ALBERT

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Description: Enhancing the accuracy of automatic eddy detection and the capability of recognizing the multi-core structures from maps of sea level anomaly Ocean Science Discussions, 10, 825-851, 2013 Author(s): J. Yi, Y. Du, Z. He, and C. Zhou Automated methods are important for automatically detecting mesoscale eddies in large volumes of altimeter data. While many algorithms have been proposed in the past, this paper presents a new method, called Hybrid Detection (HD), to enhance the eddy detection accuracy and the capability of recognizing eddies' multi-core structures from maps of sea level anomaly (SLA) by integrating the ideas of the Okubo–Weiss (OW) method and the sea-surface-height-based (SSH-based) method, two well-known eddy detection algorithms. Detection evaluation using an objective validation protocol shows that the HD method owns ~ 96.6% successful detection rate and ~ 14.2% excessive detection rate, which outperforms the OW method and other methods that identify eddies by SLA extrema and confirms the improvement in detection accuracy. The capability of recognizing multi-core structures and its significance in tracking eddies' splitting or merging events have been well illustrated by comparing with other detection algorithms and historical studies.

Description: Hydrodynamic variability based on the multi-parametric POSEIDON Pylos observatory of the south Ionian Sea Ocean Science Discussions, 10, 883-921, 2013 Author(s): D. Kassis, K. Nittis, and L. Perivoliotis The multi-platform POSEIDON-Pylos observatory of the south-east Ionian Sea operates since 2007 delivering near real time data for a variety of meteorological, water column and near seabed oceanographic parameters. It has been designed to contribute to long term monitoring of air–sea interaction and thermohaline processes of this key area of the Eastern Mediterranean where water masses of different origin interact and transform at various temporal and spatial scales. An inductive mooring line, with CTD instruments adjusted on, provides salinity, temperature and pressure real-time data down to 500 m depth. Recorded data, for the years 2008–2010, were extracted from the instruments internal logger providing more enhanced timeseries in terms of resolution and continuity. The reprocessed datasets are analyzed in combination with atmospheric, currents buoy measurements and CTD profiles obtained during maintenance visits on the site. The delayed mode analysis shows the hydrodynamic properties of the area and reveals the dynamic picture of the south Ionian upper thermocline as well as the variation of T & S in deeper layers. One can also observe seasonal atmospheric and circulation patterns, other synoptic and seasonal scale signals as well as important inter-annual variability such as a strong signal of Levantine Intermediate Waters (LIW) at intermediate depths during the spring of 2009.

Description: Microstructure measurements and estimates of entrainment in the Denmark Strait overflow plume Ocean Science Discussions, 10, 1067-1098, 2013 Author(s): V. Paka, V. Zhurbas, B. Rudels, D. Quadfasel, A. Korzh, and D. Delisi To examine processes controlling the entrainment of ambient water into the Denmark Strait overflow (DSO) plume/gravity current, measurements of turbulent dissipation rate were carried out by a quasi-free-falling (tethered) microstructure profiler (MSP). The MSP was specifically designed to collect data on dissipation-scale turbulence and fine thermohaline stratification in an ocean layer to depths of 3500 m. The task was to perform microstructure measurements in the DSO plume in the lower 300 m depth interval including the bottom mixed layer and the interfacial layer below the non-turbulent ambient water. The MSP was attached to a Rosette water sampler rack equipped with a SeaBird CTDO and an RD Instruments Lowered Acoustic Doppler Current Profiler (LADCP). At a chosen depth, the MSP was remotely released from the rack to perform measurements in a quasi-free-falling mode. Using the measured vertical profiles of dissipation, the entrainment rate as well as the bottom and interfacial stresses were estimated in the DSO plume at a location 200 km downstream of the sill at depths up to 1771 m. Dissipation-derived estimates of entrainment were found to be much smaller than bulk estimates of entrainment calculated from the downstream change of the mean properties in the plume, suggesting the lateral stirring due to meso-scale eddies rather than diapycnal mixing as the main contributor to entrainment. Dissipation-derived bottom stress estimates are argued to be roughly one-third the magnitude of those from log velocity profiles. In the interfacial layer, the Ozmidov scale calculated from turbulence dissipation rate and buoyancy frequency was found to be linearly proportional to the overturning scale extracted from conventional CTD data (the Thorpe scale), with a proportionality constant of 0.76, and a correlation coefficient of 0.77.

Description: Turbulent mixing in the seasonally-stratified western Irish Sea: a Thorpe Scale perspective Ocean Science Discussions, 10, 2141-2155, 2013 Author(s): K. L. Stansfield, M. R. Palmer, T. P. Rippeth, and J. H. Simpson The seasonal thermocline in shelf-seas represents an important biogeophysical barrier to the vertical flux of nutrients into the photic zone. Episodic weakening of this barrier plays an important role in sustaining the sub-surface chlorophyll maximum in summer and hence impacts the carbon draw-down in the seasonally-stratified zones of the shelf seas. Here we present estimates of the rate of turbulent kinetic energy dissipation inferred from microstructure shear probes and compare them with dissipation rates inferred from a standard conductivity-temperature-depth instrument and from a fast thermistor (Thorpe Scale methodology) at a site in the seasonally-stratified Irish Sea. All methods show strong dissipation rates in response to tidal stresses near the bed (order 10 −2 Wm −3 ) with qualitatively similar temporal and spatial patterns. In the interior of the water column, however, only the microstructure shear probe estimates resolve the mixing in the region of the thermocline.

Description: Assessment of MERIS ocean color data products for European seas Ocean Science Discussions, 10, 219-259, 2013 Author(s): G. Zibordi, F. Mélin, J.-F. Berthon, and E. Canuti The accuracy of marine data products from the Medium Resolution Imaging Spectrometer (MERIS) operated onboard the Envisat platform, is investigated with the aid of in situ geographically distributed measurements from different European seas. The assessment focusses on standard products from the 2012 data update commonly identified as 3rd Reprocessing. Results indicate atmospherically corrected data affected by a negative bias of several tens percent at the 413 nm center-wavelength, significantly decreasing to a few percent at 560 nm and increasing again at 665 nm. Such an underestimate at the blue center-wavelengths leads to an average overestimate of the algal-1 MERIS pigment index largely exceeding 100% for the considered European seas. A comparable overestimate is also observed for the algal-2 pigment index independently determined from top-of-atmosphere radiance through the application of neural networks.

Description: Transit and residence times in the surface Adriatic Sea as derived from drifter data and Lagrangian numerical simulations Ocean Science Discussions, 10, 197-217, 2013 Author(s): P.-M. Poulain and S. Hariri Statistics of transit and residence times in the surface Adriatic Sea, a semi-enclosed basin of the Mediterranean, are estimated from drifter data and Lagrangian numerical simulations. The results obtained from the drifters are generally underestimated given their short operating lifetimes (half life of ~ 40 days) compared to the transit and residence times. This bias can be removed by considering a large amount of numerical particles whose trajectories are integrated over a long time (750 days) with a statistical advection-diffusion model of the Adriatic surface circulation. Numerical particles indicate that the maximum transit time to exit the basin is about 216–260 days for objects released near the northern tip of the Adriatic, and that a particle entering on the eastern Otranto Channel will typically exit on the other side of the Channel after 170–185 days. A value of 150–168 days is estimated for the residence time in the Adriatic basin.

