (STF), Agulhas Current (AC) and Agulhas Return Current (ARC) controls the hydrography of southwestern Indian Ocean. Although, equator-ward migration of STF and reduction in Agulhas leakage has been reported during the last glacial period, the fate of ARC during the last glacial–interglacial cycle is not clear. Therefore, in order to understand changes in the position and strength of ARC during the last glacial–interglacial cycle, here we reconstruct hydrographic changes in the southwestern Indian Ocean from temporal variation in planktic foraminiferal abundance, stable isotopic ratio (δ 18 O) and trace metal ratio (Mg / Ca) of planktic foraminifera Globigerina bulloides in a core collected from the Agulhas Retroflection Region (ARR) in the southwestern Indian Ocean. Increased abundance of G. bulloides suggests that the productivity in the southwestern Indian Ocean increased during glacial period which confirms previous reports of high glacial productivity in the Southern Ocean. The increased productivity was likely driven by a combination of equator-ward migration of subtropical front and westerlies. Increase in relative abundance of Neogloboquadrina pachyderma Dextral suggests warming of ARR leading to strong thermocline in the southwestern Indian Ocean during the last glacial period. We suggest that the warming of Agulhas Retroflection Region was driven by strengthened ARC which shifted to the east of its present location, thus bringing warmer and saltier water to the southwestern Indian Ocean. Therefore, it is inferred that over the last glacial–interglacial cycle, the hydrography of southwestern Indian Ocean was driven by an eastward shift of retroflection region as well as migrating subtropical front.

Description: The response of the Peruvian Upwelling Ecosystem to centennial-scale global change during the last two millennia Climate of the Past Discussions, 9, 5479-5519, 2013 Author(s): R. Salvatteci, D. Gutiérrez, D. Field, A. Sifeddine, L. Ortlieb, I. Bouloubassi, M. Boussafir, H. Boucher, and F. Cetin The Tropical Pacific ocean-atmosphere system influences global climate on interannual, decadal, as well as at longer timescales. Given the uncertainties in the response of the Tropical Pacific to the ongoing greenhouse effect, it is important to assess the natural range of the Tropical Pacific climate variability in response to global natural changes, and to understand the underlying mechanisms. The Peruvian Upwelling Ecosystem (PUE) represents an ideal area to reconstruct past changes in ocean-atmosphere systems because productivity and subsurface oxygenation are strongly linked to changes in the strength of the Walker circulation. Throughout the last 2000 yr, warmer (the Roman Warm Period [RWP], the Medieval Climate Anomaly [MCA] and the Current Warm Period [CWP]), and colder (the Dark Ages Cold Period [DACP] and Little Ice Age [LIA]) intervals occurred with considerable changes around the globe. In order to reconstruct the PUE response to these climatic periods and reveal the underlying mechanisms, we use a multi-proxy approach including organic and inorganic proxies in finely laminated sediments retrieved off Pisco (~ 14° S), Peru. Our results indicate that the PUE exhibited a La Niña-like mean state during the warm periods, characterized by an intense OMZ and high marine productivity. During cold periods the PUE exhibited an El Niño-like mean state, characterized by a weak OMZ and low marine productivity. Comparing our results with other relevant paleoclimatic reconstructions revealed that changes in the strength of the Walker circulation and the expansion/contraction of the South Pacific Sub-tropical High controlled productivity and subsurface oxygenation in the PUE during the last two millennia. This indicate that large scale circulation changes are the driving forces in maintaining productivity and subsurface oxygenation off Peru at centennial time scales during the past two millennia.

Description: Cumulated insolation: a simple explanation of Milankovitch's forcing on climate changes Climate of the Past Discussions, 9, 5553-5568, 2013 Author(s): F. Marra The occurrence of the sudden melting of the ice sheets during the glacial terminations is explained in this paper as the consequence of the combined role of the minima and the maxima of mean summer insolation on the Northern Hemisphere, providing a new contribution to understand the mechanisms ruling glacial forcing. Indeed, no satisfactory answer has been provided so far to the question why one specific maximum, after a series of consecutive maxima of insolation, has the potentiality to trigger a deglaciation. The explanation proposed in this paper accounts for a pre-conditioning factor, represented by "mild" (warmer) minimum, followed by a sufficiently warm maximum as the conditions that cause the end of a glacial cycle. These conditions are realized whenever the sum of the values of each consecutive minima and maxima ("cumulated insolation") on the curve of mean summer insolation at 65° N exceeds 742 Watt m −2 . The comparison of the succession of these cumulated insolation values with the astronomically tuned Oxygen isotopes record provides a satisfactory match with the occurrence of all the glacial terminations in the last 800 ka.

Description: Chronology of Lake El'gygytgyn sediments Climate of the Past Discussions, 9, 3061-3102, 2013 Author(s): N. R. Nowaczyk, E. M. Haltia, D. Ulbricht, V. Wennrich, M. A. Sauerbrey, P. Rosén, H. Vogel, A. Francke, C. Meyer-Jacob, A. A. Andreev, and A. V. Lozhkin A 318 m long sedimentary profile drilled by the International Continental Scientific Drilling Program (ICDP) at Site 5011-1 in Lake El'gygytgyn, Far East Russian Arctic, has been analysed for its sedimentologic response to global climate modes by chrono-stratigraphic methods. The 12 km wide lake is sited in an 18 km large crater that was created by the impact of a meteorite 3.58 Ma ago. Since then sediments have been continuously deposited. For establishing their chronology, major reversals of the Earth's magnetic field provided initial tie points for the age model, confirming that the impact occurred in the earliest Gauss chron. Various stratigraphic parameters, reflecting redox conditions at the lake floor and climatic conditions in the catchment were tuned synchronously to Northern Hemisphere insolation variations and the marine oxygen isotope stack, respectively. Thus, a robust age model comprising more than 600 tie points could be defined. It could be shown that deposition of sediments in Lake El'gygytgyn occurred in concert with global climatic cycles. The upper ~160 m of sediments represent the past 3.3 Ma, equivalent to sedimentation rates of 4 to 5 cm ka −1 , whereas the lower 160 m represent just the first 0.3 Ma after the impact, equivalent to sedimentation rates in the order of 45 cm ka −1 .

Description: Late Cenozoic continuous aridification in the western Qaidam Basin: evidence from sporopollen records Climate of the Past Discussions, 9, 1485-1508, 2013 Author(s): Y. F. Miao, X. M. Fang, F. L. Wu, M. T. Cai, C. H. Song, Q. Q. Meng, and L. Xu Cenozoic climate changes in inner Asia provide a basis for understanding linkages between global cooling, the Tibetan Plateau uplift, and possibly the development of the East Asian monsoon. Based on the compiled palynological results from the western Qaidam Basin, this study reconstructed an 18 Ma record of changing vegetation and paleoclimates since the middle Miocene. Thermophilic taxa percentages were highest between 18 and 14 Ma and decreased after 14 Ma, corresponding closely with the Middle Miocene Climatic Optimum (MMCO) between 18 and 14 Ma and the following global climatic cooling. After 3.6 Ma, the thermophilic taxa percentages further decreased, showing the inevitable relations with the ice-sheets enlargement in the North Hemisphere. During the same period of time, the increase in xerophytic taxa percentages and decrease in conifers percentages imply aridification in both the basin and surrounding mountains since 18 Ma. These results indicate that global cooling mainly controlled the climate change from a relative warm-wet stage to a cold-dry stage during the late Cenozoic at the western Qaidam Basin, and that the Tibetan Plateau uplift also contributed in contrast to the East Asian summer monsoon.

Description: Post-Pliocene establishment of the present monsoonal climate in SW China: evidence from the late Pliocene Longmen megaflora Climate of the Past Discussions, 9, 1675-1701, 2013 Author(s): T. Su, F. M. B. Jacques, R. A. Spicer, Y.-S. Liu, Y.-J. Huang, Y.-W. Xing, and Z.-K. Zhou The paleoclimate of the late Pliocene Longmen flora from Yongping County located at the southeastern boundary of the Qinghai-Tibet Plateau was reconstructed using two leaf physiognomy based methods, i.e. Leaf Margin Analysis (LMA) and Climate Leaf Analysis Multivariate Program (CLAMP), to understand the paleoclimate condition and geographical pattern of monsoonal climate in southwestern China during the late Pliocene. The mean annual temperatures (MATs) estimated by LMA and CLAMP are 17.4 ± 3.3 °C and 17.4 ± 1.3 °C, respectively, compared with 15.9 °C at present. Meanwhile, the growing season precipitation (GSP) estimated by CLAMP is 1735.5 ± 217.7 mm in the Longmen flora, compared with 986.9 mm nowadays. The calculated monsoon index (MSI) of the Longmen flora is significantly lower than that of today. These results appear consistent with previous studies based on the coexistence approach (CA), and further suggest that there was a slightly warmer and much wetter climate during the late Pliocene than the present climate in western Yunnan. We conclude that the significant change of the monsoonal climate might have been resulted from the continuous uplift of mountains in western Yunnan, as well as the intensification of eastern Asian winter monsoon, both occurring concurrently in the post-Pliocene period.

Description: The transient distributions of nuclear weapon-generated tritium and its decay product 3 He in the Mediterranean Sea, 1952–2011, and their oceanographic potential Ocean Science Discussions, 10, 649-690, 2013 Author(s): W. Roether, P. Jean-Baptiste, E. Fourré, and J. Sültenfuß We present a comprehensive account of tritium and 3 He in the Mediterranean Sea since the appearance of the tritium generated by the atmospheric nuclear-weapon testing in the 1950's and early 1960's, based on essentially all available observations. Tritium in surface waters rose to 20–30 TU in 1964 (TU = 10 18 · [ 3 H]/[H]), a factor of about 100 above the natural level, and thereafter declined 30-fold up to 2011. The decline was largely due to radioactive tritium decay, which produced significant amounts of its stable daughter 3 He. We present the scheme by which we separate the tritiugenic part of 3 He and the part due to release from the sea floor (terrigenic part). We show that the tritiugenic component can be quantified throughout the Mediterranean waters, typically to a ±0.15 TU equivalent, mostly because the terrigenic part is low in 3 He. This fact makes the Mediterranean unique in offering a potential for the use of tritiugenic 3 He as a tracer. The transient distributions of the two tracers are illustrated by a number of sections spanning the entire sea and relevant features of their distributions are noted. By 2011, the 3 He concentrations in the top few hundred meters had become low, in response to the decreasing tritium concentrations combined with a flushing out by the general westward drift of these waters. Tritium- 3 He ages in Levantine Intermediate Water (LIW) were obtained repeated in time at different locations, defining transit times from the LIW source region east of Rhodes. The ages show an upward trend with the time elapsed since the surface-water tritium maximum, which arises because the repeated observations represent increasingly slower moving parts of the full transit time spectrum of LIW. The transit time dispersion found by this new application of tritium- 3 He dating is considerable. We find mean transit times of 12 ± 2 yr up to the Strait of Sicily, 18 ± 3 yr up to the Tyrrhenian Sea, and 22 ± 4 yr up into the Western Mediterranean. We furthermore present full Eastern Mediterranean sections of terrigenic 3 He and tritium- 3 He age in 1987, the latter one similarly showing an effect of the transit time dispersion. We conclude that the available tritium and 3 He data, in particular if combined with other tracer data, are useful for constraining the subsurface circulation and mixing of the Mediterranean Sea.

Description: Holocene vegetation and climate changes in central Mediterranean inferred from a high-resolution marine pollen record (Adriatic Sea) Climate of the Past Discussions, 9, 1969-2014, 2013 Author(s): N. Combourieu-Nebout, O. Peyron, V. Bout-Roumazeilles, S. Goring, I. Dormoy, S. Joannin, L. Sadori, G. Siani, and M. Magny To understand the effects of future climate change on the ecology of the central Mediterranean we can look to the impacts of long-term, millennial to centennial-scale climatic variability on vegetation in the basin. Pollen data from the Adriatic Marine core MD 90-917 allows us to reconstruct vegetation and regional climate changes over the south central Mediterranean during the Holocene. Clay mineral ratios from the same core reflect the relative contributions of riverine (illite and smectite) and eolian (kaolinite) contributions to the site, and thus act as an additional proxy with which to test precipitation changes in the Holocene. Vegetation reconstruction shows vegetation responses to the late-Glacial Preboreal oscillation, most likely driven by changes in seasonal precipitation. Pollen-inferred temperature declines during the early-mid Holocene, but increases during the mid-late Holocene, similar to southern-western Mediterranean climatic patterns during the Holocene. Several short climatic events appear in the record, indicating the sensitivity of vegetation in the region to millennial-scale variability. Reconstructed summer precipitation shows a regional maximum between 8000 and 7000 cal yr BP similar to the general pattern across southern Europe. Two important shifts in vegetation occur at 7700 and between 7500 and 7000 yr. These vegetation shifts are linked to changes in seasonal precipitation and are correlated to increased river inputs respectively from the north (7700 event) and from the central Adriatic borderlands (7500–7000 event). These results reinforce the strengths of multi-proxy analysis and provide a deeper understanding of the role of precipitation and particularly the seasonality of precipitation in mediating vegetation change in the central Mediterranean during the Holocene.

Description: Southern Hemisphere orbital forcing and its effects on CO 2 and tropical Pacific climate Climate of the Past Discussions, 9, 1869-1900, 2013 Author(s): K. Tachikawa, A. Timmermann, L. Vidal, C. Sonzogni, and O. E. Timm The western Pacific warm pool (WPWP) is an important heat source for the atmospheric circulation and influences climate conditions worldwide. Understanding its sensitivity to past radiative perturbations may help better contextualize the magnitudes and patterns of current and projected tropical climate change. Here we present a new Mg/Ca-based sea surface temperature (SST) reconstruction over the past 400 kyr from the Bismarck Sea, off Papua New Guinea, along with results from a transient earth system model simulation. Our results document the primary influence of CO 2 forcing on glacial/interglacial WPWP SSTs and secondary effects due to changes in wind-driven tropical boundary currents. In addition to the SST, deep ocean temperature reconstructions from this core are linked with Southern Ocean temperature and sea-ice variations on timescales of ~ 23 kyr. It is proposed that Southern Hemisphere insolation changes serve as pacemaker for sea-ice variations in the Southern Ocean, which in turn modulate windstress curl-driven upwelling of carbon-rich waters, hence controlling atmospheric CO 2 and tropical WPWP temperatures.

Description: A regional climate palaeosimulation for Europe in the period 1500–1990 – Part 1: Model validation Climate of the Past Discussions, 9, 1803-1839, 2013 Author(s): J. J. Gómez-Navarro, J. P. Montávez, S. Wagner, and E. Zorita We present and analyse a high-resolution regional climate palaeosimulation encompassing the European region for the period 1500–1990. We use the regional model MM5 coupled to the global model ECHO-G. Both models were driven by reconstructions of three external factors: greenhouse gas concentrations, Total Solar Irradiance and volcanic activity. The simulation has been assessed in a recent period by comparing the model results with the Climate Research Unit (CRU) database. The results show that although the regional model is tightly driven by the boundary conditions, it is able to improve the reliability of the simulations, narrowing the differences to the observations, especially in areas of complex topography. Additionally, the evolution of the spatial distributions of temperature and precipitation through the last five centuries has been analysed. The mean values of temperature reflects the influence of the external forcings but, contrary to the results obtained under climate change scenario conditions, we found that higher-order momenta of the probability distribution of seasonal temperature and precipitation are hardly affected by changes in the external forcings

Description: North–south palaeohydrological contrasts in the central Mediterranean during the Holocene: tentative synthesis and working hypotheses Climate of the Past Discussions, 9, 1901-1967, 2013 Author(s): M. Magny, N. Combourieu Nebout, J. L. de Beaulieu, V. Bout-Roumazeilles, D. Colombaroli, S. Desprat, A. Francke, S. Joannin, O. Peyron, M. Revel, L. Sadori, G. Siani, M. A. Sicre, S. Samartin, A. Simonneau, W. Tinner, B. Vannière, B. Wagner, G. Zanchetta, F. Anselmetti, E. Brugiapaglia, E. Chapron, M. Debret, M. Desmet, J. Didier, L. Essallami, D. Galop, A. Gilli, J. N. Haas, N. Kallel, L. Millet, A. Stock, J. L. Turon, and S. Wirth On the basis of a multi-proxy approach and a strategy combining lacustrine and marine records along a north–south transect, data collected in the Central Mediterranean within the framework of a collaborative project have led to reconstruction of high-resolution and well-dated palaeohydrological records and to assessment of their spatial and temporal coherency. Contrasting patterns of palaeohydrological changes have been evidenced in the Central Mediterranean: south (north) of around 40° N of latitude, the middle part of the Holocene was characterised by lake-level maxima (minima), during an interval dated to ca. 10 300–4500 cal BP to the south and 9000–4500 cal BP to the north. Available data suggest that these contrasting palaeohydrological patterns operated throughout the Holocene, both on millennial and centennial scales. Regarding precipitation seasonality, maximum humidity in the Central Mediterranean during the middle part of the Holocene was characterised by humid winters and dry summers north of ca. 40° N, and humid winters and summers south of ca. 40° N. This may explain an apparent conflict between palaeoclimatic records depending on the proxies used for reconstruction as well as the synchronous expansion of tree species taxa with contrasting climatic requirements. In addition, south of ca. 40° N, the first millennium of the Holocene was characterised by very dry climatic conditions not only in the Eastern, but also in the Central and the Western Mediterranean zones as reflected by low lake levels and delayed reforestation. These results suggest that, in addition to the influence of the Nile discharge reinforced by the African monsoon, the deposition of Sapropel 1 has been favoured (1) by an increase in winter precipitation in the northern Mediterranean borderlands, and (2) by an increase in winter and summer precipitation in the southern Mediterranean area. The climate reversal following the Holocene climate optimum appears to have been punctuated by two major climate changes around 7500 and 4500 cal BP. In the Central Mediterranean, the Holocene palaeohydrological changes developed in response to a combination of orbital, ice-sheet and solar forcing factors. The maximum humidity interval in the south-central Mediterranean started at ca. 10 300 cal BP, in correlation with the decline (1) of the possible blocking effects of the North Atlantic anticyclone linked to maximum insolation, and/or (2) of the influence of the remnant ice sheets and fresh water forcing in the North Atlantic Ocean. In the north-central Mediterranean, the lake-level minimum interval began only around 9000 cal BP when the Fennoscandian ice-sheet disappeared and a prevailing positive NAO-type circulation developed in the North Atlantic area. The major palaeohydrological oscillation around 4500–4000 cal BP may be a non-linear response to the gradual decrease, with additional key seasonal and interhemispherical changes, in insolation. On a centennial scale, the successive climatic events which punctuated the entire Holocene in the central Mediterranean coincided with cooling events associated with deglacial outbursts in the North Atlantic area and decreases in solar activity during the interval 11 700–7000 cal BP, and to a possible combination of NAO-type circulation and solar forcing since ca. 7000 cal BP onwards. Thus, regarding the centennial-scale climatic oscillations, the Mediterranean Basin appears to have been strongly linked to the North Atlantic area and affected by solar activity over the entire Holocene. In addition to model experiments, a better understanding of forcing factors and past atmospheric circulation patterns behind the Holocene palaeohydrological changes in the Mediterranean area will require further investigation to establish additional high-resolution and well-dated records in selected locations around the Mediterranean Basin and in adjacent regions. Special attention should be paid to greater precision in the reconstruction, on millennial and centennial time scales, of changes in the latitudinal location of the limit between the northern and southern palaeohydrological Mediterranean sectors, depending on (1) the intensity and/or characteristics of climatic periods/oscillations (e.g. Holocene thermal maximum versus Neoglacial, as well as, for instance, the 8.2 ka event versus the 4 ka event or the Little Ice Age), and (2) on varying geographical conditions from the western to the eastern Mediterranean areas (longitudinal gradients).

