ALBERT

Description: The Chemistry CATT-BRAMS model (CCATT-BRAMS 4.5): a regional atmospheric model system for integrated air quality and weather forecasting and research Geoscientific Model Development, 6, 1389-1405, 2013 Author(s): K. M. Longo, S. R. Freitas, M. Pirre, V. Marécal, L. F. Rodrigues, J. Panetta, M. F. Alonso, N. E. Rosário, D. S. Moreira, M. S. Gácita, J. Arteta, R. Fonseca, R. Stockler, D. M. Katsurayama, A. Fazenda, and M. Bela Coupled Chemistry Aerosol-Tracer Transport model to the Brazilian developments on the Regional Atmospheric Modeling System (CCATT-BRAMS, version 4.5) is an on-line regional chemical transport model designed for local and regional studies of atmospheric chemistry from the surface to the lower stratosphere suitable both for operational and research purposes. It includes gaseous/aqueous chemistry, photochemistry, scavenging and dry deposition. The CCATT-BRAMS model takes advantage of BRAMS-specific development for the tropics/subtropics as well as the recent availability of preprocessing tools for chemical mechanisms and fast codes for photolysis rates. BRAMS includes state-of-the-art physical parameterizations and dynamic formulations to simulate atmospheric circulations down to the meter. This on-line coupling of meteorology and chemistry allows the system to be used for simultaneous weather and chemical composition forecasts as well as potential feedback between the two. The entire system is made of three preprocessing software tools for user-defined chemical mechanisms, aerosol and trace gas emissions fields and the interpolation of initial and boundary conditions for meteorology and chemistry. In this paper, the model description is provided along with the evaluations performed by using observational data obtained from ground-based stations, instruments aboard aircrafts and retrieval from space remote sensing. The evaluation accounts for model applications at different scales from megacities and the Amazon Basin up to the intercontinental region of the Southern Hemisphere.

Description: δ 18 O water isotope in the i LOVECLIM model (version 1.0) – Part 1: Implementation and verification Geoscientific Model Development, 6, 1481-1491, 2013 Author(s): D. M. Roche A new 18 O stable water isotope scheme is developed for three components of the i LOVECLIM coupled climate model: atmospheric, oceanic and land surface. The equations required to reproduce the fractionation of stable water isotopes in the simplified atmospheric model ECBilt are developed consistently with the moisture scheme. Simplifications in the processes are made to account for the simplified vertical structure including only one moist layer. Implementation of these equations together with a passive tracer scheme for the ocean and a equilibrium fractionation scheme for the land surface leads to the closure of the (isotopic-) water budget in our climate system. Following the implementation, verification of the existence of usual δ 18 O to climatic relationships are performed for the Rayleigh distillation, the Dansgaard relationship and the δ 18 O –salinity relationship. Advantages and caveats of the approach taken are outlined. The isotopic fields simulated are shown to reproduce most expected oxygen-18–climate relationships with the notable exception of the isotopic composition in Antarctica.

Description: δ 18 O water isotope in the i LOVECLIM model (version 1.0) – Part 3: A palaeo-perspective based on present-day data–model comparison for oxygen stable isotopes in carbonates Geoscientific Model Development, 6, 1505-1516, 2013 Author(s): T. Caley and D. M. Roche Oxygen stable isotopes (δ 18 O) are among the most useful tools in palaeoclimatology/palaeoceanography. Simulation of oxygen stable isotopes allows testing how the past variability of these isotopes in water can be interpreted. By modelling the proxy directly in the model, the results can also be directly compared with the data. Water isotopes have been implemented in the global three-dimensional model of intermediate complexity i LOVECLIM, allowing fully coupled atmosphere–ocean simulations. In this study, we present the validation of the model results for present-day climate against the global database for oxygen stable isotopes in carbonates. The limitation of the model together with the processes operating in the natural environment reveal the complexity of use the continental calcite-δ 18 O signal of speleothems for a global quantitative data–model comparison exercise. On the contrary, the reconstructed surface ocean calcite-δ 18 O signal in i LOVECLIM does show a very good agreement with the late Holocene database (foraminifers) at the global and regional scales. Our results indicate that temperature and the isotopic composition of the seawater are the main control on the fossil-δ 18 O signal recorded in foraminifer shells when all species are grouped together. Depth habitat, seasonality and other ecological effects play a more significant role when individual species are considered. We argue that a data–model comparison for surface ocean calcite δ 18 O in past climates, such as the Last Glacial Maximum (≈ 21 000 yr), could constitute an interesting tool for mapping the potential shifts of the frontal systems and circulation changes throughout time. Similarly, the potential changes in intermediate oceanic circulation systems in the past could be documented by a data (benthic foraminifers)-model comparison exercise whereas future investigations are necessary in order to quantitatively compare the results with data for the deep ocean.

Description: Can we determine what controls the spatio-temporal distribution of d-excess and 17 O-excess in precipitation using the LMDZ general circulation model? Climate of the Past, 9, 2173-2193, 2013 Author(s): C. Risi, A. Landais, R. Winkler, and F. Vimeux Combined measurements of the H 2 18 O and HDO isotopic ratios in precipitation, leading to second-order parameter D-excess, have provided additional constraints on past climates compared to the H 2 18 O isotopic ratio alone. More recently, measurements of H 2 17 O have led to another second-order parameter: 17 O-excess. Recent studies suggest that 17 O-excess in polar ice may provide information on evaporative conditions at the moisture source. However, the processes controlling the spatio-temporal distribution of 17 O-excess are still far from being fully understood. We use the isotopic general circulation model (GCM) LMDZ to better understand what controls d-excess and 17 O-excess in precipitation at present-day (PD) and during the last glacial maximum (LGM). The simulation of D-excess and 17 O-excess is evaluated against measurements in meteoric water, water vapor and polar ice cores. A set of sensitivity tests and diagnostics are used to quantify the relative effects of evaporative conditions (sea surface temperature and relative humidity), Rayleigh distillation, mixing between vapors from different origins, precipitation re-evaporation and supersaturation during condensation at low temperature. In LMDZ, simulations suggest that in the tropics convective processes and rain re-evaporation are important controls on precipitation D-excess and 17 O-excess. In higher latitudes, the effect of distillation, mixing between vapors from different origins and supersaturation are the most important controls. For example, the lower d-excess and 17 O-excess at LGM simulated at LGM are mainly due to the supersaturation effect. The effect of supersaturation is however very sensitive to a parameter whose tuning would require more measurements and laboratory experiments. Evaporative conditions had previously been suggested to be key controlling factors of d-excess and 17 O-excess, but LMDZ underestimates their role. More generally, some shortcomings in the simulation of 17 O-excess by LMDZ suggest that general circulation models are not yet the perfect tool to quantify with confidence all processes controlling 17 O-excess.

Description: Assimilating water column and satellite data for marine export production estimation Geoscientific Model Development, 6, 1575-1590, 2013 Author(s): X. Yao and R. Schlitzer Recent advances in satellite retrieval methodology now allow for estimation of particular organic carbon (POC) concentration in ocean surface waters directly from satellite-based optical data. Because of the good coverage, these data reveal small-scale spatial and temporal concentration gradients and document the evolution of surface water POC as well as the underlying driving biogeochemical processes throughout the seasons. Water column nutrient data also reveal biogeochemical activity. However, because of the scarcity of data, the deduction of temporal changes of particle production and export is not possible in most parts of the ocean. Here we present first results from a new study combining both data streams, thereby exploiting the high spatio-temporal resolution of surface POC concentrations from satellite optical sensors with water column nutrient data having sparser coverage but providing information throughout the entire water column. We use a medium-resolution global model with steady-state 3-D circulation that has been optimized by fitting to a large number of hydrographic parameters and tracers, including CFCs and natural radiocarbon. Production and export of POC is allowed to vary monthly, and the magnitudes of the monthly export fluxes are determined by fitting the model to satellite POC data as well as water column nutrient data using the adjoint method. Two cases have been investigated: (1) the production rate of POC is set to be proportional to export production (EP) and the seasonal changes are assumed sinusoidal (meridionally varying amplitude and phase), and (2) the POC production rate is linked to primary production rates (literature). Both cases were run with the same initial state and model settings, and show total cost function decreases of 12 and 95%, respectively. The POC misfit term alone decreased by 75 and 99.8%. The integrated annual global POC exports of the two cases are 9.9 and 12.3 Gt C yr −1 , respectively. Overall, the remaining POC and phosphate misfits of both solutions are considered too large, and the difference fields still exhibit significant systematic geographical patterns. This indicates that the present model runs are too simplistic and do not fully explain the data. Further, more refined model setups are needed.

Description: Enhancing the representation of subgrid land surface characteristics in land surface models Geoscientific Model Development, 6, 1609-1622, 2013 Author(s): Y. Ke, L. R. Leung, M. Huang, and H. Li Land surface heterogeneity has long been recognized as important to represent in the land surface models. In most existing land surface models, the spatial variability of surface cover is represented as subgrid composition of multiple surface cover types, although subgrid topography also has major controls on surface processes. In this study, we developed a new subgrid classification method (SGC) that accounts for variability of both topography and vegetation cover. Each model grid cell was represented with a variable number of elevation classes and each elevation class was further described by a variable number of vegetation types optimized for each model grid given a predetermined total number of land response units (LRUs). The subgrid structure of the Community Land Model (CLM) was used to illustrate the newly developed method in this study. Although the new method increases the computational burden in the model simulation compared to the CLM subgrid vegetation representation, it greatly reduced the variations of elevation within each subgrid class and is able to explain at least 80% of the total subgrid plant functional types (PFTs). The new method was also evaluated against two other subgrid methods (SGC1 and SGC2) that assigned fixed numbers of elevation and vegetation classes for each model grid (SGC1: M elevation bands– N PFTs method; SGC2: N PFTs– M elevation bands method). Implemented at five model resolutions (0.1°, 0.25°, 0.5°, 1.0°and 2.0°) with three maximum-allowed total number of LRUs (i.e., N LRU of 24, 18 and 12) over North America (NA), the new method yielded more computationally efficient subgrid representation compared to SGC1 and SGC2, particularly at coarser model resolutions and moderate computational intensity ( N LRU = 18). It also explained the most PFTs and elevation variability that is more homogeneously distributed spatially. The SGC method will be implemented in CLM over the NA continent to assess its impacts on simulating land surface processes.

Description: An efficient method to generate a perturbed parameter ensemble of a fully coupled AOGCM without flux-adjustment Geoscientific Model Development, 6, 1447-1462, 2013 Author(s): P. J. Irvine, L. J. Gregoire, D. J. Lunt, and P. J. Valdes We present a simple method to generate a perturbed parameter ensemble (PPE) of a fully-coupled atmosphere-ocean general circulation model (AOGCM), HadCM3, without requiring flux-adjustment. The aim was to produce an ensemble that samples parametric uncertainty in some key variables and gives a plausible representation of the climate. Six atmospheric parameters, a sea-ice parameter and an ocean parameter were jointly perturbed within a reasonable range to generate an initial group of 200 members. To screen out implausible ensemble members, 20 yr pre-industrial control simulations were run and members whose temperature responses to the parameter perturbations were projected to be outside the range of 13.6 ± 2 °C, i.e. near to the observed pre-industrial global mean, were discarded. Twenty-one members, including the standard unperturbed model, were accepted, covering almost the entire span of the eight parameters, challenging the argument that without flux-adjustment parameter ranges would be unduly restricted. This ensemble was used in 2 experiments; an 800 yr pre-industrial and a 150 yr quadrupled CO 2 simulation. The behaviour of the PPE for the pre-industrial control compared well to ERA-40 reanalysis data and the CMIP3 ensemble for a number of surface and atmospheric column variables with the exception of a few members in the Tropics. However, we find that members of the PPE with low values of the entrainment rate coefficient show very large increases in upper tropospheric and stratospheric water vapour concentrations in response to elevated CO 2 and one member showed an implausible nonlinear climate response, and as such will be excluded from future experiments with this ensemble. The outcome of this study is a PPE of a fully-coupled AOGCM which samples parametric uncertainty and a simple methodology which would be applicable to other GCMs.

Description: GAPPARD: a computationally efficient method of approximating gap-scale disturbance in vegetation models Geoscientific Model Development, 6, 1517-1542, 2013 Author(s): M. Scherstjanoi, J. O. Kaplan, E. Thürig, and H. Lischke Models of vegetation dynamics that are designed for application at spatial scales larger than individual forest gaps suffer from several limitations. Typically, either a population average approximation is used that results in unrealistic tree allometry and forest stand structure, or models have a high computational demand because they need to simulate both a series of age-based cohorts and a number of replicate patches to account for stochastic gap-scale disturbances. The detail required by the latter method increases the number of calculations by two to three orders of magnitude compared to the less realistic population average approach. In an effort to increase the efficiency of dynamic vegetation models without sacrificing realism, we developed a new method for simulating stand-replacing disturbances that is both accurate and faster than approaches that use replicate patches. The GAPPARD (approximating GAP model results with a Probabilistic Approach to account for stand Replacing Disturbances) method works by postprocessing the output of deterministic, undisturbed simulations of a cohort-based vegetation model by deriving the distribution of patch ages at any point in time on the basis of a disturbance probability. With this distribution, the expected value of any output variable can be calculated from the output values of the deterministic undisturbed run at the time corresponding to the patch age. To account for temporal changes in model forcing (e.g., as a result of climate change), GAPPARD performs a series of deterministic simulations and interpolates between the results in the postprocessing step. We integrated the GAPPARD method in the vegetation model LPJ-GUESS, and evaluated it in a series of simulations along an altitudinal transect of an inner-Alpine valley. We obtained results very similar to the output of the original LPJ-GUESS model that uses 100 replicate patches, but simulation time was reduced by approximately the factor 10. Our new method is therefore highly suited for rapidly approximating LPJ-GUESS results, and provides the opportunity for future studies over large spatial domains, allows easier parameterization of tree species, faster identification of areas of interesting simulation results, and comparisons with large-scale datasets and results of other forest models.

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: Global and regional sea surface temperature trends during Marine Isotope Stage 11 Climate of the Past, 9, 2231-2252, 2013 Author(s): Y. Milker, R. Rachmayani, M. F. G. 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 ~18% 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 validate the CCSM3 (Community Climate System Model, version 3) predictive potential, the annual and seasonal SST anomalies recorded in a total of 74 proxy records were compared with 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: On the parallelization of atmospheric inversions of CO 2 surface fluxes within a variational framework Geoscientific Model Development, 6, 783-790, 2013 Author(s): F. Chevallier The variational formulation of Bayes' theorem allows inferring CO 2 sources and sinks from atmospheric concentrations at much higher time–space resolution than the ensemble or analytical approaches. However, it usually exhibits limited scalable parallelism. This limitation hinders global atmospheric inversions operated on decadal time scales and regional ones with kilometric spatial scales because of the computational cost of the underlying transport model that has to be run at each iteration of the variational minimization. Here, we introduce a physical parallelization (PP) of variational atmospheric inversions. In the PP, the inversion still manages a single physically and statistically consistent window, but the transport model is run in parallel overlapping sub-segments in order to massively reduce the computation wall-clock time of the inversion. For global inversions, a simplification of transport modelling is described to connect the output of all segments. We demonstrate the performance of the approach on a global inversion for CO 2 with a 32 yr inversion window (1979–2010) with atmospheric measurements from 81 sites of the NOAA global cooperative air sampling network. In this case, we show that the duration of the inversion is reduced by a seven-fold factor (from months to days), while still processing the three decades consistently and with improved numerical stability.

Description: Inclusion of ash and SO 2 emissions from volcanic eruptions in WRF-Chem: development and some applications Geoscientific Model Development, 6, 457-468, 2013 Author(s): M. Stuefer, S. R. Freitas, G. Grell, P. Webley, S. Peckham, S. A. McKeen, and S. D. Egan We describe a new functionality within the Weather Research and Forecasting (WRF) model with coupled Chemistry (WRF-Chem) that allows simulating emission, transport, dispersion, transformation and sedimentation of pollutants released during volcanic activities. Emissions from both an explosive eruption case and a relatively calm degassing situation are considered using the most recent volcanic emission databases. A preprocessor tool provides emission fields and additional information needed to establish the initial three-dimensional cloud umbrella/vertical distribution within the transport model grid, as well as the timing and duration of an eruption. From this source condition, the transport, dispersion and sedimentation of the ash cloud can be realistically simulated by WRF-Chem using its own dynamics and physical parameterization as well as data assimilation. Examples of model applications include a comparison of tephra fall deposits from the 1989 eruption of Mount Redoubt (Alaska) and the dispersion of ash from the 2010 Eyjafjallajökull eruption in Iceland. Both model applications show good coincidence between WRF-Chem and observations.

Description: PORT, a CESM tool for the diagnosis of radiative forcing Geoscientific Model Development, 6, 469-476, 2013 Author(s): A. J. Conley, J.-F. Lamarque, F. Vitt, W. D. Collins, and J. Kiehl The Parallel Offline Radiative Transfer (PORT) model is a stand-alone tool, driven by model-generated datasets, that can be used for any radiation calculation that the underlying radiative transfer schemes can perform, such as diagnosing radiative forcing. In its present distribution, PORT isolates the radiation code from the Community Atmosphere Model (CAM4) in the Community Earth System Model (CESM1). The current configuration focuses on CAM4 radiation with the constituents as represented in present-day conditions in CESM1, along with their optical properties. PORT includes an implementation of stratospheric temperature adjustment under the assumption of fixed dynamical heating, which is necessary to compute radiative forcing in addition to the more straightforward instantaneous radiative forcing. PORT can be extended to use radiative constituent distributions from other models or model simulations. Ultimately, PORT can be used with various radiative transfer models. As illustrations of the use of PORT, we perform the computation of radiative forcing from doubling of carbon dioxide, from the change of tropospheric ozone concentration from the year 1850 to 2000, and from present-day aerosols. The radiative forcing from tropospheric ozone (with respect to 1850) generated by a collection of model simulations under the Atmospheric Chemistry and Climate Model Intercomparison Project is found to be 0.34 (with an intermodel standard deviation of 0.07) W m −2 . Present-day aerosol direct forcing (relative to no aerosols) is found to be −1.3 W m −2 .