Description: Combining in-situ measurements and altimetry to estimate volume, heat and salt transport variability through the Faroe Shetland Channel Ocean Science Discussions, 10, 153-195, 2013 Author(s): B. Berx, B. Hansen, S. Østerhus, K. M. Larsen, T. Sherwin, and K. Jochumsen From 1994 to 2011, instruments measuring ocean currents (ADCPs) have been moored on a section crossing the Faroe-Shetland Channel. Together with CTD (Conductivity Temperature Depth) measurements from regular research vessel occupations, they describe the flow field and water mass structure in the channel. Here, we use these data to calculate the average volume transport and properties of the flow of warm water through the channel from the Atlantic towards the Arctic, termed the Atlantic inflow. We find the average volume transport of this flow to be 2.7 ± 0.5 Sv (1 Sv = 10 6 m 3 s −1 ) between the shelf edge on the Faroe side and the 150 m isobath on the Shetland side. The average heat transport (relative to 0 °C) was estimated to be 107 ± 21 TW and the average salt import to be 98 ± 20 × 10 6 kg s −1 . Transport values for individual months, based on the ADCP data, include a large level of variability, but can be used to calibrate sea level height data from satellite altimetry. In this way, a time series of volume transport has been generated back to the beginning of satellite altimetry in December 1992. The Atlantic inflow has a seasonal variation in volume transport that peaks around the turn of the year and has an amplitude of 0.7 Sv. The Atlantic inflow has become warmer and more saline since 1994, but no equivalent trend in volume transport was observed.

Description: Springtime contribution of dinitrogen fixation to primary production across the Mediterranean Sea Ocean Science Discussions, 10, 1-26, 2013 Author(s): E. Rahav, B. Herut, A. Levi, M. R. Mulholland, and I. Berman-Frank Dinitrogen (N 2 ) fixation rates were measured during early spring across the different provinces of Mediterranean Sea surface waters. N 2 fixation rates, measured using 15 N 2 enriched seawater, were lowest in the eastern basin and increased westward with a maximum at the Strait of Gibraltar (0.10 to 2.35 nmol N L −1 d −1 , respectively). These rates were 3–7 fold higher than N 2 fixation rates measured previously in the Mediterranean Sea during summertime. Moreover, comparisons between N 2 fixation rates measured during dark versus natural light incubations (48 h) show higher rates during dark incubations at the eastern Mediterranean stations but lower rates at the western stations. This suggests that heterotrophic diazotrophy has a significant role in the Eastern Mediterranean while autotrophic diazotrophy has a more dominant role in the Western basin.

Description: Oceanic dominance of interannual subtropical North Atlantic heat content variability Ocean Science Discussions, 10, 27-53, 2013 Author(s): M. Sonnewald, J. J.-M. Hirschi, and R. Marsh Ocean heat content varies on a range of timescales. Traditionally the atmosphere is seen to dominate the oceanic heat content variability. However, this variability can be driven either by oceanic or atmospheric heat fluxes. To diagnose the relative contributions and respective timescales, this study uses a box model forced with output from an ocean general circulation model (OGCM) to investigate the heat content variability of the upper 800 m of the subtropical North Atlantic from 26° N to 36° N. The ocean and air-sea heat flux data needed to force the box model is taken from a 19 yr (1988 to 2006) simulation performed with the 1/12° version of the OCCAM OGCM. The box model heat content is compared to the corresponding heat content in OCCAM for verification. The main goal of the study is to identify to what extent the seasonal to interannual ocean heat content variability is of atmospheric or oceanic origin. To this end, the box model is subjected to a range of scenarios forced either with the full (detrended) ocean and air-sea fluxes, or their deseasoned counterparts. Results show that in all cases, the seasonal variability is dominated by the seasonal component of the air-sea fluxes, which produce a seasonal range in mean temperature of the upper 800 m of ~ 0.42 °C. However, on longer timescales oceanic heat transport dominates, with changes of up to ~ 0.30 °C over 4 yr. The technique is subsequently applied to observational data. For the ocean heat fluxes, we use data from the RAPID program at 26° N from April 2004 to January 2011. At 36° N heat transport is inferred using a linear regression model based on the oceanic low-frequency transport in OCCAM. The air-sea flux from OCCAM is used for the period 2004 to 2006 when the RAPID timeseries and the OCCAM simulation overlap, and a climatology is used for the air-sea flux from 2006 onwards. The results confirm that on longer (〉 2 yr) timescales the ocean dominates the ocean heat content variability, which is further verified using data from the ARGO project. This work illustrates that oceanic divergence significantly impacts the ocean heat content variability on timescales relevant for applications such as seasonal hurricane forecasts.

Description: Eddy measurements, coastal turbulence and statistics in the gulf of Lions Ocean Science Discussions, 10, 55-81, 2013 Author(s): J. M. Redondo, A. Matulka, A. Platonov, E. Sekula, and P. Fraunie The advances in radar sensors may be applied to study the flow in the Region of Fresh Water Influence (ROFI) region of the ocean. The Synthetic Aperture Radar (SAR) is a useful tool that may be used to study both marine water dynamics and its pollution. Oil spills and natural slicks may be detected and processed with advanced computer techniques to reveal vortex dynamics and turbulence spectral characteristics of the complex eddy and current interaction in the ocean surface, more than 300 SAR images of the North-west Mediterranean Sea area taken between December 1996 and December 1998 were analyzed. A total of 255 eddies were detected under convenient environmental conditions and we analyzed statistically the appearance, size, shape and position of vortices in the test area. We find that the maximum size of the eddies detected near the coast is limited by the Rossby deformation radius and that there is a decrease in size in the coastal waters in the direction of the Liguro-Provenzal current with the largest eddies occurring near the cape of Rosas. Near the Rhone and Ebro rivers, high discharges also contribute to eddy forcing, coastal radar measurements confirm the SAR observations. The role of submarine canyons in the vortex generation is also confirmed due to the asymmetry of their distribution with respect to the thalwegs. It is demonstrated that useful information of a geometrical nature obtained by SAR satellite images may be used to estimate relevant dynamical parameters of coastal flows.

Description: Geostrophic currents and kinetic energies in the Black Sea estimated from merged drifter and satellite altimetry data Ocean Science Discussions, 10, 1505-1524, 2013 Author(s): M. Menna and P. -M. Poulain Drifter measurements and satellite altimetry data are merged to reconstruct the surface geostrophic circulation of the Black Sea in the period 1999–2009. This combined dataset is used to estimate pseudo-Eulerian velocity statistics for different time periods. Seasonal and interannual variability of currents and kinetic energy fields are described with particular attention to the mesoscale and sub-basin coastal eddies. The mean currents are generally stronger in winter and enhanced speeds are observed in the period 2002–2006. The most intense activity of sub-basin Batumi Eddy occurs in summer with greater speeds and dimensions in 2006 and 2008. The sub-basin Sevastopol Eddy is generated in spring from a meander of the Rim Current. Mesoscale eddies located along the Anatolia, Caucasus and Crimea coasts are permanent, quasi-permanent or intermittent features and can interact and merge with each other, showing higher values of kinetic energy.

Description: The CO 2 system in the Mediterranean Sea: a basin wide perspective Ocean Science Discussions, 10, 1447-1504, 2013 Author(s): M. Álvarez, H. Sanleón-Bartolomé, T. Tanhua, L. Mintrop, A. Luchetta, C. Cantoni, K. Schroeder, and G. Civitarese This paper provides an extensive vertical and longitudinal description of the CO 2 system variables (Total Alkalinity – TA, dissolved inorganic carbon – DIC and pH) along an East-West transect and across the Sardinia–Sicily passage in the Mediterranean Sea (MedSea) from two oceanographic cruises conducted in 2011 measuring CO 2 variables in a coordinated fashion, the RV Meteor M84/3 and the RV Urania EuroFleets 11, respectively. The over-determined CO 2 system allowed performing the first internal consistency analysis for the particularly warm, high salinity and alkalinity MedSea waters. This basin is considered a "laboratory basin" suffering dramatic changes in its oceanographic and biogeochemical conditions derived from natural and anthropogenic forces. Despite this, little is known about the CO 2 system variability in the whole basin. This work aims to be a benchmark for future studies about the CO 2 system space-time variability in the MedSea. In this sense we provide full-depth and length CO 2 distributions across the MedSea, and property – property plots showing in each sub-basin post-Eastern Mediterranean Transient (EMT) situation with regard to TA, DIC and pH.