Description: The last 7 millennia of vegetation and climate changes at Lago di Pergusa (central Sicily, Italy) Climate of the Past Discussions, 9, 2059-2094, 2013 Author(s): L. Sadori, E. Ortu, O. Peyron, G. Zanchetta, B. Vannière, M. Desmet, and M. Magny The aim of this study is to investigate climate changes and human activities under the lens of palynology. Based on a new high-resolution pollen sequence (PG2) from Lago di Pergusa (667 m a.s.l., central Sicily, Italy) covering the last 6700 yr, we propose a reconstruction of climate and landscape changes over the recent past in central Sicily. Compared to former studies from Lago di Pergusa (Sadori and Narcisi, 2001), this work provides a reconstruction of the evolution of vegetation and climate over the last millennia in central Sicily, indeed completing previous results with new data which is particularly detailed on the last 3000 yr. Joint actions of increasing dryness, climate oscillations, and human impact shaped the landscape of this privileged site. Lago di Pergusa, in fact, besides being the main inland lake of Sicily, is very sensitive to climate change and its territory was inhabited and exploited continuously since the prehistory. The lake sediments turned out to be a good observatory for the natural phenomena occurred in the last thousands of years. Results of the pollen-based study are integrated with changes in magnetic susceptibility and a tephra layer characterization. The tephra layer was shown to be related to the Sicanians' event, radiocarbon dated at 3055 ± 75 yr BP (Sadori and Narcisi, 2001). We performed palaeoclimate reconstructions by MAT and WA-PLS. Palaeoclimate reconstructions based on the core show important climate fluctuations throughout the Holocene. Climate reconstruction points out four phases of cooling and enhanced wetness in the last three millennia (2600–2000, 1650–1100, 850–550, 400–200 cal BP). This appears to be the evidence of local responses to global climate oscillations during the recent past.

Description: Dust and associated trace element fluxes in a firn core from the coastal East Antarctica and its linkages with the Southern Hemisphere climate variability over the last ~ 50 yr Climate of the Past Discussions, 9, 1841-1867, 2013 Author(s): C. M. Laluraj, M. Thamban, and K. Satheesan High-resolution records of dust and trace element fluxes were studied in a firn core from the coastal Dronning Maud Land (cDML) in East Antarctica to identify the influence of climate variability on accumulation of these components over the past ~ 50 yr. A doubling of dust deposition was observed since 1985, coinciding with a shift in the Southern Annular Mode (SAM) index to positive values and associated increase in the wind speed. Back-trajectories showed that an increase in dust deposition is associated with the air parcels originating from north-west of the site, possibly indicating its origin from the Patagonian region. Our results suggest that while multiple processes could have influenced the increased dust formation, shift in SAM had a dominant influence on its transport. It is observed that since the 1985s the strength of easterlies increased significantly over the cDML region, which could sink air and dust material to the region that were brought by the westerlies through mass compensation. The correlation between the dust flux and δ 18 O records further suggest that enhanced dust flux in the firn core occurred during periods of colder atmospheric temperature, which reduced the moisture content and increased dust fall. Interestingly, the timing and amplitude of the insoluble dust peaks matched remarkably well with the fluxes of Ba, Cr, Cu, and Zn confirming that dust was the main carrier/source of atmospheric trace elements to East Antarctica during the recent past.

Description: An inter-laboratory investigation of the Arctic sea ice biomarker proxy IP 25 in marine sediments: key outcomes and recommendations Climate of the Past Discussions, 9, 5263-5298, 2013 Author(s): S. T. Belt, T. A. Brown, L. Ampel, P. Cabedo-Sanz, K. Fahl, J. J. Kocis, G. Massé, A. Navarro-Rodriguez, J. Ruan, and Y. Xu We describe the results of an inter-laboratory investigation into the identification and quantification of the Arctic sea ice biomarker proxy IP 25 in marine sediments. 7 laboratories took part in the study, which consisted of the analysis of IP 25 in a series of sediment samples from different regions of the Arctic, sub-Arctic and Antarctic, additional sediment extracts and purified standards. The results obtained allowed 4 key outcomes to be determined. First, IP 25 was identified by all laboratories in sediments from the Canadian Arctic with inter-laboratory variation in IP 25 concentration being substantially larger than within individual laboratories. This greater variation between laboratories was attributed to the difficulty in accurately determining instrumental response factors for IP 25 , despite provision of appropriate standards. Second, the identification of IP 25 by 3 laboratories in sediment from SW Iceland that was believed to represent a blank, was interpreted as representing a better limit of detection or quantification for such laboratories, contamination or mis-identification. These alternatives could not be distinguished conclusively with the data available, although it is noted that the precision of these data was significantly poorer compared with the other IP 25 concentration measurements. Third, 3 laboratories reported the occurrence of IP 25 in a sediment sample from the Antarctic Peninsula even though this biomarker is believed to be absent from the Southern Ocean. This anomaly is attributed to a combined chromatographic and mass spectrometric interference that results from the presence of a di-unsaturated highly branched isoprenoid (HBI) pseudo-homologue of IP 25 that occurs in Antarctic sediments. Finally, data are presented that suggest that extraction of IP 25 is consistent between Automated Solvent Extraction (ASE) and sonication methods and that IP 25 concentrations based on 7-hexylnonadecane as an internal standard are comparable using these methods. Recoveries of some more unsaturated HBIs and the internal standard 9-octylheptadecene, however, were lower with the ASE procedure, possibly due to partial degradation of these more reactive chemicals as a result of higher temperatures employed with this method. For future measurements, we recommend the use of reference sediment material with known concentration(s) of IP 25 for determining and routinely monitoring instrumental response factors. Given the significance placed on the presence (or otherwise) of IP 25 in marine sediments, some further recommendations pertaining to quality control are made that should also enable the two main anomalies identified here to be addressed.

Description: Expressions of climate perturbations in western Ugandan crater lake sediment records during the last 1000 yr Climate of the Past Discussions, 9, 5183-5226, 2013 Author(s): K. Mills, D. B. Ryves, N. J. Anderson, C. L. Bryant, and J. J. Tyler Equatorial East Africa has a complex, regional patchwork of climate regimes, with multiple interacting drivers. Recent studies have focussed on large lakes and reveal signals that are smoothed in both space and time, and, whilst useful at a continental scale, are of less relevance when understanding short-term, abrupt or immediate impacts of climate and environmental changes. Smaller-scale studies have highlighted spatial complexity and regional heterogeneity of tropical palaeoenvironments in terms of responses to climatic forcing (e.g. the Little Ice Age [LIA]) and questions remain over the spatial extent and synchroneity of climatic changes seen in East African records. Sediment cores from paired crater lakes in western Uganda were examined to assess ecosystem response to long-term climate and environmental change as well as testing responses to multiple drivers using redundancy analysis. These archives provide annual to sub-decadal records of environmental change. The records from the two lakes demonstrate an individualistic response to external (e.g. climatic) drivers, however, some of the broader patterns observed across East Africa suggest that the lakes are indeed sensitive to climatic perturbations such as a dry Mediaeval Climate Anomaly (MCA; 1000–1200 AD) and a relatively drier climate during the main phase of the LIA (1500–1800 AD); though lake levels in western Uganda do fluctuate. The relationship of Ugandan lakes to regional climate drivers breaks down c. 1800 AD, when major changes in the ecosystems appear to be a response to sediment and nutrient influxes as a result of increasing cultural impacts within the lake catchments. The data highlight the complexity of individual lake response to climate forcing, indicating shifting drivers through time. This research also highlights the importance of using multi-lake studies within a landscape to allow for rigorous testing of climate reconstructions, forcing and ecosystem response.

Description: The effects of climate change on persistent organic pollutants (POPs) in the North Sea Ocean Science Discussions, 10, 1525-1557, 2013 Author(s): K. O'Driscoll, B. Mayer, J. Su, and M. Mathis The fate and cycling of two selected legacy persistent organic pollutants (POPs), PCB 153 and γ-HCH, in the North Sea in the 21st century have been modelled with combined hydrodynamic and fate and transport ocean models. To investigate the impact of climate variability on POPs in the North Sea in the 21st century, future scenario model runs for three 10 yr periods to the year 2100 using plausible levels of both in situ concentrations and atmospheric, river and open boundary inputs are performed. Since estimates of future concentration levels of POPs in the atmosphere, oceans and rivers are not available, our approach was to reutilise 2005 values in the atmosphere, rivers and at the open ocean boundaries for every year of the simulations. In this way, we attribute differences between the three 10 yr simulations to climate change only. For the HAMSOM and atmospheric forcing, results of the IPCC A1B (SRES) 21st century scenario are utilised, where surface forcing is provided by the REMO downscaling of the ECHAM5 global atmospheric model, and open boundary conditions are provided by the MPIOM global ocean model. Dry gas deposition and volatilisation of γ-HCH increase in the future relative to the present. In the water column, total mass of γ-HCH and PCB 153 remain fairly steady in all three runs. In sediment, γ-HCH increases in the future runs, relative to the present, while PCB 153 in sediment decreases exponentially in all three runs, but even faster in the future, both of which are the result of climate change. Annual net sinks exceed sources at the ends of all periods.

Description: Observations of phytoplankton spring bloom onset triggered by a density front in NW Mediterranean Ocean Science Discussions, 10, 1559-1580, 2013 Author(s): A. Olita, S. Sparnocchia, S. Cusí, L. Fazioli, R. Sorgente, J. Tintoré, and A. Ribotti Phytoplankton bloom in NW Mediterranan sea is a seasonal event that mainly occurrs in a limited area (Gulf of Lyon and Provençal basin) where this phenomenon is promoted by a cyclonic circulation, strong wind-driven mixing and subsequent spring restratification. At the southern boundary of this area a density front (North Balearic Front) separating denser waters from the lighter Modified Atlantic Waters reservoir at south is suspected to trigger weaker and earlier (late winter) blooms by (a) enhanced pumping of nutrients into the euphotic layer and (b) promoting an early restratification of the water column (by frontal instabilities). A multisensor glider round trip, equipped with CTD and fluorimeter, crossing the frontal area in February–March 2013, allowed to observe the bloom triggering after the decrease of intense wind-driven turbulent convection and mixing. Satellite imagery supports and confirms in-situ observations. It was shown that frontal activity has a relevant role in the promotion and acceleration of the dynamical restratification, with a consequent biological response in terms of primary production. Restratification is necessary preconditioning factor for bloom triggering in frontal area, net of other involved mechanism promoting the bloom as the enhanced biological pump. So, like for high-latitude fronts (Taylor and Ferrari, 2011a), also for this mid-latitude oligotrophic region front seems to promote new production by dynamically enahnced restratification inhibiting mixing. Finally, we argued that Sverdrup's Critical Depth criterion seems to apply in the northern well-mixed area, where the zeroing of heat fluxes (and related turbulent convection) does not correspond to a prompt onset of the bloom (which appeared 1 month later).

Description: Observed decline of the Atlantic Meridional Overturning Circulation 2004 to 2012 Ocean Science Discussions, 10, 1619-1645, 2013 Author(s): D. A. Smeed, G. McCarthy, S. A. Cunningham, E. Frajka-Williams, D. Rayner, W. E. Johns, C. S. Meinen, M. O. Baringer, B. I. Moat, A. Duchez, and H. L. Bryden The Atlantic Meridional Overturning Circulation (AMOC) has been observed continuously at 26° N since April 2004. The AMOC and its component parts are monitored by combining a transatlantic array of moored instruments with submarine-cable based measurements of the Gulf Stream and satellite derived Ekman transport. The time series has recently been extended to October 2012 and the results show a downward trend since 2004. From April~2008 to March 2012 the AMOC was an average of 2.7 Sv weaker than in the first four years of observation (95% confidence that the reduction is 0.3 Sv or more). Ekman transport reduced by about 0.2 Sv and the Gulf Stream by 0.5 Sv but most of the change (2.0 Sv) is due to the mid-ocean geostrophic flow. The change of the mid-ocean geostrophic flow represents a strengthening of the subtropical gyre above the thermocline. The increased southward flow of warm waters is balanced by a decrease in the southward flow of Lower North Atlantic Deep Water below 3000 m. The transport of Lower North Atlantic Deep Water slowed by 7% per year (95% confidence that the rate of slowing is greater than 2.5% per year).

Description: Evaluation of seasonal climates of the Mediterranean and nothern Africa in the CMIP5 simulations Climate of the Past Discussions, 9, 5347-5389, 2013 Author(s): A. Perez-Sanz, G. Li, P. González-Sampériz, and S. P. Harrison We analyze the spatial expression of seasonal climates of the Mediterranean and northern Africa in pre-Industrial ( piControl ) and mid-Holocene ( midHolocene , 6 ka) simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Modern observations show four distinct precipitation regimes characterized by differences in the seasonal distribution and total amount of precipitation: an equatorial band characterized by a double peak in rainfall, the monsoon zone characterized by summer rainfall, the desert characterized by low seasonality and total precipitation, and the Mediterranean zone characterized by summer drought. Most models correctly simulate the position of the Mediterranean and the equatorial climates in the piControl simulations, but over-estimate the extent of monsoon influence and underestimate the extent of desert. However, most models fail to reproduce the amount of precipitation in each zone. Model biases in the simulated magnitude of precipitation are unrelated to whether the models reproduce the correct spatial patterns of each regime. In the midHolocene , the models simulate a reduction in winter rainfall in the equatorial zone, and a northward expansion of the monsoon with a significant increase in summer and autumn rainfall. Precipitation is slightly increased in the desert, mainly in summer and autumn, with northward expansion of the monsoon. Changes in the Mediterranean are small, although there is an increase in spring precipitation consistent with palaeo-observations of increased growing-season rainfall. Comparison with reconstructions shows that most models under-estimate the mid-Holocene changes in annual precipitation, except in the equatorial zone. Biases in the piControl have only a limited influence on midHolocene anomalies in ocean-atmosphere models; carbon-cycle models show no relationship between piControl bias and midHolocene anomalies. Biases in the prediction of the midHolocene monsoon expansion are unrelated to how well the models simulate changes in Mediterranean climate.

Description: Orbital and millennial-scale environmental changes between 64 and 25 ka BP recorded in Black Sea sediments Climate of the Past Discussions, 9, 5439-5477, 2013 Author(s): L. S. Shumilovskikh, D. Fleitmann, N. R. Nowaczyk, H. Behling, F. Marret, A. Wegwerth, and H. W. Arz High-resolution pollen and dinoflagellate cyst records from marine sediment core 25-GC1 were used to reconstruct vegetation dynamics in Northern Anatolia and surface conditions of the Black Sea between 64 and 25 ka BP. During this period, the dominance of Artemisia in the pollen record indicates a steppe landscape and arid climate conditions. However, the presence of temperate and warm-temperate arboreal pollen suggests the existence of glacial refugia in Northern Anatolia. A general cooling trend towards 25 ka BP is evidenced by the decrease of Quercus and increase of Pinus . There is evidence of orbital-driven vegetation dynamics in Northern Anatolia during 64–25 ka BP with spread of steppe during precession minima (insolation maxima) and development of forests during precession maxima (insolation minima). Dansgaard–Oeschger (D–O) events are characterized by a marked increase in temperate tree pollen, indicating a spread of forests due to warm and wet conditions in Northern Anatolia. The dominance of Pyxidinopsis psilata and Spiniferites cruciformis in the dinocyst record indicates a rather brackish Black Sea during the last glacial period. The decrease of marine indicators (marine dinocysts, acritachs) at ~ 54 ka BP and increase of freshwater algae ( Pediastrum, Botryococcus ) from 32 to 25 ka BP reveals freshening of the Black Sea surface water, related to orbital-driven arid/humid phases in the region, influencing hydrology and level changes of the Black Sea. D–O interstadials are characterized by high dinocyst concentrations and calcium carbonate content, as a result of an increase in primary productivity in the Black Sea. Heinrich events show a similar impact on the environment in Northern Anatolia/Black Sea region as D–O stadials.

Description: NEMO on the shelf: assessment of the Iberia-Biscay-Ireland configuration Ocean Science Discussions, 10, 83-151, 2013 Author(s): C. Maraldi, J. Chanut, B. Levier, N. Ayoub, P. De Mey, G. Reffray, F. Lyard, S. Cailleau, M. Drévillon, E. A. Fanjul, M. G. Sotillo, and P. Marsaleix The Iberia-Biscay-Ireland (IBI) system serves one of the 7 MyOcean "Monitoring and Forecasting Centres". A high resolution simulation covering the IBI region is set-up over July 2007–February 2009. The NEMO (Nucleus for European Modelling of the Ocean) model is used with a 1/36° horizontal resolution and 50 z-levels in the vertical. New developments have been incorporated in NEMO to make it suitable to open- as well as coastal-ocean modelling. In this paper, we pursue three main objectives: (1) give an overview of the model configuration used for the simulations; (2) give a broad-brush account of one particular aspect of this work, namely consistency verification; this type of validation is conducted upstream of the implementation of the system before it is used for production and routinely validated; it is meant to guide model development in identifying gross deficiencies in the modelling of several key physical processes; (3) show that such a regional modelling system has potential as a complement to patchy observations (an integrated approach) to give information on non-observed physical quantities and to provide links between observations by identifying broader-scale patterns and processes. We concentrate on the year 2008. We first provide domain-wide consistency verification results in terms of barotropic tides, transports, sea surface temperature and stratification. We then focus on two dynamical sub-regions: the Celtic shelves and the Bay of Biscay slope and deep regions. The model-data consistency is checked for variables and processes such as tidal currents, tidal fronts, internal tides, residual elevation. We also examine the representation in the model of a seasonal pattern of the Bay of Biscay circulation: the warm extension of the Iberian Poleward Current along the northern Spanish coast (Navidad event) in winter 2007–2008.

Description: Vegetation responses to interglacial warming in the Arctic, examples from Lake El'gygytgyn, northeast Siberia Climate of the Past Discussions, 9, 245-267, 2013 Author(s): A. V. Lozhkin and P. M. Anderson Palynological data from Lake El'gygytgyn reveal responses of plant communities to a range of climatic conditions that can help assess the possible impact of global warming on arctoboreal ecosystems. Vegetation associated with climatic optima suggests two types of interglacial responses: one is dominated by deciduous taxa (the postglacial thermal maximum (PGTM) and marine isotope stage (MIS5)) and the second by evergreen conifers (MIS11, MIS31). The MIS11 forests show a similarity to Picea-Larix-Betula-Alnus forests of Siberia. While dark coniferous forest also characterizes MIS31, the pollen taxa show an affinity to the modern boreal forest of the lower Amur valley in the Russian Far East. Despite vegetation differences during the thermal maxima, all four glacial-interglacial transitions are alike, being dominated by deciduous woody taxa. Initially Betula shrub tundra established and was replaced by tundra with tree-sized shrubs (PGTM), Betula woodland (MIS5), or Betula-Larix (MIS11, MIS31) forest. The consistent occurrence of deciduous forest and/or high shrub tundra in all interglaciations as they approach or achieve maximum warmth underscores the significance of this biome for modeling efforts. The El'gygytgyn data also suggest the possible elimination or massive reduction of arctic plant communities under extreme warm-earth scenarios.

Description: The effect of precipitation seasonality on Eemian ice core isotope records from Greenland Climate of the Past Discussions, 9, 269-296, 2013 Author(s): W. J. van de Berg, M. R. van den Broeke, E. van Meijgaard, and F. Kaspar The previous interglacial (Eemian, 130–114 kyr BP) had a mean sea level highstand 4 to 7 m above the current level, and, according to climate proxies, a 2 to 6 K warmer Arctic summer climate. Greenland ice cores extending back into the Eemian show a reduced depletion in δ 18 O of about 3‰ for this period, which suggests a significant warming of several degrees over the Greenland ice sheet. Since the depletion in δ 18 O depends, among other factors, on the condensation temperature of the precipitation, we analyze climatological processes other than mean temperature changes that influence condensation temperature, using output of the regional climate model RACMO2. This model is driven by ERA-40 reanalysis and ECHO-G GCM boundaries for present-day, preindustrial and Eemian climate. The processes that affect the condensation temperature of the precipitation are analyzed using 6-hourly model output. Our results show that changes in precipitation seasonality can cause significant changes of up to 2 K in the condensation temperature that are unrelated to changes in mean temperature.

Description: Why could ice ages be unpredictable? Climate of the Past Discussions, 9, 1053-1098, 2013 Author(s): M. Crucifix It is commonly accepted that the variations of Earth's orbit and obliquity control the timing of Pleistocene glacial-interglacial cycles. Evidence comes from power spectrum analysis of palaeoclimate records and from inspection of the timing of glacial and deglacial transitions. However, we do not know how tight this control is. Is it, for example, conceivable that random climatic fluctuations could cause a delay in deglaciation, bad enough to skip a full precession or obliquity cycle and subsequently modify the sequence of ice ages? To address this question, seven previously published conceptual models of ice ages are analysed by reference to the notion of generalised synchronisation. Insight is being gained by comparing the effects of the astronomical forcing with idealised forcings composed of only one or two periodic components. In general, the richness of the astronomical forcing allows for synchronisation over a wider range of parameters, compared to periodic forcing. Hence, glacial cycles may conceivably have remained paced by the astronomical forcing throughout the Pleistocene. However, all the models examined here also show a range of parameters for which the structural stability of the ice age dynamics is weak. This means that small variations in parameters or random fluctuations may cause significant shifts in the succession of ice ages if the system were effectively in that parameter range. Whether or not the system has strong structural stability depends on the amplitude of the effects associated with the astronomical forcing, which significantly differ across the different models studied here. The possibility of synchronisation on eccentricity is also discussed and it is shown that a high Rayleigh number on eccentricity, as recently found in observations, is no guarantee of reliable synchronisation.