Description: Skill and reliability of climate model ensembles at the Last Glacial Maximum and mid-Holocene Climate of the Past, 9, 811-823, 2013 Author(s): J. C. Hargreaves, J. D. Annan, R. Ohgaito, A. Paul, and A. Abe-Ouchi Paleoclimate simulations provide us with an opportunity to critically confront and evaluate the performance of climate models in simulating the response of the climate system to changes in radiative forcing and other boundary conditions. Hargreaves et al. (2011) analysed the reliability of the Paleoclimate Modelling Intercomparison Project, PMIP2 model ensemble with respect to the MARGO sea surface temperature data synthesis (MARGO Project Members, 2009) for the Last Glacial Maximum (LGM, 21 ka BP). Here we extend that work to include a new comprehensive collection of land surface data (Bartlein et al., 2011), and introduce a novel analysis of the predictive skill of the models. We include output from the PMIP3 experiments, from the two models for which suitable data are currently available. We also perform the same analyses for the PMIP2 mid-Holocene (6 ka BP) ensembles and available proxy data sets. Our results are predominantly positive for the LGM, suggesting that as well as the global mean change, the models can reproduce the observed pattern of change on the broadest scales, such as the overall land–sea contrast and polar amplification, although the more detailed sub-continental scale patterns of change remains elusive. In contrast, our results for the mid-Holocene are substantially negative, with the models failing to reproduce the observed changes with any degree of skill. One cause of this problem could be that the globally- and annually-averaged forcing anomaly is very weak at the mid-Holocene, and so the results are dominated by the more localised regional patterns in the parts of globe for which data are available. The root cause of the model-data mismatch at these scales is unclear. If the proxy calibration is itself reliable, then representativity error in the data-model comparison, and missing climate feedbacks in the models are other possible sources of error.

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: Stable isotopic evidence of El Niño-like atmospheric circulation in the Pliocene western United States Climate of the Past, 9, 903-912, 2013 Author(s): M. J. Winnick, J. M. Welker, and C. P. Chamberlain Understanding how the hydrologic cycle has responded to warmer global temperatures in the past is especially important today as concentrations of CO 2 in the atmosphere continue to increase due to human activities. The Pliocene offers an ideal window into a climate system that has equilibrated with current atmospheric p CO 2 . During the Pliocene the western United States was wetter than modern, an observation at odds with our current understanding of future warming scenarios, which involve the expansion and poleward migration of the subtropical dry zone. Here we compare Pliocene oxygen isotope profiles of pedogenic carbonates across the western US to modern isotopic anomalies in precipitation between phases of the El Niño–Southern Oscillation (ENSO). We find that when accounting for seasonality of carbonate formation, isotopic changes through the late Pliocene match modern precipitation isotopic anomalies in El Niño years. Furthermore, isotopic shifts through the late Pliocene mirror changes through the early Pleistocene, which likely represents the southward migration of the westerly storm track caused by growth of the Laurentide ice sheet. We propose that the westerly storm track migrated northward through the late Pliocene with the development of the modern cold tongue in the east equatorial Pacific, then returned southward with widespread glaciation in the Northern Hemisphere – a scenario supported by terrestrial climate proxies across the US. Together these data support the proposed existence of background El Niño-like conditions in western North America during the warm Pliocene. If the earth behaves similarly with future warming, this observation has important implications with regard to the amount and distribution of precipitation in western North America.

Description: Mass-movement and flood-induced deposits in Lake Ledro, southern Alps, Italy: implications for Holocene palaeohydrology and natural hazards Climate of the Past, 9, 825-840, 2013 Author(s): A. Simonneau, E. Chapron, B. Vannière, S. B. Wirth, A. Gilli, C. Di Giovanni, F. S. Anselmetti, M. Desmet, and M. Magny High-resolution seismic profiles and sediment cores from Lake Ledro combined with soil and riverbed samples from the lake's catchment area are used to assess the recurrence of natural hazards (earthquakes and flood events) in the southern Italian Alps during the Holocene. Two well-developed deltas and a flat central basin are identified on seismic profiles in Lake Ledro. Lake sediments have been finely laminated in the basin since 9000 cal. yr BP and frequently interrupted by two types of sedimentary events (SEs): light-coloured massive layers and dark-coloured graded beds. Optical analysis (quantitative organic petrography) of the organic matter present in soil, riverbed and lacustrine samples together with lake sediment bulk density and grain-size analysis illustrate that light-coloured layers consist of a mixture of lacustrine sediments and mainly contain algal particles similar to the ones observed in background sediments. Light-coloured layers thicker than 1.5 cm in the main basin of Lake Ledro are synchronous to numerous coeval mass-wasting deposits remoulding the slopes of the basin. They are interpreted as subaquatic mass-movements triggered by historical and pre-historical regional earthquakes dated to AD 2005, AD 1891, AD 1045 and 1260, 2545, 2595, 3350, 3815, 4740, 7190, 9185 and 11 495 cal. yr BP. Dark-coloured SEs develop high-amplitude reflections in front of the deltas and in the deep central basin. These beds are mainly made of terrestrial organic matter (soils and lignocellulosic debris) and are interpreted as resulting from intense hyperpycnal flood event. Mapping and quantifying the amount of soil material accumulated in the Holocene hyperpycnal flood deposits of the sequence allow estimating that the equivalent soil thickness eroded over the catchment area reached up to 5 mm during the largest Holocene flood events. Such significant soil erosion is interpreted as resulting from the combination of heavy rainfall and snowmelt. The recurrence of flash flood events during the Holocene was, however, not high enough to affect pedogenesis processes and highlight several wet regional periods during the Holocene. The Holocene period is divided into four phases of environmental evolution. Over the first half of the Holocene, a progressive stabilization of the soils present through the catchment of Lake Ledro was associated with a progressive reforestation of the area and only interrupted during the wet 8.2 event when the soil destabilization was particularly important. Lower soil erosion was recorded during the mid-Holocene climatic optimum (8000–4200 cal. yr BP) and associated with higher algal production. Between 4200 and 3100 cal. yr BP, both wetter climate and human activities within the drainage basin drastically increased soil erosion rates. Finally, from 3100 cal. yr BP to the present-day, data suggest increasing and changing human land use.

Description: Using data assimilation to investigate the causes of Southern Hemisphere high latitude cooling from 10 to 8 ka BP Climate of the Past, 9, 887-901, 2013 Author(s): P. Mathiot, H. Goosse, X. Crosta, B. Stenni, M. Braida, H. Renssen, C. J. Van Meerbeeck, V. Masson-Delmotte, A. Mairesse, and S. Dubinkina From 10 to 8 ka BP (thousand years before present), paleoclimate records show an atmospheric and oceanic cooling in the high latitudes of the Southern Hemisphere. During this interval, temperatures estimated from proxy data decrease by 0.8 °C over Antarctica and 1.2 °C over the Southern Ocean. In order to study the causes of this cooling, simulations covering the early Holocene have been performed with the climate model of intermediate complexity LOVECLIM constrained to follow the signal recorded in climate proxies using a data assimilation method based on a particle filtering approach. The selected proxies represent oceanic and atmospheric surface temperature in the Southern Hemisphere derived from terrestrial, marine and glaciological records. Two mechanisms previously suggested to explain the 10–8 ka BP cooling pattern are investigated using the data assimilation approach in our model. The first hypothesis is a change in atmospheric circulation, and the second one is a cooling of the sea surface temperature in the Southern Ocean, driven in our experimental setup by the impact of an increased West Antarctic melting rate on ocean circulation. For the atmosphere hypothesis, the climate state obtained by data assimilation produces a modification of the meridional atmospheric circulation leading to a 0.5 °C Antarctic cooling from 10 to 8 ka BP compared to the simulation without data assimilation, without congruent cooling of the atmospheric and sea surface temperature in the Southern Ocean. For the ocean hypothesis, the increased West Antarctic freshwater flux constrainted by data assimilation (+100 mSv from 10 to 8 ka BP) leads to an oceanic cooling of 0.7 °C and a strengthening of Southern Hemisphere westerlies (+6%). Thus, according to our experiments, the observed cooling in Antarctic and the Southern Ocean proxy records can only be reconciled with the reconstructions by the combination of a modified atmospheric circulation and an enhanced freshwater flux.

Description: Model sensitivity to North Atlantic freshwater forcing at 8.2 ka Climate of the Past, 9, 955-968, 2013 Author(s): C. Morrill, A. N. LeGrande, H. Renssen, P. Bakker, and B. L. Otto-Bliesner We compared four simulations of the 8.2 ka event to assess climate model sensitivity and skill in responding to North Atlantic freshwater perturbations. All of the simulations used the same freshwater forcing, 2.5 Sv for one year, applied to either the Hudson Bay (northeastern Canada) or Labrador Sea (between Canada's Labrador coast and Greenland). This freshwater pulse induced a decadal-mean slowdown of 10–25% in the Atlantic Meridional Overturning Circulation (AMOC) of the models and caused a large-scale pattern of climate anomalies that matched proxy evidence for cooling in the Northern Hemisphere and a southward shift of the Intertropical Convergence Zone. The multi-model ensemble generated temperature anomalies that were just half as large as those from quantitative proxy reconstructions, however. Also, the duration of AMOC and climate anomalies in three of the simulations was only several decades, significantly shorter than the duration of ~150 yr in the paleoclimate record. Possible reasons for these discrepancies include incorrect representation of the early Holocene climate and ocean state in the North Atlantic and uncertainties in the freshwater forcing estimates.

Description: A mid-Holocene climate reconstruction for eastern South America Climate of the Past, 9, 2117-2133, 2013 Author(s): L. F. Prado, I. Wainer, C. M. Chiessi, M.-P. Ledru, and B. Turcq The mid-Holocene (6000 calibrated years before present) is a key period in palaeoclimatology because incoming summer insolation was lower than during the late Holocene in the Southern Hemisphere, whereas the opposite happened in the Northern Hemisphere. However, the effects of the decreased austral summer insolation over South American climate have been poorly discussed by palaeodata syntheses. In addition, only a few of the regional studies have characterised the mid-Holocene climate in South America through a multiproxy approach. Here, we present a multiproxy compilation of mid-Holocene palaeoclimate data for eastern South America. We compiled 120 palaeoclimatological datasets, which were published in 84 different papers. The palaeodata analysed here suggest a water deficit scenario in the majority of eastern South America during the mid-Holocene if compared to the late Holocene, with the exception of northeastern Brazil. Low mid-Holocene austral summer insolation caused a reduced land–sea temperature contrast and hence a weakened South American monsoon system circulation. This scenario is represented by a decrease in precipitation over the South Atlantic Convergence Zone area, saltier conditions along the South American continental margin, and lower lake levels.

Description: Downscaling a global climate model to simulate climate change over the US and the implication on regional and urban air quality Geoscientific Model Development, 6, 1429-1445, 2013 Author(s): M. Trail, A. P. Tsimpidi, P. Liu, K. Tsigaridis, Y. Hu, A. Nenes, and A. G. Russell Climate change can exacerbate future regional air pollution events by making conditions more favorable to form high levels of ozone. In this study, we use spectral nudging with the Weather Research and Forecasting (WRF) model to downscale NASA earth system GISS modelE2 results during the years 2006 to 2010 and 2048 to 2052 over the contiguous United States in order to compare the resulting meteorological fields from the air quality perspective during the four seasons of five-year historic and future climatological periods. GISS results are used as initial and boundary conditions by the WRF regional climate model (RCM) to produce hourly meteorological fields. The downscaling technique and choice of physics parameterizations used are evaluated by comparing them with in situ observations. This study investigates changes of similar regional climate conditions down to a 12 km by 12 km resolution, as well as the effect of evolving climate conditions on the air quality at major US cities. The high-resolution simulations produce somewhat different results than the coarse-resolution simulations in some regions. Also, through the analysis of the meteorological variables that most strongly influence air quality, we find consistent changes in regional climate that would enhance ozone levels in four regions of the US during fall (western US, Texas, northeastern, and southeastern US), one region during summer (Texas), and one region where changes potentially would lead to better air quality during spring (Northeast). Changes in regional climate that would enhance ozone levels are increased temperatures and stagnation along with decreased precipitation and ventilation. We also find that daily peak temperatures tend to increase in most major cities in the US, which would increase the risk of health problems associated with heat stress. Future work will address a more comprehensive assessment of emissions and chemistry involved in the formation and removal of air pollutants.

Description: The SOCOL version 3.0 chemistry–climate model: description, evaluation, and implications from an advanced transport algorithm Geoscientific Model Development, 6, 1407-1427, 2013 Author(s): A. Stenke, M. Schraner, E. Rozanov, T. Egorova, B. Luo, and T. Peter We present the third generation of the coupled chemistry–climate model (CCM) SOCOL (modeling tools for studies of SOlar Climate Ozone Links). The most notable modifications compared to the previous model version are (1) the dynamical core has been updated with the fifth generation of the middle-atmosphere general circulation model MA-ECHAM (European Centre/HAMburg climate model), and (2) the advection of the chemical species is now calculated by a mass-conserving and shape-preserving flux-form transport scheme instead of the previously used hybrid advection scheme. The whole chemistry code has been rewritten according to the ECHAM5 infrastructure and transferred to Fortran95. In contrast to its predecessors, SOCOLvs3 is now fully parallelized. The performance of the new SOCOL version is evaluated on the basis of transient model simulations (1975–2004) with different horizontal (T31 and T42) resolutions, following the approach of the CCMVal-1 model validation activity. The advanced advection scheme significantly reduces the artificial loss and accumulation of tracer mass in regions with strong gradients that was observed in previous model versions. Compared to its predecessors, SOCOLvs3 generally shows more realistic distributions of chemical trace species, especially of total inorganic chlorine, in terms of the mean state, but also of the annual and interannual variability. Advancements with respect to model dynamics are for example a better representation of the stratospheric mean state in spring, especially in the Southern Hemisphere, and a slowdown of the upward propagation in the tropical lower stratosphere. Despite a large number of improvements model deficiencies still remain. Examples include a too-fast vertical ascent and/or horizontal mixing in the tropical stratosphere, the cold temperature bias in the lowermost polar stratosphere, and the overestimation of polar total ozone loss during Antarctic springtime.

Description: Stable water isotopes in the coupled atmosphere–land surface model ECHAM5-JSBACH Geoscientific Model Development, 6, 1463-1480, 2013 Author(s): B. Haese, M. Werner, and G. Lohmann In this study we present first results of a new model development, ECHAM5-JSBACH-wiso, where we have incorporated the stable water isotopes H 2 18 O and HDO as tracers in the hydrological cycle of the coupled atmosphere–land surface model ECHAM5-JSBACH. The ECHAM5-JSBACH-wiso model was run under present-day climate conditions at two different resolutions (T31L19, T63L31). A comparison between ECHAM5-JSBACH-wiso and ECHAM5-wiso shows that the coupling has a strong impact on the simulated temperature and soil wetness. Caused by these changes of temperature and the hydrological cycle, the δ 18 O in precipitation also shows variations from −4‰ up to 4‰. One of the strongest anomalies is shown over northeast Asia where, due to an increase of temperature, the δ 18 O in precipitation increases as well. In order to analyze the sensitivity of the fractionation processes over land, we compare a set of simulations with various implementations of these processes over the land surface. The simulations allow us to distinguish between no fractionation, fractionation included in the evaporation flux (from bare soil) and also fractionation included in both evaporation and transpiration (from water transport through plants) fluxes. While the isotopic composition of the soil water may change for δ 18 O by up to +8&permil:, the simulated δ 18 O in precipitation shows only slight differences on the order of ±1‰. The simulated isotopic composition of precipitation fits well with the available observations from the GNIP (Global Network of Isotopes in Precipitation) database.

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: Glacial fluctuations of the Indian monsoon and their relationship with North Atlantic climate: new data and modelling experiments Climate of the Past, 9, 2135-2151, 2013 Author(s): C. Marzin, N. Kallel, M. Kageyama, J.-C. Duplessy, and P. Braconnot Several paleoclimate records such as from Chinese loess, speleothems or upwelling indicators in marine sediments present large variations of the Asian monsoon system during the last glaciation. Here, we present a new record from the northern Andaman Sea (core MD77-176) which shows the variations of the hydrological cycle of the Bay of Bengal. The high-resolution record of surface water δ 18 O dominantly reflects salinity changes and displays large millennial-scale oscillations over the period 40 000 to 11 000 yr BP. Their timing and sequence suggests that events of high (resp. low) salinity in the Bay of Bengal, i.e. weak (resp. strong) Indian monsoon, correspond to cold (resp. warm) events in the North Atlantic and Arctic, as documented by the Greenland ice core record. We use the IPSL_CM4 Atmosphere-Ocean coupled General Circulation Model to study the processes that could explain the teleconnection between the Indian monsoon and the North Atlantic climate. We first analyse a numerical experiment in which such a rapid event in the North Atlantic is obtained under glacial conditions by increasing the freshwater flux in the North Atlantic, which results in a reduction of the intensity of the Atlantic meridional overturning circulation. This freshwater hosing results in a weakening of the Indian monsoon rainfall and circulation. The changes in the continental runoff and local hydrological cycle are responsible for an increase in salinity in the Bay of Bengal. This therefore compares favourably with the new sea water δ 18 O record presented here and the hypothesis of synchronous cold North Atlantic and weak Indian monsoon events. Additional sensitivity experiments are produced with the LMDZ atmospheric model to analyse the teleconnection mechanisms between the North Atlantic and the Indian monsoon. The changes over the tropical Atlantic are shown to be essential in triggering perturbations of the subtropical jet over Africa and Eurasia, that in turn affect the intensity of the Indian monsoon. These relationships are also found to be valid in additional coupled model simulations in which the Atlantic meridional overturning circulation (AMOC) is forced to resume.

Description: δ 18 O water isotope in the i LOVECLIM model (version 1.0) – Part 2: Evaluation of model results against observed δ 18 O in water samples Geoscientific Model Development, 6, 1493-1504, 2013 Author(s): D. M. Roche and T. Caley The H 2 18 O stable isotope was previously introduced in the three coupled components of the earth system model i LOVECLIM: atmosphere, ocean and vegetation. The results of a long (5000 yr) pre-industrial equilibrium simulation are presented and evaluated against measurement of H 2 18 O abundance in present-day water for the atmospheric and oceanic components. For the atmosphere, it is found that the model reproduces the observed spatial distribution and relationships to climate variables with some merit, though limitations following our approach are highlighted. Indeed, we obtain the main gradients with a robust representation of the Rayleigh distillation but caveats appear in Antarctica and around the Mediterranean region due to model limitation. For the oceanic component, the agreement between the modelled and observed distribution of water δ 18 O is found to be very good. Mean ocean surface latitudinal gradients are faithfully reproduced as well as the mark of the main intermediate and deep water masses. This opens large prospects for the applications in palaeoclimatic context.