Description: A practical scheme to introduce explicit tidal forcing into OGCM Ocean Science Discussions, 10, 473-517, 2013 Author(s): K. Sakamoto, H. Tsujino, H. Nakano, M. Hirabara, and G. Yamanaka A practical scheme is proposed to introduce tides explicitly into ocean general circulation models (OGCM). In this scheme, barotropic linear response to the tidal forcing is calculated by the time differential equations modified for ocean tides, instead of the original barotropic equations of OGCM. This allows usage of various parameterizations specified for tides, such as the self attraction/loading (SAL) effect and energy dissipation due to internal tides, without unintentional violation of the original dynamical balances in OGCM. Meanwhile, secondary nonlinear effects of tides, e.g. excitation of internal tides and advection by tidal currents, are fully represented in the framework of the original OGCM equations. That is, this scheme drives OGCM by the barotropic linear tidal currents which are predicted progressively by a well-tuned tide model, instead of the equilibrium tide potential, without large additional numerical costs. We incorporated this scheme into Meteorological Research Institute Community Ocean Model and executed test experiments with a low-resolution global model. The results showed that the model can simulate both of non-tidal circulations and tidal motion simultaneously. Owing to usage of tidal parameterizations such as a SAL term, a root mean square error in the tidal heights was as small as 10.0 cm, which is comparable to tide models tuned elaborately. In addition, analysis of speed and energy of the barotropic tidal currents was consistent with past tide studies. The model also showed active excitement of internal tides and tidal mixing. Their impacts should be examined using a model with a finer resolution in future, since explicit and precise introduction of tides into OGCM is a significant step toward upgrade of ocean modeling.

Description: Transport of AABW through the Kane Gap, tropical NE Atlantic Ocean Science Discussions, 10, 539-553, 2013 Author(s): E. G. Morozov, R. Yu. Tarakanov, and H. van Haren We study low-frequency flow of Antarctic Bottom Water through the Kane Gap (9° N) in the Atlantic. The measurements in the Kane Gap include five visits with CTD sections in 2009–2012 and a year-long record of currents using three AquaDopp current-meters. We found an alternating regime of flow, which changes direction several times during a year. The velocities reach 0.21 m s −1 . The transport of Antarctic Bottom Water ( 〈 1.9 °C) based on the mooring and LADCP data varies by ± 0.3 Sv.

Description: Monitoring ocean heat content from the current generation of global ocean observing systems Ocean Science Discussions, 10, 923-949, 2013 Author(s): K. von Schuckmann, J.-B. Sallée, D. Chambers, P.-Y. Le Traon, C. Cabanes, F. Gaillard, S. Speich, and M. Hamon Variations in the world's ocean heat storage and its associated volume changes are a key factor to gauge global warming and to assess the Earth's energy budget. It is also directly link to sea level change, which has a direct impact on coastal populations. Understanding and monitoring heat and sea level change is therefore one of the major legacies of current global ocean observing systems. In this study, we present an inter-comparison of the three of these global ocean observing systems: the ocean temperature/salinity network Argo, the gravimeter GRACE and the satellite altimeters. Their consistency is investigated at global and regional scale during the period 2005–2010 of overlapping time window of re-qualified data. These three datasets allow closing the recent global ocean sea level budget within uncertainties. However, sampling inconsistencies need to be corrected for an accurate budget at global scale. The Argo network allows estimating global ocean heat content and global sea level and reveals a positive change of 0.5 ± 0.1W m −2 and 0.5 ± 0.1 mm yr −1 over the last 8 yr (2005–2012). Regional inter-comparison of the global observing systems highlights the importance of specific ocean basins for the global estimates. Specifically, the Indonesian Archipelago appears as a key region for the global ocean variability. Both the large regional variability and the uncertainties in the current observing systems, prevent us to shed light, from the global sea level perspective, on the climatically important deep ocean changes. This emphasises, once more, the importance of continuing sustained effort in measuring the deep ocean from ship platforms and by setting up a much needed automated deep-Argo network.

Description: Experiments with the Secchi disk Ocean Science Discussions, 10, 1833-1893, 2013 Author(s): E. Aas, J. Høkedal, and K. Sørensen The Secchi depth and its relationships to other properties of the sea water in the Oslofjord–Skagerrak area have been investigated. White and black disks of different sizes have been applied, and the Secchi depth has been observed with the naked eye, through colour filters and with a water telescope. Spectral luminances and illuminances have been calculated from recordings of radiance and irradiance. A theoretical expression for the Secchi depth has been tested against field observations, and statistical relationships between Secchi depths and attenuation coefficients have been determined. Effects of size, colour filters, sun glitter and ship shadow have been quantified. The possibility to estimate quanta irradiance, chlorophyll a and total suspended material has also been studied.

Description: On the tides and resonances of Hudson Bay and Hudson Strait Ocean Science Discussions, 10, 2053-2083, 2013 Author(s): D. J. Webb The resonances of Hudson Bay, Foxe Basin and Hudson Strait are investigated using a~linear shallow water numerical model. The region is of particular interest because it is the most important region of the world ocean for dissipating tidal energy. The model shows that the semi-diurnal tides of the region are dominated by four nearby overlapping resonances. It shows that these not only affect Ungava Bay, a region of extreme tidal range, but they also extend far into Foxe Basin and Hudson Bay and appear to be affected by the geometry of those regions. The results also indicate that it is the four resonances acting together which make the region such an important area for dissipating tidal energy.

Description: Adapting to life: ocean biogeochemical modelling and adaptive remeshing Ocean Science Discussions, 10, 1997-2051, 2013 Author(s): J. Hill, E. E. Popova, D. A. Ham, M. D. Piggott, and M. Srokosz An outstanding problem in biogeochemical modelling of the ocean is that many of the key processes occur intermittently at small scales, such as the sub-mesoscale, that are not well represented in global ocean models. As an example, state-of-the-art models give values of primary production approximately two orders of magnitude lower than those observed in the ocean's oligotrophic gyres, which cover a third of the Earth's surface. This is partly due to their failure to resolve sub-mesoscale phenomena, which play a significant role in nutrient supply. Simply increasing the resolution of the models may be an inefficient computational solution to this problem. An approach based on recent advances in adaptive mesh computational techniques may offer an alternative. Here the first steps in such an approach are described, using the example of a~simple vertical column (quasi 1-D) ocean biogeochemical model. We present a novel method of simulating ocean biogeochemical behaviour on a vertically adaptive computational mesh, where the mesh changes in response to the biogeochemical and physical state of the system throughout the simulation. We show that the model reproduces the general physical and biological behaviour at three ocean stations (India, Papa and Bermuda) as compared to a high-resolution fixed mesh simulation and to observations. The simulations capture both the seasonal and inter-annual variations. The use of an adaptive mesh does not increase the computational error, but reduces the number of mesh elements by a factor of 2–3, so reducing computational overhead. We then show the potential of this method in two case studies where we change the metric used to determine the varying mesh sizes in order to capture the dynamics of chlorophyll at Bermuda and sinking detritus at Papa. We therefore demonstrate adaptive meshes may provide a~suitable numerical technique for simulating seasonal or transient biogeochemical behaviour at high spatial resolution whilst minimising computational cost.

Description: First air–sea gas exchange laboratory study at hurricane wind speeds Ocean Science Discussions, 10, 1971-1996, 2013 Author(s): K. E. Krall and B. Jähne In a pilot study conducted in October and November 2011, air–sea gas transfer velocities of the two sparingly soluble trace gases hexafluorobenzene and 1,4-difluorobenzene were measured in the unique High-Speed Wind-Wave Tank at Kyoto University, Japan. This air–sea interaction facility is capable of producing hurricane strength wind speeds of up to u 10 =67 m s −1 . This constitutes the first lab study of gas transfer at such high wind speeds. The measured transfer velocities k 600 spanned two orders of magnitude, lying between 11 cm h −1 and 1180 cm h −1 with the latter being the highest ever measured wind induced gas transfer velocity. The measured gas transfer velocities are in agreement with the only available dataset at hurricane wind speeds (McNeil and D'Asaro, 2007). The disproportionately large increase of the transfer velocities found at highest wind speeds indicates a new regime of air–sea gas transfer, which is characterized by strong wave breaking, enhanced turbulence and bubble cloud entrainment. It was found that tracers spanning a wide range of solubilities and diffusivities are needed to separate the effects of enhanced surface area and turbulence due to breaking waves from the effects of bubble and spray mediated gas transfer.