Description: Impact of the Indonesian throughflow on Agulhas leakage Ocean Science Discussions, 10, 353-391, 2013 Author(s): D. Le Bars, H. A. Dijkstra, and W. P. M. De Ruijter Using ocean models of different complexity we show that opening the Indonesian Passage between the Pacific and the Indian Ocean increases the input of Indian Ocean water into the South Atlantic via the Agulhas leakage. In a strongly eddying global ocean model this response results from an increased Agulhas Current transport and a constant proportion of Agulhas retroflection south of Africa. The leakage increases through an increased frequency of ring shedding events. In an idealized two-layer and flat-bottom eddy resolving model, the proportion of the Agulhas Current transport that retroflects is (for a wide range of wind stress forcing) not affected by an opening of the Indonesian Passage. A linear ocean model is not able to explain this behavior which reveals the importance of mixed barotropic/baroclinic instabilities in controlling the Agulhas leakage.

Description: On the shelf resonances of the English Channel and Irish Sea Ocean Science Discussions, 10, 393-433, 2013 Author(s): D. J. Webb The resonances of the English Channel and Irish Sea are investigated using the methods of Webb (2012) together with an Arakawa C-grid model of the region under study. In the semi-diurnal tidal band, the high tides of the Bristol Channel and Gulf of St. Malo are shown to be due to two shelf resonances which strongly couple the two regions. In the diurnal band, the response is complicated by the presence of continental shelf waves.

Description: East Asian monsoon climate simulated in the PlioMIP Climate of the Past Discussions, 9, 1135-1164, 2013 Author(s): R. Zhang, Q. Yan, Z. S. Zhang, D. Jiang, B. L. Otto-Bliesner, A. M. Haywood, D. J. Hill, A. M. Dolan, C. Stepanek, G. Lohmann, C. Contoux, F. Bragg, W.-L. Chan, M. A. Chandler, A. Jost, Y. Kamae, A. Abe-Ouchi, G. Ramstein, N. A. Rosenbloom, L. Sohl, and H. Ueda Based on the simulations with fifteen climate models in the Pliocene Model Intercomparison Project (PlioMIP), the regional climate of East Asia (focusing on China) during the mid-Pliocene is investigated in this study. Compared to the pre-industrial, the multi-model ensemble mean (MMM) of all models shows the East Asian summer wind (EASW) largely strengthens in monsoon China, and the East Asian winter wind (EAWW) strengthens in south monsoon China but slightly weakens in north monsoon China in mid-Pliocene. The MMM of all models also illustrates a warmer and wetter mid-Pliocene climate in China. The simulated weakened mid-Pliocene EAWW in north monsoon China and intensified EASW in monsoon China agree well with geological reconstructions. However, the model-model discrepancy in simulating mid-Pliocene East Asian monsoon climate, in particular EAWW, should be further addressed in the future work of PlioMIP.

Description: A reconstruction of radiocarbon production and total solar irradiance from the Holocene 14 C and CO 2 records: implications of data and model uncertainties Climate of the Past Discussions, 9, 1165-1235, 2013 Author(s): R. Roth and F. Joos Past atmospheric CO 2 concentrations reconstructed from polar ice cores combined with its Δ 14 C signature as conserved in tree-rings provide important information both on the cycling of carbon as well as the production of radiocarbon ( Q ) in the atmosphere. The latter is modulated by changes in the strength of the magnetic field enclosed in the solar wind and is a proxy for past changes in solar activity. We perform transient carbon-cycle simulations spanning the past 21 kyr using Bern3D-LPX, a fully featured Earth System Model of Intermediate Complexity (EMIC) with a 3-D ocean, sediment and a dynamic vegetation model. Using the latest atmospheric IntCal09/SHCal04 radiocarbon records, we reconstruct the Holocene radiocarbon fluxes and the total production rate. Our carbon-cycle based modern estimate of Q ≈ 1.7 atoms cm −2 s −1 is lower than previously reported by Masarik and Beer (2009) and more in line with Kovaltsov et al. (2012). Q is then translated into the solar modulation potential (Φ) using the latest geomagnetic field reconstruction and linked to a recent reanalysis of early instrumental data. In contrast to earlier reconstructions, our record suggests that periods of high solar activity (〉600 MeV) were quite common not only in recent millennia but throughout the Holocene. Solar activity in our decadally-smoothed record is during 28% of the time higher than the modern average of 650 MeV during the past 9 ka. But due to considerable uncertainties in the normalization of Φ to instrumental data, the absolute value of Φ remains weakly constrained. Further, our simulations with a spatially resolved model (taking the interhemispheric Δ 14 C gradient into account) show that reconstructions that rely on the Northern Hemisphere 14 C record only are biased towards low values during the Holocene. Notable deviations on decadal-to-centennial time scales are also found in comparison with earlier reconstructions. In a last step, past total solar irradiance (TSI) is quantified using a recently published Φ-TSI relationship yielding small changes in Holocene TSI of order 1 W m −2 with a Maunder Minimum irradiance reduction of 0.85 ± 0.17 W m −2 . Future extension of TSI using autoregressive modeling suggest a declining solar activity in the next decades towards average Holocene conditions. Past TSI changes are finally translated into changes in surfaces atmospheric temperature (SAT) by forcing the Bern3D-LPX model with our new TSI record, yielding SAT anomalies of less than 0.1 K.

Description: Northward advection of Atlantic water in the eastern Nordic Seas over the last 3000 yr: a coccolith investigation of volume transport and surface water changes Climate of the Past Discussions, 9, 1259-1295, 2013 Author(s): C. V. Dylmer, J. Giraudeau, F. Eynaud, K. Husum, and A. De Vernal Three marine sediment cores distributed along the Norwegian (MD95-2011), Barents Sea (JM09-KA11-GC), and Svalbard (HH11-134-BC) continental margins have been investigated in order to reconstruct changes in the poleward flow of Atlantic Waters (AW) and in the nature of upper surface water masses within the eastern Nordic Seas over the last 3000 yr. These reconstructions are based on a limited set of coccolith proxies: the abundance ratio between Emiliania huxleyi and Coccolithus pelagicus , an index of Atlantic vs. Polar-Arctic surface water masses; and Gephyrocapsa muellerae , a drifted coccolith species from the temperate North Atlantic, whose abundance changes are related to variations in the volume transport of the North Atlantic Current and its northernmost extension (the West Spitsbergen Current – WSC) off western Svalbard. The entire investigated area, from 66 to 77° N, was affected by an overall increase in volume flow of AW from 3000 cal yr BP to Present. The long-term modulation of westerlies strength and location which are essentially driven by the dominant mode of the North Atlantic Oscillation (NAO), is thought to explain the observed dynamics of poleward AW flow. The same mechanism also reconciles the recorded opposite zonal shifts in the location of the Arctic Front between the area off western Norway and the Barents Sea-eastern Fram Strait region. The Little Ice Age was governed by deteriorating conditions, with Arctic/Polar waters dominating in the surface off western Svalbard and western Barents Sea, possibly associated with both severe sea-ice conditions and a strongly reduced AW volume flow. A sudden short pulse of resumed high WSC flow interrupted this cold spell in eastern Fram Strait from 330 to 410 cal yr BP, with a a magnitude only surpassed by the one which characterizes the Modern Period. Our dataset not only confirms the high amplitude warming of surface waters at the turn of the 19th century off western Svalbard, it also shows that such a warming was primarily induced by an excess volume flow of AW which stands as unprecedented over the last 3000 yr.

Description: On the Milankovitch sensitivity of the Quaternary deep-sea record Climate of the Past Discussions, 9, 1237-1257, 2013 Author(s): W. H. Berger The response of the climate system to external forcing has become an item of prime interest in the context of global warming, especially with respect to the rate of melting land-based ice masses. The deep-sea record of ice-age climate change has been useful in assessing the sensitivity of the climate system to such forcing, notably to orbital forcing, which is well-known for the last several million years. When comparing response and forcing, one finds that sensitivity varies greatly through time, apparently in dependence on the state of the system. The changing stability of ice masses presumably is the underlying cause for the changing state of the system. A buildup of vulnerable ice masses within the latest Tertiary, when going into the ice ages, is conjectured to cause a stepwise increase of climate variability since the early Pliocene.

Description: Simulation of the Indian monsoon and its variability during the last millennium Climate of the Past Discussions, 9, 703-740, 2013 Author(s): S. Polanski, B. Fallah, S. Prasad, and U. Cubasch The general circulation model ECHAM5 has been used to simulate the Indian monsoon and its variability during the Medieval Warm Period (MWP; 900–1100 AD), the Little Ice Age (LIA; 1515–1715 AD) and for recent climate (REC; 1800–2000 AD). The focus is on the analysis of external drivers and internal feedbacks leading to extreme rainfall events over India from interannual to multidecadal time scale. An evaluation of spatiotemporal monsoon patterns with present-day observation data is in agreement with other state-of-the-art monsoon modeling studies. The simulated monsoon intensity on multidecadal time scale is weakened (enhanced) in summer (winter) due to colder (warmer) SSTs in the Indian Ocean. Variations in solar insolation are the main drivers for these SST anomalies, verified by very strong temporal anticorrelations between Total Solar Irradiance and All-India-Monsoon-Rainfall in summer monsoon months. The external solar forcing is coupled and overlain by internal climate modes of the ocean (ENSO and IOD) with asynchronous intensities and lengths of periods. In addition, the model simulations have been compared with a relative moisture index derived from paleoclimatic reconstructions based on various proxies and archives in India. In this context, the Lonar record in Central India has been highlighted and evaluated the first time. The simulated relative annual rainfall anomalies in comparison to present-day climate are in agreement (disagreement) with the reconstructed moisture index for MWP (LIA) climate. In order to investigate the interannual monsoon variability with respect to monsoon failures, dry summer monsoon composites for 30-yr-long periods of MWP, LIA and REC have been further analysed. Within dry years of LIA, the summer rainfall over India and surrounding oceans is less than in MWP indicating stronger drying conditions due to a stronger summer solar insolation forcing coupled with variations in ENSO. To quantify the ECHAM5 simulated long-term drought conditions within Monsoon Asia, the Palmer Drought Severity Index has been additionally estimated for recent climate showing strong pattern correlation between global SST anomalies and EOF variability signal of the drought index, whereas the temporal relationship is weak.

Description: Peak glacial 14 C ventilation ages suggest major draw-down of carbon into the abyssal ocean Climate of the Past Discussions, 9, 925-965, 2013 Author(s): M. Sarnthein, B. Schneider, and P. M. Grootes Ice core records demonstrate a glacial-interglacial atmospheric CO 2 increase of ~ 100 ppm. A transfer of ~ 530 Gt C is required to produce the deglacial rise of carbon in the atmosphere and terrestrial biosphere. This amount is usually ascribed to oceanic carbon release, although the actual mechanisms remained elusive, since an adequately old and carbon-enriched deep-ocean reservoir seemed unlikely. Here we present a new, though still fragmentary, ocean-wide 14 C dataset showing that during the Last Glacial Maximum (LGM) and Heinrich Stadial 1 (HS-1) the 14 C age difference between ocean deep waters and the atmosphere exceeded the modern values by up to 1500 14 C yr, in the extreme reaching 5100 yr. Below 2000 m depth the 14 C ventilation age of modern ocean waters is directly linked to the concentration of dissolved inorganic carbon (DIC). We assume that the range of regression slopes of DIC vs. Δ 14 C remained constant for LGM times, which implies that an average LGM aging by ~ 600 14 C yr corresponded to a global rise by ~ 85–115 μmol DIC kg −1 in the deep ocean. Thus, the prolonged residence time of ocean deep waters indeed made it possible to absorb an additional ~ 730–980 Gt DIC, ~ 1/3 of which transferred from intermediate waters. We infer that LGM deep-water O 2 dropped to suboxic values of 〈 10 μmol kg −1 in the Atlantic sector of the Southern ocean, possibly also in the subpolar North Pacific. The transfer of aged deep-ocean carbon to the atmosphere and the ocean-atmosphere exchange are sufficient to account for the 190-‰ drop in atmospheric 14 C during the so-called HS-1 "Mystery Interval".

Description: Volcanic forcing for climate modeling: a new microphysics-based dataset covering years 1600–present Climate of the Past Discussions, 9, 967-1012, 2013 Author(s): F. Arfeuille, D. Weisenstein, H. Mack, E. Rozanov, T. Peter, and S. Brönnimann As the understanding and representation of the impacts of volcanic eruptions on climate have improved in the last decades, uncertainties in the stratospheric aerosol forcing from large eruptions are now not only linked to visible optical depth estimates on a global scale but also to details on the size, latitude and altitude distributions of the stratospheric aerosols. Based on our understanding of these uncertainties, we propose a new model-based approach to generating a volcanic forcing for General-Circulation-Model (GCM) and Chemistry-Climate-Model (CCM) simulations. This new volcanic forcing, covering the 1600–present period, uses an aerosol microphysical model to provide a realistic, physically consistent treatment of the stratospheric sulfate aerosols. Twenty-six eruptions were modeled individually using the latest available ice cores aerosol mass estimates and historical data on the latitude and date of eruptions. The evolution of aerosol spatial and size distribution after the sulfur dioxide discharge are hence characterized for each volcanic eruption. Large variations are seen in hemispheric partitioning and size distributions in relation to location/date of eruptions and injected SO 2 masses. Results for recent eruptions are in good agreement with observations. By providing accurate amplitude and spatial distributions of shortwave and longwave radiative perturbations by volcanic sulfate aerosols, we argue that this volcanic forcing may help refine the climate model responses to the large volcanic eruptions since 1600. The final dataset consists of 3-D values (with constant longitude) of spectrally resolved extinction coefficients, single scattering albedos and asymmetry factors calculated for different wavelength bands upon request. Surface area densities for heterogeneous chemistry are also provided.

Description: Inter-annual tropical Pacific climate variability in an isotope-enabled CGCM: implications for interpreting coral stable oxygen isotope records of ENSO Climate of the Past Discussions, 9, 741-773, 2013 Author(s): T. Russon, A. W. Tudhope, G. C. Hegerl, M. Collins, and J. Tindall Water isotope-enabled coupled atmosphere/ocean climate models allow for exploration of the relative contributions to coral stable oxygen isotope (δ 18 O coral ) variability arising from Sea Surface Temperature (SST) and the isotopic composition of seawater (δ 18 O sw ). The unforced behaviour of the isotope-enabled HadCM3 Coupled General Circulation Model affirms that the extent to which inter-annual δ 18 O sw variability contributes to that in model δ 18 O coral is strongly spatially dependent, ranging from being negligible in the eastern equatorial Pacific to accounting for 50% of δ 18 O coral variance in parts of the western Pacific. In these latter cases, a significant component of the inter-annual δ 18 O sw variability is correlated to that in SST, meaning that local calibrations of the effective local δ 18 O coral –SST relationships are likely to be essential. Furthermore, the relationship between δ 18 O sw and SST in the central and western equatorial Pacific is non-linear, such that the interpretation of model δ 18 O coral in the context of a linear dependence on SST alone may lead to overestimation (by up to 20%) of the SST anomalies associated with large El-Niño events. Intra-model evaluation of a salinity-based pseudo-coral approach shows that such an approach captures the first-order features of the model δ 18 O sw behaviour. However, the utility of the pseudo-corals is limited by the extent of spatial variability seen within the modelled slopes of the temporal salinity–δ 18 O sw relationship.

Description: Using paleo-climate comparisons to constrain future projections in CMIP5 Climate of the Past Discussions, 9, 775-835, 2013 Author(s): G. A. Schmidt, J. D. Annan, P. J. Bartlein, B. I. Cook, E. Guilyardi, J. C. Hargreaves, S. P. Harrison, M. Kageyama, A. N. LeGrande, B. Konecky, S. Lovejoy, M. E. Mann, V. Masson-Delmotte, C. Risi, D. Thompson, A. Timmermann, L.-B. Tremblay, and P. Yiou We present a description of the theoretical framework and "best practice" for using the paleo-climate model component of the Coupled Model Intercomparison Project (Phase 5) (CMIP5) to constrain future projections of climate using the same models. The constraints arise from measures of skill in hindcasting paleo-climate changes from the present over 3 periods: the Last Glacial Maximum (LGM) (21 thousand years before present, ka), the mid-Holocene (MH) (6 ka) and the Last Millennium (LM) (850–1850 CE). The skill measures may be used to validate robust patterns of climate change across scenarios or to distinguish between models that have differing outcomes in future scenarios. We find that the multi-model ensemble of paleo-simulations is adequate for addressing at least some of these issues. For example, selected benchmarks for the LGM and MH are correlated to the rank of future projections of precipitation/temperature or sea ice extent to indicate that models that produce the best agreement with paleoclimate information give demonstrably different future results than the rest of the models. We also find that some comparisons, for instance associated with model variability, are strongly dependent on uncertain forcing timeseries, or show time dependent behaviour, making direct inferences for the future problematic. Overall, we demonstrate that there is a strong potential for the paleo-climate simulations to help inform the future projections and urge all the modeling groups to complete this subset of the CMIP5 runs.

Description: Global and regional sea surface temperature trends during Marine Isotope Stage 11 Climate of the Past Discussions, 9, 837-890, 2013 Author(s): Y. Milker, R. Rachmayani, M. Weinkauf, M. Prange, M. Raitzsch, M. Schulz, and M. Kučera The Marine Isotope Stage (MIS) 11 (424–374 ka) was characterized by a protracted deglaciation and an unusually long climatic optimum. It remains unclear to what degree the climate development during this interglacial reflects the unusually weak orbital forcing or greenhouse gas trends. Previously, arguments about the duration and timing of the MIS11 climatic optimum and about the pace of the deglacial warming were based on a small number of key records, which appear to show regional differences. In order to obtain a global signal of climate evolution during MIS11, we compiled a database of 78 sea surface temperature (SST) records from 57 sites spanning MIS11, aligned these individually on the basis of benthic ( N = 28) or planktonic ( N = 31) stable oxygen isotope curves to a common time-frame and subjected 48 of them to an Empirical Orthogonal Function (EOF) analysis. The analysis revealed a high commonality among all records, with the principal SST trend explaining almost 49% of the variability. This trend indicates that on the global scale, the surface ocean underwent rapid deglacial warming during Termination V, in pace with carbon dioxide rise, followed by a broad SST optimum centered at ~ 410 kyr. The second EOF, which explained 19% of the variability, revealed the existence of a different SST trend, characterized by a delayed onset of the temperature optimum during MIS11 at ~ 398 kyr, followed by a prolonged warm period lasting beyond 380 kyr. This trend is most consistently manifested in the mid-latitude North Atlantic and Mediterranean Sea and is here attributed to the strength of the Atlantic meridional overturning circulation. A sensitivity analysis indicates that these results are robust to record selection and to age-model uncertainties of up to 3–6 kyr, but more sensitive to SST seasonal attribution and SST uncertainties 〉 1 °C. In order to assess the effect of orbital forcing on MIS11 SST trends, the annual and seasonal SST anomalies recorded in a total of 74 proxy records were compared with CCSM3 (Community Climate System Model, version 3) runs for three time slices representing orbital configuration extremes during the peak interglacial of MIS11. The modeled SST anomalies are characterized by a significantly lower variance compared to the reconstructions. Nevertheless, significant correlations between proxy and model data are found in comparisons on the seasonal basis, indicating that the model captures part of the long-term variability induced by astronomical forcing, which appears to have left a detectable signature in SST trends.