Description: Mid-Holocene ocean and vegetation feedbacks over East Asia Climate of the Past, 9, 2153-2171, 2013 Author(s): Z. Tian and D. Jiang Mid-Holocene ocean and vegetation feedbacks over East Asia are investigated by a set of numerical experiments performed with the version 4 of the Community Climate System Model (CCSM4). With reference to the pre-industrial period, most of the mid-Holocene annual and seasonal surface-air temperature and precipitation changes are found to result from a direct response of the atmosphere to insolation forcing, while dynamic ocean and vegetation modulate regional climate of East Asia to some extent. Because of its thermal inertia, the dynamic ocean induced an additional warming of 0.2 K for the annual mean, 0.5 K in winter (December–February), 0.0003 K in summer (June–August), and 1.0 K in autumn (September–November), but a cooling of 0.6 K in spring (March–May) averaged over China, and it counteracted (amplified) the direct effect of insolation forcing for the annual mean and in winter and autumn (spring) for that period. The dynamic vegetation had an area-average impact of no more than 0.4 K on the mid-Holocene annual and seasonal temperatures over China, with an average cooling of 0.2 K for the annual mean. On the other hand, ocean feedback induced a small increase of precipitation in winter (0.04 mm day −1 ) and autumn (0.05 mm day −1 ), but a reduction for the annual mean (0.14 mm day −1 ) and in spring (0.29 mm day −1 ) and summer (0.34 mm day −1 ) over China, while it also suppressed the East Asian summer monsoon rainfall. The effect of dynamic vegetation on the mid-Holocene annual and seasonal precipitation was comparatively small, ranging from −0.03 mm day −1 to 0.06 mm day −1 averaged over China. In comparison, the CCSM4 simulated annual and winter cooling over China agrees with simulations within the Paleoclimate Modeling Intercomparison Project (PMIP), but the results are contrary to the warming reconstructed from multiple proxy data for the mid-Holocene. Ocean feedback narrows this model–data mismatch, whereas vegetation feedback plays an opposite role but with a level of uncertainty.

Description: The Rock Geochemical Model (RokGeM) v0.9 Geoscientific Model Development, 6, 1543-1573, 2013 Author(s): G. Colbourn, A. Ridgwell, and T. M. Lenton A new model of terrestrial rock weathering – the Rock Geochemical Model (RokGeM) – was developed for incorporation into the GENIE Earth System modelling framework. In this paper we describe the model. We consider a range of previously devised parameterizations, ranging from simple dependencies on global mean temperature following Berner et al. (1983), to spatially explicit dependencies on run-off and temperature (GKWM, Bluth and Kump, 1994; GEM-CO2, Amiotte-Suchet et al., 2003) – fields provided by the energy-moisture balance atmosphere model component in GENIE. Using long-term carbon cycle perturbation experiments, we test the effects of a wide range of model parameters, including whether or not the atmosphere was "short-circuited" in the carbon cycle; the sensitivity and feedback strength of temperature and run-off on carbonate and silicate weathering; different river-routing schemes; 0-D (global average) vs. 2-D (spatially explicit) weathering schemes; and the lithology dependence of weathering. Included are details of how to run the model and visualize the results.

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: Water-soluble organic carbon in snow and ice deposited at Alpine, Greenland, and Antarctic sites: a critical review of available data and their atmospheric relevance Climate of the Past, 9, 2195-2211, 2013 Author(s): M. Legrand, S. Preunkert, B. Jourdain, J. Guilhermet, X. Fa{ï}n, I. Alekhina, and J. R. Petit While it is now recognized that organic matter dominates the present-day atmospheric aerosol load over continents, its sources remain poorly known. The studies of organic species or organic fractions trapped in ice cores may help to overcome this lack of knowledge. Available data on the dissolved (or total) organic carbon (DOC or TOC) content of snow and ice often appear largely inconsistent, and, until now, no critical review has been conducted to understand the causes of these inconsistencies. To draw a more consistent picture of the organic carbon amount present in solid precipitation that accumulates on cold glaciers, we here review available data and, when needed, complete the data set with analyses of selected samples. The different data sets are then discussed by considering the age (modern versus pre-industrial, Holocene versus Last glacial Maximum) and type (surface snow, firn, or ice) of investigated samples, the deployed method, and the applied contamination control. Finally, the OC (DOC or TOC) levels of Antarctic, Greenland, and Alpine ice cores are compared and discussed with respect to natural (biomass burning, vegetation emissions) and anthropogenic sources (fossil fuel combustion) contributing to atmospheric OC aerosol.

Description: An approach to computing direction relations between separated object groups Geoscientific Model Development, 6, 1591-1599, 2013 Author(s): H. Yan, Z. Wang, and J. Li Direction relations between object groups play an important role in qualitative spatial reasoning, spatial computation and spatial recognition. However, none of existing models can be used to compute direction relations between object groups. To fill this gap, an approach to computing direction relations between separated object groups is proposed in this paper, which is theoretically based on gestalt principles and the idea of multi-directions. The approach firstly triangulates the two object groups, and then it constructs the Voronoi diagram between the two groups using the triangular network. After this, the normal of each Voronoi edge is calculated, and the quantitative expression of the direction relations is constructed. Finally, the quantitative direction relations are transformed into qualitative ones. The psychological experiments show that the proposed approach can obtain direction relations both between two single objects and between two object groups, and the results are correct from the point of view of spatial cognition.

Description: A method to represent ozone response to large changes in precursor emissions using high-order sensitivity analysis in photochemical models Geoscientific Model Development, 6, 1601-1608, 2013 Author(s): G. Yarwood, C. Emery, J. Jung, U. Nopmongcol, and T. Sakulyanontvittaya Photochemical grid models (PGMs) are used to simulate tropospheric ozone and quantify its response to emission changes. PGMs are often applied for annual simulations to provide both maximum concentrations for assessing compliance with air quality standards and frequency distributions for assessing human exposure. Efficient methods for computing ozone at different emission levels can improve the quality of ozone air quality management efforts. This study demonstrates the feasibility of using the decoupled direct method (DDM) to calculate first- and second-order sensitivity of ozone to anthropogenic NO x and VOC emissions in annual PGM simulations at continental scale. Algebraic models are developed that use Taylor series to produce complete annual frequency distributions of hourly ozone at any location and any anthropogenic emission level between zero and 100%, adjusted independently for NO x and VOC. We recommend computing the sensitivity coefficients at the midpoint of the emissions range over which they are intended to be applied, in this case with 50% anthropogenic emissions. The algebraic model predictions can be improved by combining sensitivity coefficients computed at 10 and 50% anthropogenic emissions. Compared to brute force simulations, algebraic model predictions tend to be more accurate in summer than winter, at rural than urban locations, and with 100% than zero anthropogenic emissions. Equations developed to combine sensitivity coefficients computed with 10 and 50% anthropogenic emissions are able to reproduce brute force simulation results with zero and 100% anthropogenic emissions with a mean bias of less than 2 ppb and mean error of less than 3 ppb averaged over 22 US cities.

Description: Major dust events in Europe during marine isotope stage 5 (130–74 ka): a climatic interpretation of the "markers" Climate of the Past, 9, 2213-2230, 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 ka 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 ka, 88–86 ka, and 78.5–77 ka. These dates are determined by considering the OSL ages with their errors measured on the studied sequence, and the comparison with Greenland ice-core and European speleothem chronologies. 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: A Lagrangian advection scheme with shape matrix (LASM) for solving advection problems Geoscientific Model Development, 7, 2951-2968, 2014 Author(s): L. Dong, B. Wang, and L. Liu A new Lagrangian advection scheme with shape matrix (LASM) is proposed to take advantage of the extreme low numerical diffusion of the Lagrangian methods. The tracer is discretized into finite parcels, which move along the downstream trajectories. Different from other Lagrangian schemes, the parcel shape is simulated explicitly by a linear transformation matrix. By doing so, the aliasing error in the Lagrangian schemes is largely reduced without introducing substantial interparcel mixing in the pure advection stage, because the flow information will be respected when remapping tracer density onto the fixed model grids. An adaptive interparcel mixing algorithm is constructed to ensure the validity of the linear approximation of the parcel shape, where the mixing is only triggered when it is necessary and resembles the physical mixing. The total tracer mass on the parcels is conserved exactly. The new scheme is validated by using several test cases.

Description: Global representation of tropical cyclone-induced ocean thermal changes using Argo data – Part 2: Estimating air–sea heat fluxes and ocean heat content changes Ocean Science Discussions, 11, 2907-2937, 2014 Author(s): L. Cheng, J. Zhu, and R. L. Sriver We use Argo temperature data to examine changes in ocean heat content (OHC) and air–sea heat fluxes induced by tropical cyclones (TC)s on a global scale. A footprint technique that analyzes the vertical structure of cross-track thermal responses along all storm tracks during the period 2004–2012 is utilized (see part I). We find that TCs are responsible for 1.87 PW (11.05 W m −2 when averaging over the global ocean basin) of heat transfer annually from the global ocean to the atmosphere during storm passage (0–3 days) on a global scale. Of this total, 1.05 ± 0.20 PW (4.80 ± 0.85 W m −2 ) is caused by Tropical storms/Tropical depressions (TS/TD) and 0.82 ± 0.21 PW (6.25 ± 1.5 W m −2 ) is caused by hurricanes. Our findings indicate that ocean heat loss by TCs may be a substantial missing piece of the global ocean heat budget. Net changes in OHC after storm passage is estimated by analyzing the temperature anomalies during wake recovery following storm events (4–20 days after storm passage) relative to pre-storm conditions. Results indicate the global ocean experiences a 0.75 ± 0.25 PW (5.98 ± 2.1W m −2 ) net heat gain annually for hurricanes. In contrast, under TS/TD conditions, ocean experiences 0.41 ± 0.21 PW (1.90 ± 0.96 W m −2 ) net ocean heat loss, suggesting the overall oceanic thermal response is particularly sensitive to the intensity of the event. The net ocean heat uptake caused by all storms is 0.34 PW.

Description: Laminated sediments in the Bering Sea reveal atmospheric teleconnections to Greenland climate on millennial to decadal timescales during the last deglaciation Climate of the Past, 10, 2215-2236, 2014 Author(s): H. Kuehn, L. Lembke-Jene, R. Gersonde, O. Esper, F. Lamy, H. Arz, G. Kuhn, and R. Tiedemann During the last glacial termination, the upper North Pacific Ocean underwent dramatic and rapid changes in oxygenation that lead to the transient intensification of oxygen minimum zones (OMZs), recorded by the widespread occurrence of laminated sediments on circum-Pacific continental margins. We present a new laminated sediment record from the mid-depth (1100 m) northern Bering Sea margin that provides insight into these deglacial OMZ maxima with exceptional, decadal-scale detail. Combined ultrahigh-resolution micro-X-ray-fluorescence (micro-XRF) data and sediment facies analysis of laminae reveal an alternation between predominantly terrigenous and diatom-dominated opal sedimentation. The diatomaceous laminae are interpreted to represent spring/summer productivity events related to the retreating sea ice margin. We identified five laminated sections in the deglacial part of our site. Lamina counts were carried out on these sections and correlated with the Bølling–Allerød and Preboreal phases in the North Greenland Ice Core (NGRIP) oxygen isotope record, indicating an annual deposition of individual lamina couplets (varves). The observed rapid decadal intensifications of anoxia, in particular within the Bølling–Allerød, are tightly coupled to short-term warm events through increases in regional export production. This dependence of laminae formation on warmer temperatures is underlined by a correlation with published Bering Sea sea surface temperature records and δ 18 O data of planktic foraminifera from the Gulf of Alaska. The rapidity of the observed changes strongly implies a close atmospheric teleconnection between North Pacific and North Atlantic regions. We suggest that concomitant increases in export production and subsequent remineralization of organic matter in the Bering Sea, in combination with oxygen-poor waters entering the Being Sea, drove down oxygen concentrations to values below 0.1 mL L −1 and caused laminae preservation. Calculated benthic–planktic ventilation ages show no significant variations throughout the last deglaciation, indicating that changes in formation rates or differing sources of North Pacific mid-depth waters are not prime candidates for strengthening the OMZ at our site. The age models established by our correlation procedure allow for the determination of calendar age control points for the Bølling–Allerød and the Preboreal that are independent of the initial radiocarbon-based chronology. Resulting surface reservoir ages range within 730–990 yr during the Bølling–Allerød, 800–1100 yr in the Younger Dryas, and 765–775 yr for the Preboreal.

Description: Implication of methodological uncertainties for mid-Holocene sea surface temperature reconstructions Climate of the Past, 10, 2237-2252, 2014 Author(s): I. Hessler, S. P. Harrison, M. Kucera, C. Waelbroeck, M.-T. Chen, C. Anderson, A. de Vernal, B. Fréchette, A. Cloke-Hayes, G. Leduc, and L. Londeix We present and examine a multi-sensor global compilation of mid-Holocene (MH) sea surface temperatures (SST), based on Mg/Ca and alkenone palaeothermometry and reconstructions obtained using planktonic foraminifera and organic-walled dinoflagellate cyst census counts. We assess the uncertainties originating from using different methodologies and evaluate the potential of MH SST reconstructions as a benchmark for climate-model simulations. The comparison between different analytical approaches (time frame, baseline climate) shows the choice of time window for the MH has a negligible effect on the reconstructed SST pattern, but the choice of baseline climate affects both the magnitude and spatial pattern of the reconstructed SSTs. Comparison of the SST reconstructions made using different sensors shows significant discrepancies at a regional scale, with uncertainties often exceeding the reconstructed SST anomaly. Apparent patterns in SST may largely be a reflection of the use of different sensors in different regions. Overall, the uncertainties associated with the SST reconstructions are generally larger than the MH anomalies. Thus, the SST data currently available cannot serve as a target for benchmarking model simulations. Further evaluations of potential subsurface and/or seasonal artifacts that may contribute to obscure the MH SST reconstructions are urgently needed to provide reliable benchmarks for model evaluations.

Description: Sensitivity of the Mediterranean sea level to atmospheric pressure and free surface elevation numerical formulation in NEMO Geoscientific Model Development, 7, 3001-3015, 2014 Author(s): P. Oddo, A. Bonaduce, N. Pinardi, and A. Guarnieri The sensitivity of the dynamics of the Mediterranean Sea to atmospheric pressure and free surface elevation formulation using NEMO (Nucleus for European Modelling of the Ocean) was evaluated. Four different experiments were carried out in the Mediterranean Sea using filtered or explicit free surface numerical schemes and accounting for the effect of atmospheric pressure in addition to wind and buoyancy fluxes. Model results were evaluated by coherency and power spectrum analysis with tide gauge data. We found that atmospheric pressure plays an important role for periods shorter than 100 days. The free surface formulation is important to obtain the correct ocean response for periods shorter than 30 days. At frequencies higher than 15 days −1 the Mediterranean basin's response to atmospheric pressure was not coherent and the performance of the model strongly depended on the specific area considered. A large-amplitude seasonal oscillation observed in the experiments using a filtered free surface was not evident in the corresponding explicit free surface formulation case, which was due to a phase shift between mass fluxes in the Gibraltar Strait and at the surface. The configuration with time splitting and atmospheric pressure always performed best; the differences were enhanced at very high frequencies.

Description: Gaseous chemistry and aerosol mechanism developments for version 3.5.1 of the online regional model, WRF-Chem Geoscientific Model Development, 7, 2557-2579, 2014 Author(s): S. Archer-Nicholls, D. Lowe, S. Utembe, J. Allan, R. A. Zaveri, J. D. Fast, Ø. Hodnebrog, H. Denier van der Gon, and G. McFiggans We have made a number of developments to the Weather, Research and Forecasting model coupled with Chemistry (WRF-Chem), with the aim of improving model prediction of trace atmospheric gas-phase chemical and aerosol composition, and of interactions between air quality and weather. A reduced form of the Common Reactive Intermediates gas-phase chemical mechanism (CRIv2-R5) has been added, using the Kinetic Pre-Processor (KPP) interface, to enable more explicit simulation of VOC degradation. N 2 O 5 heterogeneous chemistry has been added to the existing sectional MOSAIC aerosol module, and coupled to both the CRIv2-R5 and existing CBM-Z gas-phase schemes. Modifications have also been made to the sea-spray aerosol emission representation, allowing the inclusion of primary organic material in sea-spray aerosol. We have worked on the European domain, with a particular focus on making the model suitable for the study of nighttime chemistry and oxidation by the nitrate radical in the UK atmosphere. Driven by appropriate emissions, wind fields and chemical boundary conditions, implementation of the different developments are illustrated, using a modified version of WRF-Chem 3.4.1, in order to demonstrate the impact that these changes have in the Northwest European domain. These developments are publicly available in WRF-Chem from version 3.5.1 onwards.

Description: Oxygen stable isotopes during the Last Glacial Maximum climate: perspectives from data–model ( i LOVECLIM) comparison Climate of the Past, 10, 1939-1955, 2014 Author(s): T. Caley, D. M. Roche, C. Waelbroeck, and E. Michel We use the fully coupled atmosphere–ocean three-dimensional model of intermediate complexity i LOVECLIM to simulate the climate and oxygen stable isotopic signal during the Last Glacial Maximum (LGM, 21 000 years). By using a model that is able to explicitly simulate the sensor (δ 18 O), results can be directly compared with data from climatic archives in the different realms. Our results indicate that i LOVECLIM reproduces well the main feature of the LGM climate in the atmospheric and oceanic components. The annual mean δ 18 O in precipitation shows more depleted values in the northern and southern high latitudes during the LGM. The model reproduces very well the spatial gradient observed in ice core records over the Greenland ice sheet. We observe a general pattern toward more enriched values for continental calcite δ 18 O in the model at the LGM, in agreement with speleothem data. This can be explained by both a general atmospheric cooling in the tropical and subtropical regions and a reduction in precipitation as confirmed by reconstruction derived from pollens and plant macrofossils. Data–model comparison for sea surface temperature indicates that i LOVECLIM is capable to satisfyingly simulate the change in oceanic surface conditions between the LGM and present. Our data–model comparison for calcite δ 18 O allows investigating the large discrepancies with respect to glacial temperatures recorded by different microfossil proxies in the North Atlantic region. The results argue for a strong mean annual cooling in the area south of Iceland and Greenland between the LGM and present (〉 6 °C), supporting the foraminifera transfer function reconstruction but in disagreement with alkenones and dinocyst reconstructions. The data–model comparison also reveals that large positive calcite δ 18 O anomaly in the Southern Ocean may be explained by an important cooling, although the driver of this pattern is unclear. We deduce a large positive δ 18 Osw anomaly for the north Indian Ocean that contrasts with a large negative δ 18 Osw anomaly in the China Sea between the LGM and the present. This pattern may be linked to changes in the hydrological cycle over these regions. Our simulation of the deep ocean suggests that changes in δ 18 Osw between the LGM and the present are not spatially homogeneous. This is supported by reconstructions derived from pore fluids in deep-sea sediments. The model underestimates the deep ocean cooling thus biasing the comparison with benthic calcite δ 18 O data. Nonetheless, our data–model comparison supports a heterogeneous cooling of a few degrees (2–4 °C) in the LGM Ocean.