Description: Antarctic Circumpolar Transport and the Southern Mode: a model investigation of interannual to decadal time scales Ocean Science Discussions, 10, 2085-2107, 2013 Author(s): C. W. Hughes, J. Williams, A. C. Coward, and B. A. de Cuevas It is well-established that, at periods shorter than a year, variations in Antarctic Circumpolar Transport are reflected in a barotropic mode, known as the Southern Mode, in which sea level and bottom pressure varies coherently around Antarctica. Here, we use two multidecadal ocean model runs to investigate the behaviour of the Southern Mode at time scales on which density changes become important, leading to a baroclinic component to the adjustment. We find that the concept of a Southern Mode in bottom pressure remains valid, and remains a direct measure of the circumpolar transport, with changes at the northern boundary playing only a small role even on decadal time scales. However, at periods longer than about 5 yr, density changes start to play a role, leading to a surface intensification of the vertical profile of the transport. We also find that barotropic currents on the continental slope account for a significant fraction of the variability, and produce surface intensification in the meridional-integral flow. The role of density variations results in a sea level signal which, although reflecting transport changes at all time scales, has a ratio of sea level to transport which becomes larger at longer time scales. This means that any long-term transport monitoring strategy based on present measurement systems must involve multiplying the observed quantity by a factor which depends on frequency.

Description: Simulated melt rates for the Totten and Dalton ice shelves Ocean Science Discussions, 10, 2109-2140, 2013 Author(s): D. E. Gwyther, B. K. Galton-Fenzi, J. R. Hunter, and J. L. Roberts The Totten Glacier drains a large proportion of the East Antarctic ice sheet, much of it marine based (grounded below sea level), and is rapidly losing mass. It has been suggested that this mass loss is driven by changes in oceanic forcing; however, the details of the ice-ocean interaction are unknown. Here we present results from an ice shelf-ocean model of the region that includes the Totten, Moscow University and Dalton Ice Shelves, based on the Regional Oceanic Modeling System for the period 1992–2007. Simulated area-averaged basal melt rates (net basal mass loss) for the Totten and Dalton ice shelves are 9.1 m ice yr −1 (44.5 Gt ice yr −1 ) and 10.1 m ice yr −1 (46.6 Gt ice yr −1 ), respectively. The melting of the ice shelves varies strongly on seasonal and interannual timescales. Basal melting (mass loss) from the Totten ice shelf spans a range of 5.7 m ice yr −1 (28 Gt ice yr −1 ) on interannual timescales and 3.4 m ice yr −1 (17 Gt ice yr −1 ) on seasonal timescales. This study links basal melt of the Totten and Dalton ice shelves to warm water intrusions across the continental shelf break and atmosphere-ocean heat exchange. Totten ice shelf melting is high when the nearby Dalton polynya interannual strength is below average, and vice versa. Melting of the Dalton ice shelf is primarily controlled by the strength of warm water intrusions across the Dalton Rise and into the ice shelf cavity. During periods of strong westwards coastal current flow, Dalton melt water flows directly under the Totten ice shelf further reducing melting. This is the first such modelling study of this region, providing a valuable framework for directing future observational and modelling efforts.

Description: The role of subpolar deep water formation and Nordic Seas overflows in simulated multidecadal variability of the Atlantic overturning Ocean Science Discussions, 10, 1895-1931, 2013 Author(s): K. Lohmann, J. H. Jungclaus, D. Matei, J. Mignot, M. Menary, H. R. Langehaug, J. Ba, Y. Gao, O. H. Otterå, W. Park, and S. Lorenz We investigate the respective role of variations in subpolar deep water formation and Nordic Seas overflows for the decadal to multidecadal variability of the Atlantic meridional overturning circulation (AMOC). This is done by analysing long (order of 1000 yr) control simulations with five coupled climate models as well as sensitivity experiments performed with one of the models, in which we suppress the variability of either subpolar deep water formation or Nordic Seas overflows. For all models, the maximum influence of variations in subpolar deep water formation is found at about 45° N, while the maximum influence of variations in Nordic Seas overflows is rather found at 55° N to 60° N. Regarding the two overflow branches, the influence of variations in the Denmark Strait overflow is, for all models, substantially larger than that of variations in the overflow across the Iceland–Scotland–Ridge. The influence of variations in subpolar deep water formation is, on multi-model average, larger than that of variations in the Denmark Strait overflow. This is true both at 45° N, where the maximum standard deviation of decadal to multidecadal AMOC variability is located for all but one model, and at the more classical latitude of 30° N. At 30° N, variations in subpolar deep water formation and Denmark Strait overflow explain, on multi-model average, about half and one third respectively of the decadal to multidecadal AMOC variance.

Description: Intrinsic variability of the Antarctic Circumpolar Current system: low- and high-frequency fluctuations of the Argentine Basin flow Ocean Science Discussions, 10, 1933-1969, 2013 Author(s): G. Sgubin, S. Pierini, and H. A. Dijkstra In this paper, the variability of the Antarctic Circumpolar Current system produced by purely intrinsic nonlinear oceanic mechanisms is studied through a sigma-coordinate ocean model, implemented in a large portion of the Southern Ocean at an eddy-permitting resolution under steady surface heat and momentum fluxes. The mean transport through Drake Passage and the structure of the main Antarctic Circumpolar Current fronts are well reproduced by the model. Intrinsic variability is found to be particularly intense in the Subantarctic Front and in the Argentine Basin, on which further analysis is focused. The low-frequency variability at interannual time scales is related to bimodal behavior of the Zapiola Anticyclone, with transitions between a strong and collapsed anticyclonic circulation in substantial agreement with altimeter observations. Variability on smaller time scales shows clear evidence of topographic Rossby-mode propagation along the eastern and southern flanks of the Zapiola Rise and of mesoscale eddies, also in agreement with satellite altimeter observations. The analysis of the relationship between the low- and high-frequency variability suggests possible mechanisms of mutual interaction.

Description: Decadal variability of heat content in South China Sea inferred from observation data and an ocean data assimilation product Ocean Science Discussions, 10, 1329-1342, 2013 Author(s): W. Song, J. Lan, Q. Liu, and D. Wang Using an observation dataset of temperature and the Simple Ocean Data Assimilation (SODA), the decadal variability of upper ocean heat content (0–400 m; hereafter, OHC) in the South China Sea (SCS) was investigated for the period from 1958 to 2007. Decadal variability was identified as the dominant mode of upper OHC besides the seasonal cycle. According to deceasing or increasing OHC, four periods were chosen to discuss detailed processes behind OHC variability in the SCS; the four periods are 1958–1968, 1969–1981, 1982–1992, and 1993–2003. Results show that advection was the major factor for decreasing (increasing) OHC during 1958–1968 (1968–1981). During 1982–1992 and 1993–2003, the net surface heat flux was the main contributor to the variability of OHC besides the advection. The OHC, advection and net surface heat flux had significant rising tendencies during 1992–2003. The spatial characteristics of OHC variability and heat budget in the Luzon Strait, west of Luzon Island, and Xisha warm eddy region were also discussed in this paper.