Description: Large spatial variations in coastal 14 C reservoir age – a case study from the Baltic Sea Climate of the Past Discussions, 9, 891-923, 2013 Author(s): B. C. Lougheed, H. L. Filipsson, and I. Snowball Coastal locations are highly influenced by input from freshwater river runoff, including sources of terrestrial carbon, which can be expected to modify the 14 C reservoir age, or R(t) , associated with marine water. In this Baltic Sea case study, pre-bomb museum collection mollusc shells of known calendar age, from 30 locations across a strategic salinity transect of the Baltic Sea, were analysed for 14 C, δ 13 C and δ 18 O. R(t) was calculated for all 30 locations. Seven locations, of which six are within close proximity of the coast, were found to have relatively higher R(t) values, indicative of hard-water effects. δ 13 C aragonite values were found to be indicative of hard-water influence only for certain locations, suggesting the possibility of different sources of old carbon in different locations. Whenever possible, the Macoma genus of mollusc was selected from the museum collections, in order to exclude species specific reservoir age effects as much as possible. When the Macoma samples are exclusively considered, and samples from hard-water locations excluded, a statistically significant correlation between Macoma R(t) and average salinity is found, indicating a two end-member linear mixing model between 14 C marine and 14 C runoff . A map of Baltic Sea Macoma aragonite R(t) for the late 19th and early 20th centuries is produced. Such a map can provide an estimate for contemporary Baltic Sea Macoma R(t) , although one must exercise caution when applying such estimates back in time or to 14 C dates obtained from different sample material. A statistically significant correlation is also found between δ 18 O aragonite and Macoma R(t) , suggesting that δ 18 O aragonite can be used to estimate Macoma palaeo- R(t) . The results of this Baltic Sea case study, which show that R(t) is affected by hydrographic conditions and local carbon inputs, have important consequences for other coastal and estuarine locations, where R(t) is also likely to significantly vary on spatial and temporal bases.

Description: Investigation of saline water intrusions into the Curonian Lagoon (Lithuania) and two-layer flow in the Klaipėda Strait using finite element hydrodynamic model Ocean Science Discussions, 10, 321-352, 2013 Author(s): P. Zemlys, C. Ferrarin, G. Umgiesser, S. Gulbinskas, and D. Bellafiore This work is focused on the application of a modelling system to simulate 3-D interaction between the Curonian Lagoon and the Baltic Sea coastal waters and to reflect spatio-temporal dynamics of marine waters in the Curonian Lagoon. The model system is based on the finite element program package SHYFEM which can be used to resolve the hydrodynamic equations in lagoons, coastal seas, estuaries and lakes. The results of a one year 3-D model simulation with real weather and hydrological forcing show that the saline water intrusions from the sea through Klaipėda Strait are gradually decreasing with distance from the sea and become negligible (average annual salinity about 0.5 ‰) at a~distance of about 20 km to the south of Kiaulės Nugara island. Analyses of the simulation results also show this area being highly heterogeneous according to the vertical salinity distribution. While in the deeper Klaipėda Strait (harbour waterway) differences in average salinity between near bottom and surface layers varies in the range 2–2.5 ‰, in the rest of the Curonian Lagoon it is less than 0.1 ‰. Analyses of the simulation results confirmed the presence of a two-directional flow that from time to time changes to either saline water one-directional flow to the Curonian Lagoon or fresh water one-directional flow to the sea. Two-directional flow duration decreases with a distance from sea entrance in Klaipėda Strait from around 180 days yr −1 close to the sea entrance to 50 days yr −1 just behind Kiaulės Nugara island. One-directional outflow duration is increasing with a distance from the sea entrance from 100 to 225 days yr −1 . One-directional inflow duration occurs in the range 85–100 days yr −1 . The analysis of the ratio of buoyancy layer thickness to water depth ( h b /H) and the Wedderburn number showed three main flow regimes in the strait, identifying the main importance of wind action in the along-strait direction. Absence of wind or cross-strait wind regimes allow the maintenance of an two-layer flow typical of estuarine dynamics.

Description: The Irish famine of 1740–1741: causes and effects Climate of the Past Discussions, 9, 1013-1052, 2013 Author(s): S. Engler, J. Luterbacher, F. Mauelshagen, and J. Werner This paper advances the current debate on causes and effects of famines. Since Sen's food entitlement decline theory emerged in the 1980's, climate and environmental factors are widely excluded in famine analysis. Studying the causation and the processes of famines as well as the adaptations to it before the 20th century will enhance modern famine theories and lead to a rethinking of the role of climate/environmental aspects in current research. In our case study, the "Famine Vulnerability Analysis Model" (FVAM) serves as an explanatory model and will open up new perspectives on famines. Special emphasis will be put on the Europe-wide crises of 1740–1741, with a focus on the famine of the "great frost" in Ireland. The interaction of demographic, political, economic and environmental aspects is characteristic in this famine.

Description: A Last Glacial Maximum world-ocean simulation at eddy-permitting resolution – Part 1: Experimental design and basic evaluation Climate of the Past Discussions, 9, 297-328, 2013 Author(s): M. Ballarotta, L. Brodeau, J. Brandefelt, P. Lundberg, and K. Döös Most state-of-the-art climate models include a coarsely resolved oceanic component, which has difficulties in capturing detailed dynamics, and therefore eddy-permitting/eddy-resolving simulations have been developed to reproduce the observed World Ocean. In this study, an eddy-permitting numerical experiment is conducted to simulate the global ocean state for a period of the Last Glacial Maximum (LGM, ~ 26 500 to 19 000 yr ago) and to investigate the improvements due to taking into account these higher spatial scales. The ocean general circulation model is forced by a 49-yr sample of LGM atmospheric fields constructed from a quasi-equilibrated climate-model simulation. The initial state and the bottom boundary condition conform to the Paleoclimate Modelling Intercomparison Project (PMIP) recommendations. Before evaluating the model efficiency in representing the paleo-proxy reconstruction of the surface state, the LGM experiment is in this first part of the investigation, compared with a present-day eddy-permitting hindcast simulation as well as with the available PMIP results. It is shown that the LGM eddy-permitting simulation is consistent with the quasi-equilibrated climate-model simulation, but large discrepancies are found with the PMIP model analyses, probably due to the different equilibration states. The strongest meridional gradients of the sea-surface temperature are located near 40° N and S, this due to particularly large North-Atlantic and Southern-Ocean sea-ice covers. These also modify the locations of the convection sites (where deep-water forms) and most of the LGM Conveyor Belt circulation consequently takes place in a thinner layer than today. Despite some discrepancies with other LGM simulations, a glacial state is captured and the eddy-permitting simulation undertaken here yielded a useful set of data for comparisons with paleo-proxy reconstructions.

Description: Petrophysical characterization of the lacustrine sediment succession drilled in Lake El'gygytgyn, Far East Russian Arctic Climate of the Past Discussions, 9, 351-391, 2013 Author(s): A. C. Gebhardt, A. Francke, J. Kück, M. Sauerbrey, F. Niessen, V. Wennrich, and M. Melles Seismic profiles of Far East Russian Lake El'gygytgyn which was formed by a meteorite impact some 3.6 million years ago show a stratified sediment succession that can be separated into Subunits Ia and Ib at approximately 167 m below lake floor (= ∼ 3.17 Ma). The former is well-stratified, while the latter is acoustically more massive. The sediments are intercalated with frequent mass movement deposits mainly in the proximal parts, while the distal part is almost free of such deposits at least in the upper part. In spring 2009, a long core drilled in the lake center within the framework of the International Continental Scientific Drilling Program (ICDP) penetrated the entire lacustrine sediment succession down to ~ 320 m below lake floor and about 200 m further into the meteorite-impact related bedrock. Downhole logging data down to 390 m below lake floor show that the bedrock and the lacustrine part of the core differ largely in their petrophysical characteristics. The contact between the bedrock and the lacustrine sediments is not abrupt, but rather transitional with a mixture of impact-altered bedrock clasts in a lacustrine matrix with varying percentages. Physical and chemical proxies measured on the cores can be used to divide the lacustrine part into five different clusters. These can be plotted in a redox-condition vs. input type diagram with total organic carbon content and magnetic susceptibility values indicating anoxic or oxic conditions and with the Si/Ti ratio representing more clastic or more biogenic input. Plotting the clusters in this diagram allows identifying clusters that represent glacial phases (Cluster I), super interglacials (Cluster II), and interglacial phases (Clusters III and IV).

Description: Vegetation and climate development on the North American Atlantic Coastal Plain from 33 to 13 million years ago (IODP Expedition 313) Climate of the Past Discussions, 9, 6551-6603, 2013 Author(s): U. Kotthoff, D. R. Greenwood, F. M. G. McCarthy, K. Müller-Navarra, and S. P. Hesselbo We have investigated the palynology of sediment cores from Sites M0027 and M0029 of IODP Expedition 313 on the New Jersey shallow shelf, east coast of North America, spanning an age range of 33 to 13 million years before present. Additionally, a pollen assemblage from the Pleistocene was examined. The palynological results were statistically analyzed and complemented with pollen-based quantitative climate reconstructions. Transport-related bias of the pollen assemblages was identified via analysis of the ratio of terrestrial to marine palynomorphs, and considered when interpreting palaeovegetation and palaeoclimate from the pollen data. Results indicate that from the early Oligocene to the middle Miocene, the hinterland vegetation of the New Jersey shelf was characterized by oak-hickory forests in the lowlands and conifer-dominated vegetation in the highlands. The Oligocene witnessed several expansions of conifer forest, probably related to cooling events. The pollen-based climate data imply an increase in annual temperatures from ~12 °C to more than 15 °C during the Oligocene. The Mi-1 cooling event at the onset of the Miocene is reflected by an expansion of conifers and an annual temperature decrease by almost 3 °C, from 15 °C to 12.5 °C around 23 million years before present. Particularly low annual temperatures are also recorded for an interval around ~20 million years before present, which probably reflects the Mi-1aa cooling event. Generally, the Miocene ecosystem and climate conditions were very similar to those of the Oligocene in the hinterland of the New Jersey shelf. Miocene grasslands, as known from other areas in the USA during that time period, are not evident for the hinterland of the New Jersey shelf. Surprisingly, the palaeovegetation data for the hinterland of the New Jersey shelf do not show extraordinary changes during the Mid-Miocene climatic optimum at ~15 million years before present, except for a minor increase in deciduous-evergreen mixed forest taxa and a decrease in swamp forest taxa. Pollen-based annual temperature reconstructions show average annual temperatures of ~14 °C during the Mid-Miocene climatic optimum. We conclude that vegetation and regional climate in the hinterland of the New Jersey shelf did not react as sensitively to Oligocene and Miocene climate changes as other regions in North America or Europe. An additional explanation for the relatively low regional temperatures reconstructed for the Mid-Miocene climatic optimum could be an uplift of the Appalachian Mountains during the Miocene. The Pleistocene pollen assemblage probably derives from the Marine Isotope Chron 7 or 5e and shows climate conditions similar to present-day.

Description: The surface thermal signature and air–sea coupling over the Agulhas rings propagating in the South Atlantic Ocean interior Ocean Science Discussions, 10, 2327-2361, 2013 Author(s): J. M. A. C. Souza, B. Chapron, and E. Autret The surface signature of the Agulhas rings propagating across the South Atlantic Ocean is observed based on 3 independent datasets: TMI/AMSR-E satellite sea surface temperature, Argo profiling floats and a merged winds product derived from scatterometer observations and reanalysis results. A persistent pattern of cold (negative) SST anomalies in the eddy core, with warm (positive) anomalies at the boundary is revealed. This pattern contrasts with the classical idea of a warm core anti-cyclone. Taking advantage of a moving reference frame corresponding to the altimetry-detected Agulhas rings, modifications of the surface winds by the ocean induced currents and SST gradients are evaluated using satellite SST and wind observations. As obtained, the averaged stationary thermal expression and mean eddy-induced circulation are coupled to the marine atmospheric boundary layer, leading to surface wind anomalies. Consequently, an average Ekman pumping associated with these mean surface wind variations is consistently emerging. This average Ekman pumping is found to very well explain the SST anomaly signatures of the detected Agulhas rings. Particularly, this mechanism seems to be the key factor determining that these anti-cyclonic eddies exhibit stationary imprints of cold SST anomalies near their core centers. A residual phase with the maximum SSH anomaly and wind speed anomaly is found to the right of the mean wind direction, apparently maintaining a coherent stationary thermal expression coupled to the marine atmospheric boundary layer.

Description: Meridional transport of salt in the global ocean from an eddy-resolving model Ocean Science Discussions, 10, 2293-2326, 2013 Author(s): A. M. Treguier, J. Deshayes, J. Le Sommer, C. Lique, G. Madec, T. Penduff, J.-M. Molines, B. Barnier, R. Bourdalle-Badie, and C. Talandier The meridional transport of salt is computed in a global eddy-resolving numerical model (1/12° resolution) in order to improve our understanding of the ocean salinity budget. A methodology is proposed that allows a global analysis of the salinity balance in relation with surface water fluxes, without defining a "freshwater anomaly" based on an arbitrary reference salinity. The method consists in a decomposition of the meridional transport into (i) the transport by the time-longitude-depth mean velocity, (ii) time-mean velocity recirculations and (iii) transient eddy perturbations. Water is added (rainfall) or removed (evaporation) at the ocean surface at different latitudes, which creates convergences and divergences of mass tranport with maximum and minimum values close to ±1 Sv. The resulting meridional velocity effects a net transport of salt at each latitude (±30 Sv PSU), which is balanced by the time-mean recirculations and by the net effect of eddy salinity-velocity correlations. This balance ensures that the total meridional transport of salt is close to zero, a necessary condition to maintain a quasi-stationary salinity distribution. Our model confirms that the eddy salt transport cannot be neglected: it is comparable to the transport by the time-mean recirculation (up to 15 Sv PSU) at the poleward and equatorial boundaries of the subtropical gyres. Two different mechanisms are found: eddy contributions are localized in intense currents such as the Kuroshio at the poleward boundary of the subtropical gyres, while they are distributed across the basins at the equatorward boundaries. Closer to the equator, salinity-velocity correlations are mainly due to the seasonal cycle and large scale perturbations such as tropical instability waves.

Description: Upper ocean response to two collocated typhoons Ocean Science Discussions, 10, 2255-2292, 2013 Author(s): D. B. Baranowski, P. J. Flatau, S. Chen, and P. G. Black The atmospheric wind stress forcing and the oceanic response are examined for the period between 15 September 2008 and 6 October 2008, during which two typhoons, Hagupit and Jangmi passed through the same region of the Western Pacific at Saffir–Simpson intensity categories one and three, respectively. A three-dimensional oceanic mixed layer model is compared against the remote sensing observations as well as high repetition Argo float data. Numerical model simulations suggested that magnitude of the cooling caused by the second typhoon, Jangmi, would have been significantly larger if the ocean had not already been influenced by the first typhoon, Hagupit. It is estimated that the temperature anomaly behind Jangmi would have been about 0.4 °C larger in both cold wake and left side of the track. The numerical simulations suggest that the magnitude and position of Jangmi's cold wake depends on the precursor state of the ocean as well as lag between typhoons. Based on sensitivity experiments we show that temperature anomaly difference between "single typhoon" and "two typhoons" as well as magnitude of the cooling strongly depends on the value of inertial current decay time parameter. Thus, the magnitude of the observed cooling depends also on the amount of kinetic energy in the upper ocean. This paper indicates that studies of ocean-atmosphere tropical cyclone interaction will benefit from denser, high repetition Argo float measurements.

Description: Ventilation of the Mediterranean Sea constrained by multiple transient tracer measurements Ocean Science Discussions, 10, 1647-1705, 2013 Author(s): T. Stöven and T. Tanhua Ventilation is the prime pathway for ocean surface perturbations, such as temperature anomalies, to be relayed to the ocean interior. It is also the conduit for gas exchange between atmosphere and ocean and thus the mechanism whereby, for instance, the interior ocean is oxygenated and enriched in anthropogenic carbon. The ventilation of the Mediterranean Sea is fast in comparison to the world ocean and has large temporal variability, so that quantification of Mediterranean Sea ventilation rates is challenging and very relevant for Mediterranean oceanography and biogeochemistry. Here we present transient tracer data from a field-campaign in April 2011 that sampled a unique suite of transient tracers (SF 6 , CFC-12, tritium and 3 He) in all major basins of the Mediterranean. We apply the Transit Time Distribution (TTD) model to the data which then constrain the mean age, the ratio of the advective/diffusive transport mechanism, and the presence, or not, of more than one significant (for ventilation) water mass. We find that the eastern part of the Eastern Mediterranean can be reasonable described with a one dimensional Inverse Gaussian (1IG) TTD, and thus constrained with two independent tracers. The ventilation of the Ionian Sea and the Western Mediterranean can only be constrained by a multidimensional TTD. We approximate the ventilation with a two-dimensional Inverse Gaussian (2IG) TTD for these areas and demonstrate one way of constraining a 2IG-TTD from the available transient tracer data. The deep water in the Ionian Sea has higher mean ages than the deep water of the Levantine Basin despite higher transient tracer concentrations. This is partly due to the deep water of Adriatic origin having more diffusive properties in the transport and formation, i.e. a high ratio of diffusion over advection, compared to the deep water of Aegean Sea origin that still dominates the deep Levantine Basin deep water after the Eastern Mediterranean Transient (EMT) in the early 1990s. We also show that the deep Western Mediterranean has approximately 40% contribution of recently ventilated deep water from the Western Mediterranean Transition (WMT) event of the mid-2000s. The deep water has higher transient tracer concentrations than the mid-depth water, but the mean age is similar.

Description: 10 Be in late deglacial climate simulated by ECHAM5-HAM – Part 2: Isolating the solar signal from 10 Be deposition Climate of the Past Discussions, 9, 5627-5657, 2013 Author(s): U. Heikkilä, X. Shi, S. J. Phipps, and A. M. Smith This study investigates the effect of deglacial climate on the deposition of the solar proxy 10 Be globally, and at two specific locations, the GRIP site at Summit, Central Greenland, and the Law Dome site in coastal Antarctica. The deglacial climate is represented by three 30 yr time slice simulations of 10 000 BP (years before present = 1950 CE), 11 000 BP and 12 000 BP, compared with a preindustrial control simulation. The model used is the ECHAM5-HAM atmospheric aerosol–climate model, driven with sea surface temperatures and sea ice cover simulated using the CSIRO Mk3L coupled climate system model. The focus is on isolating the 10 Be production signal, driven by solar variability, from the weather or climate driven noise in the 10 Be deposition flux during different stages of climate. The production signal varies on lower frequencies, dominated by the 11yr solar cycle within the 30 yr time scale of these experiments. The climatic noise is of higher frequencies. We first apply empirical orthogonal functions (EOF) analysis to global 10 Be deposition on the annual scale and find that the first principal component, consisting of the spatial pattern of mean 10 Be deposition and the temporally varying solar signal, explains 64% of the variability. The following principal components are closely related to those of precipitation. Then, we apply ensemble empirical decomposition (EEMD) analysis on the time series of 10 Be deposition at GRIP and at Law Dome, which is an effective method for adaptively decomposing the time series into different frequency components. The low frequency components and the long term trend represent production and have reduced noise compared to the entire frequency spectrum of the deposition. The high frequency components represent climate driven noise related to the seasonal cycle of e.g. precipitation and are closely connected to high frequencies of precipitation. These results firstly show that the 10 Be atmospheric production signal is preserved in the deposition flux to surface even during climates very different from today's both in global data and at two specific locations. Secondly, noise can be effectively reduced from 10 Be deposition data by simply applying the EOF analysis in the case of a reasonably large number of available data sets, or by decomposing the individual data sets to filter out high-frequency fluctuations.

Description: Testing long-term summer temperature reconstruction based on maximum density chronologies obtained by reanalysis of tree-ring datasets from northernmost Sweden and Finland Climate of the Past Discussions, 9, 5659-5700, 2013 Author(s): V. V. Matskovsky and S. Helama Here we analysed the maximum latewood density (MXD) chronologies of two published tree-ring datasets: from Torneträsk region in northernmost Sweden (TORN, Melvin et al., 2013) and from northern Fennoscandia (FENN, Esper et al., 2012). We paid particular attention to the MXD low-frequency variations to reconstruct long-term summer (June–August, JJA) temperature history. We used published methods of tree-ring standardization: regional curve (RC) standardization, combined with signal-free (SF) implementation. Comparisons with a single-RC (RC1) and multiple-RC (RC2) were also carried out. We develop a novel method of standardization, the correction (C) implementation to SF (hence, RC1SFC or RC2SFC), tailored for detection of pure low-frequency signal in tree-ring chronologies. In this method, the error in RC1SF (or RC2SF) chronology, is analytically assessed and extracted to produce a RC1SFC or RC2SFC chronology. In TORN, the RC1SF chronology shows higher correlation with summer temperature (JJA) than RC1SFC, whereas in FENN the temperature signals of RC1SF chronology is improved by correction implementation (RC1SFC). The highest correlation between differently standardized chronologies for two datasets is obtained using FENN-RC2SFC and TORN-RC1 chronologies. Focusing on lowest frequencies, the importance of correction becomes obvious as the chronologies become progressively more correlative with RC1SFC and RC2SFC implementations. Subsampling the FENN data (which presents a higher number of samples than TORN dataset) to the chronology sample size of TORN data shows that the chronologies consistently bifurcate during the 7th, 9th, 17th and 20th centuries. We used the two MXD datasets to reconstruct summer temperature variations over the period −48–2010 calendar years. Our new reconstruction shows multi-decadal to multi-centennial variability with changes in the amplitude of the summer temperature of 2.6 °C in average during the Common Era.