Description: Dynamics of turbulent western boundary currents at low latitude in a shallow water model Ocean Science Discussions, 11, 2461-2493, 2014 Author(s): C. Q. C. Akuetevi and A. Wirth The dynamics of low latitude turbulent western boundary currents crossing the equator is considered using numerical results from integrations of a reduced gravity shallow-water model. For viscosity values of 1000 m 2 s −1 and more, the boundary layer dynamics compares well to the analytical Munk-layer solution. When the viscosity is reduced, the boundary layer becomes turbulent and coherent structures in form of anticyclonic eddies, bursts (violent detachments of the viscous sub-layer) and dipoles appear. Three distinct boundary layers emerge, the viscous sub-layer, the advective boundary layer and the extended boundary layer. The first is characterized by a dominant vorticity balance between the viscous transport and the advective transport of vorticity. The second by a balance between the advection of planetary vorticity and the advective transport of relative vorticity. The extended boundary layer is the area to which turbulent motion from the boundary extends. The scaling of the three boundary layer thicknesses with viscosity is evaluated. Characteristic scales of the dynamics and dissipation are determined. A pragmatic approach to determine the eddy viscosity diagnostically for coarse resolution numerical models is proposed.

Description: A strategy for GIS-based 3-D slope stability modelling over large areas Geoscientific Model Development, 7, 2969-2982, 2014 Author(s): M. Mergili, I. Marchesini, M. Alvioli, M. Metz, B. Schneider-Muntau, M. Rossi, and F. Guzzetti GIS-based deterministic models may be used for landslide susceptibility mapping over large areas. However, such efforts require specific strategies to (i) keep computing time at an acceptable level, and (ii) parameterize the geotechnical data. We test and optimize the performance of the GIS-based, 3-D slope stability model r.slope.stability in terms of computing time and model results. The model was developed as a C- and Python-based raster module of the open source software GRASS GIS and considers the 3-D geometry of the sliding surface. It calculates the factor of safety (FoS) and the probability of slope failure ( P f ) for a number of randomly selected potential slip surfaces, ellipsoidal or truncated in shape. Model input consists of a digital elevation model (DEM), ranges of geotechnical parameter values derived from laboratory tests, and a range of possible soil depths estimated in the field. Probability density functions are exploited to assign P f to each ellipsoid. The model calculates for each pixel multiple values of FoS and P f corresponding to different sliding surfaces. The minimum value of FoS and the maximum value of P f for each pixel give an estimate of the landslide susceptibility in the study area. Optionally, r.slope.stability is able to split the study area into a defined number of tiles, allowing parallel processing of the model on the given area. Focusing on shallow landslides, we show how multi-core processing makes it possible to reduce computing times by a factor larger than 20 in the study area. We further demonstrate how the number of random slip surfaces and the sampling of parameters influence the average value of P f and the capacity of r.slope.stability to predict the observed patterns of shallow landslides in the 89.5 km 2 Collazzone area in Umbria, central Italy.

Description: Deciphering the spatio-temporal complexity of climate change of the last deglaciation: a model analysis Climate of the Past, 7, 591-602, 2011 Author(s): D. M. Roche, H. Renssen, D. Paillard, and G. Levavasseur Understanding the sequence of events occuring during the last major glacial to interglacial transition (21 ka BP to 9 ka BP) is a challenging task that has the potential to unveil the mechanisms behind large scale climate changes. Though many studies have focused on the understanding of the complex sequence of rapid climatic change that accompanied or interrupted the deglaciation, few have analysed it in a more theoretical framework with simple forcings. In the following, we address when and where the first significant temperature anomalies appeared when using slow varying forcing of the last deglaciation. We used here coupled transient simulations of the last deglaciation, including ocean, atmosphere and vegetation components to analyse the spatial timing of the deglaciation. To keep the analysis in a simple framework, we did not include freshwater forcings that potentially cause rapid climate shifts during that time period. We aimed to disentangle the direct and subsequent response of the climate system to slow forcing and moreover, the location where those changes are more clearly expressed. In a data – modelling comparison perspective, this could help understand the physically plausible phasing between known forcings and recorded climatic changes. Our analysis of climate variability could also help to distinguish deglacial warming signals from internal climate variability. We thus are able to better pinpoint the onset of local deglaciation, as defined by the first significant local warming and further show that there is a large regional variability associated with it, even with the set of slow forcings used here. In our model, the first significant hemispheric warming occurred simultaneously in the North and in the South and is a direct response to the obliquity forcing.

Description: Calculating the water and heat balances of the Eastern Mediterranean basin using ocean modelling and available meteorological, hydrological, and ocean data Ocean Science Discussions, 8, 1301-1338, 2011 Author(s): M. Shaltout and A. Omstedt This paper analyses the Eastern Mediterranean water and heat balances over a 52-yr period. The modelling uses a process-oriented approach resolving the one-dimensional equations of momentum, heat, and salt conservation, with turbulence modelled using a two-equation model. The exchange through the Sicily Channel connecting the Eastern and Western basins is calculated from satellite altimeter data. The results illustrates that calculated surface temperature and salinity follow the reanalysed data well and with biases of −0.4 °C and −0.004, respectively. Monthly and yearly temperature and salinity cycles are also satisfactory simulated. Reanalysed data and calculated water mass structure and heat balance components are in good agreement, indicating that the air-sea interaction and the turbulent mixing are realistically simulated. The study illustrates that the water balance in the Eastern Mediterranean basin is controlled by the difference between inflows/outflows through the Sicily Channel and by the net precipitation rates. The heat balance is controlled by the heat loss from the water surface, sun radiation into the sea, and heat flow through the Sicily Channel, the first two displaying both climate trends. An annual net heat loss of approximately 9 W m −2 was balanced by net heat in flow through the Sicily Channel.

Description: Extraction of spatial-temporal rules from mesoscale eddies in the South China Sea Based on rough set theory Ocean Science Discussions, 8, 1261-1300, 2011 Author(s): Y. Du, X. Fan, Z. He, F. Su, C. Zhou, H. Mao, and D. Wang In this paper, a rough set theory is introduced to represent spatial-temporal relationships and extract the corresponding rules from typical mesoscale-eddy states in the South China Sea (SCS). Three decision attributes are adopted in this study, which make the approach flexible in retrieving spatial-temporal rules with different features. Spatial-temporal rules of typical states in the SCS are extracted as three decision attributes, which then are confirmed by the previous works. The results demonstrate that this approach is effective in extracting spatial-temporal rules from typical mesoscale-eddy states, and therefore provides a powerful approach to forecasts in the future. Spatial-temporal rules in the SCS indicate that warm eddies following the rules are generally in the southeastern and central SCS around 2000 m isobaths in winter. Their intensity and vorticity are weaker than those of cold eddies. They usually move a shorter distance. By contrast, cold eddies are in 2000 m-deeper regions of the southwestern and northeastern SCS in spring and fall. Their intensity and vorticity are strong. Usually they move a long distance. In winter, a few rules are followed by cold eddies in the northern tip of the basin and southwest of Taiwan Island rather than warm eddies, indicating cold eddies may be well-regulated in the region. Several warm-eddy rules are achieved west of Luzon Island, indicating warm eddies may be well-regulated in the region as well. Otherwise, warm and cold eddies are distributed not only in the jet flow off southern Vietnam induced by intraseasonal wind stress in summer-fall, but also in the northern shallow water, which should be a focus of future study.

Description: Tracer distribution in the Pacific Ocean following a release off Japan – what does an oceanic general circulation model tell us? Ocean Science Discussions, 8, 1441-1466, 2011 Author(s): H. Dietze and I. Kriest In the aftermath of an earthquake and tsunami on 11 March 2011 considerable amounts of radioactive materials were accidentally released into the sea off Fukushima-Daiichi, Japan. This study uses a three-dimensional eddy-resolving oceanic general circulation model to explore potential pathways of a tracer, similar to 137 Cs, from the coast to the open ocean. Results indicate that enhanced concentrations meet a receding spring bloom offshore and that the area of enhanced concentrations offshore is strongly determined by surface mixed layer dynamics. However, huge uncertainties remain. Among them are the realism of the simulated cross-shelf transport and apparently inconsistent estimates of the particle reactivity of 137 Cs which are discussed in a brief literature review. We argue that a comprehensive set of 137 Cs measurements, including sites offshore, could be a unique opportunity to both evaluate and advance the evaluation of oceanic general circulation models.

Description: Usefulness of high resolution coastal models for operational oil spill forecast: the Full City accident Ocean Science Discussions, 8, 1467-1504, 2011 Author(s): G. Broström, A. Carrasco, L. R. Hole, S. Dick, F. Janssen, J. Mattsson, and S. Berger Oil spill modeling is considered to be an important decision support system (DeSS) useful for remedial action in case of accidents, as well as for designing the environmental monitoring system that is frequently set up after major accidents. Many accidents take place in coastal areas implying that low resolution basin scale ocean models is of limited use for predicting the trajectories of an oil spill. In this study, we target the oil spill in connection with the Full City accident on the Norwegian south coast and compare three different oil spill models for the area. The result of the analysis is that all models do a satisfactory job. The "standard" operational model for the area is shown to have severe flaws but including an analysis based on a higher resolution model (1.5 km resolution) for the area the model system show results that compare well with observations. The study also shows that an ensemble using three different models is useful when predicting/analyzing oil spill in coastal areas.

Description: The last deglaciation: timing the bipolar seesaw Climate of the Past, 7, 671-683, 2011 Author(s): J. B. Pedro, T. D. van Ommen, S. O. Rasmussen, V. I. Morgan, J. Chappellaz, A. D. Moy, V. Masson-Delmotte, and M. Delmotte Precise information on the relative timing of north-south climate variations is a key to resolving questions concerning the mechanisms that force and couple climate changes between the hemispheres. We present a new composite record made from five well-resolved Antarctic ice core records that robustly represents the timing of regional Antarctic climate change during the last deglaciation. Using fast variations in global methane gas concentrations as time markers, the Antarctic composite is directly compared to Greenland ice core records, allowing a detailed mapping of the inter-hemispheric sequence of climate changes. Consistent with prior studies the synchronized records show that warming (and cooling) trends in Antarctica closely match cold (and warm) periods in Greenland on millennial timescales. For the first time, we also identify a sub-millennial component to the inter-hemispheric coupling. Within the Antarctic Cold Reversal the strongest Antarctic cooling occurs during the pronounced northern warmth of the Bølling. Warming then resumes in Antarctica, potentially as early as the Intra-Allerød Cold Period, but with dating uncertainty that could place it as late as the onset of the Younger Dryas stadial. There is little-to-no time lag between climate transitions in Greenland and opposing changes in Antarctica. Our results lend support to fast acting inter-hemispheric coupling mechanisms, including recently proposed bipolar atmospheric teleconnections and/or rapid bipolar ocean teleconnections.

Description: Impact of combining GRACE and GOCE gravity data on ocean circulation estimates Ocean Science Discussions, 8, 1535-1573, 2011 Author(s): T. Janjić, J. Schröter, R. Savcenko, W. Bosch, A. Albertella, R. Rummel, and O. Klatt In this work we examine the impact of assimilation of multi-mission-altimeter data and the GRACE/GOCE gravity fields into the finite element ocean model (FEOM), with the focus on the Southern Ocean circulation. In order to do so, we use the geodetic approach for obtaining the dynamical ocean topography (DOT), that combines the multi-mission-altimeter data and the GRACE/GOCE gravity fields, and requires that both fields be spectrally consistent. The spectral consistency is achieved by filtering of the sea surface height and the geoid using profile approach. Combining the GRACE and GOCE data, a considerably shorter filter length resolving more DOT details can be used. In order to specify the spectrally consistent geodetic DOT we applied the Jekeli-Wahr filter corresponding to 241 km, 121 km, 97 km and 81 km halfwidths for the GRACE/GOCE based gravity field model GOCO01S and to the sea surface. More realistic features of the ocean assimilation were obtained in the Weddel gyre area due to increased resolution of the data fields, particularly for temperature field at the 800 m depth compared to Argo data.

Description: Reply to Henriksson et al.'s comment on "Using multiple observationally-based constraints to estimate climate sensitivity" by Annan and Hargreaves (2010) Climate of the Past, 7, 587-589, 2011 Author(s): J. D. Annan and J. C. Hargreaves Henriksson et al. (2010), hereafter HALTL10, criticize Annan and Hargreaves (2006a) (AH06) primarily on the grounds that we assumed that different sources of data were conditionally independent given the climate sensitivity. While we consider this approximation to have been a reasonable one under the circumstances (and provided arguments to justify this approach), we also acknowledged its importance in our original paper and performed several sensitivity analyses. The alternative calculations presented by HALTL10 appear to strengthen rather than contradict our conclusion. HALTL10 additionally criticize Annan and Hargreaves (2009) (AH09) for proposing a Cauchy type prior (as an alternative to the use of a uniform prior, which was widespread up to that time) "without sufficient support", and further claim that anticipated economic damages were used as a means of selecting the prior. We are surprised by these claims, especially considering that the proposed prior was justified at some length both on the basis of both the "Charney report" (National Research Council, 1979) and basic physical arguments, and also in light of our elementary demonstration of the pathological failings of the most commonly-used alternative. Thus, these claims are factually incorrect.

Description: Methane variations on orbital timescales: a transient modeling experiment Climate of the Past, 7, 635-648, 2011 Author(s): T. Y. M. Konijnendijk, S. L. Weber, E. Tuenter, and M. van Weele Methane (CH 4 ) variations on orbital timescales are often associated with variations in wetland coverage, most notably in the summer monsoon areas of the Northern Hemisphere. Here we test this assumption by simulating orbitally forced variations in global wetland emissions, using a simple wetland distribution and CH 4 emissions model that has been run on the output of a climate model (CLIMBER-2) containing atmosphere, ocean and vegetation components. The transient climate modeling simulation extends over the last 650 000 yr and includes variations in land-ice distribution and greenhouse gases. Tropical temperature and global vegetation are found to be the dominant controls for global CH 4 emissions and therefore atmospheric concentrations. The relative importance of wetland coverage, vegetation coverage, and emission temperatures depends on the specific climatic zone (boreal, tropics and Indian/Asian monsoon area) and timescale (precession, obliquity and glacial-interglacial timescales). Despite the low spatial resolution of the climate model and crude parameterizations for methane production and release, simulated variations in CH 4 emissions agree well with those in measured concentrations, both in their time series and spectra. The simulated lags between emissions and orbital forcing also show close agreement with those found in measured data, both on the precession and obliquity timescale. We find causal links between atmospheric CH 4 concentrations and tropical temperatures and global vegetation, but only covariance between monsoon precipitation and CH 4 concentrations. The primary importance of the first two factors explains the lags found in the CH 4 record from ice cores. Simulation of the dynamical vegetation response to climate variation on orbital timescales would be needed to reduce the uncertainty in these preliminary attributions.

Description: Wind forcing of salinity anomalies in the Denmark Strait overflow Ocean Science Discussions, 8, 1403-1440, 2011 Author(s): S. Hall, S. R. Dye, K. J. Heywood, and M. R. Wadley The overflow of dense water from the Nordic Seas to the North Atlantic through Denmark Strait is an important part of the global thermohaline circulation. The salinity of the overflow plume has been measured by an array of current meters across the continental slope off the coast of Angmagssalik, southeast Greenland since September 1998. During 2004 the salinity of the overflow plume changed dramatically, with the entire width of the array (70 km) freshening between January 2004 and July 2004, with a significant negative salinity anomaly of about 0.06 in May. The event in May represents a fresh anomaly of over 3 standard deviations from the mean since recording began in 1998. We show that the OCCAM 1/12° Ocean General Circulation Model not only reproduces the 2004 freshening event ( r =0.96, p

Description: Semi-Lagrangian methods in air pollution models Geoscientific Model Development, 4, 511-541, 2011 Author(s): A. B. Hansen, J. Brandt, J. H. Christensen, and E. Kaas Various semi-Lagrangian methods are tested with respect to advection in air pollution modeling. The aim is to find a method fulfilling as many of the desirable properties by Rasch andWilliamson (1990) and Machenhauer et al. (2008) as possible. The focus in this study is on accuracy and local mass conservation. The methods tested are, first, classical semi-Lagrangian cubic interpolation, see e.g. Durran (1999), second, semi-Lagrangian cubic cascade interpolation, by Nair et al. (2002), third, semi-Lagrangian cubic interpolation with the modified interpolation weights, Locally Mass Conserving Semi-Lagrangian (LMCSL), by Kaas (2008), and last, semi-Lagrangian cubic interpolation with a locally mass conserving monotonic filter by Kaas and Nielsen (2010). Semi-Lagrangian (SL) interpolation is a classical method for atmospheric modeling, cascade interpolation is more efficient computationally, modified interpolation weights assure mass conservation and the locally mass conserving monotonic filter imposes monotonicity. All schemes are tested with advection alone or with advection and chemistry together under both typical rural and urban conditions using different temporal and spatial resolution. The methods are compared with a current state-of-the-art scheme, Accurate Space Derivatives (ASD), see Frohn et al. (2002), presently used at the National Environmental Research Institute (NERI) in Denmark. To enable a consistent comparison only non-divergent flow configurations are tested. The test cases are based either on the traditional slotted cylinder or the rotating cone, where the schemes' ability to model both steep gradients and slopes are challenged. The tests showed that the locally mass conserving monotonic filter improved the results significantly for some of the test cases, however, not for all. It was found that the semi-Lagrangian schemes, in almost every case, were not able to outperform the current ASD scheme used in DEHM with respect to accuracy.