Description: The instability of diffusive convection and its implication for the thermohaline staircases in the deep Arctic Ocean Ocean Science Discussions, 10, 1343-1366, 2013 Author(s): S.-Q. Zhou, L. Qu, Y.-Z. Lu, and X.-L. Song In the present study, the classical description of diffusive convection is updated to interpret the instability of diffusive interfaces and the dynamical evolution of the bottom layer in the deep Arctic Ocean. In the new consideration of convective instability, both the background salinity stratification and rotation are involved. The critical Rayleigh number of diffusive convection is found to vary from 10 3 to 10 11 in the deep Arctic Ocean as well as in other oceans and lakes. In such a wide range of conditions, the interface-induced thermal Rayleigh number is indicated to be consistent with the critical Rayleigh number of diffusive convection. In most regions, background salinity stratification is found to be the main hindrance to the occurrence of convecting layers. With the new parameterization, it is predicted that the maximum thickness of the bottom layer is 1051 m, which is close to the observed value of 929 m. And the evolution time of the bottom layer is predicted to be of the same order as that based on 14 C isolation age estimation.

Description: Temporal variations of zooplankton biomass in the Ligurian Sea inferred from long time series of ADCP data Ocean Science Discussions, 10, 1367-1404, 2013 Author(s): R. Bozzano, E. Fanelli, S. Pensieri, P. Picco, and M. E. Schiano Three years of 300 kHz ADCP data collected in the central Ligurian Sea are analyzed to investigate the variability of the zooplankton biomass and the Diel Vertical Migrations (DVM) in the upper thermocline. After a pre-processing aimed at avoiding the slant range attenuation, hourly volume backscattering strength time series are obtained. Despite the lack of concurrent net samples collection, different migration patterns are identified and their temporal variability examined by means of time-frequency analysis. The effect of changes in the environmental condition is also investigated. Highest zooplankton biomasses are observed in April–May just after the peak of surface primary production in March–April. The main migration pattern points to a "nocturnal" migration with zooplankton organisms occurring deeper in the water column during the day and shallower at night. Also twilight migration is highlighted during this study. The largest migrations are recorded in November–December, corresponding to lowest backscattering strength values and are likely attributable to larger and more active organisms (i.e. euphausiids and mesopelagic fish). The results suggest further applications of the historical ADCP time series available.

Description: Changes in ventilation of the Mediterranean Sea during the past 25 yr Ocean Science Discussions, 10, 1405-1445, 2013 Author(s): A. Schneider, T. Tanhua, W. Roether, and R. Steinfeldt The Mediterranean Sea has a fast overturning circulation and the deep water masses are well ventilated in comparison to the deep waters of the world ocean. Significant changes in the overturning circulation has been observed during the last few decades, the most prominent phenomena being the Eastern Mediterranean Transient (EMT) in the early 1990s and the Western Mediterranean Transit (WMT) near the mid of the decade following. During both of these events unusually large amounts of deep water were formed, and in the case of the EMT, the deep water formation area shifted from the Adriatic to the Aegean Sea. This variability is important to understand and to monitor, because ventilation is the main process to propagate surface perturbations, such as uptake of anthropogenic CO 2 , into the ocean interior. Here we synthesize a unique collection of transient tracer (CFC-12, SF 6 and tritium) data from nine cruises conducted between 1987 and 2011 and use these data to determine temporal variability of Mediterranean ventilation. We also discuss biases and technical problems with transient tracer-based ages arising from their different input histories over time; particularly in the case of time-dependent ventilation. We observe a period of stagnation in the deep eastern (Levantine) basin after it was ventilated by the EMT so that the age of the deep water is increasing with time. In the Ionian Sea, on the other hand, we see evidence of increased ventilation after year 2001, indicating the restarted deep water formation in the Adriatic Sea. This is also reflected in the increasing age of the Cretan Sea deep water and decreasing age of Adriatic Sea deep water since the end of the 1980s. In the western Mediterranean deep basin we see the massive input of recently ventilated waters during the WMT. This signal is not yet apparent in the Tyrrhenian Sea, where the ventilation seems to be fairly constant since the EMT. Also the western Alboran Sea does not show any temporal trends in ventilation.

Description: Estuarine circulation reversals and related rapid changes in winter near-bottom oxygen conditions in the Gulf of Finland, Baltic Sea Ocean Science Discussions, 10, 727-762, 2013 Author(s): T. Liblik, J. Laanemets, U. Raudsepp, J. Elken, and I. Suhhova The reversal of estuarine circulation caused by southwesterly wind forcing may lead to vanishing of stratification and oxygenation of deep layers during the cold season in the Gulf of Finland. Six CTD + oxygen transects (130 km long, 10 stations) were conducted along the thalweg from the western border to the central gulf (21 December 2011–8 May 2012). Two bottom-mounted ADCP, near the western border and in the central gulf were installed. A CTD with dissolved oxygen sensor was deployed close to the western ADCP. Periods with typical estuarine circulation were characterized by strong stratification; high salinity, hypoxic conditions and inflow to the gulf in the near bottom layer. Two circulation reversals were observed, in December–January and February–March. The first well-developed reversal event caused the vanishing of stratification and oxygen concentrations that were almost over 270 μmol L −1 (6 mL L −1 ) in the entire water column along the transect; and lasted for about 1.5 months. Shifts from estuarine circulation to reversed circulation and vice versa were both associated with strong currents (up to 40 cm s −1 ) in the deep layer. In the western area of study, near-bottom oxygen conditions strongly depended on salt wedge intrusions (hypoxic water) from the NE Baltic Proper, while in the eastern part good oxygen conditions caused by reversals remained for a few months. Change from oxygenated to hypoxic conditions in the entrance area to the Gulf might occur very rapidly, within less than a day.

Description: The circulation of Icelandic waters – a modelling study Ocean Science Discussions, 10, 763-824, 2013 Author(s): K. Logemann, J. Ólafsson, Á. Snorrason, H. Valdimarsson, and G. Marteinsdóttir The three-dimensional flow, temperature and salinity fields of the North Atlantic including the Arctic Ocean covering the time period 1992 to 2006 are simulated with the numerical ocean model CODE. The model reveals several new insights and previously unknown structures which help us to clarify open questions on the regional oceanography of Icelandic waters. These relate to the structure and geographical distribution of the coastal current, the primary forcing of the North Icelandic Irminger Current (NIIC), the path of the Atlantic Water south-east of Iceland and the structure of the North Icelandic Jet (NIJ). The model's adaptively refined computational mesh has a maximum resolution of 1 km horizontal and 2.5 m vertical in Icelandic waters. CTD profiles from this region and the river discharge of 46 Icelandic watersheds, computed by the hydrological model WaSiM, are assimilated into the simulation. The model realistically reproduces the established elements of the circulation around Iceland. However, analysis of the simulated mean flow field also provides further insights. It suggests a distinct freshwater-induced coastal current that only exists along the south-west and west coasts which is accompanied by a counter-directed undercurrent. The simulated transport of Atlantic Water over the Icelandic shelf takes place in a symmetrical system of two currents, with the established NIIC over the north-western and northern shelf, and a current over the southern and south-eastern shelf herein called the South Icelandic Current (SIC). Both currents are driven by topographically induced distortions of the Arctic Front's barotropic pressure field. The SIC is simulated to be an upstream precursor of the Faroe Current (FC). The recently discovered North Icelandic Jet (NIJ) also features in the model predictions and is found to be forced by the baroclinic pressure field of the Arctic Front, to originate east of the Kolbeinsey Ridge and to have a volume transport of around 1.5 Sv within northern Denmark Strait. The simulated multi-annual mean Atlantic Water transport of the NIIC increased by 85% during 1992 to 2006, whereas the corresponding NIJ transport decreased by 27%. Based on our model results we propose a new and further differentiated circulation scheme of Icelandic waters whose details may inspire future observational oceanography studies.