Description: Persistent decadal-scale rainfall variability in the tropical South Pacific Convergence Zone through the past six centuries Climate of the Past Discussions, 9, 5593-5625, 2013 Author(s): C. R. Maupin, J. W. Partin, C.-C. Shen, T. M. Quinn, K. Lin, F. W. Taylor, J. L. Banner, K. Thirumalai, and D. J. Sinclair Observations and reconstructions of decadal-scale climate variability are necessary to place predictions of future global climate change into temporal context (Goddard et al., 2012). This is especially true for decadal-scale climate variability that originates in the Pacific Ocean (Deser et al., 2004; Dong and Lu, 2013). We focus here on the western tropical Pacific (Solomon Islands; ~ 9.5° S, ~ 160° E), a region directly influenced by: the South Pacific Convergence Zone (SPCZ), the West Pacific Warm Pool (WPWP), the Pacific Walker Circulation (PWC), and the Hadley Circulation. We calibrate δ 18 O variations in a fast growing stalagmite to local rainfall amount and produce a 600 yr record of rainfall variability from the zonally oriented, tropical portion of the SPCZ. We present evidence for large (~ 1.5 m), persistent and decade(s)-long shifts in total annual rainfall amount in the Solomon Islands since 1416 ± 5 CE. The timing of the decadal changes in rainfall inferred from the 20th century portion of the stalagmite δ 18 O record coincide with previously identified decadal shifts in Pacific ocean-atmosphere behavior (Clement et al., 2011; Deser et al., 2004). The 600 yr Solomons stalagmite δ 18 O record indicates that decadal oscillations in rainfall are a robust characteristic of SPCZ-related climate variability, which has important implications to water resource management in this region.

Description: Transformation of organic carbon, trace element, and organo-mineral colloids in the mixing zone of the largest European Arctic river Ocean Science Discussions, 10, 1707-1764, 2013 Author(s): O. S. Pokrovsky, L. S. Shirokova, J. Viers, V. V. Gordeev, V. P. Shevchenko, A. V. Chupakov, T. Y. Vorobieva, F. Candaudap, C. Casseraund, A. Lanzanova, and C. Zouiten The estuarine behavior of organic carbon (OC) and trace elements (TE) was studied for the largest European sub-Arctic river, which is the Severnaya Dvina; this river is a deltaic estuary covered in ice during several hydrological seasons: summer (July 2010, 2012) and winter (March 2009) baseflow, and the November–December 2011 ice-free period. Colloidal forms of OC and TE were assessed using three pore size cutoff (1, 10, and 50 kDa) using an in-situ dialysis procedure. Conventionally dissolved ( 〈 0.22 μm) fractions demonstrated clear conservative behavior for Li, B, Na, Mg, K, Ca, Sr, Mo, Rb, Cs, and U during the mixing of freshwater with the White Sea; a significant (up to a factor of 10) concentration increase occurs with increases in salinity. Si and OC also displayed conservative behavior but with a pronounced decrease of concentration seawards. Rather conservative behavior, but with much smaller changes in concentration (variation within ±30%) over a full range of salinities, was observed for Ti, Ni, Cr, As, Co, Cu, Ga, Y, and heavy REE. Strong non-conservative behavior with coagulation/removal at low salinities ( 〈 5‰) was exhibited by Fe, Al, Zr, Hf, and light REE. Finally, certain divalent metals exhibited non-conservative behavior with a concentration gain at low (~2–5‰, Ba, Mn) or intermediate (~10–15‰, Ba, Zn, Pb, Cd) salinities, which is most likely linked to TE desorption from suspended matter or sediment outflux. The most important result of this study is the elucidation of the behavior of the "truly" dissolved low molecular weight LMW 〈 1 kDa fraction containing Fe, OC, and a number of insoluble elements. The concentration of the LMW fraction either remains constant or increases its relative contribution to the overall dissolved ( 〈 0.22 μm) pool as the salinity increases. Similarly, the relative proportion of colloidal (1 kDa–0.22 μm) pool for the OC and insoluble TE bound to ferric colloids systematically decreased seaward, with the largest decrease occurring at low ( 〈 5‰) salinities. Overall, the observed decrease of the colloidal fraction may be related to the coagulation of organo-ferric colloids at the beginning of the mixing zone and therefore the replacement of the HMW 1 kDa–0.22 μm portion by the LMW 〈 1 kDa fraction. These patterns are highly reproducible across different sampling seasons, suggesting significant enrichment of the mixing zone by the most labile (and potentially bioavailable) fraction of the OC, Fe and insoluble TE. The size fractionation of the colloidal material during estuarine mixing reflects a number of inorganic and biological processes, the relative contribution of which to element speciation varies depending on the hydrological stage and time of year. In particular, LMW 〈 1 kDa ligand production in the surface horizons of the mixing zone may be linked to heterotrophic mineralization of allochthonous DOM and/or photodestruction. Given the relatively low concentration of particulate vs. dissolved load of most trace elements, desorption from the river suspended material was less pronounced than in other rivers in the world. As a result, the majority of dissolved components exhibited either a conservative (OC and related elements such as divalent metals) or non-conservative, coagulation-controlled (Fe, Al, and insoluble TE associated with organo-ferric colloids) behavior. The climate warming in high latitudes is likely to intensify the production of LMW 〈 1 kDa organic ligands and the associated TE; therefore, the delivery of potentially bioavailable trace metal micronutrients from the land to the ocean may increase.

Description: The influence of atmospheric circulation on the mid-Holocene climate of Europe: a data-model comparison Climate of the Past Discussions, 9, 5569-5592, 2013 Author(s): A. Mauri, B. A. S. Davis, P. M. Collins, and J. O. Kaplan The atmospheric circulation is a key area of uncertainty in climate model simulations of future climate change, especially in mid-latitude regions such as Europe where atmospheric dynamics have a significant role in climate variability. It has been proposed that the mid-Holocene was characterized in Europe by a stronger westerly circulation in winter comparable with a more positive AO/NAO, and a weaker westerly circulation in summer caused by anti-cyclonic blocking near Scandinavia. Model simulations indicate at best only a weakly positive AO/NAO, whilst changes in summer atmospheric circulation have not been widely investigated. Here we use a new pollen-based reconstruction of European mid-Holocene climate to investigate the role of atmospheric circulation in explaining the spatial pattern of seasonal temperature and precipitation anomalies. We find that the footprint of the anomalies is entirely consistent with those from modern analogue atmospheric circulation patterns associated with a strong westerly circulation in winter (positive AO/NAO) and a weak westerly circulation in summer (positive SCAND). We find little agreement between the reconstructed anomalies and those from a climate model simulation, which as with most model simulations shows a much greater sensitivity to local radiative forcing from top-of-the-atmosphere changes in solar insolation. Our findings are consistent with data-model comparisons on contemporary timescales that indicate that models underestimate the role of atmospheric circulation in climate change, whilst also highlighting the importance of atmospheric dynamics in explaining interglacial warming.

Description: Environmental and climatic changes in Central Chilean Patagonia since the Late Glacial (Mallín El Embudo, 44° S) Climate of the Past Discussions, 9, 5747-5784, 2013 Author(s): M. E. de Porras, A. Maldonado, F. A. Quintana, A. Martel-Cea, O. Reyes, and C. Méndez Multi-millennial environmental and climatic changes in Central Chilean Patagonia (44–49° S) during the Last Glacial–Interglacial cycle have been of particular interest as changes in the position and strength of the Southern Westerlies are the major forcing factor conditioning the environmental dynamics. Recent attempts to reconstruct regional environmental and climatic signals from Central Chilean Patagonia reveal some discrepancies and unclear issues among the records. This paper presents the 13 ka pollen and charcoal records from Mallín El Embudo (44°40' S; 71°42' W) located in the deciduous Nothofagus forest in the middle Río Cisnes valley. The paper aims to (1) establish the timing and magnitude of local vegetation changes and fire activity since the Late Glacial and (2) integrate these results at the regional scale in order to discuss the discrepancies and depict the Central Chilean Patagonia environmental and climatic dynamics since Late Glacial. Open landscapes dominated by grasses associated with scattered Nothofagus forest patches dominated middle Río Cisnes valley between 13–11.2 ka suggesting low effective moisture but also reflecting that landscape configuration after glacial retreat was still ongoing. At 11.2 ka, a sudden development of an open and quite dynamic Nothofagus forest probably associated to the synchronous high fire activity occurred suggesting a rise in effective moisture. Since 9.5 ka, the record reflects the presence of a closed Nothofagus forest related to higher/similar effective moisture conditions than before but under an unmarked precipitation seasonality. The forest experienced a slight canopy opening since 5.7 ka, probably due to slightly drier conditions than before followed by a sudden change around 4.2 ka associated with fire and volcanic disturbances. The recovery of an open Nothofagus forest related to slight wetter conditions (similar to present) occurred around 2 ka and persisted under highly variable climatic conditions up to 0.1 ka when massive forest burning and logging due to European settlements occurred. Central Chilean Patagonian climatic and environmental changes at millennial-centennial time scales since Late Glacial were driven by changes in the Southern Westerlies latitudinal shift and/or intensity but during the Late Holocene fire, volcanism and humans arise as major forcings contributing to environmental dynamics.

Description: Uncertainties in the modelled CO 2 threshold for Antarctic glaciation Climate of the Past Discussions, 9, 5701-5745, 2013 Author(s): E. Gasson, D. J. Lunt, R. DeConto, A. Goldner, M. Heinemann, M. Huber, A. N. LeGrande, D. Pollard, N. Sagoo, M. Siddall, and A. Winguth A frequently cited atmospheric CO 2 threshold for the onset of Antarctic glaciation of ~ 780 ppmv is based on a study using an ice sheet model and the GENESIS climate model. Proxy records suggest that atmospheric CO 2 concentrations passed through this threshold across the Eocene–Oligocene transition ~ 34 Ma. However, atmospheric CO 2 concentrations may have been close to this threshold earlier than this transition, which is used by some to suggest the possibility of Antarctic ice sheets during the Eocene. Here we investigate the climate model dependency of the threshold for Antarctic glaciation by performing offline ice sheet model simulations using the climate from a number of different climate models (HadCM3L, CCSM3, CESM1.0, GENESIS, FAMOUS, ECHAM5 and GISS_ER). These climate simulations are sourced from a number of independent studies, as such the boundary conditions, which are poorly constrained during the Eocene, are not identical between simulations. The results of this study suggest that the atmospheric CO 2 threshold for Antarctic glaciation is highly dependent on the climate model used and the climate model configuration. A large discrepancy between the climate model and ice sheet model grids for some simulations leads to a strong sensitivity to the lapse rate parameter. However, with the exception of HadCM3L and its reduced complexity version FAMOUS, the simulations suggest the growth of an intermediate sized ice sheet (〉 25 m sea level equivalent) for atmospheric CO 2 concentrations in the range of 560–920 ppmv, which is consistent with previous studies.

Description: A seasonality trigger for carbon injection at the Paleocene-Eocene thermal maximum Climate of the Past Discussions, 9, 5837-5854, 2013 Author(s): J. S. Eldrett, D. R. Greenwood, M. Polling, H. Brinkhuis, and A. Sluijs The Paleocene-Eocene thermal maximum (PETM) represents a ~170 kyr episode of anomalous global warmth ~56 Ma ago. The PETM is associated with rapid and massive injections of 13 C-depleted carbon into the ocean-atmosphere system reflected as a prominent negative carbon isotope excursion (CIE) in sedimentary components. Earth's surface and deep ocean waters warmed by ~5 °C, of which part may have occurred prior to the CIE. However, few records document continental climatic trends and changes in seasonality have not been documented. Here we present the first high-resolution vegetation reconstructions for the PETM, based on bioclimatic analysis of terrestrially-derived spore and pollen assemblages preserved in an expanded section from the Central North Sea. Our data indicate reductions in boreal conifers and an increase in mesothermal to megathermal taxa, reflecting a shift towards wetter and warmer climate. We also record an increase in summer temperatures, greater in magnitude than the rise in mean annual temperature changes. Within the CIE, vegetation varies significantly with initial increases in epiphytic and climbing ferns, and development of extensive wetlands, followed by abundance of Carya spp. indicative of broadleaf forest colonization. Critically, the change in vegetation we report occurs prior to the CIE, and is concomitant with anomalous marine ecological change, as represented by the occurrence of Apectodinium augustum . This suggests that amplifications of seasonal extremes triggered carbon injection.

Description: Numerical modelling of physical processes governing larval transport in the Southern North Sea Ocean Science Discussions, 10, 1765-1806, 2013 Author(s): M. C. H. Tiessen, L. Fernard, T. Gerkema, J. van der Molen, P. Ruardij, and H. W. van der Veer A three-dimensional hydrodynamic model (GETM) was coupled with a particle tracking routine (GITM) to study the inter-annual variability in transport paths of particles in the North Sea and English Channel. For validation, a comparison with observed drifter trajectories was made. The aim was to investigate to what extent variability in the hydrodynamic conditions alone (reflecting passive particle transport) contributed to inter-annual variability in transport of eggs and larvae. In this idealized study, no a-priori selection of spawning grounds or periods was made and no active behaviour (vertical migration) or mortality were included. Egg and larval development towards coastal nursery areas was based solely on sea water temperature, while settlement areas were defined by a threshold water depth. Results showed strong inter-annual variability in drift direction and distance, caused by a combination of wind speed and direction. Strong inter-annual variability was observed both in absolute amount of settlement in coastal areas, as well as in the relative importance of the different areas. Settlement in the western Dutch Wadden Sea not only showed inter-annual variability, but patterns were also variable within each year and revealed seasonal changes in the origin of particles.

Description: Regional climate model simulations for Europe at 6 k and 0.2 k yr BP: sensitivity to changes in anthropogenic deforestation Climate of the Past Discussions, 9, 5785-5836, 2013 Author(s): G. Strandberg, E. Kjellström, A. Poska, S. Wagner, M.-J. Gaillard, A.-K. Trondman, A. Mauri, H. J. B. Birks, A. E. Bjune, B. A. S. Davis, R. Fyfe, T. Giesecke, L. Kalnina, M. Kangur, J. O. Kaplan, W. O. van der Knaap, U. Kokfelt, P. Kuneš, M. Latałowa, L. Marquer, F. Mazier, A. B. Nielsen, B. Smith, H. Seppä, and S. Sugita This study aims to evaluate the direct effects of anthropogenic deforestation on simulated climate at two contrasting periods in the Holocene, ~6 k BP and ~0.2 k BP in Europe. We apply RCA3, a regional climate model with 50 km spatial resolution, for both time periods, considering three alternative descriptions of the past vegetation: (i) potential natural vegetation (V) simulated by the dynamic vegetation model LPJ-GUESS, (ii) potential vegetation with anthropogenic land cover (deforestation) as simulated by the HYDE model (V + H), and (iii) potential vegetation with anthropogenic land cover as simulated by the KK model (V + K). The KK model estimates are closer to a set of pollen-based reconstructions of vegetation cover than the HYDE model estimates. The climate-model results show that the simulated effects of deforestation depend on both local/regional climate and vegetation characteristics. At ~6 k BP the extent of simulated deforestation in Europe is generally small, but there are areas where deforestation is large enough to produce significant differences in summer temperatures of 0.5–1 °C. At ~0.2 k BP, simulated deforestation is much more extensive than previously assumed, in particular according to the KK model. This leads to significant temperature differences in large parts of Europe in both winter and summer. In winter, deforestation leads to lower temperatures because of the differences in albedo between forested and unforested areas, particularly in the snow-covered regions. In summer, deforestation leads to higher temperatures in central and eastern Europe since evapotranspiration from unforested areas is lower than from forests. Summer evaporation is already limited in the southernmost parts of Europe under potential vegetation conditions and, therefore, cannot become much lower. Accordingly, the albedo effect dominates also in summer, which implies that deforestation causes a decrease in temperatures. Differences in summer temperature due to deforestation range from −1 °C in south-western Europe to +1 °C in eastern Europe. The choice of anthropogenic land cover estimate has a significant influence on the simulated climate, but uncertainties in palaeoclimate proxy data for the two time periods do not allow for a thorough comparison with climate model results.

Description: Eddy length scales and the Rossby radius in the Arctic Ocean Ocean Science Discussions, 10, 1807-1831, 2013 Author(s): A. J. G. Nurser and S. Bacon The first (and second) baroclinic deformation (or Rossby) radii are presented and discussed north of ~60° N, focusing on deep basins and shelf seas in the high Arctic Ocean, the Nordic Seas, Baffin Bay, Hudson Bay and the Canadian Arctic Archipelago, derived from high-resolution ice-ocean general circulation model output. Comparison of the model output with measured results shows that low values of the Rossby radius (in shallow water) and high values (in the Canada Basin) are accurately reproduced, while intermediate values (in the region of the Makarov and Amundsen Basins) are overestimated. In the high Arctic Ocean, the first Rossby radius increases from ~5 km in the Nansen Basin to ~15 km in the central Canadian Basin. In the shelf seas and elsewhere, values are low (1–7 km), reflecting weak density stratification, shallow water, or both. Seasonality only strongly impacts the Rossby radii in shallow seas where winter homogenisation of the water column can reduce it to the order of 100 m. We also offer an interpretation and explanation of the observed scales of Arctic Ocean eddies.

Description: Terrigenous input off northern South America driven by changes in Amazonian climate and the North Brazil Current retroflection during the last 250 ka Climate of the Past Discussions, 9, 5855-5898, 2013 Author(s): A. Govin, C. M. Chiessi, M. Zabel, A. O. Sawakuchi, D. Heslop, T. Hörner, Y. Zhang, and S. Mulitza We investigate changes in the delivery and oceanic transport of Amazon sediments related to terrestrial climate variations over the last 250 ka. We present high-resolution geochemical records from four marine sediment cores located between 5 and 12° N along the northern South American margin. The Amazon River is the sole source of terrigenous material for sites at 5 and 9° N, while the core at 12° N receives a mixture of Amazon and Orinoco detrital particles. Using an endmember unmixing model, we estimated the relative proportions of Amazon Andean material ("%-Andes", at 5 and 9° N) and of Amazon material ("%-Amazon", at 12° N) within the terrigenous fraction. The %-Andes and %-Amazon records exhibit significant precessional variations over the last 250 ka that are more pronounced during interglacials in comparison to glacial times. High %-Andes values observed during periods of high austral summer insolation reflect the increased delivery of suspended sediments by Andean tributaries and enhanced Amazonian precipitation, in agreement with western Amazonian speleothem records. However, low %-Amazon values obtained at 12° N during the same periods seem to contradict the increased delivery of Amazon sediments. We propose that reorganisations in surface ocean currents modulate the northwestward transport of Amazon material. In agreement with published records, the seasonal North Brazil Current retroflection is intensified (or prolonged in duration) during cold substages of the last 250 ka (which correspond to intervals of high DJF or low JJA insolation) and deflects eastward the Amazon sediment and freshwater plume.

Description: Comparison of N. Atlantic heat storage estimates during the Argo period (1999–2010) Ocean Science Discussions, 10, 2363-2398, 2013 Author(s): N. C. Wells, M. Couldrey, and V. O. Ivchenko Ocean heat storage is an essential component of the climate system and there is considerable interest in its accurate evaluation. There are a number of heat storage products produced by many different groups. These products are derived from Argo as well as other platforms, for example XBT and CTD, in the last decade. Here we compare two heat storage estimates for the North Atlantic 0–2000 m from 10° to 70° N. One derived solely from Argo data whilst the other is derived from Argo and other platforms. It is found that there is a positive trend in heat storage over the period 1999–2010. This trend is influenced by a strong air–sea interaction event in 2009–2010, and this reduces the upward trend 1999–2008 identified previously. Both data sets are consistent with each other for the layer 0–1000 m on a timescale of beyond 1 yr. There are significant differences at sub-annual time scales and in the layer 1000–2000 m.