Description: GEOCLIM reloaded (v 1.0): a new coupled earth system model for past climate change Geoscientific Model Development, 4, 451-481, 2011 Author(s): S. Arndt, P. Regnier, Y. Goddéris, and Y. Donnadieu We present a new version of the coupled Earth system model GEOCLIM. The new release, GEOCLIM reloaded (v 1.0), links the existing atmosphere and weathering modules to a novel, temporally and spatially resolved model of the global ocean circulation, which provides a physical framework for a mechanistic description of the marine biogeochemical dynamics of carbon, nitrogen, phosphorus and oxygen. The ocean model is also coupled to a fully formulated, vertically resolved diagenetic model. GEOCLIM reloaded is thus a unique tool to investigate the short- and long-term feedbacks between climatic conditions, continental inputs, ocean biogeochemical dynamics and diagenesis. A complete and detailed description of the resulting Earth system model and its new features is first provided. The performance of GEOCLIM reloaded is then evaluated by comparing steady-state simulation under present-day conditions with a comprehensive set of oceanic data and existing global estimates of bio-element cycling in the pelagic and benthic compartments.

Description: Mesoscale variability of water masses in the Arabian Sea as revealed by ARGO floats Ocean Science Discussions, 8, 1369-1402, 2011 Author(s): X. Carton and P. L'Hegaret By analysing ARGO float data over the last four years, some aspects of the mesoscale variability of water masses in the Arabian Sea are described. The Red Sea Water outflow is strong in the Southwestern Gulf of Aden, in particular when a cyclonic gyre predominates in this region. Salinities of 36.5 and temperatures of 16 °C are found there between 600 and 1000 m depths. The Red Sea Water is more dilute in the eastern part of the Gulf, and fragments of this water mass can be advected offshore across the gulf or towards its northern coast by the regional gyres. The Red Sea Water outflow is also detected along the northeastern coast of Socotra, and fragments of RSW are found between one and three degrees of latitude north of this island. In the whole Gulf of Aden, the correlation between the deep motions of the floats and the SSH measured by altimetry is strong, at regional scale. The finer scale details of the float trajectories are more often related to the anomalous water masses that they encounter. The Persian Gulf Water (PGW) is found in the float profiles near Ras ash Sharbatat (near 57° E, 18° N), again with 36.5 in salinity and about 18–19 °C in temperature. These observations were achieved in winter when the southwestward monsoon currents can advect PGW along the South Arabian coast. Fragments of PGW are found in the Arabian Sea between 18 and 20° N and 63 and 65° E, showing that this water mass can escape the Gulf of Oman southeastward, in particular during summer.

Description: The CSIRO Mk3L climate system model version 1.0 – Part 1: Description and evaluation Geoscientific Model Development, 4, 483-509, 2011 Author(s): S. J. Phipps, L. D. Rotstayn, H. B. Gordon, J. L. Roberts, A. C. Hirst, and W. F. Budd The CSIRO Mk3L climate system model is a coupled general circulation model, designed primarily for millennial-scale climate simulations and palaeoclimate research. Mk3L includes components which describe the atmosphere, ocean, sea ice and land surface, and combines computational efficiency with a stable and realistic control climatology. This paper describes the model physics and software, analyses the control climatology, and evaluates the ability of the model to simulate the modern climate. Mk3L incorporates a spectral atmospheric general circulation model, a z -coordinate ocean general circulation model, a dynamic-thermodynamic sea ice model and a land surface scheme with static vegetation. The source code is highly portable, and has no dependence upon proprietary software. The model distribution is freely available to the research community. A 1000-yr climate simulation can be completed in around one-and-a-half months on a typical desktop computer, with greater throughput being possible on high-performance computing facilities. Mk3L produces realistic simulations of the larger-scale features of the modern climate, although with some biases on the regional scale. The model also produces reasonable representations of the leading modes of internal climate variability in both the tropics and extratropics. The control state of the model exhibits a high degree of stability, with only a weak cooling trend on millennial timescales. Ongoing development work aims to improve the model climatology and transform Mk3L into a comprehensive earth system model.

Description: A coupled climate model simulation of Marine Isotope Stage 3 stadial climate Climate of the Past, 7, 649-670, 2011 Author(s): J. Brandefelt, E. Kjellström, J.-O. Näslund, G. Strandberg, A. H. L. Voelker, and B. Wohlfarth We present a coupled global climate model (CGCM) simulation, integrated for 1500 yr to quasi-equilibrium, of a stadial (cold period) within Marine Isotope Stage 3 (MIS 3). The simulated Greenland stadial 12 (GS12; ~44 ka BP) annual global mean surface temperature ( T s ) is 5.5 °C lower than in the simulated recent past (RP) climate and 1.3 °C higher than in the simulated Last Glacial Maximum (LGM; 21 ka BP) climate. The simulated GS12 is evaluated against proxy data and previous modelling studies of MIS3 stadial climate. We show that the simulated MIS 3 climate, and hence conclusions drawn regarding the dynamics of this climate, is highly model-dependent. The main findings are: (i) Proxy sea surface temperatures (SSTs) are higher than simulated SSTs in the central North Atlantic, in contrast to earlier simulations of MIS 3 stadial climate in which proxy SSTs were found to be lower than simulated SST. (ii) The Atlantic Meridional Overturning Circulation (AMOC) slows down by 50 % in the GS12 climate as compared to the RP climate. This slowdown is attained without freshwater forcing in the North Atlantic region, a method used in other studies to force an AMOC shutdown. (iii) El-Niño-Southern Oscillation (ENSO) teleconnections in mean sea level pressure (MSLP) are significantly modified by GS12 and LGM forcing and boundary conditions. (iv) Both the mean state and variability of the simulated GS12 is dependent on the equilibration. The annual global mean T s only changes by 0.10 °C from model years 500–599 to the last century of the simulation, indicating that the climate system may be close to equilibrium already after 500 yr of integration. However, significant regional differences between the last century of the simulation and model years 500–599 exist. Further, the difference between simulated and proxy SST is reduced from model years 500–599 to the last century of the simulation. The results of the ENSO variability analysis is also shown to depend on the equilibration.

Description: Using dissolved oxygen concentrations to determine mixed layer depths in the Bellingshausen Sea Ocean Science Discussions, 8, 1505-1533, 2011 Author(s): K. Castro-Morales and J. Kaiser Concentrations of oxygen (O 2 ) and other dissolved gases in the oceanic mixed layer are often used to calculate air-sea gas exchange fluxes; for example, in the context of net and gross biological production estimates. The mixed layer depth ( z mix ) may be defined using criteria based on temperature or density differences to a reference depth near the ocean surface. However, temperature criteria fail in regions with strong haloclines such as the Southern Ocean where heat, freshwater and momentum fluxes interact to establish mixed layers. Moreover, the time scales of air-sea exchange differ for gases and heat, so that z mix defined using O 2 may be different to z mix defined using temperature or density. Here, we propose to define an O 2 -based mixed layer depth, z mix (O 2 ), as the depth where the relative difference between the O 2 concentration and a reference value at a depth equivalent to 10 dbar equals 0.5 %. This definition was established by numerical analysis of O 2 profiles in coastal areas of the Southern Ocean and corroborated by visual inspection. Comparisons of z mix (O 2 ) with z mix based on potential temperature differences, i.e. z mix (Δ θ = 0.2 °C) and z mix (Δ θ = 0.5 °C), and potential density differences, i.e. z mix (Δ σ θ = 0.03 kg m −3 ) and z mix (Δ σ θ = 0.125 kg m −3 ), showed that z mix (O 2 ) closely follows z mix (Δ σ θ = 0.03 kg m −3 ). Further comparisons with published z mix climatologies and z mix derived from World Ocean Atlas 2005 data were also performed. To establish z mix for use with biological production estimates in the absence of O 2 profiles, we suggest using z mix (Δ σ θ = 0.03 kg m −3 ), which is also the basis for the climatology by de Boyer Montégut et al. (2004).

Description: N/P ratio of nutrient uptake in the Baltic Sea Ocean Science Discussions, 8, 1233-1259, 2011 Author(s): Z. Wan, L. Jonasson, and H. Bi The N/P ratio of nutrient uptake, i.e., the ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) taken by primary producers, varies in different basins and in different seasons in the Baltic Sea. The N/P ratio of nutrient alteration fore and after spring blooms is not same as the N/P ratio of nutrient uptake, but the former can be regarded as an indicator for the later in the Baltic Sea. Based on the observed N/P ratio of nutrient alteration, we hypothesize a non-Redfield N/P ratio of nutrient uptake. The 3D-ecosystem model ERGOM coupled with the circulation model DMI-BSHcmod was used to test the hypothesis. When the Redfield ratio was used in the model, the DIP surplus after spring blooms was too high and resulted in the overly growth of cyanobacteria and too much nitrogen fixation. When the non-Redfield ratio was used in the model, the corresponding problem tended to disappear. In summary, we show that: (1) the Redfield N/P ratio of nutrient uptake in the Baltic Sea tends to be too high; (2) a lower N/P ratio 10:1 appears to work better than the Redfield value; and (3) the N/P ratio of nutrient uptake in the Baltic Proper during spring blooms is around 6:1.

Description: Spectrophotometric high-precision seawater pH determination for use in underway measuring systems Ocean Science Discussions, 8, 1339-1367, 2011 Author(s): S. Aßmann, C. Frank, and A. Kötzinger Autonomous sensors are required for a comprehensive documentation of the changes in the marine carbon system and thus to differentiate between its natural variability and anthropogenic impacts. Spectrophotometric determination of pH – a key variable of the seawater carbon system – is particularly suited to achieve precise and drift-free measurements. However, available spectrophotometric instruments are not suitable for integration into automated measurement systems (e.g. FerryBox) since they do not meet the major requirements of reliability, stability, robustness and moderate cost. Here we report on the development and testing of a new indicator-based pH sensor that meets all of these requirements. This sensor can withstand the rough conditions during long-term deployments on ships of opportunities and is applicable on the open ocean as well as in coastal waters with complex background and highly variable conditions. The sensor uses a high resolution CCD spectrometer as detector connected via optical fibers to a custom-made cuvette designed to reduce the impact of air bubbles. The sample temperature can be precisely adjusted (25 °C ± 0.006 °C) using computer-controlled power supplies and Peltier elements thus avoiding the widely used water bath. The overall setup achieves a measurement frequency of 1 min −1 with a precision of ± 0.0007 pH units and an average offset of +0.0018 pH units to a pH reference during shipboard operation. Application of this sensor allows monitoring of seawater pH in autonomous underway systems, providing a key variable for characterization and understanding the marine carbon system.

Description: The early Eocene equable climate problem revisited Climate of the Past, 7, 603-633, 2011 Author(s): M. Huber and R. Caballero The early Eocene "equable climate problem", i.e. warm extratropical annual mean and above-freezing winter temperatures evidenced by proxy records, has remained as one of the great unsolved problems in paleoclimate. Recent progress in modeling and in paleoclimate proxy development provides an opportunity to revisit this problem to ascertain if the current generation of models can reproduce the past climate features without extensive modification. Here we have compiled early Eocene terrestrial temperature data and compared with climate model results using a consistent and rigorous methodology. We test the hypothesis that equable climates can be explained simply as a response to increased greenhouse gas forcing within the framework of the atmospheric component of the Community Climate System Model (version 3), a climate model in common use for predicting future climate change. We find that, with suitably large radiative forcing, the model and data are in general agreement for annual mean and cold month mean temperatures, and that the pattern of high latitude amplification recorded by proxies can be largely, but not perfectly, reproduced.

Description: Medusa-1.0: a new intermediate complexity plankton ecosystem model for the global domain Geoscientific Model Development, 4, 381-417, 2011 Author(s): A. Yool, E. E. Popova, and T. R. Anderson The ongoing, anthropogenically-driven changes to the global ocean are expected to have significant consequences for plankton ecosystems in the future. Because of the role that plankton play in the ocean's "biological pump", changes in abundance, distribution and productivity will likely have additional consequences for the wider carbon cycle. Just as in the terrestrial biosphere, marine ecosystems exhibit marked diversity in species and functional types of organisms. Predicting potential change in plankton ecosystems therefore requires the use of models that are suited to this diversity, but whose parameterisation also permits robust and realistic functional behaviour. In the past decade, advances in model sophistication have attempted to address diversity, but have been criticised for doing so inaccurately or ahead of a requisite understanding of underlying processes. Here we introduce MEDUSA-1.0 ( M odel of E cosystem D ynamics, nutrient U tilisation, S equestration and A cidification), a new "intermediate complexity" plankton ecosystem model that expands on traditional nutrient-phytoplankton-zooplankton-detritus (NPZD) models, and remains amenable to global-scale evaluation. MEDUSA-1.0 includes the biogeochemical cycles of nitrogen, silicon and iron, broadly structured into "small" and "large" plankton size classes, of which the "large" phytoplankton class is representative of a key phytoplankton group, the diatoms. A full description of MEDUSA-1.0's state variables, differential equations, functional forms and parameter values is included, with particular attention focused on the submodel describing the export of organic carbon from the surface to the deep ocean. MEDUSA-1.0 is used here in a multi-decadal hindcast simulation, and its biogeochemical performance evaluated at the global scale.

Description: PREP-CHEM-SRC – 1.0: a preprocessor of trace gas and aerosol emission fields for regional and global atmospheric chemistry models Geoscientific Model Development, 4, 419-433, 2011 Author(s): S. R. Freitas, K. M. Longo, M. F. Alonso, M. Pirre, V. Marecal, G. Grell, R. Stockler, R. F. Mello, and M. Sánchez Gácita The preprocessor PREP-CHEM-SRC presented in the paper is a comprehensive tool aiming at preparing emission fields of trace gases and aerosols for use in atmospheric-chemistry transport models. The considered emissions are from the most recent databases of urban/industrial, biogenic, biomass burning, volcanic, biofuel use and burning from agricultural waste sources. For biomass burning, emissions can be also estimated directly from satellite fire detections using a fire emission model included in the tool. The preprocessor provides emission fields interpolated onto the transport model grid. Several map projections can be chosen. The inclusion of these emissions in transport models is also presented. The preprocessor is coded using Fortran90 and C and is driven by a namelist allowing the user to choose the type of emissions and the databases.

Description: A pre-operational 3-D variational data assimilation system in the North/Baltic Sea Ocean Science Discussions, 8, 1131-1160, 2011 Author(s): S. Y. Zhuang, W. W. Fu, and J. She This paper describes the implementation and evaluation of a pre-operational three dimensional variational (3DVAR) data assimilation system for the North/Baltic Sea. The univariate analysis for temperature and salinity is applied in a 3DVAR scheme in which the horizontal component of the background error covariance is modeled by an isotropic recursive filter (IRF) and the vertical component is represented by dominant Empirical Orthogonal Functions (EOFs) of the background error. Observations of temperature and salinity ( T / S ) profiles in the North/Baltic Sea are assimilated in the year of 2005. Effectiveness of the data assimilation scheme is assessed by comparison with the control run that no assimilation is done. The statistical analysis indicates that the model simulation is significantly improved with the 3DVAR scheme. On average, the root mean square error (RMSE) of temperature and salinity is reduced by 0.2 °C and 0.25 psu in the North/Baltic Sea. In addition, the bias of temperature and salinity is also decreased by 0.1 °C and 0.2 psu, respectively. Starting from an analyzed initial state, one month simulation without assimilation is carried out with the aim of examining the persistence of the initial impact. It is shown that the assimilated initial state can impact the model simulation for nearly two weeks. The influence on salinity is more pronounced than temperature.

Description: The construction of a Central Netherlands temperature Climate of the Past, 7, 527-542, 2011 Author(s): G. van der Schrier, A. van Ulden, and G. J. van Oldenborgh The Central Netherlands Temperature (CNT) is a monthly daily mean temperature series constructed from homogenized time series from the centre of the Netherlands. The purpose of this series is to offer a homogeneous time series representative of a larger area in order to study large-scale temperature changes. It will also facilitate a comparison with climate models, which resolve similar scales. From 1906 onwards, temperature measurements in the Netherlands have been sufficiently standardized to construct a high-quality series. Long time series have been constructed by merging nearby stations and using the overlap to calibrate the differences. These long time series and a few time series of only a few decades in length have been subjected to a homogeneity analysis in which significant breaks and artificial trends have been corrected. Many of the detected breaks correspond to changes in the observations that are documented in the station metadata. This version of the CNT, to which we attach the version number 1.1, is constructed as the unweighted average of four stations (De Bilt, Winterswijk/Hupsel, Oudenbosch/Gilze-Rijen and Gemert/Volkel) with the stations Eindhoven and Deelen added from 1951 and 1958 onwards, respectively. The global gridded datasets used for detecting and attributing climate change are based on raw observational data. Although some homogeneity adjustments are made, these are not based on knowledge of local circumstances but only on statistical evidence. Despite this handicap, and the fact that these datasets use grid boxes that are far larger then the area associated with that of the Central Netherlands Temperature, the temperature interpolated to the CNT region shows a warming trend that is broadly consistent with the CNT trend in all of these datasets. The actual trends differ from the CNT trend up to 30 %, which highlights the need to base future global gridded temperature datasets on homogenized time series.

Description: Winter and summer blocking variability in the North Atlantic region – evidence from long-term observational and proxy data from southwestern Greenland Climate of the Past, 7, 543-555, 2011 Author(s): N. Rimbu and G. Lohmann We investigate the interannual and decadal variability of the North Atlantic atmospheric blocking frequency and distribution in connection with long-term observational and proxy records from southwestern Greenland. It is shown that warm (cold) conditions in southwestern Greenland during winter are related with high (low) blocking activity in the Greenland-Scandinavian region. The pattern of winter temperature-blocking variability is more complex than the blocking pattern associated to the North Atlantic Oscillation (NAO). We find, furthermore, that a North Atlantic blocking index is significantly correlated with seasonally resolved stable isotope records from Greenland ice cores. Both suggest a possible reconstruction of blocking variability in this region. During summer, high (low) blocking activity in the Euro-Atlantic region is associated with cold (warm) conditions in southwestern Greenland. We conclude that historical temperature records, as well as proxy data from Greenland, can be used to obtain information related to interannual and multidecadal variation of winter and summer blocking during past periods.