Description: From satellite altimetry to Argo and operational oceanography: three revolutions in oceanography Ocean Science Discussions, 10, 1127-1167, 2013 Author(s): P. Y. Le Traon The launch of the US/French mission Topex/Poseidon (T/P) (CNES/NASA) in August 1992 was the start of a revolution in oceanography. For the first time, a very precise altimeter system optimized for large scale sea level and ocean circulation observations was flying. T/P alone could not observe the mesoscale circulation. In the 1990s, the ESA satellites ERS-1/2 were flying simultaneously with T/P. Together with my CLS colleagues, we demonstrated that we could use T/P as a reference mission for ERS-1/2 and bring the ERS-1/2 data to an accuracy level comparable to T/P. Near real time high resolution global sea level anomaly maps were then derived. These maps have been operationally produced as part of the SSALTO/DUACS system for the last 15 yr. They are now widely used by the oceanographic community and have contributed to a much better understanding and recognition of the role and importance of mesoscale dynamics. Altimetry needs to be complemented with global in situ observations. In the end of the 90s, a major international initiative was launched to develop Argo, the global array of profiling floats. This has been an outstanding success. Argo floats now provide the most important in situ observations to monitor and understand the role of the ocean on the earth climate and for operational oceanography. This is a second revolution in oceanography. The unique capability of satellite altimetry to observe the global ocean in near real time at high resolution and the development of Argo were essential to the development of global operational oceanography, the third revolution in oceanography. The Global Ocean Data Assimilation Experiment (GODAE) was instrumental in the development of the required capabilities. This paper provides an historical perspective on the development of these three revolutions in oceanography which are very much interlinked. This is not an exhaustive review and I will mainly focus on the contributions we made together with many colleagues and friends.

Description: Interannual correlations between sea surface temperature and concentration of chlorophyll pigment off Punta Eugenia, Baja California during different remote forcing conditions Ocean Science Discussions, 10, 853-882, 2013 Author(s): H. Herrera-Cervantes, S. E. Lluch-Cota, D. B. Lluch-Cota, and G. Gutiérrez-de-Velasco Interannual correlation between satellite-derived sea surface temperature (SST) and surface chlorophyll a (Chl a ) are examined in the coastal upwelling zone off Punta Eugenia on the west coast of the Baja California Peninsula, area identified as intense biological productivity and oceanographic transition between mid-latitude and tropical ocean conditions. We used empirical orthogonal functions (EOF) analysis separately and jointly on the two fields from 1997 through 2007, a time period dominated by different remote forcing; ENSO conditions (weak, moderate and strong) and the largest intrusion of subarctic water reported in the last 50 yr. Coastal Upwelling Index anomalies (CUI) and the Multivariate ENSO Index (MEI) were used to identify the influence of local (wind stress) and remote (ENSO) forcing over the interannual variability of both variables. The individual EOF 1 analysis showed the greater variability of SST and Chl a offshore, their corresponding amplitude time series presented the highest peaks during the intrusion of subartic water (2002–2004) and were significantly correlated with the MEI ( R SST ≈ 0.68, R Chl a ≈ −0.30, P 〈 0.001) and moderately correlated with the CUI ( R SST ≈ −0.4, R Chl a ≈ 0.25, P 〈 0.001), showing similar trends. The joint EOF 1 and the SST–Chl a correlations pattern show the area where both variables covary tightly; a band near to the coast with the largest correlations ( R 〉 |0.4|) mainly regulated by ENSO cycles. This was revealed when we calculate the homogeneous correlations for the periods El Niño–La Niña and the intrusion of subartic water. Both, SST and Chl a showed higher coupling and two distinct physical-biological responses; on average ENSO influence were clearly along the coast mostly in SST while the subarctic water influence, were observed offshore mostly in Chl a . We found a coastal chlorophyll bloom correlated strongly with high wind stress anomalies that reach the coast off Punta Eugenia during spring and summer 2002 and continued its presence during 2003 which showed an enrichment pattern similar to that observed at high latitudes (∼40° N). This observation may provide an explanation of why Punta Eugenia is one of the most important biological action centers.

Description: Tidal variability of the motion in the Strait of Otranto Ocean Science Discussions, 10, 435-472, 2013 Author(s): L. Ursella, V. Kovačević, and M. Gačić Various current data, collected in the Strait of Otranto during the period 1994–2007, have been analysed with the aim of describing the characteristics of the tidal motions and their contribution to the total flow variance. The principal tidal constituents in the area were the semi-diurnal (M2) and the diurnal (K1), with the latter one predominant. The total flow was, in general, more energetic along the flanks than in the middle of the Strait. Specifically, it was most energetic over the western shelf and in the upper layer along the eastern flank. In spite of the generally low velocities (a few cm s −1 ) of the principal tidal constituents, the tidal variance has a pattern similar to that of the total flow variance, that is, it was large over the western shelf and low in the middle. The proportion of non-tidal (comprising the inertial and sub-inertial low-frequency bands) to tidal flow variances was quite variable in both time and space. The contribution of the low-frequency motions predominated over the tidal and inertial ones in the eastern portion of the strait during the major part of the year, particularly in the upper and intermediate layers. In the deep, near-bottom, layer the variance was evenly distributed between the low frequency, diurnal and semi-diurnal bands. A prominent exception was observed near the western shelf break during the summer season when the contribution of the tidal signal alone to the total variance reached 77%. This high contribution was mainly due to the intensification of the diurnal signal at that location in the proximity of both the surface and bottom layers (velocities of about 10 cm s −1 ). Local wind and sea level data were analysed and compared with the flow to find the possible origin of this diurnal intensification. Having excluded the sea-breeze impact on the intensification of the diurnal tidal signal, the most likely cause remains the generation of the topographically trapped internal waves and the diurnal resonance in the tidal response. These waves were sometimes generated by the barotropic tidal signal in the presence of summer stratification. The effect was seen only in the presence of the topographic slope change. This phenomenon may stimulate the diapycnal mixing during the stratified season and enhance ventilation of the near-bottom layers.

Description: The Mediterranean Sea system: a review and an introduction to the special issue Ocean Science Discussions, 10, 581-617, 2013 Author(s): T. Tanhua, D. Hainbucher, K. Schröder, V. Cardin, M. Álvarez, and G. Civitarese The Mediterranean is a semi-enclosed sea characterized by high salinities, temperatures and densities. The net evaporation exceeds the precipitation, driving an anti-estuarine circulation through the Strait of Gibraltar, contributing to very low nutrient concentrations. The Mediterranean Sea has an active overturning circulation, one shallow cell that communicates directly with the Atlantic Ocean, and two deep overturning cells, one in each of the two main basins. It is surrounded by populated areas and is thus sensitive to anthropogenic forcing. Several dramatic changes in the oceanographic and biogeochemical conditions have been observed during the past several decades, emphasising the need to better monitor and understand the changing conditions and their drivers. During 2011 three oceanographic cruises were conducted in a coordinated fashion in order to produce base-line data of important physical and biogeochemical parameters that can be compared to historic data and be used as reference for future observational campaigns. In this article we provide information on the Mediterranean Sea oceanographic situation, and present a short review that will serve as background information for the special issue in Ocean Science on "Physical, chemical and biological oceanography of the Mediterranean Sea". An important contribution of this article is the set of figures showing the large-scale distributions of physical and chemical properties along the full length of the Mediterranean Sea.

Description: Mechanisms of AMOC variability simulated by the NEMO model Ocean Science Discussions, 10, 619-648, 2013 Author(s): V. N. Stepanov and K. Haines We have investigated dominant mechanisms of the Atlantic Meridional Overturning Circulation (AMOC) variability at 26.5° N (without the Ekman component) on monthly timescales using 1° and 1/4° NEMO model data. All data were detrended and the seasonal cycle removed. The spatial lead-lag correlations of different hydrodynamic fields with the AMOC time series were calculated. The analysis shows that the AMOC depends on the strength of wind over the North Atlantic on different time scales. At ∼ 1 yr the January–June difference of mean sea level pressure between high and mid-latitudes in the North Atlantic defines (according to different model runs) 35–50% of the annual AMOC variability. At interannual time scales ∼ 4 yr after strong (weak) winds over the North Atlantic the AMOC transport becomes higher (lower) by means of an increase (a decrease) in deep water formation in the North Atlantic subpolar gyre. The analysis of the 1/4° NEMO model shows that about 30% of the AMOC variability is due to density changes in the top 1000 m in the Labrador and Irminger seas occurring about 4 yr early.