Description: Model-data comparison and data assimilation of mid-Holocene Arctic sea-ice concentration Climate of the Past Discussions, 9, 6515-6549, 2013 Author(s): F. Klein, H. Goosse, A. Mairesse, and A. de Vernal The consistency between a new quantitative reconstruction of Arctic sea-ice concentration based on dinocyst assemblages and the results of climate models has been investigated for the mid-Holocene. The comparison shows that the simulated sea-ice changes are weaker and spatially more homogeneous than the recorded ones. Furthermore, although the model-data agreement is relatively good in some regions such as the Labrador Sea, the skill of the models at local scale is low. The response of the models follows mainly the increase in summer insolation at large scale. This is modulated by changes in atmospheric circulation leading to differences between regions in the models that are albeit smaller than in the reconstruction. Performing simulations with data assimilation using the model LOVECLIM amplifies those regional differences, mainly through a reduction of the southward winds in the Barents Sea and an increase in the westerly winds in the Canadian Basin of the Arctic. This leads to an increase in the ice concentration in the Barents and Chukchi Seas and a better agreement with the reconstructions. This underlines the potential role of atmospheric circulation to explain the reconstructed changes during the Holocene.

Description: Spring-time zooplankton size structure over the continental shelf of the Bay of Biscay Ocean Science Discussions, 10, 2207-2254, 2013 Author(s): P. Vandromme, E. Nogueira, M. Huret, Á Lopez-Urrutia, G. González-Nuevo González, M. Sourisseau, and P. Petitgas Linking lower to higher trophic levels requires a special focus on the pivotal role played by mid-trophic levels, i.e. the zooplankton. One of the most relevant information on zooplankton in term of fluxes of matter lies in its size structure. We present here an extensive dataset of size measurements covering part of the western European shelf and slope, from the Galician coast to the Ushant front, during springs from 2005 to 2012. Zooplankton size spectra were estimated using both measurements carried out in situ by the Laser-Optical Plankton Counter (LOPC, 816 records) and WP2 net (200 μm mesh size) samples scanned following the ZooScan methodology and image analysis (a total of 89 samples were analyzed). The LOPC counts and sizes all particles in the range 100 to 2000 μm of spherical equivalent diameter (ESD) whereas the WP2/ZooScan allows the counting, sizing and identification of zooplankton from ~400 μm ESD. The difference between the LOPC (all particles) and the WP2/ZooScan (zooplankton only) is assumed to provide the size distribution of non-living particles whose descriptors are further related to a set of explanatory variables (including physical, biological and geographic descriptors). A statistical correction based on these explanatory variables is then applied to LOPC measurements to removed the part due to non-living particles and estimate zooplankton size spectra. This extensive data set provides a new look at regional and inter-annual variability of the pelagic ecosystem of the Bay of Biscay.

Description: Evaluation of MERIS products from Baltic Sea coastal waters rich in CDOM Ocean Science Discussions, 10, 2157-2206, 2013 Author(s): J. M. Beltrán-Abaunza, S. Kratzer, and C. Brockmann In this study, retrievals of the medium resolution imaging spectrometer (MERIS) reflectances and water quality products using 4 different coastal processing algorithms freely available are assessed by comparison against sea-truthing data. The study is based on a pair-wise comparison using processor-dependent quality flags for the retrieval of valid common macro-pixels. This assessment is required in order to ensure the reliability of monitoring systems based on MERIS data, such as the Swedish coastal and lake monitoring system ( http.vattenkvalitet.se ). The results show that the pre-processing with the Improved Contrast between Ocean and Land (ICOL) processor, correcting for adjacency effects, improve the retrieval of spectral reflectance for all processors, Therefore, it is recommended that the ICOL processor should be applied when Baltic coastal waters are investigated. Chlorophyll was retrieved best using the FUB (Free University of Berlin) processing algorithm, although overestimations in the range 18–26.5%, dependent on the compared pairs, were obtained. At low chlorophyll concentrations ( 〈 2.5 mg m −3 ), random errors dominated in the retrievals with the MEGS (MERIS ground segment processor) processor. The lowest bias and random errors were obtained with MEGS for suspended particulate matter, for which overestimations in te range of 8–16% were found. Only the FUB retrieved CDOM (Coloured Dissolved Organic Matter) correlate with in situ values. However, a large systematic underestimation appears in the estimates that nevertheless may be corrected for by using a~local correction factor. The MEGS has the potential to be used as an operational processing algorithm for the Himmerfjärden bay and adjacent areas, but it requires further improvement of the atmospheric correction for the blue bands and better definition at relatively low chlorophyll concentrations in presence of high CDOM attenuation.

Description: A new method for continuous measurements of oceanic and atmospheric N 2 O, CO and CO 2 : performance of off-axis integrated cavity output spectroscopy (OA-ICOS) coupled to non-dispersive infrared detection (NDIR) Ocean Science Discussions, 10, 1281-1327, 2013 Author(s): D. L. Arévalo-Martínez, M. Beyer, M. Krumbholz, I. Piller, A. Kock, T. Steinhoff, A. Körtzinger, and H. W. Bange A new system for continuous, highly-resolved oceanic and atmospheric measurements of N 2 O , CO and CO 2 is described. The system is based upon off-axis integrated cavity output spectroscopy (OA-ICOS) and a non-dispersive infrared analyzer (NDIR) both coupled to a Weiss-type equilibrator. Performance of the combined setup was evaluated by testing its precision, accuracy, long-term stability, linearity and response time. Furthermore, the setup was tested during two oceanographic campaigns in the equatorial Atlantic Ocean in order to explore its potential for autonomous deployment onboard voluntary observing ships (VOS). Improved equilibrator response times for N 2 O (2.5 min) and CO (45 min) were achieved in comparison to response times from similar chamber designs used by previous studies. High stability of the OA-ICOS analyzer was demonstrated by low optimal integration times of 2 and 4 min for N 2 O and CO respectively, as well as detection limits of 〈 40 ppt and precision better than 0.3 ppb Hz −1/2 . Results from a direct comparison of the method presented here and well-established discrete methods for oceanic N 2 O and CO 2 measurements showed very good consistency. The favorable agreement between underway atmospheric N 2 O, CO and CO 2 measurements and monthly means at Ascension Island (7.96° S 14.4° W) further suggests a reliable operation of the underway setup in the field. The potential of the system as an improved platform for measurements of trace gases was explored by using continuous N 2 O and CO 2 data to characterize the development of the seasonal equatorial upwelling in the Atlantic Ocean during two R/V Maria S. Merian cruises. A similar record of high-resolution CO measurements was simultaneously obtained offering for the first time the possibility of a comprehensive view on the distribution and emissions of these climate relevant gases on the area. The relatively simple underway N 2 O/CO/CO 2 setup is suitable for long-term deployment on board of research and commercial vessels although potential sources of drift such as cavity temperature and further technical improvements towards automation still need to be addressed.

Description: Black shale deposition during Toarcian super-greenhouse driven by sea level Climate of the Past Discussions, 9, 4365-4384, 2013 Author(s): M. Hermoso, F. Minoletti, and P. Pellenard One of the most elusive aspects of the Toarcian Oceanic Anoxic Event (T-OAE) is the paradox between carbon isotopes that indicate intense global primary productivity and organic carbon burial at a global scale, and the delayed expression of anoxia in Europe. During the earliest Toarcian, no black shales were deposited in the European epicontinental seaways, and most organic carbon enrichment of the sediments postdated the T-OAE (defined by the overarching positive trend in the carbon isotopes). In the present studied, we have attempted to establish a sequence stratigraphy framework for Early Toarcian deposits recovered from a core drilled in the Paris Basin using a combination of mineralogical (quartz and clay relative abundance) and geochemical (Si, Zr, Ti and Al) measurements. Combined with the evolution in redox sensitive elements (Fe, V and Mo), the data suggest that expression of anoxia was hampered in European epicontinental seas during most of the T-OAE due to insufficient water depth that prevented stratification of the water column. Only the first stratigraphic occurrence of black shales in Europe corresponds to the "global" event. This interval is characterised by 〉 10% Total Organic Carbon (TOC) content that contains relatively low concentration of molybdenum compared to subsequent black shale horizons. Additionally, this first black shale occurrence is coeval with the record of the major negative Carbon Isotope Excursion (CIE), likely corresponding to a period of transient greenhouse intensification likely due to massive injection of carbon into the Atmosphere–Ocean system. As a response to enhanced weathering and riverine run-off, increased fresh water supply to the basin may have promoted the development of full anoxic conditions through haline stratification of the water column. In contrast, post T-OAE black shales were restricted to epicontinental seas (higher Mo to TOC ratios) during a period of relative high sea level, and carbon isotopes returning to pre-T-OAE values. Comparing palaeoredox proxies with the inferred sequence stratigraphy for Sancerre suggests that episodes of short-term organic carbon enrichment were primarily driven by third-order sea level changes. These black shales exhibit remarkably well-expressed higher-frequency cyclicities in the concentration of redox-sensitive elements such as iron or vanadium whose nature has still to be determined through cyclostratigraphic analysis.

Description: The East Asian winter monsoon variability in response to precession and inter-hemispheric heat balance Climate of the Past Discussions, 9, 4229-4261, 2013 Author(s): M. Yamamoto, H. Sai, M.-T. Chen, and M. Zhao The response of Asian monsoon variability to orbital forcing is still unclear, and all hypotheses are controversial. We present a record of the sea surface temperature difference (ΔSST) between the South China Sea and the other Western Pacific Warm Pool regions as a proxy for the intensity of the Asian winter monsoon, because the winter cooling of the South China Sea is caused by the cooling of surface water at the northern margin and the southward advection of cooled water due to winter monsoon winds. The ΔSST showed significant precession cycles during the last 150 kyr. In the precession cycle, the maximum winter monsoon intensity shown by the ΔSST corresponded to the May perihelion and was delayed behind the maximum ice volume. The East Asian winter monsoon was anti-phase with the Indian summer monsoon and the summer monsoon precipitation in central Japan. The timing of the maximum phase of the East Asian winter monsoon was different from previous results in terms of the March perihelion (ice volume maxima) and June perihelion (minimum of Northern Hemisphere winter insolation). We infer that the variation of the East Asian winter monsoon was caused by a physical mechanism of inter-hemispheric heat balance. The East Asian winter monsoon was intensified by the Northern Hemisphere cooling, which was caused by the combined effect of cooling by the ice volume forcing and the decrease in winter insolation, or by decreased heat transfer from the Southern Hemisphere to the Northern Hemisphere owing to the weak Indian summer monsoon at the May perihelion.

Description: Trace elements and cathodoluminescence of detrital quartz in Arctic marine sediments – a new ice-rafted debris provenance proxy Climate of the Past Discussions, 9, 4145-4189, 2013 Author(s): A. Müller and J. Knies The records of ice-rafted debris (IRD) provenance in the North Atlantic – Barents Sea allow the reconstruction of the spatial and temporal changes of ice-flow drainage patterns during glacial and deglacial periods. In this study a new approach to characterisation of the provenance of detrital quartz grains in the fraction 〉 500 μm of marine sediments offshore of Spitsbergen is introduced, utilizing scanning electron microscope backscattered electron and cathodoluminescence (CL) imaging, combined with laser ablation inductively-coupled plasma mass spectrometry. Based on their micro-inclusions, CL and trace element characteristics the investigated IRD grains can be classified into five distinct populations. Three of the populations are indicative of potential IRD provenance provinces in the Storfjord area including Barentsøya and Egdeøya. The results imply that under modern (interglacial) conditions IRD deposition along the western Spitsbergen margin is mainly governed by the East Svalbard Current controlling the ice-drift pattern. The presence of detrital quartz from local provinces, however, indicates that variations in IRD supply from western Spitsbergen may be quantified as well. In this pilot study it is demonstrated that this new approach applied on Arctic continental margin sediments, bears a considerable potential for the definition of the sources of IRD and thus of spatial/temporal changes in ice-flow drainage patterns during glacial/interglacial cycles.

Description: NGRIP temperature reconstruction from 10 to 120 kyr b2k Climate of the Past Discussions, 9, 4099-4143, 2013 Author(s): P. Kindler, M. Guillevic, M. Baumgartner, J. Schwander, A. Landais, and M. Leuenberger In order to reconstruct Greenland NGRIP temperature, measurements of δ 15 N from the beginning of the Holocene to Dansgaard–Oeschger (DO) event 8 have been performed. Together with previously measured and mostly published δ 15 N data, we are now able to present for the first time a NGRIP temperature reconstruction for the whole last glacial period (beginning of the Holocene back to 120 kyr) including every DO event based on δ 15 N isotope measurements using a firn densification and heat diffusion model. The detected temperature rises at DO events range from 5 °C (DO 25) up to 16.5 °C (DO 11), ± 3 °C. To bring measured and modelled data into agreement, we had to reduce the accumulation rate given by the ss09sea06bm time scale in some periods significantly, especially during the last glacial maximum (LGM). A comparison between reconstructed temperature and δ 18 O ice data confirms that the isotopic composition of the stadial was strongly influenced by seasonality. We continuously calculated α (δ 18 O ice to temperature sensitivity) on a 10 kyr running time window. α variations show an anticorrelation with obliquity, in agreement with a simple Rayleigh distillation model, and moreover seem to be influenced by Northern Hemisphere ice sheet volume.

Description: Physical forcing and physical/biochemical variability of the Mediterranean Sea: a review of unresolved issues and directions for future research Ocean Science Discussions, 10, 1205-1280, 2013 Author(s): P. Malanotte-Rizzoli, V. Artale, G. L. Borzelli-Eusebi, S. Brenner, G. Civitarese, A. Crise, J. Font, M. Gacic, N. Kress, S. Marullo, E. Ozsoy, M. Ribera d'Alcalà, W. Roether, K. Schroeder, S. Sofianos, T. Tanhua, A. Theocharis, M. Alvarez, Y. Ashkenazy, A. Bergamasco, V. Cardin, S. Carniel, F. D'Ortenzio, E. Garcia-Ladona, J. M. Garcia-Lafuente, A. Gogou, M. Gregoire, D. Hainbucher, H. Kontoyannis, V. Kovacevic, E. Krasakapoulou, G. Krokos, A. Incarbona, M. G. Mazzocchi, M. Orlic, A. Pascual, P.-M. Poulain, A. Rubino, J. Siokou-Frangou, E. Souvermezoglou, M. Sprovieri, I. Taupier-Letage, J. Tintoré, and G. Triantafyllou The importance of the Mediterranean Sea for the world ocean has long been recognized. First, the Mediterranean sea has a profound impact on the Atlantic ocean circulation and, consequently, on the global thermohaline conveyor belt. Maps of the Mediterranean salty water tongue exiting from the Gibraltar strait at intermediate depths and spreading throughout the Atlantic interior are well known since the 1950s. Through direct pathways to the Atlantic polar regions or through indirect mixing processes, the salty Mediterranean water preconditions the deep convection cells of the polar Atlantic. There the North Atlantic Deep Water is formed which successively spreads throughout the world ocean constituting the core of the global thermohaline circulation. Even more importantly, the Mediterranean Sea is a laboratory basin for the investigation of processes of global importance, being much more amenable to observational surveys because of its location in mid-latitude and its dimensions. Both the western and eastern basins in fact possess closed thermohaline circulations analogous to the global conveyor belt. A unique upper layer open thermohaline cell connects the eastern to the western basin and, successively, to the north Atlantic through the Gibraltar strait. In it, the Atlantic water entering into Gibraltar in the surface layer, after travelling to the easternmost Levantine basin, is transformed into one of the saltiest water masses through air–sea heat and moisture fluxes. This is the salty water which, crossing the entire basin in the opposite direction below the surface Atlantic water, finally exits from the Gibraltar strait at mid-depths. Both the western and eastern basins are endowed with deep/intermediate convection cells analogous to the polar Atlantic deep convection cells or to the intermediate mode water ones. Deep and intermediate water masses are therefore formed in different sites of the entire basin. Because of their easily accessible locations, these convection cells are much more amenable to direct observational surveys and mooring arrays. An ubiquitous, energetic mesoscale and sub-mesoscale eddy field is superimposed to and interacts with the sub-basin scale, wind-driven gyres that characterize the upper thermocline circulation. Three different scales of motion are therefore superimposed producing a richness of interaction processes which typify similar interactions in unexplored ocean regions. Both wide and narrow shelves are present separated by steep continental slopes from the deep interiors. Cross-shelf fluxes of physical as well biogeochemical parameters are crucial in determining the properties of the shallow versus deep local ecosystems and their trophic chain. Most importantly, the Mediterranean Sea is a basin of contrasting ecosystems, from the strongly oligotrophic deep interiors to the fully eutrophic northern Adriatic characterized by recurrent, anomalous algal blooms and related anoxia events. This review focuses on the identification of the major unresolved scientific issues and wants also to provide directions for future research which may lead to the formulation of interdisciplinary, collaborative implementation plans to address these issues both theoretically and observationally.

Description: A parameter model of gas exchange for the seasonal sea ice zone Ocean Science Discussions, 10, 1169-1204, 2013 Author(s): B. Loose, W. R. McGillis, D. Perovich, C. J. Zappa, and P. Schlosser Carbon budgets for the polar oceans require better constraint on air-sea gas exchange in the sea ice zone (SIZ). Here, we utilize recent advances in the theory of turbulence, mixing and air-sea flux in the ice-ocean boundary layer (IOBL) to formulate a simple model for gas exchange when the surface ocean is partially covered by sea ice. The gas transfer velocity ( k ) is related to shear-driven and convection-driven turbulence in the aqueous mass boundary layer, and to the mean-squared wave slope at the air–sea interface. We use the model to estimate k along the drift track of Ice-Tethered Profilers (ITPs) in the Arctic. Individual estimates of daily-averaged k from ITP drifts ranged between 1.1 and 22 m d −1 , and the fraction of open water ( f ) ranged from 0 to 0.83. Converted to area-weighted effective transfer velocities ( k eff ), the minimum value of k eff was 10 −5 m d −1 near f = 0 with values exceeding k eff = 5 m d −1 at f = 0.4. The largest values of k occurred during the periods when ice cover around the ITP was changing rapidly; either in advance or retreat. The model indicates that effects from shear and convection in the sea ice zone contribute an additional 40% to the magnitude of k eff , beyond what would be predicted from an estimate of k eff based solely upon a windspeed parameterization. Although the ultimate scaling relationship for gas exchange in the sea ice zone will require validation in laboratory and field studies, the basic parameter model described here demonstrates that it is feasible to formulate estimates of k based upon properties of the IOBL using data sources that presently exist.

Description: The impact of early Holocene Arctic Shelf flooding on climate in an atmosphere–ocean–sea–ice model Climate of the Past Discussions, 9, 4191-4227, 2013 Author(s): M. Blaschek and H. Renssen Glacial terminations are characterized by a strong rise in sea level related to melting ice sheets. This rise in sea level is not uniform all over the world, because regional effects (uplift and subsidence of coastal zones) are superimposed on global trends. During the early Holocene the Siberian Shelf became flooded before 7.5 ka BP and the coastline reached modern-day high stands at 5 ka BP. This area is currently known as a sea–ice production area and contributes significantly to the sea–ice exported from the Arctic through the Fram Strait. This leads to the following hypothesis: during times of rising sea levels, shelves become flooded, increasing sea–ice production on these shelves, increasing sea–ice volume and export through Fram Strait and causing the sea–ice extent to advance in the Nordic Seas, yielding cooler and fresher sea surface conditions. We have tested this hypothesis in an ocean–sea–ice–atmosphere coupled model of intermediate complexity (LOVECLIM). Our results of an early Holocene Siberian Shelf flooding show that in our model the Northern Hemisphere sea–ice production is increased (15%) and that the Northern Hemisphere sea–ice extent increases (14%) contrary to our hypothesis with lower sea–ice export through Fram Strait (−15%). The reason of this unexpected behaviour has its origin in a weakened polar vortex, induced by the land-ocean changes due to the shelf flooding, and a resulting decrease of zonality in the Nordic Seas pressure regime. Hence the winter Greenland high and the Icelandic low strengthen, yielding stronger winds on both sides of the Nordic Seas. Increased winds along the East Greenland Current support local sea–ice production and transport towards the south, resulting in a wider sea–ice cover and a southward shift of convection areas. The overall strength of the Atlantic Meridional Overturning Circulation is reduced by 4% and the heat transport in the Atlantic basin by 7%, resulting in an annual cooling pattern over the Nordic Seas by up to −4 °C. We find that the flooding of the Siberian shelf as a result of an orbital induced warming, causing Northern Hemisphere ice sheets to melt and global sea level to rise, causes a Nordic Seas cooling feedback opposed to this warming.