Description: Numerical simulation and decomposition of kinetic energies in the Central Mediterranean Sea: insight on mesoscale circulation and energy conversion Ocean Science Discussions, 8, 1161-1214, 2011 Author(s): R. Sorgente, A. Olita, P. Oddo, L. Fazioli, and A. Ribotti The spatial and temporal variability of eddy and mean kinetic energy of the Central Mediterranean Sea has been investigated, from January 2008 to December 2010, by mean of a numerical simulation mainly to quantify the mesoscale dynamics and their relationships with physical forcing. In order to understand the energy redistribution processes, the baroclinic energy conversion has been analysed, suggesting hypotheses about the drivers of the mesoscale activity in this area. The ocean model used is based on the Princeton Ocean Model implemented at 1/32° horizontal resolution. Surface momentum and buoyancy fluxes are interactively computed by mean of standard bulk formulae using predicted model Sea Surface Temperature and atmospheric variables provided by the European Centre for Medium Range Weather Forecast operational analyses. At its lateral boundaries the model is one-way nested within the Mediterranean Forecasting System operational products. The model domain has been subdivided in four sub-regions: Sardinia channel and southern Tyrrhenian Sea, Sicily channel, eastern Tunisian shelf and Libyan Sea. Temporal evolution of eddy and mean kinetic energy has been analysed, on each of the four sub-regions composing the model domain, showing different behaviours. On annual scales and within the first 5 m depth, the eddy kinetic energy represents approximately the 60 % of the total kinetic energy over the whole domain, confirming the strong mesoscale nature of the surface current flows in this area. The analyses show that the model well reproduces the path and the temporal behaviour of the main known sub-basin circulation features. New mesoscale structures have been also identified, from numerical results and direct observations, for the first time as the Pantelleria Vortex and the Medina Gyre. The classical the kinetic energy decomposition (eddy and mean) allowed to depict and to quantify the stable and fluctuating parts of the circulation in the region, and to differentiate the four sub-regions as function of relative and absolute strength of the mesoscale activity. Furthermore the Baroclinic Energy Conversion term shows that in the Sardinia Channel the mesoscale activity, due to baroclinic instabilities, is significantly larger than in the other sub-regions, while a negative sign of the energy conversion, meaning a transfer of energy from the Eddy Kinetic Energy to the Eddy Available Potential Energy, has been recorded only for the surface layers of the Sicily Channel during summer.

Description: Modelling of the anthropogenic tritium transient and its decay product helium-3 in the Mediterranean Sea using a high-resolution regional model Ocean Science Discussions, 11, 2691-2732, 2014 Author(s): M. Ayache, J.-C. Dutay, P. Jean-Baptiste, K. Beranger, T. Arsouze, J. Beuvier, J. Palmieri, B. Le-vu, and W. Roether This numerical study provides the first simulation of the anthropogenic tritium invasion and its decay product helium-3 ( 3 He) in the Mediterranean Sea. The simulation covers the entire tritium ( 3 H) transient generated by the atmospheric nuclear-weapon tests performed in the 1950s and early 1960s and run till 2011. Tritium, helium-3 and their derived age estimates are particularly suitable for studying intermediate and deep-water ventilation and spreading of water masses at intermediate/deep levels. The simulation is made using a high resolution regional model NEMO-MED12 forced at the surface with prescribed tritium evolution derived from observations. The simulation is compared to measurements of tritium and helium-3 performed along large-scale transects in the Mediterranean Sea during the last few decades on cruises of Meteor M5/6, M31/1, M44/4, M51/2, M84/3, and Poseidon 234. The results show that the input function used for the tritium, generates a realistic distribution of the main hydrographic features of the Mediterranean Sea circulation. In the eastern basin, the results highlight the weak formation of Adriatic Deep Water in the model, which explains its weak contribution to the Eastern Mediterranean Deep Water in the Ionian sub-basin. It produces a realistic representation of the Eastern Mediterranean Transient signal, simulating a deep-water formation in the Aegean sub-basin at the beginning of the 1993, with a realistic timing of deep-water renewal in the eastern basin. In the western basin, the unusual intense deep convection event of winter 2005 in the Gulf of Lions during the Western Mediterranean Transition is simulated. However the spreading of the recently ventilated deep water toward the South is too weak. The ventilation and spreading of the Levantine Intermediate Water from the eastern basin toward the western basin is simulated with realistic tracer-age distribution compared to observation-based estimates.

Description: Tropical troposphere to stratosphere transport of carbon monoxide and long-lived trace species in the Chemical Lagrangian Model of the Stratosphere (CLaMS) Geoscientific Model Development, 7, 2895-2916, 2014 Author(s): R. Pommrich, R. Müller, J.-U. Grooß, P. Konopka, F. Ploeger, B. Vogel, M. Tao, C. M. Hoppe, G. Günther, N. Spelten, L. Hoffmann, H.-C. Pumphrey, S. Viciani, F. D'Amato, C. M. Volk, P. Hoor, H. Schlager, and M. Riese Variations in the mixing ratio of trace gases of tropospheric origin entering the stratosphere in the tropics are of interest for assessing both troposphere to stratosphere transport fluxes in the tropics and the impact of these transport fluxes on the composition of the tropical lower stratosphere. Anomaly patterns of carbon monoxide (CO) and long-lived tracers in the lower tropical stratosphere allow conclusions about the rate and the variability of tropical upwelling to be drawn. Here, we present a simplified chemistry scheme for the Chemical Lagrangian Model of the Stratosphere (CLaMS) for the simulation, at comparatively low numerical cost, of CO, ozone, and long-lived trace substances (CH 4 , N 2 O, CCl 3 F (CFC-11), CCl 2 F 2 (CFC-12), and CO 2 ) in the lower tropical stratosphere. For the long-lived trace substances, the boundary conditions at the surface are prescribed based on ground-based measurements in the lowest model level. The boundary condition for CO in the lower troposphere (below about 4 km) is deduced from MOPITT measurements. Due to the lack of a specific representation of mixing and convective uplift in the troposphere in this model version, enhanced CO values, in particular those resulting from convective outflow are underestimated. However, in the tropical tropopause layer and the lower tropical stratosphere, there is relatively good agreement of simulated CO with in situ measurements (with the exception of the TROCCINOX campaign, where CO in the simulation is biased low ≈10–15 ppbv). Further, the model results (and therefore also the ERA-Interim winds, on which the transport in the model is based) are of sufficient quality to describe large scale anomaly patterns of CO in the lower stratosphere. In particular, the zonally averaged tropical CO anomaly patterns (the so called "tape recorder" patterns) simulated by this model version of CLaMS are in good agreement with observations, although the simulations show a too rapid upwelling compared to observations as a consequence of the overestimated vertical velocities in the ERA-Interim reanalysis data set. Moreover, the simulated tropical anomaly patterns of N 2 O are in good agreement with observations. In the simulations, anomaly patterns of CH 4 and CFC-11 were found to be very similar to those of N 2 O; for all long-lived tracers, positive anomalies are simulated because of the enhanced tropical upwelling in the easterly shear phase of the quasi-biennial oscillation.

Description: Sensitivity of East African savannah vegetation to historical moisture-balance variation Climate of the Past, 10, 2067-2080, 2014 Author(s): I. Ssemmanda, V. Gelorini, and D. Verschuren Fossil pollen records provide key insight into the sensitivity of terrestrial ecosystems to climate change. However, tracing vegetation response to relatively modest historical climate fluctuations is often complicated by the overriding signature of anthropogenic landscape disturbance. Here we use high-resolution pollen data from a ~200-year lake-sediment record in open wooded savannah of Queen Elizabeth National Park (southwestern Uganda) to assess the sensitivity of the tropical lowland grassland–forest transition to historical, decade-scale moisture-balance fluctuations. Specifically we trace vegetation response to three episodes of higher average rainfall dated to the 1820s–1830s, ca. 1865–1890 and from 1962 to around 2000. Our pollen data indeed reveal a sequence of three wet periods, separated by two drier periods. During the inferred wetter episodes we find increases in the percent pollen abundance of trees and shrubs from moist semi-deciduous forest ( Allophylus , Macaranga , Alchornea , Celtis ), riparian forest ( Phoenix reclinata ) and wooded savannah ( Acalypha , Rhus -type vulgaris , Combretaceae/Melastomataceae) as well as taxa common in the local rift-valley grasslands ( Acacia , Ficus ), together creating strong temporary reductions in Poaceae pollen (to 45–55% of the terrestrial pollen sum). During intervening dry periods, Poaceae pollen attained values of 65–75%, and dryland herbs such as Commelina , Justicia -type odora and Chenopodiaceae expanded at the expense of Asteraceae, Solanum -type, Swertia usambarensis -type, and (modestly so) Urticaceae. Noting that the overall richness of arboreal taxa remained high but their combined abundance low, we conclude that the landscape surrounding Lake Chibwera has been an open wooded savannah throughout the past 200 years, with historical moisture-balance variation exerting modest effects on local tree cover (mostly the abundance of Acacia and Ficus ) and the occurrence of damp soil areas promoting Phoenix reclinata . The strong apparent expansion of true forest trees during wet episodes can be explained partly by enhanced pollen input via a temporarily activated upland stream. Pollen from exotic trees and cultural indicators appears from the 1970s onwards, but their combined influence fails to mask the signature of natural vegetation dynamics in the pollen record.

Description: CREST (Climate REconstruction SofTware): a probability density function (PDF)-based quantitative climate reconstruction method Climate of the Past, 10, 2081-2098, 2014 Author(s): M. Chevalier, R. Cheddadi, and B. M. Chase Several methods currently exist to quantitatively reconstruct palaeoclimatic variables from fossil botanical data. Of these, probability density function (PDF)-based methods have proven valuable as they can be applied to a wide range of plant assemblages. Most commonly applied to fossil pollen data, their performance, however, can be limited by the taxonomic resolution of the pollen data, as many species may belong to a given pollen type. Consequently, the climate information associated with different species cannot always be precisely identified, resulting in less-accurate reconstructions. This can become particularly problematic in regions of high biodiversity. In this paper, we propose a novel PDF-based method that takes into account the different climatic requirements of each species constituting the broader pollen type. PDFs are fitted in two successive steps, with parametric PDFs fitted first for each species and then a combination of those individual species PDFs into a broader single PDF to represent the pollen type as a unit. A climate value for the pollen assemblage is estimated from the likelihood function obtained after the multiplication of the pollen-type PDFs, with each being weighted according to its pollen percentage. To test its performance, we have applied the method to southern Africa as a regional case study and reconstructed a suite of climatic variables (e.g. winter and summer temperature and precipitation, mean annual aridity, rainfall seasonality). The reconstructions are shown to be accurate for both temperature and precipitation. Predictable exceptions were areas that experience conditions at the extremes of the regional climatic spectra. Importantly, the accuracy of the reconstructed values is independent of the vegetation type where the method is applied or the number of species used. The method used in this study is publicly available in a software package entitled CREST (Climate REconstruction SofTware) and will provide the opportunity to reconstruct quantitative estimates of climatic variables even in areas with high geographical and botanical diversity.

Description: Evaluation of the ECHAM family radiation codes performance in the representation of the solar signal Geoscientific Model Development, 7, 2859-2866, 2014 Author(s): T. Sukhodolov, E. Rozanov, A. I. Shapiro, J. Anet, C. Cagnazzo, T. Peter, and W. Schmutz Solar radiation is the main source of energy for the Earth's atmosphere and in many respects defines its composition, photochemistry, temperature profile and dynamics. The magnitude of the solar irradiance variability strongly depends on the wavelength, making difficult its representation in climate models. Due to some deficiencies in the applied radiation codes, several models fail to show a clear response in middle stratospheric heating rates to solar spectral irradiance variability; therefore, it is important to evaluate model performance in this respect before doing multiple runs. In this work we evaluate the performance of three generations of ECHAM (4, 5 and 6) solar radiation schemes by a comparison with the reference high-resolution libRadtran code. We found that all original ECHAM radiation codes miss almost all solar signals in the heating rates in the mesosphere. In the stratosphere the two-band ECHAM4 code (E4) has an almost negligible radiative response to solar irradiance changes and the six-band ECHAM5 code (E5c) reproduces only about half of the reference signal, while representation in the ECHAM6 code (E6) is better – it misses a maximum of about 15% in the upper stratosphere. On the basis of the comparison results we suggest necessary improvements to the ECHAM family codes by the inclusion of available parameterizations of the heating rate due to absorption by oxygen (O 2 ) and ozone (O 3 ). Improvement is presented for E5c and E6, and both codes, with the introduced parameterizations, represent the heating rate response to the spectral solar irradiance variability simulated with libRadtran much better without a substantial increase in computer time. The suggested parameterizations are recommended to be applied in the middle-atmosphere version of the ECHAM-5 and 6 models for the study of the solar irradiance influence on climate.

Description: GEOtop 2.0: simulating the combined energy and water balance at and below the land surface accounting for soil freezing, snow cover and terrain effects Geoscientific Model Development, 7, 2831-2857, 2014 Author(s): S. Endrizzi, S. Gruber, M. Dall'Amico, and R. Rigon GEOtop is a fine-scale grid-based simulator that represents the heat and water budgets at and below the soil surface. It describes the three-dimensional water flow in the soil and the energy exchange with the atmosphere, considering the radiative and turbulent fluxes. Furthermore, it reproduces the highly non-linear interactions between the water and energy balance during soil freezing and thawing, and simulates the temporal evolution of the water and energy budgets in the snow cover and their effect on soil temperature. Here, we present the core components of GEOtop 2.0 and demonstrate its functioning. Based on a synthetic simulation, we show that the interaction of processes represented in GEOtop 2.0 can result in phenomena that are significant and relevant for applications involving permafrost and seasonally frozen soils, both in high altitude and latitude regions.

Description: The North American Carbon Program Multi-scale Synthesis and Terrestrial Model Intercomparison Project – Part 2: Environmental driver data Geoscientific Model Development, 7, 2875-2893, 2014 Author(s): Y. Wei, S. Liu, D. N. Huntzinger, A. M. Michalak, N. Viovy, W. M. Post, C. R. Schwalm, K. Schaefer, A. R. Jacobson, C. Lu, H. Tian, D. M. Ricciuto, R. B. Cook, J. Mao, and X. Shi Ecosystems are important and dynamic components of the global carbon cycle, and terrestrial biospheric models (TBMs) are crucial tools in further understanding of how terrestrial carbon is stored and exchanged with the atmosphere across a variety of spatial and temporal scales. Improving TBM skills, and quantifying and reducing their estimation uncertainties, pose significant challenges. The Multi-scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP) is a formal multi-scale and multi-model intercomparison effort set up to tackle these challenges. The MsTMIP protocol prescribes standardized environmental driver data that are shared among model teams to facilitate model–model and model–observation comparisons. This paper describes the global and North American environmental driver data sets prepared for the MsTMIP activity to both support their use in MsTMIP and make these data, along with the processes used in selecting/processing these data, accessible to a broader audience. Based on project needs and lessons learned from past model intercomparison activities, we compiled climate, atmospheric CO 2 concentrations, nitrogen deposition, land use and land cover change (LULCC), C3 / C4 grasses fractions, major crops, phenology and soil data into a standard format for global (0.5° × 0.5° resolution) and regional (North American: 0.25° × 0.25° resolution) simulations. In order to meet the needs of MsTMIP, improvements were made to several of the original environmental data sets, by improving the quality, and/or changing their spatial and temporal coverage, and resolution. The resulting standardized model driver data sets are being used by over 20 different models participating in MsTMIP. The data are archived at the Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC, http://daac.ornl.gov ) to provide long-term data management and distribution.

Description: The atmospheric chemistry box model CAABA/MECCA-3.0 Geoscientific Model Development, 4, 373-380, 2011 Author(s): R. Sander, A. Baumgaertner, S. Gromov, H. Harder, P. Jöckel, A. Kerkweg, D. Kubistin, E. Regelin, H. Riede, A. Sandu, D. Taraborrelli, H. Tost, and Z.-Q. Xie We present version 3.0 of the atmospheric chemistry box model CAABA/MECCA. In addition to a complete update of the rate coefficients to the most recent recommendations, a number of new features have been added: chemistry in multiple aerosol size bins; automatic multiple simulations reaching steady-state conditions; Monte-Carlo simulations with randomly varied rate coefficients within their experimental uncertainties; calculations along Lagrangian trajectories; mercury chemistry; more detailed isoprene chemistry; tagging of isotopically labeled species. Further changes have been implemented to make the code more user-friendly and to facilitate the analysis of the model results. Like earlier versions, CAABA/MECCA-3.0 is a community model published under the GNU General Public License.

Description: The Aegean Sea marine security decision support system Ocean Science Discussions, 8, 1025-1053, 2011 Author(s): L. Perivoliotis, G. Krokos, K. Nittis, and G. Korres As part of the integrated ECOOP (European Coastal Sea Operational observing and Forecasting System) project, HCMR upgraded the already existing standalone Oil Spill Forecasting System for the Aegean Sea, initially developed for the Greek Operational Oceanography System (POSEIDON), into an active element of the European Decision Support System (EuroDeSS). The system is accessible through a user friendly web interface where the case scenarios can be fed into the oil spill drift model component, while the synthetic output contains detailed information about the distribution of oil spill particles and the oil spill budget and it is provided both in text based ECOOP common output format and as a series of sequential graphics. The main development steps that were necessary for this transition were the modification of the forcing input data module in order to allow the import of other system products which are usually provided in standard formats such as NetCDF and the transformation of the model's calculation routines to allow use of current, density and diffusivities data in z instead of sigma coordinates. During the implementation of the Aegean DeSS, the system was used in operational mode in order support the Greek marine authorities in handling a real accident that took place in North Aegean area. Furthermore, the introduction of common input and output files by all the partners of EuroDeSS extended the system's interoperability thus facilitating data exchanges and comparison experiments.