Description: Intercomparison of the Charnock and CORE bulk wind stress formulations for coastal ocean modelling Ocean Science Discussions, 10, 519-537, 2013 Author(s): J. M. Brown, L. O. Amoudry, F. M. Mercier, and A. J. Souza The accurate parameterisation of momentum and heat transfer across the air-sea interface is vital for realistic simulation of the atmosphere-ocean system. In many modelling applications accurate representation of the wind stress is required to numerically reproduce surge, coastal ocean circulation, surface waves, turbulence and mixing. Different formulations can be implemented and impact the accuracy of: the instantaneous and long-term residual circulation; the surface mixed layer; and the generation of wave-surge conditions. This, in turn, affects predictions of storm impact, sediment pathways, and coastal resilience to climate change. The specific numerical formulation needs careful selection to ensure the accuracy of the simulation. Two wind stress formulae widely used in respectively the ocean circulation and the storm surge communities are studied with focus on an application to the NW region of the UK. Model-observation validation is performed at two nearshore and one estuarine ADCP stations in Liverpool Bay, a hypertidal region of freshwater influence with vast intertidal areas. The period of study covers both calm and extreme conditions to fully test the robustness of the 10 m wind stress component of the Common Ocean Reference Experiment (CORE) bulk formulae and the Charnock relation. In this coastal application a realistic barotropic-baroclinic simulation of the circulation and surge elevation is setup, demonstrating greater accuracy occurs when using the Charnock relation for surface wind stress.

Description: Biogeography of planktonic microbial communities across the whole Mediterranean Sea Ocean Science Discussions, 10, 291-319, 2013 Author(s): F. Mapelli, M. M. Varela, M. Barbato, R. Alvariño, M. Fusi, M. Álvarez, G. Merlino, D. Daffonchio, and S. Borin The M84/3 cruise recently held onboard of R/V Meteor represented a great and rare opportunity for the scientific community to realize a multidisciplinary survey on the whole Mediterranean Sea. In this context, molecular microbiology investigation, realized by applying Automated Ribosomal Intergenic Sequence Analysis (ARISA) and microscope evaluation of prokaryotic abundance, were performed on seawater samples aiming to identify the environmental factors driving planktonic bacterial community composition across both vertical and longitudinal transects. Prokaryotic abundance decreased along with depth in all the stations and presented similar values in sub-surface, meso- and bathypelagic layers across the whole Mediterranean basin. On the contrary, peculiar bacterial assemblages were selected along a longitudinal transect in the surface layers of the eastern and western sub-basins. Sharp vertical profiling of the bacterial communities was observed only considering the boundary of the water column, while the study of bacterial β-diversity at finer scale across the water column displayed higher variability at the intermediate layers. Nonetheless, different physico-chemical factors were significantly related to microbial zonation both according to geographic position and across the water column in the whole Mediterranean Sea. These results demonstrated that bacterial diversity is putatively correlated to different water masses across the water column of the complex hydrographical systems of the eastern and western Mediterranean sub-basins.

Description: An optical model for deriving the spectral particulate backscattering coefficients in clear and turbid coastal waters Ocean Science Discussions, 10, 261-290, 2013 Author(s): S. P. Tiwari and P. Shanmugam An optical model is developed based on the diffuse attenuation coefficient ( K d ) to estimate particulate backscattering coefficients b bp (λ) in clear and turbid coastal waters. A large in-situ data set is used to establish robust relationships between b bp (530) and b bp (555) and K d (490) using an efficient nonlinear least square method which uses the Trust-Region algorithm with Bisquare weights scheme to adjust the coefficients. These relationships are obtained with good correlation coefficients ( R 2 = 0.786 and 0.790), low Root Mean Square Error (RMSE = 0.00076 and 0.00072) and 95% confidence bounds. The new model is tested with two independent data sets such as the NOMAD SeaWiFS Match-ups and OOXIX IOP algorithm workshop evaluation data set (Version 2.0w APLHA). Results show that the new model makes good retrievals of b bp at all key wavelengths (from 412–683 nm), with statistically significant improvements over other inversion models. Thus, the new model has the potential to improve our knowledge of particulate matters and their optical variability in both clear and turbid coastal waters.

Description: Sources of 21st century regional sea level rise along the coast of North-West Europe Ocean Science Discussions, 10, 2433-2459, 2013 Author(s): T. Howard, A. K. Pardaens, J. A. Lowe, J. Ridley, R. T. W. L. Hurkmans, J. L. Bamber, G. Spada, and D. Vaughan Changes in both global and regional mean sea level, and changes in the magnitude of extreme flood heights, are the result of a combination of several distinct contributions most, but not all, of which are associated with climate change. These contributions include effects in the solid earth, gravity field, changes in ocean mass due to ice-loss from ice sheets and glaciers, thermal expansion, alterations in ocean circulation driven by climate change and changing freshwater fluxes, and the intensity of surge tides. Due to the diverse range of models required to simulate these systems, the contributions to sea-level change have usually been discussed in isolation rather than as a fully-coupled system. Focusing on the coastline of Northwest Europe, we consider all these processes and their relative impact on 21st century regional mean sea levels and extreme flood height. As far as possible our projections of change are derived from process-based models forced by the A1B emissions scenario to provide a self-consistent comparison of the contributions. We address uncertainty by considering both a mid-range and an illustrative high-end combination of the different components. For our mid-range ice-loss scenario we find that thermal expansion of seawater is the dominant contributor to change in sea level by 2100. However, the projected contribution to extreme sea level, due to changes in storminess alone, is significant and in places is comparable to the global mean contribution of thermal expansion. For example, under the A1B emissions scenario, by 2100, change in storminess contributes around 15 cm to the increase in projected height of the 50 yr storm surge on the west coast of the Jutland Peninsula, compared with a contribution of around 22 cm due to thermal expansion. An illustrative combination of our high-end projections suggests increases in the 50 yr return level of 86 cm at Sheerness, 95 cm at Roscoff, 106 cm at Esbjerg, and 67 cm at Bergen. The notable regional differences between these locations arise from differences in rate of vertical land movement and changes in storminess.

Description: Hydrographic situation during cruise M84/3 and P414 (spring 2011) in the Mediterranean Sea Ocean Science Discussions, 10, 2399-2432, 2013 Author(s): D. Hainbucher, A. Rubino, V. Cardin, T. Tanhua, K. Schroeder, and M. Bensi Hydrography and large scale circulation observed in the Mediterranean Sea during the M84/3 and P414 cruises (April and June 2011, respectively) are presented. In contrast to most of the recent expeditions, which were limited to special areas of the basin, these two cruises, especially the M84/3, offered the opportunity of delineating a quasi-synoptic picture of the distribution of the relevant physical parameters through the whole Mediterranean. A section was observed from the Lebanese coast up to the Strait of Gibraltar. The focus of our analysis are the water mass properties, also in the context of the recently observed variability, and a comparison between the velocity fields observed using a vessel-mounted ADCP and those calculated from the observed density fields. Overall, a distribution of temperature, salinity, and geostrophic velocities emerges, which seems far from that observed before the beginning of the so-called "Eastern Mediterranean Transient", a major climatic shift in the hydrography and circulation of the Mediterranean Sea occurred at the end of 1980s. Here, our focus is a discussion of the observed water mass properties analysed through T – S diagrams and through an Optimum Multiparameter (OMP) analysis. Additionally, ADCP velocities are compared to geostrophic calculations.