Description: One plausible reason for the change in ENSO characteristics in the 2000s Ocean Science Discussions, 10, 951-984, 2013 Author(s): V. N. Stepanov It is well known that El Niño Southern Oscillation (ENSO) causes floods, droughts and the collapse of fisheries, therefore forecasting of ENSO is an important task in climate researches. Variations in the equatorial warm water volume of the tropical Pacific and wind variability in the western equatorial Pacific has been considered to be a good ENSO predictor. However, in the 2000s, the interrelationship between these two characteristics and ENSO onsets became weak. This article attempts to find some plausible explanation for this. The results presented here demonstrate a possible link between the variability of atmospheric conditions over the Southern Ocean and their impact on the ocean circulation leading to the amplifying/triggering of ENSO events. It is shown that the variability of the atmospheric conditions upstream of Drake Passage can strongly influence ENSO events. The interrelationship between ENSO and variability in the equatorial warm water volume of the equatorial Pacific, together with wind variability in the western equatorial Pacific has recently weakened. It can be explained by the fact that the process occurred in the Southern Ocean recently became a major contributor amplifying ENSO events (in comparison with the processes of interaction between the atmosphere and the ocean in the tropics of the Pacific). Likely it is due to a warmer ocean state observed from the end of the 1990s that led to smaller atmospheric variability in the tropics and insignificant their changes in the Southern Ocean.

Description: Frontal structures in the West Spitsbergen Current margins Ocean Science Discussions, 10, 985-1030, 2013 Author(s): W. Walczowski The structures of the hydrographic fronts separating the Atlantic origin waters from ambient waters in the northern Nordic Seas are discussed. Flows of the western and eastern branches of the West Spitsbergen Current create the Atlantic domain borders and maintain these fronts. The work is based on previous research and on investigations in the project DAMOCLES (Developing Arctic Modeling and Observational Capabilities for Long-term Environmental Studies). Most of the observational data were collected during the R/V Oceania cruises. The main focus of the paper is put on the western border of the Atlantic domain – the Arctic Front, along- and transfrontal transports, the front instability and variability. The baroclinic instability and advection of baroclinic eddies which occurs due to this instability were found as the main transfrontal transport processes. Most of the Atlantic Water transported by the western branch recirculates west and southward. The eastern branch of the West Spitsbergen Current provides most of the Atlantic Water entering the Arctic Ocean. Both processes are very important for the Arctic and global Thermohaline Circulation.

Description: A brief history of ice core science over the last 50 yr Climate of the Past Discussions, 9, 3711-3767, 2013 Author(s): J. Jouzel For about 50 yr, ice cores have provided a wealth of information about past climatic and environmental changes. Ice cores from Greenland, Antarctica and other glaciers, now emcompass a variety of timescales. However, the longer time scales (e.g. at least back to the Last Glacial period) are covered by deep ice cores the number of which is still very limited, seven from Greenland, with only one providing an undisturbed record of a part of the Last Interglacial Period, and a dozen from Antarctica with the longest record covering the last 800 000 yr. This article aims to summarize this successful adventure initiated by a few pioneers and their teams and to review key scientific results in focusing on climate (in particular water isotopes) and climate related (e.g. greenhouse gases) reconstructions. Future research is well taken into account by the four projects defined by IPICS. However it remains a challenge to get an intact record of the Last Interglacial in Greenland and to extend the Antarctic record through the mid-Pleistocene transition, if possible back to 1.5 Myr.

Description: Turbulent dispersion properties from a model simulation of the western Mediterranean Ocean Science Discussions, 10, 1099-1125, 2013 Author(s): H. Nefzi, D. Elhmaidi, and X. Carton Using a high resolution primitive equation model of the western Mediterranean Sea, we analyzed the dispersion properties of a set of homogeneously distributed, passive particle pairs. These particles were initially separated by different distances D 0 ( D 0 = 5.55, 11.1 and 16.5 km), and were seeded in the model at initial depths of 44 and 500 m. This realistic ocean model, which reproduces the main features of the regional circulation, puts in evidence the three well-known regimes of relative dispersion. The first regime due to the chaotic advection at small scales, lasts only a few days (3 days at 44 m depth, a duration comparable with the integral time scale) and the relative dispersion is then exponential. In the second regime, extending from 3 to 20 days, the relative dispersion has a power law t α where α tends to 3 as D 0 becomes small. In the third regime, a linear growth of the relative dispersion is observed starting from the twentieth day. For the relative diffusivity, the D 2 growth is followed by the Richardson regime D 4/3 . At large scales, where particle velocities are decorrelated, the relative diffusivity is constant. At 500 m depth, the integral time scale increases (〉 4 days) and the intermediate regime becomes narrower than that at 44 m depth due to weaker effect of vortices (this effect decreases with depth). The turbulent properties become less intermittent and more homogeneous and the Richardson law takes place.

Description: Past freeze and thaw cycling in the margin of the El'gygytgyn Crater deduced from a 141 m long permafrost record Climate of the Past Discussions, 9, 3993-4034, 2013 Author(s): G. Schwamborn, H. Meyer, L. Schirrmeister, and G. Fedorov Past permafrost thaw and freeze has destabilised the basin slopes of Lake El'gygytgyn in the northeastern Eurasian Arctic. This has probably promoted the release of mass movements from the lake edge to the deeper basin as known from frequently occurring turbidite layers in the lake sediment column. The continuous sediment record from the Arctic spans the last 3.6 Ma and for much of this time permafrost dynamics and lake level changes likely have played a crucial role for sediment delivery to the lake. Changes in the ground ice hydrochemical composition (pH, δ 18 O, δD, electrical conductivity, Na + , Mg 2+ , Ca 2+ , K + , HCO 3 − , Cl − , SO 4 − ) of a 141 m long permafrost record from the western crater plain are examined to reconstruct repeated freeze and thaw cycles at the lake edge. Stable water isotope and major ion records of ground ice in the permafrost reflect both a synsedimentary palaeo-precipitation signal preserved in the near-surface permafrost (0.0 m to 9.1 m core depth) and a postdepositional record of talik thawing and refreezing in deeper layers of the core (9.1 to 141.0 m core depth). The lake marginal permafrost dynamics were controlled by lake level changes that episodically flooded the surfaces and induced thaw in the underlying frozen ground. At least three cycles of freeze and thaw during marine isotope stage (MIS) 7, possibly MIS 5, and the Allerød (AD) are identified and the hydrochemical data point to a vertical and horizontal talik refreezing through time.

Description: 10 Be in last deglacial climate simulated by ECHAM5-HAM – Part 1: Climatological influences on 10 Be deposition Climate of the Past Discussions, 9, 3681-3709, 2013 Author(s): U. Heikkilä, S. J. Phipps, and A. M. Smith Reconstruction of solar irradiance has only been possible for the Holocene so far. During the last deglaciation two solar proxies ( 10 Be and 14 C) deviate strongly, both of them being influenced by climatic changes in a different way. This work addresses the climate influence on 10 Be deposition by means of ECHAM5-HAM atmospheric aerosol-climate model simulations, forced by sea surface temperatures and sea ice extent created by the coupled climate system model CSIRO Mk3L. Three time slice simulations were performed during the last deglaciation: 10 000 BP ("10k"), 11 000 BP ("11k") and 12 000 BP ("12k"), each 30 yr long. The same 10 Be production rate was used in each simulation to isolate the impact of climate on 10 Be deposition. The changes are found to follow roughly the reduction in the greenhouse gas concentrations within the simulations. The 10k and 11k simulations produce a surface cooling which is symmetrically amplified in the 12k simulation. The precipitation rate is only slightly reduced at high latitudes, but there is a northward shift in the polar jet in the Northern Hemisphere and the stratospheric westerly winds are significantly weakened. These changes occur where the sea ice change is largest in the deglaciation simulations. This leads to a longer residence time of 10 Be in the stratosphere by 30 (10k and 11k) to 80 (12k) days, heavily increasing the atmospheric concentrations. Furthermore the shift of westerlies in the troposphere leads to an increase of tropospheric 10 Be concentrations, especially at high latitudes. The contribution of dry deposition generally increases, but decreases where sea ice changes are largest. In total, the 10 Be deposition rate changes by no more than 20% at mid- to high latitudes, but by up to 50% in the tropics. We conclude that on "long" time scales (a year to a few years), climatic influences on 10 Be deposition remain small even though atmospheric concentrations can vary significantly. Averaged over a longer period all 10 Be produced has to be deposited by mass conservation. This dominates over any climatic influences on 10 Be deposition. Snow concentrations, however, do not follow mass conservation and can potentially be impacted more by climate due to precipitation changes. Quantifying the impact of deglacial climate modulation on 10 Be in terms of preserving the solar signal locally is analysed in an accompanying paper (Heikkilä et al., 2013).

Description: Equilibrator-based measurements of dissolved nitrous oxide in the surface ocean using an integrated cavity output laser absorption spectrometer Ocean Science Discussions, 10, 1031-1065, 2013 Author(s): I. Grefe and J. Kaiser A laser-based analyser for nitrous oxide, carbon monoxide and water vapour was coupled to an equilibrator for continuous high-resolution dissolved gas measurements in the surface ocean. Results for nitrous oxide measurements from laboratory tests and field deployments are presented here. Short-term precision for 10 s-average N 2 O mole fractions at an acquisition rate of 1 Hz was better than 0.2 nmol mol −1 for standard gases and equilibrator measurements. The same precision was achieved for replicate standard gas analyses within 1 h of each other. The accuracy of the equilibrator measurements was verified by comparison with purge-and-trap GC-MS measurements of N 2 O concentrations in discrete samples from the Southern Ocean and showed agreement to within the 2% measurement uncertainty of the GC-MS method. Measured atmospheric N 2 O mole fractions agreed with AGAGE values to within 0.4%. The equilibrator response time to concentration changes in water was 142 to 203 s, depending on the headspace flow rate. The system was tested at sea during a north-to-south transect of the Atlantic Ocean. While the subtropical gyres were slightly undersaturated, the equatorial region was a source of nitrous oxide to the atmosphere. The ability to measure at high temporal and spatial resolution revealed sub-mesoscale variability in dissolved N 2 O concentrations. The magnitude of the observed saturation is in agreement with published data. Mean sea-to-air fluxes in the tropical and subtropical Atlantic ranged between −1.6 and 0.11 μmol m −2 d −1 and confirm that the subtropical Atlantic is not an important source region for N 2 O to the atmosphere, compared to average global fluxes of 0.6 to 2.4 μmol m −2 d −1 . The system can be easily modified for autonomous operation on voluntary observing ships (VOS). Further work should include an interlaboratory comparison exercise with other methods of dissolved N 2 O analyses.

Description: Greenland accumulation and its connection to the large-scale atmospheric circulation in ERA-Interim and paleo-climate simulations Climate of the Past Discussions, 9, 3825-3870, 2013 Author(s): N. Merz, C. C. Raible, H. Fischer, V. Varma, M. Prange, and T. F. Stocker Accumulation and aerosol chemistry records from Greenland ice cores offer the potential to reconstruct variability in Northern Hemisphere atmospheric circulation over the last millennia. However, an important prerequisite for a reconstruction is the stable relationship between local accumulation at the ice core site with the respective circulation pattern throughout the reconstruction period. We address this stability issue by using a comprehensive climate model and performing time-slice simulations for the present, the pre-industrial, the early Holocene and the last glacial maximum (LGM). The relationships between accumulation, precipitation and atmospheric circulation are investigated on on various time-scales. The analysis shows that the relationship between local accumulation on the Greenland ice sheet and the large-scale circulation undergoes a significant seasonal cycle. As the weights of the individual seasons change, annual mean accumulation variability is not necessarily related to the same atmospheric circulation patterns during the different climate states. Within a season, local Greenland accumulation variability is indeed linked to a consistent circulation pattern, which is observed for all studied climate periods, even for the LGM, however these circulation patterns are specific for different regions on the Greenland ice sheet. The simulated impact of orbital forcing and changes in the ice-sheet topography on accumulation exhibits strong spatial variability emphasizing that accumulation records from different ice core sites cannot be expected to look alike since they include a distinct local signature. Accumulation changes between different climate periods are dominated by changes in the amount of snowfall and are driven by both thermodynamic and dynamic factors. The thermodynamic impact determines the strength of the hydrological cycle, and warmer temperatures are generally accompanied by an increase in Greenland precipitation. Dynamical drivers of accumulation changes are the large-scale circulation and the local orography having a distinct influence on the local flow characteristic and hence the amount of precipitation deposited in any Greenland region.

Description: Multi-decadal to-century NAO-like marine climate oscillations across the Denmark Strait (~66° N) over the last 2000 cal yr BP Climate of the Past Discussions, 9, 3871-3917, 2013 Author(s): J. T. Andrews and A. E. Jennings In the area of Denmark Strait (~66° N) the two modes of the North Atlantic Oscillation (NAO) are expressed in changes of the northward flux of Atlantic Water and the southward advection of Polar Water in the East Iceland Current. Proxies from marine cores along an environmental gradient from extensive to little or no drift ice, capture low frequency NAO-like variations over the last 2000 cal yr BP. Key proxies are the weight% of calcite, a measure of surface water stratification and nutrient supply, the weight% of quartz, a measure of drift ice transport, and grain-size. Records from Nansen and Kangerlussuaq fjords show variable ice-rafted debris (IRD) records but have distinct mineralogy associated with differences in the fjord catchment bedrock. High-resolution detrended records from Kangerlussuaq Trough show abrupt, significant multi-decadal changes (72 and 56 yr for calcite, and 94 and 65 yr for quartz), and parallel the 2000 yr Arctic summer temperature reconstructions. The calcite minimum occurred ca. 1550 AD whereas the quartz maxima occurred 200 yr earlier. Changes in calcite wt% from N and SW Iceland show similar abrupt changes to those in Kangerlussuaq Trough with an abrupt calcite peak ~1320 AD. Quartz values increased at two N Iceland sites in the last 500 yr whereas values declined in the East Greenland site.

Description: Persistent millennial-scale link between Greenland climate and northern Pacific Oxygen Minimum Zone under interglacial conditions Climate of the Past Discussions, 9, 3919-3952, 2013 Author(s): O. Cartapanis, K. Tachikawa, O. E. Romero, and E. Bard The intensity and/or extent of the northeastern Pacific Oxygen Minimum Zone (OMZ) varied in-phase with the Northern Hemisphere high latitude climate on millennial timescales during the last glacial period, indicating the presence of atmospheric and oceanic connections under glacial conditions. While millennial variability was observed for both the Greenland ice core and the northern Atlantic during the last interglacial period, the relationship with the northeastern Pacific OMZ has not yet been observed under warm interglacial conditions. Here we present a~new geochemical dataset, spanning the past 120 kyr, for major components (terrigenous fraction, marine organic matter, biogenic opal, and carbonates) by X-ray fluorescence scanning alongside with biological productivity and redox sensitive trace element content (Mo, Ni, Cd) of sediment core MD02-2508 at 23° N, retrieved from the northern limit of the modern OMZ. We evidenced high biological productivity from opal based on Si/Ti, and from Cd/Al, and Ni/Al ratios during last interglacial period. Highly resolved opal reconstruction presents millennial variability corresponding to all the Dansgaard–Oeschger interstadial events over the last interglacial, while the Mo/Al ratio indicates reduced oxygenation during these intervals. Extremely high opal content during warm interstadials suggests high diatom productivity. Despite the different climatic and oceanic background between glacial and interglacial periods, rapid variability in the northeastern Pacific OMZ seems to be tightly related to Northern Hemisphere high latitude climate mainly via atmospheric processes.

Description: Investigating the consistency between proxies and between proxies and models using data assimilation: a mid-Holocene case study Climate of the Past Discussions, 9, 3953-3991, 2013 Author(s): A. Mairesse, H. Goosse, P. Mathiot, H. Wanner, and S. Dubinkina The mid-Holocene (6 thousand years before present) is a key period to study the consistency between model results and proxy data as it corresponds to a standard test for models and a reasonable number of proxy records are available. Taking advantage of this relatively large amount of information, we have first compared a compilation of 50 air and sea surface temperature reconstructions with the results of three simulations performed with general circulation models and one carried out with LOVECLIM, a model of intermediate complexity. The conclusions derived from this analysis confirm that models and data agree on the large-scale spatial pattern but underestimate the magnitude of some observed changes and that large discrepancies are observed at the local scale. To further investigate the origin of those inconsistencies, we have constrained LOVECLIM to follow the signal recorded by the proxies selected in the compilation using a data assimilation method based on a particle filter. In one simulation, all the 50 proxies are used while in the other two, only the continental or oceanic proxies constrains the model results. This assimilation improves the consistency between model results and the reconstructions. In particular, this is achieved in a robust way in all the experiments through a strengthening of the westerlies at mid-latitude that warms up the Northern Europe. Furthermore, the comparison of the LOVECLIM simulations with and without data assimilation has also objectively identified 16 proxies whose reconstructed signal is either incompatible with the one recorded by some other proxies or with model physics.

Description: Biogeochemical properties and diagenetic changes during the past 3.6 Ma recorded by FTIR spectroscopy in the sediment record of Lake El'gygytgyn, Far East Russian Arctic Climate of the Past Discussions, 9, 2489-2515, 2013 Author(s): C. Meyer-Jacob, H. Vogel, M. Melles, and P. Rosén A number of studies have shown that Fourier transform infrared spectroscopy (FTIRS) can be applied to quantitatively assess lacustrine sediment constituents. In this study, we developed calibration models based on FTIRS for the quantitative determination of biogenic silica (BSi; n = 420; gradient: 0.9–56.5%), total organic carbon (TOC; n = 309; gradient: 0.02–2.89%), and total inorganic carbon (TIC; n = 153; gradient: 0.01–1.46%) in a 318 m long sediment record with a basal age of 3.6 Ma from Lake El'gygytgyn, Far East Russian Arctic. The developed partial least squares regression (PLSR) models yield high cross-validated (CV) R 2 CV = 0.85–0.91 and low root mean square error of cross-validation (RMSECV) (2.1–4.3% of the gradient for the different properties). The FTIRS-inferred concentrations of BSi, TOC, and TIC provide an initial insight into the climatic and environmental evolution at Lake El'gygytgyn throughout the late Pliocene and Quaternary showing a considerably high bioproductivity in the lake ecosystem between ~ 3.27–3.54 Ma during the early Pliocene warm period. Moreover, we found that the recorded FTIR spectra contain information on sample burial depth as a result of diagenetic changes (dehydration/dehydroxilation) of certain mineral phases. Despite the indicated post-depositional processes, the calibration models yield good statistical performances showing that general FTIRS models can be developed for several hundred meters long records extending several million years back in time. Our results highlight FTIRS to be a rapid, cost-effective alternative to conventional methods for quantification of biogeochemical properties.

Description: High-resolution glacial and deglacial record of atmospheric methane by continuous-flow and laser spectrometer analysis along the NEEM ice core Climate of the Past Discussions, 9, 2517-2556, 2013 Author(s): J. Chappellaz, C. Stowasser, T. Blunier, D. Baslev-Clausen, E.J. Brook, R. Dallmayr, X. Faïn, J.E. Lee, L.E. Mitchell, O. Pascual, D. Romanini, J. Rosen, and S. Schüpbach The Greenland NEEM (North Greenland Eemian Ice Drilling) operation in 2010 provided the first opportunity to combine trace-gas measurements by laser spectroscopic instruments and continuous-flow analysis along a freshly drilled ice core in a field based setting. We present the resulting atmospheric methane (CH 4 ) record covering the time period from 107.7 to 9.5 ka b2k (thousand years before 2000 AD). Companion discrete CH 4 measurements are required to transfer the laser spectroscopic data from a relative to an absolute scale. However, even on a relative scale, the high-resolution CH 4 dataset significantly improves our knowledge of past atmospheric methane concentration changes. New significant sub-millennial-scale features appear during interstadials and stadials, generally associated with similar changes in water isotopic ratios of the ice, a proxy for local temperature. In addition to the mid-point of Dansgaard/Oeschger (D/O) CH 4 transitions usually used for cross-dating, sharp definition of the start and end of these events brings precise depth markers (with ±20 cm uncertainty) for further cross-dating with other ice core or paleo records, e.g. speleothems. The method also provides an estimate of CH 4 rates of change. The onsets of D/O events in the methane signal show a more rapid rate of change than their endings. The rate of CH 4 increase associated with the onsets of D/O events progressively declines from 1.7 to 0.6 ppbv yr −1 in the course of Marine Isotope Stage 3. The largest observed rate of increase takes place at the onset of D/O event #21 and reaches 2.5 ppbv yr −1 .