Description: How well can we derive Global Ocean Indicators from Argo data? Ocean Science Discussions, 8, 999-1024, 2011 Author(s): K. von Schuckmann and P.-Y. Le Traon Argo deployments began in the year 2000 and by November 2007 the array was 100 % complete, covering the global ocean from the surface down to 2000 m depth. In this study, Argo temperature and salinity measurements during the period 2005 to 2010 are used to develop a revised estimation of Global Ocean Indicators (GOIs) such as heat content variability, freshwater content and steric height. These revised indices are based on a simple box averaging scheme using a weighted mean. They include a proper estimation of the errors due to data handling methods and climatology uncertainties. A global ocean heat content change (OHC) trend of 0.55 ± 0.1 W m −2 is estimated over the time period 2005–2010. Similarly, a global steric sea level (GSSL) rise of 0.69 ± 0.14 mm yr −1 is observed. The global ocean freshwater content (OFC) trend is barely significant. Results show that there is significant interannual variability at global scale, especially for global OFC. Annual mean GOIs from the today's Argo samling can be derived with an accuracy of ±0.10 cm for GSSL, ±0.21 × 10 8 J m −2 for global OHC, and ±700 km 3 for global OFC. Long-term trends (15 yr) of GOIs based on the complete Argo sampling (10–1500 m depth) can be performed with an accuracy of about ±0.03 mm yr −1 for steric rise, ±0.02 W m −2 for ocean warming and ±20 km 3 yr −1 for global OFC trends – under the assumption that no systematic errors remain in the observing system.

Description: Silicon pool dynamics and biogenic silica export in the Southern Ocean, inferred from Si-isotopes Ocean Science Discussions, 8, 639-674, 2011 Author(s): F. Fripiat, A.-J. Cavagna, F. Dehairs, S. Speich, L. André, and D. Cardinal Water column silicon isotopic signatures (δ 30 Si) of silicic acid (Si(OH) 4 ) in the Southern Ocean were measured along a meridional transect from South Africa (Subtropical Zone) down to 57° S (northern Weddell Gyre). These data are the first reported for a summer transect across the whole Antarctic Circumpolar Current (ACC). δ 30 Si variations are large in the upper 1000 m, reflecting the effect of the silica pump superimposed upon meridional transfer across the ACC: the transport of Antarctic surface waters northward by a net Ekman drift and their convergence and mixing with warmer upper-ocean Si-depleted waters to the north. Using Si isotopic signatures, we determined different mixing interfaces between ACC water masses: the Antarctic Surface Water (AASW), the Antarctic Intermediate Water (AAIW), and the thermoclines in the low latitude areas. The residual silicic acid concentrations of end-members control the δ 30 Si alteration of the mixing products. With the exception of AASW, all mixing interfaces have a highly Si-depleted mixed layer end-member. These processes deplete the silicic acid AASW concentration across the different interfaces northward without significantly changing the AASW δ 30 Si. By comparing our new results with a previous study in the Australian sector we show that during the circumpolar transport of the ACC eastward, there is a slight but significant Si-isotopic lightening of the silicic acid pools from the Atlantic to the Australian sectors. This results either from the dissolution of biogenic silica in the deeper layers and/or from an isopycnal mixing with the deep water masses in the different oceanic basins: North Atlantic Deep Water in the Atlantic, and Indian Ocean deep water in the Indo-Australian sector. This eastward lightening is further transmitted to the subsurface waters, representing mixing interfaces between the surface and deeper layers. Using the Si-isotopic constraint, we estimate for the Greenwich Meridian a net biogenic silica production which should be representative of the annual export, at 4.5 ± 1.1 and 1.5 ± 0.4 mol Si m −2 for the Antarctic Zone and Polar Front Zone, respectively, in agreement with previous estimations. The summertime Si-supply into the mixed layer via vertical mixing was also assessed at 1.5 ± 0.4 and 0.1 ± 0.5 mol Si m −2 , respectively.

Description: Quality control of automated hyperspectral remote sensing measurements from a seaborne platform Ocean Science Discussions, 8, 613-638, 2011 Author(s): S. P. Garaba, M. R. Wernand, and O. Zielinski In this study four data quality flags are presented for automated and unmanned above-water hyperspectral optical measurements collected underway in the North Sea, The Minch, Irish Sea and Celtic Sea in April/May 2009. Coincident to these optical measurements a DualDome D12 (Mobotix, Germany) camera system was used to capture sea surface and sky images. The first three flags are based on meteorological conditions, to select erroneous incoming solar irradiance ( E S ) taken during dusk, dawn, before significant incoming solar radiation could be detected or under rainfall. Furthermore, the relative azimuthal angle of the optical sensors to the sun is used to identify possible sunglint free sea surface zones. A total of 629 spectra remained after applying the meteorological masks (first three flags). Based on this dataset, a fourth flag for sunglint was generated by analysing and evaluating water leaving radiance ( L W ) and remote sensing reflectance ( R RS ) spectral behaviour in the presence and absence of sunglint salient in the simultaneously available sea surface images. Spectra conditions satisfying "mean L W (700–950 nm) 〈 2 mW m −2 nm −1 Sr −1 " or alternatively "minimum R RS (700–950 nm) 〈 0.010 Sr −1 ", mask the most measurements affected by sunglint, providing efficient flagging of sunglint in automated quality control. It is confirmed that valid optical measurements can be performed 0° ≤ Φ ≤ 360° although 90° ≤ Φ ≤ 135° is recommended.

Description: Numerical modelling of POC yearly dynamics in the southern Baltic under variable scenarios of nutrients, light and temperature Ocean Science Discussions, 8, 675-700, 2011 Author(s): L. Dzierzbicka-Glowacka, K. Kuliński, A. Maciejewska, J. Jakacki, and J. Pempkowiak This paper presents various scenarios of the particulate organic carbon (POC) in the southern Baltic Sea. The study is based on a one-dimensional Particulate Organic Carbon model (1-D POC). Mathematically, the pelagic variables of 1-D POC model are described by a second-order partial differential equations of the diffusion type with biogeochemical sources and sinks. The POC concentration is determined as the sum of phytoplankton, zooplankton and dead organic matter (detritus) concentrations. The temporal changes in the phytoplankton biomass are caused by primary production, mortality, grazing by zooplankton and sinking. The zooplankton biomass is affected by ingestion, excretion, faecal production, mortality, and carnivorous grazing. The changes in the pelagic detritus concentration are determined by input of: dead phytoplankton and zooplankton, natural mortality of predators, faecal pellets, and sinks: sedimentation, zooplankton grazing and biochemical decomposition. The 1-D POC model was used to simulate temporal dynamics of POC in the southern Baltic Sea (Gdansk Deep, Bornholm Deep and Gotland Deep) under scenarios characterized by different temperature, nutrients and light. Daily, monthly, seasonal and annual variabilities of POC in the upper water layer are presented for the different scenarios. The starting-point of the numerical simulations was assumed as average values of the investigated pelagic variables for 1965–1998 period. Two- to three-fold increases of POC concentrations in late spring were revealed as well as the shift towards postponed maximum POC concentration. It is speculated that, due to POC increase, oxygenation of under-halocline water layer will decrease, while supply of food to organisms from higher trophic level should increase.

Description: The global ocean circulation on a retrograde rotating earth Climate of the Past, 7, 487-499, 2011 Author(s): V. Kamphuis, S. E. Huisman, and H. A. Dijkstra To understand the three-dimensional ocean circulation patterns that have occurred in past continental geometries, it is crucial to study the role of the present-day continental geometry and surface (wind stress and buoyancy) forcing on the present-day global ocean circulation. This circulation, often referred to as the Conveyor state, is characterised by an Atlantic Meridional Overturning Circulation (MOC) with a deep water formation at northern latitudes and the absence of such a deep water formation in the North Pacific. This MOC asymmetry is often attributed to the difference in surface freshwater flux: the Atlantic as a whole is a basin with net evaporation, while the Pacific receives net precipitation. This issue is revisited in this paper by considering the global ocean circulation on a retrograde rotating earth, computing an equilibrium state of the coupled atmosphere-ocean-land surface-sea ice model CCSM3. The Atlantic-Pacific asymmetry in surface freshwater flux is indeed reversed, but the ocean circulation pattern is not an Inverse Conveyor state (with deep water formation in the North Pacific) as there is relatively weak but intermittently strong deep water formation in the North Atlantic. Using a fully-implicit, global ocean-only model the stability properties of the Atlantic MOC on a retrograde rotating earth are also investigated, showing a similar regime of multiple equilibria as in the present-day case. These results indicate that the present-day asymmetry in surface freshwater flux is not the most important factor setting the Atlantic-Pacific salinity difference and, thereby, the asymmetry in the global MOC.

Description: A comprehensive set of benchmark tests for a land surface model of simultaneous fluxes of water and carbon at both the global and seasonal scale Geoscientific Model Development, 4, 255-269, 2011 Author(s): E. Blyth, D. B. Clark, R. Ellis, C. Huntingford, S. Los, M. Pryor, M. Best, and S. Sitch Evaluating the models we use in prediction is important as it allows us to identify uncertainties in prediction as well as guiding the priorities for model development. This paper describes a set of benchmark tests that is designed to quantify the performance of the land surface model that is used in the UK Hadley Centre General Circulation Model (JULES: Joint UK Land Environment Simulator). The tests are designed to assess the ability of the model to reproduce the observed fluxes of water and carbon at the global and regional spatial scale, and on a seasonal basis. Five datasets are used to test the model: water and carbon dioxide fluxes from ten FLUXNET sites covering the major global biomes, atmospheric carbon dioxide concentrations at four representative stations from the global network, river flow from seven catchments, the seasonal mean NDVI over the seven catchments and the potential land cover of the globe (after the estimated anthropogenic changes have been removed). The model is run in various configurations and results are compared with the data. A few examples are chosen to demonstrate the importance of using combined use of observations of carbon and water fluxes in essential in order to understand the causes of model errors. The benchmarking approach is suitable for application to other global models.

Description: A computational method for determining XBT depths Ocean Science Discussions, 8, 1777-1802, 2011 Author(s): J. Stark, J. Gorman, M. Hennessey, F. Reseghetti, J. Willis, J. Lyman, and J. Abraham A new technique for determining the depth of expendable bathythermographs (XBTs) is developed. This new method combines a forward-stepping calculation which incorporates all of the forces on the XBT devices during their descent. Of particular note are drag forces which are calculated using a new drag coefficient expression. That expression, obtained entirely from computational fluid dynamic modeling, accounts for local variations in the ocean environment. Consequently, the method allows for accurate determination of depths for any local temperature environment. The results, which are entirely based on numerical simulation, are compared with an experimental descent of an LM-Sippican T-5 XBT. It is found that the calculated depths differ by less than 3 % from depth estimates using the industry standard FRE. Furthermore, the differences decrease with depth. The computational model allows an investigation of the fluid patterns along the outer surface of the probe as well as in the interior channel. The simulations take account of complex flow phenomena such as laminar-turbulent transition and flow separation.

Description: Holocene vegetation and biomass changes on the Tibetan Plateau – a model-pollen data comparison Climate of the Past, 7, 881-901, 2011 Author(s): A. Dallmeyer, M. Claussen, U. Herzschuh, and N. Fischer Results of a transient numerical experiment performed in a coupled atmosphere-ocean-vegetation model with orbital forcing alone are compared to pollen-based vegetation reconstructions covering the last 6000 yr from four representative sites on the Tibetan Plateau. Causes of the vegetation change and consequences of the biomass storage are analysed. In general, simulated and reconstructed vegetation trends at each site are in good agreement. Both methods reveal a general retreat of the biomass-rich vegetation that is particularly manifested in a strong decrease of forests. However, model and reconstructions often differ with regard to the climatic factors causing the vegetation change at each site. The reconstructions primarily identify decreasing summer monsoon precipitation and changes in the temperature of the warm season as the responsible mechanisms for the vegetation shift. In the model, the land cover change mainly originates from differences in warm/cold seasonal temperatures and only to a lesser extent from precipitation changes. According to the model results, the averaged forest fraction on the Plateau shrinks by almost one-third from mid-Holocene (41.4 %) to present-day (28.3 %). Shrubs, whose fraction is quadrupled at present-day (12.3 %), replace most of this forest. Grass fraction increases from 38.1 % during the mid-Holocene to 42.3 % at present-day. This land cover change results in a decrease of living biomass by 0.62 kgC m −2 . Total biomass on the Tibetan Plateau decreases by 1.9 kgC m −2 , i.e. approx. 6.64 PgC are released due to the natural land cover change.

Description: Weakened atmospheric energy transport feedback in cold glacial climates Climate of the Past, 7, 1061-1073, 2011 Author(s): I. Cvijanovic, P. L. Langen, and E. Kaas The response of atmospheric energy transport during Northern Hemisphere cooling and warming from present day (PD) and Last Glacial Maximum (LGM) conditions is investigated using sea surface temperature anomalies derived from a freshwater hosing experiment. The present day climate shows enhanced sensitivity of the atmospheric mid-latitude energy transport compared to that of the LGM, suggesting its ability to reorganize more easily and thereby dampen high latitude temperature anomalies that may arise from changes in the oceanic transport. This effect is found to be a result of both the atmospheric and surface flux response. The increased PD transport sensitivity relative to that of the LGM is linked to a stronger dry static energy transport response which, in turn, is mainly driven by larger changes in the transient eddy heat flux. In comparison, changes in mid-latitude latent heat transport play a minor role in the overall transport sensitivity.

Description: A new version of the CNRM Chemistry-Climate Model, CNRM-CCM: description and improvements from the CCMVal-2 simulations Geoscientific Model Development, 4, 873-900, 2011 Author(s): M. Michou, D. Saint-Martin, H. Teyssèdre, A. Alias, F. Karcher, D. Olivié, A. Voldoire, B. Josse, V.-H. Peuch, H. Clark, J. N. Lee, and F. Chéroux This paper presents a new version of the Météo-France CNRM Chemistry-Climate Model, so-called CNRM-CCM. It includes some fundamental changes from the previous version (CNRM-ACM) which was extensively evaluated in the context of the CCMVal-2 validation activity. The most notable changes concern the radiative code of the GCM, and the inclusion of the detailed stratospheric chemistry of our Chemistry-Transport model MOCAGE on-line within the GCM. A 47-yr transient simulation (1960–2006) is the basis of our analysis. CNRM-CCM generates satisfactory dynamical and chemical fields in the stratosphere. Several shortcomings of CNRM-ACM simulations for CCMVal-2 that resulted from an erroneous representation of the impact of volcanic aerosols as well as from transport deficiencies have been eliminated. Remaining problems concern the upper stratosphere (5 to 1 hPa) where temperatures are too high, and where there are biases in the NO 2 , N 2 O 5 and O 3 mixing ratios. In contrast, temperatures at the tropical tropopause are too cold. These issues are addressed through the implementation of a more accurate radiation scheme at short wavelengths. Despite these problems we show that this new CNRM CCM is a useful tool to study chemistry-climate applications.

Description: A vertical-mode decomposition to investigate low-frequency internal motion across the Atlantic at 26° N Ocean Science Discussions, 8, 2047-2100, 2011 Author(s): Z. B. Szuts, J. R. Blundell, M. P. Chidichimo, and J. Marotzke Hydrographic data from full-depth moorings maintained by the RAPID/MOCHA project that span the Atlantic at 26° N are decomposed into vertical modes, in order to give a dynamical framework for interpreting the observed fluctuations. Vertical modes at each mooring are fit to pressure perturbations using a Gauss-Markov inversion. Away from boundaries, the vertical structure is almost entirely described by the first baroclinic mode, as confirmed by high correlation between the original signal and reconstructions using only the first baroclinic mode. These first baroclinic motions are also highly coherent with altimetric sea surface height (SSH). On both the western and eastern boundaries, however, the decomposition contains significant variance at higher modes, and there is a corresponding decrease in the agreement between SSH and either the original signal or the first baroclinic mode reconstruction. At the boundaries, the transport fluctuations described by the first baroclinic mode represent less than 10% of the variance of the full transport signal. At the eastern boundary, a linear combination of many baroclinic modes is required to explain the observed vertical density profile of the seasonal cycle, a result that is consistent with the oceanic response to wind-forcing not propagating far from the eastern boundary.

Description: MIROC-ESM 2010: model description and basic results of CMIP5-20c3m experiments Geoscientific Model Development, 4, 845-872, 2011 Author(s): S. Watanabe, T. Hajima, K. Sudo, T. Nagashima, T. Takemura, H. Okajima, T. Nozawa, H. Kawase, M. Abe, T. Yokohata, T. Ise, H. Sato, E. Kato, K. Takata, S. Emori, and M. Kawamiya An earth system model (MIROC-ESM 2010) is fully described in terms of each model component and their interactions. Results for the CMIP5 (Coupled Model Intercomparison Project phase 5) historical simulation are presented to demonstrate the model's performance from several perspectives: atmosphere, ocean, sea-ice, land-surface, ocean and terrestrial biogeochemistry, and atmospheric chemistry and aerosols. An atmospheric chemistry coupled version of MIROC-ESM (MIROC-ESM-CHEM 2010) reasonably reproduces transient variations in surface air temperatures for the period 1850–2005, as well as the present-day climatology for the zonal-mean zonal winds and temperatures from the surface to the mesosphere. The historical evolution and global distribution of column ozone and the amount of tropospheric aerosols are reasonably simulated in the model based on the Representative Concentration Pathways' (RCP) historical emissions of these precursors. The simulated distributions of the terrestrial and marine biogeochemistry parameters agree with recent observations, which is encouraging to use the model for future global change projections.