Description: Evaluating climate field reconstruction techniques using improved emulations of real-world conditions Climate of the Past Discussions, 9, 3015-3060, 2013 Author(s): J. Wang, J. Emile-Geay, D. Guillot, J. E. Smerdon, and B. Rajaratnam Pseudoproxy experiments (PPEs) have become an essential framework for evaluating paleoclimate reconstruction methods. Most existing PPE studies assume constant proxy availability through time and uniform proxy quality across the pseudoproxy network. Real multi-proxy networks are, however, marked by pronounced disparities in proxy quality, and a steep decline in proxy availability back in time, either of which may have large effects on reconstruction skill. Additionally, an investigation of a real-world global multi-proxy network suggests that proxies are not exclusively indicators of local climate; rather, many are indicative of large-scale teleconnections. A suite of PPEs constructed from a millennium-length general circulation model simulation is thus designed to mimic these various real-world characteristics. The new pseudoproxy network is used to evaluate four climate field reconstruction (CFR) techniques: truncated total least square embedded within the regularized EM algorithm (RegEM-TTLS), the Mann et al. (2009) implementation of RegEM-TTLS (M09), canonical correlation analysis (CCA), and Gaussian graphical models embedded within RegEM (GraphEM). Each method's risk properties are also assessed via a 100-member noise ensemble. Contrary to expectation, it is found that reconstruction skill does not vary monotonically with proxy availability, but rather is a function of the type of climate variability (forced events vs. internal variability). The use of realistic spatiotemporal pseudoproxy characteristics also exposes large inter-method differences. Despite the comparable fidelity in reconstructing the global mean temperature, spatial skill varies considerably between CFR techniques. Both GraphEM and CCA efficiently exploit teleconnections, and produce consistent reconstructions across the ensemble. RegEM-TTLS and M09 appear advantageous for reconstructions on highly noisy data, but are subject to larger stochastic variations across different realizations of pseudoproxy noise. Results collectively highlight the importance of designing realistic pseudoproxy networks and implementing multiple noise realizations of PPEs. The results also underscore the difficulty in finding the proper bias-variance tradeoff for jointly optimizing the spatial skill of CFRs and the fidelity of the global mean reconstructions.

Description: On the low frequency component of the ENSO-Indian Monsoon relationship; a paired proxy perspective Climate of the Past Discussions, 9, 3103-3123, 2013 Author(s): M. Berkelhammer, A. Sinha, M. Mudelsee, H. Cheng, K. Yoshimura, and J. Biswas There are a number of clear examples in the instrumental period where positive El Niño events were coincident with a severely weakened summer monsoon over India (ISM). ENSO's influence on the Indian Monsoon has therefore remained the centerpiece of various predictive schemes of ISM rainfall for over a century. The teleconnection between the monsoon and ENSO has undergone a protracted weakening since the late 1980's suggesting the strength of ENSO's influence on the monsoon may vary considerably on multidecadal timescales. The recent weakening has specifically prompted questions as to whether this shift represents a natural mode of climate variability or a fundamental change in ENSO and/or ISM dynamics due to anthropogenic warming. The brevity of empirical observations and large systematic errors in the representation of these two systems in state-of-the-art general circulation models hamper efforts to reliably assess the low frequency nature of this dynamical coupling under varying climate forcings. Here we place the 20th century ENSO-Monsoon relationship in a millennial context by assessing the phase angle between the two systems across the time spectrum using a continuous tree-ring ENSO reconstruction from North America and a speleothem oxygen isotope (δ 18 O) based reconstruction of the ISM. The results suggest that in the high-frequency domain (≤ 15 yr), El Niño (La Niña) events persistently lead to a weakened (strengthened) monsoon consistent with the observed relationship between the two systems during the instrumental period. However, in the low frequency domain (≥ 60 yr), periods of strong monsoon are, in general, coincident with periods of enhanced ENSO variance. This relationship is opposite to which would be predicted dynamically and leads us to conclude that ENSO is not pacing the prominent multidecadal variability that has characterized the ISM over the last millennium.

Description: Paleo Agulhas rings enter the subtropical gyre during the penultimate deglaciation Climate of the Past Discussions, 9, 2095-2114, 2013 Author(s): P. Scussolini and E. van Sebille A maximum in the strength of Agulhas Leakage has been registered at the interface between Indian and South Atlantic oceans during glacial Termination II (T II), presumably transporting the salt and heat necessary to maintain the Atlantic Meridional Overturning Circulation (AMOC) at rates similar to the present day. However, it was never shown whether these were effectively incorporated in the South Atlantic gyre, or whether they retroflected into the Indian and/or Southern Oceans. To solve this question, we investigate the presence of paleo Agulhas rings from a sediment core on the central Walvis Ridge, almost 1800 km farther into the Atlantic basin than previously studied. Analysis of a 20 yr dataset from a global ocean circulation model allows us to relate density perturbations, at the depth of the thermocline, to the passage of individual rings over the core site. Using this relation from the numerical model as the basis for a proxy, we generate a time series of δ 18 O variability of Globorotalia truncatulinoides single specimens, revealing high levels of pycnocline depth variability at the site, suggesting enhanced numbers of Agulhas rings moving into the South Atlantic gyre around and before T II. Our record closely follows the published quantifications of Agulhas Leakage from the east of the Cape Basin, and thus shows that Indian Ocean waters entered the South Atlantic circulation. This provides crucial support to the view of a prominent role of the Agulhas Leakage in the shift from a glacial to an interglacial mode of AMOC.

Description: A high resolution record of atmospheric carbon dioxide and its stable carbon isotopic composition from the penultimate glacial maximum to the glacial inception Climate of the Past Discussions, 9, 2015-2057, 2013 Author(s): R. Schneider, J. Schmitt, P. Köhler, F. Joos, and H. Fischer The reconstruction of the stable carbon isotope evolution in atmospheric CO 2 (δ 13 C atm ), as archived in Antarctic ice cores, bears the potential to disentangle the contributions of the different carbon cycle fluxes causing past CO 2 variations. Here we present a highly resolved record of δ 13 C atm before, during and after the Marine Isotope Stage 5.5 (155 000 to 105 000 yr BP). The record was derived with a well established sublimation method using ice from the EPICA Dome C (EDC) and the Talos Dome ice cores in East Antarctica. We find an 0.4‰ offset between the mean δ 13 C atm level in the Penultimate (~140 000 yr BP) and Last Glacial Maximum (~22 000 yr BP), which can be explained by either (i) changes in the isotopic composition or (ii) intensity of the carbon input fluxes to the combined ocean/atmosphere carbon reservoir or (iii) by long-term peat buildup. Our isotopic data suggest that the carbon cycle evolution along Termination II and the subsequent interglacial was controlled by essentially the same processes as during the last 24 000 yr, but with different phasing and magnitudes. Furthermore, a 5000 yr lag in the CO 2 decline relative to EDC temperatures is confirmed during the glacial inception at the end of MIS 5.5 (120 000 yr BP). Based on our isotopic data this lag can be explained by terrestrial carbon release and carbonate compensation.

Description: Tidally-induced lateral dispersion of the Storfjorden overflow plume Ocean Science Discussions, 10, 691-726, 2013 Author(s): F. Wobus, G. I. Shapiro, J. M. Huthnance, M. A. M. Maqueda, and Y. Aksenov We investigate the flow of brine-enriched shelf water from Storfjorden (Svalbard) into Fram Strait and onto the Western Svalbard Shelf using a regional setup of NEMO-SHELF, a 3-D numerical ocean circulation model. The model is set up with realistic bathymetry, atmospheric forcing, open boundary conditions and tides. The model has 3 km horizontal resolution and 50 vertical levels in the s h -coordinate system which is specially designed to resolve bottom boundary layer processes. In a series of modelling experiments we focus on the influence of tides on the propagation of the dense water plume by comparing results from tidal and non-tidal model runs. Comparisons of non-tidal to tidal simulations reveal a hotspot of tidally-induced horizontal diffusion leading to the lateral dispersion of the plume at the southernmost headland of Spitsbergen which is in close proximity to the plume path. As a result the lighter fractions in the diluted upper layer of the plume are drawn into the shallow coastal current that carries Storfjorden water onto the Western Svalbard Shelf, while the dense bottom layer continues to sink down the slope. This bifurcation of the plume into a diluted shelf branch and a dense downslope branch is enhanced by tidally-induced shear dispersion at the headland. Tidal effects at the headland are shown to cause a net reduction in the downslope flux of Storfjorden water into deep Fram Strait. This finding contrasts previous results from observations of a dense plume on a different shelf without abrupt topography.

Description: Coupling of eastern and western subpolar North Atlantic: salt transport in the Irminger Current Ocean Science Discussions, 10, 555-579, 2013 Author(s): A. Born, T. F. Stocker, and A. B. Sandø Salt transport in the Irminger Current and thus the coupling between eastern and western subpolar North Atlantic plays an important role for climate variability across a wide range of time scales. High-resolution ocean modeling and observations indicate that salinities in the eastern subpolar North Atlantic decrease with enhanced circulation of the North Atlantic subpolar gyre (SPG). This has led to the perception that a stronger SPG also transports less salt westward. In this study, we analyze a regional ocean model and a comprehensive global coupled climate model, and show that a stronger SPG transports more salt in the Irminger Current irrespective of lower salinities in its source region. The additional salt converges in the Labrador Sea and the Irminger Basin by eddy transports, increases surface salinity in the western SPG, and favors more intense deep convection. This is part of a positive feedback mechanism with potentially large implications for climate variability and predictability.

Description: A comparative study of large scale atmospheric circulation in the context of future scenario (RCP4.5) and past warmth (Mid Pliocene) Climate of the Past Discussions, 9, 1449-1483, 2013 Author(s): Y. Sun, G. Ramstein, C. Contoux, and T. Zhou The Pliocene climate (3.3 ~ 3.0 Ma) is often considered as the last sustained warm period with close enough geographic configurations compared to the present one and associated with atmospheric CO 2 concentration (405 ± 50 ppm) higher than the modern level. It is therefore suggested that the warm Pliocene climate may provide a plausible scenario for the future climate warming with the important advantage, that for mid-Pliocene, many marine and continental data are available. To investigate this issue, we selected RCP4.5 scenario, one of the current available future projections, to compare the pattern of tropical atmospheric response with past warm mid-Pliocene climate. We performed three OAGCM simulations (RCP4.5 scenario, mid-Pliocene and present day simulation) with the IPSL-CM5A model and investigated atmospheric tropical dynamics through Hadley and Walker cell responses to warmer conditions. Our results show that there is a damping of the Hadley cell intensity in the northern tropics and an increase in both subtropics. Moreover, northern and southern Hadley cells expand poleward. The response of Hadley cell is stronger for RCP4.5 scenario than for mid-Pliocene, but in very good agreement with the fact the atmospheric CO 2 concentration is higher in future scenario than mid-Pliocene (543 versus 405 ppm). Concerning the response of the Walker cell, we showed that, despite very large similarities, there are also some differences. i.e. the common features are for both scenarios: weakening of the ascending branch, leading to a suppression of the precipitation over the western tropical Pacific. The response of Walker cell is stronger in RCP4.5 scenario than mid-Pliocene but also depicts some major difference as an eastward shift of the rising branch of Walker cell in future scenario compared to the mid-Pliocene. In this paper, we explain the dynamics of the Hadley and Walker cell, and show that despite minor discrepancy, mid-Pliocene is certainly an interesting analogue for future climate changes in the tropical areas.

Description: Re-evaluation of the age model for North Atlantic Ocean Site 982 – arguments for a return to the original chronology Climate of the Past Discussions, 9, 2217-2233, 2013 Author(s): K. T. Lawrence, I. Bailey, and M. E. Raymo Recently, the veracity of the published chronology for the Pliocene section of North Atlantic Ocean Drilling Program Site 982 was called into question. Here, we examine the robustness of the original age model as well as the proposed age model revision. The proposed revision is predicated on an apparent mis-identification of the depth to the Gauss-Matuyama (G/M) polarity chronozone reversal boundary (2.581 Ma) based on preliminary shipboard paleomagnetic data and offers a new chronology which includes a hiatus between ~ 3.2 and 3 Ma. However, an even more accurate shore-based, u-channel-derived polarity chronozone stratigraphy for the past ~ 2.7 Ma supports the shipboard composite stratigraphy and demonstrates that the original estimate of the depth of the G/M reversal in the Site 982 record is correct. Thus, the main justification forwarded to support the revised chronology is not valid. We demonstrate that the proposed revision results in a pronounced anomaly in sedimentation rates proximal to the proposed hiatus, erroneous assignment of marine-isotope stages in the Site 982 Pliocene benthic stable oxygen isotope stratigraphy, and a markedly worse correlation of proxy records between this site and other regional paleoclimate data. We conclude that the original chronology for Site 982 is a far more accurate age-model than that which arises from the published revision. We strongly recommend the use of the original chronology for all future work at Site 982.

Description: Major dust events in Europe during marine isotope stage 5 (130–74 ka): a climatic interpretation of the "markers" Climate of the Past Discussions, 9, 2235-2276, 2013 Author(s): D.-D. Rousseau, M. Ghil, G. Kukla, A. Sima, P. Antoine, M. Fuchs, C. Hatté, F. Lagroix, M. Debret, and O. Moine At present, major dust storms are occurring at mid-latitudes in the Middle East and Asia, as well as at low latitudes in northern Africa and in Australia. Western Europe, though, does not experience such dramatic climate events, except for some African dust reaching it from the Sahara. This modern situation is of particular interest, in the context of future climate projections, since the present interglacial is usually interpreted, in this context, as an analog of the warm Eemian interval. European terrestrial records show, however, major dust events during the penultimate interglacial and early glacial. These events are easily observed in loess records by their whitish-color deposits, which lie above and below dark chernozem paleosols in Central European records of Marine Isotope Stage (MIS) 5 age. We describe here the base of the Dolni Vestonice (DV) loess sequence, Czech Republic, as the reference of such records. The dust is deposited during intervals that are characterized by poor vegetation – manifested by high δ 13 C values and low magnetic susceptibility – while the fine sand and clay in the deposits shows grain sizes that are clearly different from the overlying pleniglacial loess deposits. Some of these dust events have been previously described as "Markers" or Marker Silts (MS) by one of us (G. Kukla), and are dated at about 111–109 and 93–92 ka, with a third and last one slightly visible at about 75–73 ka. Other events correspond to the loess material of Kukla's cycles, and are described as eolian silts (ES); they are observed in the same DV sequence and are dated at about 106–105, 88–86, and 78.5–77 ka. The fine eolian deposits mentioned above, MS as well as ES, correspond to short events that lasted about 2 ka; they are synchronous with re-advances of the polar front over the North Atlantic, as observed in marine sediment cores. These deposits also correlate with important changes observed in European vegetation. Some ES and MS events appear to be coeval with significant dust peaks recorded in the Greenland ice cores, while others are not. This decoupling between the European eolian and Greenland dust depositions is of considerable interest, as it differs from the fully glacial situation, in which the Eurasian loess sedimentation mimics the Greenland dust record. Previous field observations supported an interpretation of MS events as caused by continental dust storms. We show here, by a comparison with speleothems of the same age found in the northern Alps, that different atmospheric-circulation modes seem to be responsible for the two categories of dust events, MS vs. ES.

Description: On the effect of orbital forcing on mid-Pliocene climate, vegetation and ice sheets Climate of the Past Discussions, 9, 1703-1734, 2013 Author(s): M. Willeit, A. Ganopolski, and G. Feulner We present results from modeling of the mid-Pliocene warm period (3.3–3 million years ago) using the Earth system model of intermediate complexity CLIMBER-2 analyzing the effect of changes in boundary conditions as well as of orbital forcing on climate. Firstly we performed equilibrium experiments following PlioMIP (Pliocene Model Intercomparison Project) protocol with a CO 2 concentration of 405 ppm, reconstructed mid-Pliocene orography and vegetation and a present day orbital configuration. Simulated global Pliocene warming is about 2.5 °C, fully consistent with results of atmosphere-ocean general circulation model simulations performed for the same modeling setup. A factor separation analysis attributes 1.5 °C warming to CO 2 , 0.3 °C to orography, 0.2 °C to ice sheets and 0.4 °C to vegetation. Transient simulations for the entire mid-Pliocene warm period with time-dependent orbital forcing as well as interactive ice sheets and vegetation give a global warming varying within the range 1.9–2.8 °C. Ice sheet and vegetation feedbacks in synergy act as amplifiers of the orbital forcing transforming seasonal insolation variations into an annual mean temperature signal. The effect of orbital forcing is more significant at high latitudes, especially during summer, when the warming over land varies in the wide range from 0–10 °C. The modeled ice sheet extent and vegetation distribution also show significant temporal variations. Modeled and reconstructed data for Northern Hemisphere sea surface temperatures and vegetation distribution show the best agreement if the reconstructions are assumed to be representative for the "warmest" periods during the orbital cycles. This suggests that low-resolution Pliocene paleoclimate reconstructions can reflect not only the impact of increased CO 2 concentrations and topography changes but also the effect of orbital forcing. Therefore, the climate (Earth system) sensitivity estimates from Pliocene reconstructions which do not account for the effect of orbital forcing can be biased toward high values.

Description: A 1500 yr warm-season temperature record from varved Lago Plomo, Northern Patagonia (47° S) and implications for the Pacific Decadal Oscillation (PDO) Climate of the Past Discussions, 9, 1771-1801, 2013 Author(s): J. Elbert, M. Jacques-Coper, M. Van Daele, R. Urrutia, and M. Grosjean High-resolution records of calibrated proxy data for the past 2000 yr are fundamental to place current changes into the context of pre-industrial natural forced and unforced variability. Although the need for regional spatially explicit comprehensive reconstructions is widely recognized, the proxy data sources are still scarce, particularly for the Southern Hemisphere and South America. We provide a 1500 yr long warm season temperature record from varved Lago Plomo, a proglacial lake of the Northern Patagonian Ice field in southern Chile (46°59' S, 72°52' W, 203 m). The thickness of the bright summer sediment layer relative to the dark winter layer (measured as total brightness; % reflectance 400–730 nm) is calibrated against warm season SONDJF temperature (1900–2009; r = 0.58, p (aut) = 0.056, RE = 0.52; CE = 0.15, RMSEP = 0.28 °C; five-year triangular filtered data). In Lago Plomo, warm summer temperatures lead to enhanced glacier melt and suspended sediment transport, which results in a thicker light summer layer and to brighter sediments (% total brightness). Although Patagonia shows pronounced regional differences in decadal temperature trends and variability, the 1500 yr temperature reconstruction from Lago Plomo compares favourably with other regional/continental temperature records but also emphasizes significant regional differences for which no data and information existed so far. The reconstruction shows pronounced sub-decadal–multi-decadal variability with cold phases in the 5th, 7th and 9th centuries, during parts of the Little Ice Age chronozone (16th and 18th centuries) and in the beginning of the 20th century. The most prominent warm phase is the 19th century which is as warm as the second half of the 20th century, emphasizing a delayed recent global warming in the Southern Hemisphere. The comparison between winter precipitation and summer temperature (inter-seasonal coupling) from Lago Plomo reveals alternating phases with parallel and contrasting decadal trends of winter precipitation and summer temperature and positive and negative running correlations (winterPP;summerTT) . In the 20th century the trend of this correlation changes at 1920, 1945 and 1975 AD, and the phases with positive (negative) correlations inferred from the lake sediments are also found as a regional robust pattern in reanalysis data, and coincide with the changes of the instrumental PDO index. Enhanced circumpolar flow around 60° S is proposed for positive phases of PDO which leads to the reversed coupling and contrasting decadal trends of winter precipitation and summer temperature during PDO positive phases. Our reconstruction of the inter-seasonal coupling back to 1530 AD reproduces many features of existing PDO reconstructions from the Pacific suggesting that Lago Plomo provides a record for the regional expression of the PDO in Patagonia.