Description: Uncertainties in modelling CH 4 emissions from northern wetlands in glacial climates: the role of vegetation parameters Climate of the Past, 7, 1075-1087, 2011 Author(s): C. Berrittella and J. van Huissteden Marine Isotope Stage 3 (MIS 3) interstadials are marked by a sharp increase in the atmospheric methane (CH 4 ) concentration, as recorded in ice cores. Wetlands are assumed to be the major source of this CH 4 , although several other hypotheses have been advanced. Modelling of CH 4 emissions is crucial to quantify CH 4 sources for past climates. Vegetation effects are generally highly generalized in modelling past and present-day CH 4 fluxes, but should not be neglected. Plants strongly affect the soil-atmosphere exchange of CH 4 and the net primary production of the vegetation supplies organic matter as substrate for methanogens. For modelling past CH 4 fluxes from northern wetlands, assumptions on vegetation are highly relevant since paleobotanical data indicate large differences in Last Glacial (LG) wetland vegetation composition as compared to modern wetland vegetation. Besides more cold-adapted vegetation, Sphagnum mosses appear to be much less dominant during large parts of the LG than at present, which particularly affects CH 4 oxidation and transport. To evaluate the effect of vegetation parameters, we used the PEATLAND-VU wetland CO 2 /CH 4 model to simulate emissions from wetlands in continental Europe during LG and modern climates. We tested the effect of parameters influencing oxidation during plant transport ( f ox ), vegetation net primary production (NPP, parameter symbol P max ), plant transport rate ( V transp ), maximum rooting depth ( Z root ) and root exudation rate ( f ex ). Our model results show that modelled CH 4 fluxes are sensitive to f ox and Z root in particular. The effects of P max , V transp and f ex are of lesser relevance. Interactions with water table modelling are significant for V transp . We conducted experiments with different wetland vegetation types for Marine Isotope Stage 3 (MIS 3) stadial and interstadial climates and the present-day climate, by coupling PEATLAND-VU to high resolution climate model simulations for Europe. Experiments assuming dominance of one vegetation type ( Sphagnum vs. Carex vs. Shrubs ) show that Carex -dominated vegetation can increase CH 4 emissions by 50% to 78% over Sphagnum -dominated vegetation depending on the modelled climate, while for shrubs this increase ranges from 42% to 72%. Consequently, during the LG northern wetlands may have had CH 4 emissions similar to their present-day counterparts, despite a colder climate. Changes in dominant wetland vegetation, therefore, may drive changes in wetland CH 4 fluxes, in the past as well as in the future.

Description: The Middle Miocene climate as modelled in an atmosphere-ocean-biosphere model Climate of the Past, 7, 1169-1188, 2011 Author(s): M. Krapp and J. H. Jungclaus We present simulations with a coupled atmosphere-ocean-biosphere model for the Middle Miocene 15 million years ago. The model is insofar more consistent than previous models because it captures the essential interactions between ocean and atmosphere and between atmosphere and vegetation. The Middle Miocene topography, which alters both large-scale ocean and atmospheric circulations, causes a global warming of 0.7 K compared to present day. Higher than present-day CO 2 levels of 480 and 720 ppm cause a global warming of 2.8 and 4.9 K. The associated water vapour feedback enhances the greenhouse effect which leads to a polar amplification of the warming. These results suggest that higher than present-day CO 2 levels are necessary to drive the warm Middle Miocene climate, also because the dynamic vegetation model simulates a denser vegetation which is in line with fossil records. However, we do not find a flatter than present-day equator-to-pole temperature gradient as has been suggested by marine and terrestrial proxies. Instead, a compensation between atmospheric and ocean heat transport counteracts the flattening of the temperature gradient. The acclaimed role of the large-scale ocean circulation in redistributing heat cannot be supported by our results. Including full ocean dynamics, therefore, does not solve the problem of the flat temperature gradient during the Middle Miocene.

Description: Corrigendum to "Upper ocean climate of the Eastern Mediterranean Sea during the Holocene Insolation Maximum – a model study" published in Clim. Past, 7, 1103–1122, 2011 Climate of the Past, 7, 1149-1168, 2011 Author(s): F. Adloff, U. Mikolajewicz, M. Kučera, R. Grimm, E. Maier-Reimer, G. Schmiedl, and K.-C. Emeis Nine thousand years ago (9 ka BP), the Northern Hemisphere experienced enhanced seasonality caused by an orbital configuration close to the minimum of the precession index. To assess the impact of this "Holocene Insolation Maximum" (HIM) on the Mediterranean Sea, we use a regional ocean general circulation model forced by atmospheric input derived from global simulations. A stronger seasonal cycle is simulated by the model, which shows a relatively homogeneous winter cooling and a summer warming with well-defined spatial patterns, in particular, a subsurface warming in the Cretan and western Levantine areas. The comparison between the SST simulated for the HIM and a reconstruction from planktonic foraminifera transfer functions shows a poor agreement, especially for summer, when the vertical temperature gradient is strong. As a novel approach, we propose a reinterpretation of the reconstruction, to consider the conditions throughout the upper water column rather than at a single depth. We claim that such a depth-integrated approach is more adequate for surface temperature comparison purposes in a situation where the upper ocean structure in the past was different from the present-day. In this case, the depth-integrated interpretation of the proxy data strongly improves the agreement between modelled and reconstructed temperature signal with the subsurface summer warming being recorded by both model and proxies, with a small shift to the south in the model results. The mechanisms responsible for the peculiar subsurface pattern are found to be a combination of enhanced downwelling and wind mixing due to strengthened Etesian winds, and enhanced thermal forcing due to the stronger summer insolation in the Northern Hemisphere. Together, these processes induce a stronger heat transfer from the surface to the subsurface during late summer in the western Levantine; this leads to an enhanced heat piracy in this region, a process never identified before, but potentially characteristic of time slices with enhanced insolation.

Description: Glacial-interglacial vegetation dynamics in South Eastern Africa coupled to sea surface temperature variations in the Western Indian Ocean Climate of the Past, 7, 1209-1224, 2011 Author(s): L. M. Dupont, T. Caley, J.-H. Kim, I. Castañeda, B. Malaizé, and J. Giraudeau Glacial-interglacial fluctuations in the vegetation of South Africa might elucidate the climate system at the edge of the tropics between the Indian and Atlantic Oceans. However, vegetation records covering a full glacial cycle have only been published from the eastern South Atlantic. We present a pollen record of the marine core MD96-2048 retrieved by the Marion Dufresne from the Indian Ocean ∼120 km south of the Limpopo River mouth. The sedimentation at the site is slow and continuous. The upper 6 m (spanning the past 342 Ka) have been analysed for pollen and spores at millennial resolution. The terrestrial pollen assemblages indicate that during interglacials, the vegetation of eastern South Africa and southern Mozambique largely consisted of evergreen and deciduous forests. During glacials open mountainous scrubland dominated. Montane forest with Podocarpus extended during humid periods was favoured by strong local insolation. Correlation with the sea surface temperature record of the same core indicates that the extension of mountainous scrubland primarily depends on sea surface temperatures of the Agulhas Current. Our record corroborates terrestrial evidence of the extension of open mountainous scrubland (including fynbos-like species of the high-altitude Grassland biome) for the last glacial as well as for other glacial periods of the past 300 Ka.

Description: Using synoptic type analysis to understand New Zealand climate during the Mid-Holocene Climate of the Past, 7, 1189-1207, 2011 Author(s): D. Ackerley, A. Lorrey, J. A. Renwick, S. J. Phipps, S. Wagner, S. Dean, J. Singarayer, P. Valdes, A. Abe-Ouchi, R. Ohgaito, and J. M. Jones Diagnosing the climate of New Zealand from low-resolution General Circulation Models (GCMs) is notoriously difficult due to the interaction of the complex topography and the Southern Hemisphere (SH) mid-latitude westerly winds. Therefore, methods of downscaling synoptic scale model data for New Zealand are useful to help understand past climate. New Zealand also has a wealth of palaeoclimate-proxy data to which the downscaled model output can be compared, and to provide a qualitative method of assessing the capability of GCMs to represent, in this case, the climate 6000 yr ago in the Mid-Holocene. In this paper, a synoptic weather and climate regime classification system using Empirical Orthogonal Function (EOF) analysis of GCM and reanalysis data was used. The climate regimes are associated with surface air temperature and precipitation anomalies over New Zealand. From the analysis in this study, we find at 6000 BP that increased trough activity in summer and autumn led to increased precipitation, with an increased north-south pressure gradient ("zonal events") in winter and spring leading to drier conditions. Opposing effects of increased (decreased) temperature are also seen in spring (autumn) in the South Island, which are associated with the increased zonal (trough) events; however, the circulation induced changes in temperature are likely to have been of secondary importance to the insolation induced changes. Evidence from the palaeoclimate-proxy data suggests that the Mid-Holocene was characterized by increased westerly wind events in New Zealand, which agrees with the preference for trough and zonal regimes in the models.

Description: Past surface temperatures at the NorthGRIP drill site from the difference in firn diffusion of water isotopes Climate of the Past, 7, 1327-1335, 2011 Author(s): S. B. Simonsen, S. J. Johnsen, T. J. Popp, B. M. Vinther, V. Gkinis, and H. C. Steen-Larsen A new ice core paleothermometer is introduced based on the temperature dependent diffusion of the stable water isotopes in the firn. A new parameter called differential diffusion length is defined as the difference between the diffusion length of the two stable water isotopologues 2 H 1 H 16 O and 1 H 2 18 O. A model treatment of the diffusion process of the firn and the ice is presented along with a method of retrieving the diffusion signal from the ice core record of water isotopes using spectral methods. The model shows how the diffusion process is highly dependent on the inter-annual variations in the surface temperatures. It results in a diffusion length longer than if the firn was isothermal. The longer diffusion length can be explained by the strong nonlinearly behaviour of the saturation pressure over ice in the range of the surface temperature fluctuations. The method has been tested on δ 18 O and δD measurements, spanning the transition from the last glacial to the holocene, from the NorthGRIP ice core. The surface temperature reconstruction based on the differential diffusion resembles other temperature reconstructions for the NorthGRIP ice core. However, the Allerød warming is seen to be significantly warmer than observed in other ice core based temperature reconstructions. The mechanisms behind this behaviour are not fully understood. The method shows the need of an expansion of high resolution stable water isotope datasets from ice cores. However, the new ice core paleothermometer presented here will give valuable insight into past climate, through the physical process of isotope diffusion in the firn column of ice sheets.

Description: Arctic Ocean circulation and variability – advection and external forcing encounter constraints and local processes Ocean Science Discussions, 8, 2313-2376, 2011 Author(s): B. Rudels The first hydrographic data from the Arctic Ocean, the section from the Laptev Sea to the passage between Greenland and Svalbard obtained by Nansen on the drift by Fram 1893–1896, aptly illustrate the main features of Arctic Ocean oceanography and indicate possible processes active in transforming the water masses in the Arctic Ocean. Many, perhaps most, of these processes were identified already by Nansen, who put his mark on almost all subsequent research in the Arctic Ocean. Here we shall revisit some key questions and follow how our understanding has evolved from the early 20th century to present. What questions, if any, can now be regarded as solved and which remain still open? Five different but connected topics will be discussed: (1) The low salinity surface layer and the storage and export of freshwater. (2) The vertical heat transfer from the Atlantic water to sea ice and to the atmosphere. (3) The circulation and mixing of the two Atlantic inflow branches. (4) The formation and circulation of deep and bottom waters in the Arctic Ocean. (5) The exchanges through Fram Strait. Foci will be on the potential effects of increased freshwater input and reduced sea ice export on the freshwater storage and residence time in the Arctic Ocean, on the deep waters of the Makarov Basin and on the circulation and relative importance of the two inflows, over the Barents Sea and through Fram Strait, for the distribution of heat in the intermediate layers of the Arctic Ocean.

Description: Towards an online-coupled chemistry-climate model: evaluation of trace gases and aerosols in COSMO-ART Geoscientific Model Development, 4, 1077-1102, 2011 Author(s): C. Knote, D. Brunner, H. Vogel, J. Allan, A. Asmi, M. Äijälä, S. Carbone, H. D. van der Gon, J. L. Jimenez, A. Kiendler-Scharr, C. Mohr, L. Poulain, A. S. H. Prévôt, E. Swietlicki, and B. Vogel The online-coupled, regional chemistry transport model COSMO-ART is evaluated for periods in all seasons against several measurement datasets to assess its ability to represent gaseous pollutants and ambient aerosol characteristics over the European domain. Measurements used in the comparison include long-term station observations, satellite and ground-based remote sensing products, and complex datasets of aerosol chemical composition and number size distribution from recent field campaigns. This is the first time these comprehensive measurements of aerosol characteristics in Europe are used to evaluate a regional chemistry transport model. We show a detailed analysis of the simulated size-resolved chemical composition under different meteorological conditions. Mean, variability and spatial distribution of the concentrations of O 3 and NO x are well reproduced. SO 2 is found to be overestimated, simulated PM 2.5 and PM 10 levels are on average underestimated, as is AOD. We find indications of an overestimation of shipping emissions. Time evolution of aerosol chemical composition is captured, although some biases are found in relative composition. Nitrate aerosol components are on average overestimated, and sulfates underestimated. The accuracy of simulated organics depends strongly on season and location. While strongly underestimated during summer, organic mass is comparable in spring and autumn. We see indications for an overestimated fractional contribution of primary organic matter in urban areas and an underestimation of SOA at many locations. Aerosol number concentrations compare well with measurements for larger size ranges, but overestimations of particle number concentration with factors of 2–5 are found for particles smaller than 50 nm. Size distribution characteristics are often close to measurements, but show discrepancies at polluted sites. Suggestions for further improvement of the modeling system consist of the inclusion of a revised secondary organic aerosols scheme, aqueous-phase chemistry and improved aerosol boundary conditions. Our work sets the basis for subsequent studies of aerosol characteristics and climate impacts with COSMO-ART, and highlights areas where improvements are necessary for current regional modeling systems in general.

Description: Fire history in western Patagonia from paired tree-ring fire-scar and charcoal records Climate of the Past, 8, 451-466, 2012 Author(s): A. Holz, S. Haberle, T. T. Veblen, R. De Pol-Holz, and J. Southon Fire history reconstructions are typically based on tree ages and tree-ring fire scars or on charcoal in sedimentary records from lakes or bogs, but rarely on both. In this study of fire history in western Patagonia (47–48° S) in southern South America (SSA) we compared three sedimentary charcoal records collected in bogs with tree-ring fire-scar data collected at 13 nearby sample sites. We examined the temporal and spatial correspondence between the two fire proxies and also compared them to published charcoal records from distant sites in SSA, and with published proxy reconstructions of regional climate variability and large-scale climate modes. Two of our three charcoal records record fire activity for the last 4 ka yr and one for the last 11 ka yr. For the last ca. 400 yr, charcoal accumulation peaks tend to coincide with high fire activity in the tree-ring fire scar records, but the charcoal records failed to detect some of the fire activity recorded by tree rings. Potentially, this discrepancy reflects low-severity fires that burn in herbaceous and other fine fuels without depositing charcoal in the sedimentary record. Periods of high fire activity tended to be synchronous across sample areas, across proxy types, and with proxy records of regional climatic variability as well as major climate drivers. Fire activity throughout the Holocene in western Patagonia has responded to regional climate variation affecting a broad region of southern South America that is teleconnected to both tropical- and high-latitude climate drivers-El Niño-Southern Oscillation and the Southern Annular Mode. An early Holocene peak in fire activity pre-dates any known human presence in our study area, and consequently implicates lightning as the ignition source. In contrast, the increased fire activity during the 20th century, which was concomitantly recorded by charcoal from all the sampled bogs and at all fire-scar sample sites, is attributed to human-set fires and is outside the range of variability characteristic of these ecosystems over many centuries and probably millennia.

Description: Exploring errors in paleoclimate proxy reconstructions using Monte Carlo simulations: paleotemperature from mollusk and coral geochemistry Climate of the Past, 8, 433-450, 2012 Author(s): M. Carré, J. P. Sachs, J. M. Wallace, and C. Favier Quantitative reconstructions of the past climate statistics from geochemical coral or mollusk records require quantified error bars in order to properly interpret the amplitude of the climate change and to perform meaningful comparisons with climate model outputs. We introduce here a more precise categorization of reconstruction errors, differentiating the error bar due to the proxy calibration uncertainty from the standard error due to sampling and variability in the proxy formation process. Then, we propose a numerical approach based on Monte Carlo simulations with surrogate proxy-derived climate records. These are produced by perturbing a known time series in a way that mimics the uncertainty sources in the proxy climate reconstruction. A freely available algorithm, MoCo, was designed to be parameterized by the user and to calculate realistic systematic and standard errors of the mean and the variance of the annual temperature, and of the mean and the variance of the temperature seasonality reconstructed from marine accretionary archive geochemistry. In this study, the algorithm is used for sensitivity experiments in a case study to characterize and quantitatively evaluate the sensitivity of systematic and standard errors to sampling size, stochastic uncertainty sources, archive-specific biological limitations, and climate non-stationarity. The results of the experiments yield an illustrative example of the range of variations of the standard error and the systematic error in the reconstruction of climate statistics in the Eastern Tropical Pacific. Thus, we show that the sample size and the climate variability are the main sources of the standard error. The experiments allowed the identification and estimation of systematic bias that would not otherwise be detected because of limited modern datasets. Our study demonstrates that numerical simulations based on Monte Carlo analyses are a simple and powerful approach to improve the understanding of the proxy records. We show that the standard error for the climate statistics linearly increases with the climate variability, which means that the accuracy of the error estimated by MoCo is limited by the climate non-stationarity.

Description: Identifying the causes of differences in ozone production from the CB05 and CBMIV chemical mechanisms Geoscientific Model Development, 5, 257-268, 2012 Author(s): R. D. Saylor and A. F. Stein An investigation was conducted to identify the mechanistic differences between two versions of the carbon bond gas-phase chemical mechanism (CB05 and CBMIV) which consistently lead to larger ground-level ozone concentrations being produced in the CB05 version of the National Air Quality Forecasting Capability (NAQFC) modeling system even though the two parallel forecast systems utilize the same meteorology and base emissions and similar initial and boundary conditions. Box models of each of the mechanisms as they are implemented in the NAQFC were created and a set of 12 sensitivity simulations was designed. The sensitivity simulations independently probed the conceptual mechanistic differences between CB05 and CBMIV and were exercised over a 45-scenario simulation suite designed to emulate the wide range of chemical regimes encountered in a continental-scale atmospheric chemistry model. Results of the sensitivity simulations indicate that two sets of reactions that were included in the CB05 mechanism, but which were absent from the CBMIV mechanism, are the primary causes of the greater ozone production in the CB05 version of the NAQFC. One set of reactions recycles the higher organic peroxide species of CB05 (ROOH), resulting in additional photochemically reactive products that act to produce additional ozone in some chemical regimes. The other set of reactions recycles reactive nitrogen from less reactive forms back to NO 2 , increasing the effective NO x concentration of the system. In particular, the organic nitrate species (NTR), which was a terminal product for reactive nitrogen in the CBMIV mechanism, acts as a reservoir species in CB05 to redistribute NO x from major source areas to potentially NO x -sensitive areas where additional ozone may be produced in areas remote from direct NO x sources.