Description: Reconciling the north–south density difference scaling for the Meridional Overturning Circulation strength with geostrophy Ocean Science Discussions, 10, 2461-2479, 2013 Author(s): A. A. Cimatoribus, S. Drijfhout, and H. A. Dijkstra Since the formulation of the Stommel two-box model for the meridional overturning circulation (MOC), various theoretical and conceptual models for the MOC emerged based on scaling the MOC strength with the north south density difference. At the same time the MOC should obey geostrophic balance with an east-west density difference. Scaling with the north south density gradient seems to violate the common assumption of geostrophic balance for the large-scale circulation, which implies that the pressure gradient is orthogonal to the flow. In this brief report, we report on the results of a series of numerical simulations in an idealized ocean basin (with a zonally periodic channel at its southern end). The simulations performed with different surface forcing conditions indicate that the meridional and zonal density gradients, important for the MOC strength, are in fact related to each other through the stratification located at the northern end of the periodic channel. The results suggest that the water properties at the northern end of the periodic channel play a crucial role in setting the MOC strength, possibly explaining the sensitivity of climate models to the conditions in this area.

Description: Changes in both global and regional mean sea level, and changes in the magnitude of extreme flood heights, are the result of a combination of several distinct contributions most, but not all, of which are associated with climate change. These contributions include effects in the solid earth, gravity field, changes in ocean mass due to ice-loss from ice sheets and glaciers, thermal expansion, alterations in ocean circulation driven by climate change and changing freshwater fluxes, and the intensity of surge tides. Due to the diverse range of models required to simulate these systems, the contributions to sea-level change have usually been discussed in isolation rather than as a fully-coupled system. Focusing on the coastline of Northwest Europe, we consider all these processes and their relative impact on 21st century regional mean sea levels and extreme flood height. As far as possible our projections of change are derived from process-based models forced by the A1B emissions scenario to provide a self-consistent comparison of the contributions. We address uncertainty by considering both a mid-range and an illustrative high-end combination of the different components. For our mid-range ice-loss scenario we find that thermal expansion of seawater is the dominant contributor to change in sea level by 2100. However, the projected contribution to extreme sea level, due to changes in storminess alone, is significant and in places is comparable to the global mean contribution of thermal expansion. For example, under the A1B emissions scenario, by 2100, change in storminess contributes around 15 cm to the increase in projected height of the 50 yr storm surge on the west coast of the Jutland Peninsula, compared with a contribution of around 22 cm due to thermal expansion. An illustrative combination of our high-end projections suggests increases in the 50 yr return level of 86 cm at Sheerness, 95 cm at Roscoff, 106 cm at Esbjerg, and 67 cm at Bergen. The notable regional differences between these locations arise from differences in rate of vertical land movement and changes in storminess.

Description: Since the formulation of the Stommel two-box model for the meridional overturning circulation (MOC), various theoretical and conceptual models for the MOC emerged based on scaling the MOC strength with the north south density difference. At the same time the MOC should obey geostrophic balance with an east-west density difference. Scaling with the north south density gradient seems to violate the common assumption of geostrophic balance for the large-scale circulation, which implies that the pressure gradient is orthogonal to the flow. In this brief report, we report on the results of a series of numerical simulations in an idealized ocean basin (with a zonally periodic channel at its southern end). The simulations performed with different surface forcing conditions indicate that the meridional and zonal density gradients, important for the MOC strength, are in fact related to each other through the stratification located at the northern end of the periodic channel. The results suggest that the water properties at the northern end of the periodic channel play a crucial role in setting the MOC strength, possibly explaining the sensitivity of climate models to the conditions in this area.

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

Description: Hydrography and large scale circulation observed in the Mediterranean Sea during the M84/3 and P414 cruises (April and June 2011, respectively) are presented. In contrast to most of the recent expeditions, which were limited to special areas of the basin, these two cruises, especially the M84/3, offered the opportunity of delineating a quasi-synoptic picture of the distribution of the relevant physical parameters through the whole Mediterranean. A section was observed from the Lebanese coast up to the Strait of Gibraltar. The focus of our analysis are the water mass properties, also in the context of the recently observed variability, and a comparison between the velocity fields observed using a vessel-mounted ADCP and those calculated from the observed density fields. Overall, a distribution of temperature, salinity, and geostrophic velocities emerges, which seems far from that observed before the beginning of the so-called "Eastern Mediterranean Transient", a major climatic shift in the hydrography and circulation of the Mediterranean Sea occurred at the end of 1980s. Here, our focus is a discussion of the observed water mass properties analysed through T–S diagrams and through an Optimum Multiparameter (OMP) analysis. Additionally, ADCP velocities are compared to geostrophic calculations.

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

Description: It is well-established that, at periods shorter than a year, variations in Antarctic Circumpolar Transport are reflected in a barotropic mode, known as the Southern Mode, in which sea level and bottom pressure varies coherently around Antarctica. Here, we use two multidecadal ocean model runs to investigate the behaviour of the Southern Mode at time scales on which density changes become important, leading to a baroclinic component to the adjustment. We find that the concept of a Southern Mode in bottom pressure remains valid, and remains a direct measure of the circumpolar transport, with changes at the northern boundary playing only a small role even on decadal time scales. However, at periods longer than about 5 yr, density changes start to play a role, leading to a surface intensification of the vertical profile of the transport. We also find that barotropic currents on the continental slope account for a significant fraction of the variability, and produce surface intensification in the meridional-integral flow. The role of density variations results in a sea level signal which, although reflecting transport changes at all time scales, has a ratio of sea level to transport which becomes larger at longer time scales. This means that any long-term transport monitoring strategy based on present measurement systems must involve multiplying the observed quantity by a factor which depends on frequency.

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

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

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

Description: In a pilot study conducted in October and November 2011, air–sea gas transfer velocities of the two sparingly soluble trace gases hexafluorobenzene and 1,4-difluorobenzene were measured in the unique High-Speed Wind-Wave Tank at Kyoto University, Japan. This air–sea interaction facility is capable of producing hurricane strength wind speeds of up to u10=67 m s−1. This constitutes the first lab study of gas transfer at such high wind speeds. The measured transfer velocities k600 spanned two orders of magnitude, lying between 11 cm h−1 and 1180 cm h−1 with the latter being the highest ever measured wind induced gas transfer velocity. The measured gas transfer velocities are in agreement with the only available dataset at hurricane wind speeds (McNeil and D'Asaro, 2007). The disproportionately large increase of the transfer velocities found at highest wind speeds indicates a new regime of air–sea gas transfer, which is characterized by strong wave breaking, enhanced turbulence and bubble cloud entrainment. It was found that tracers spanning a wide range of solubilities and diffusivities are needed to separate the effects of enhanced surface area and turbulence due to breaking waves from the effects of bubble and spray mediated gas transfer.

Description: The Totten Glacier drains a large proportion of the East Antarctic ice sheet, much of it marine based (grounded below sea level), and is rapidly losing mass. It has been suggested that this mass loss is driven by changes in oceanic forcing; however, the details of the ice-ocean interaction are unknown. Here we present results from an ice shelf-ocean model of the region that includes the Totten, Moscow University and Dalton Ice Shelves, based on the Regional Oceanic Modeling System for the period 1992–2007. Simulated area-averaged basal melt rates (net basal mass loss) for the Totten and Dalton ice shelves are 9.1 m ice yr−1 (44.5 Gt ice yr−1) and 10.1 m ice yr−1 (46.6 Gt ice yr−1), respectively. The melting of the ice shelves varies strongly on seasonal and interannual timescales. Basal melting (mass loss) from the Totten ice shelf spans a range of 5.7 m ice yr−1 (28 Gt ice yr−1) on interannual timescales and 3.4 m ice yr−1 (17 Gt ice yr−1) on seasonal timescales. This study links basal melt of the Totten and Dalton ice shelves to warm water intrusions across the continental shelf break and atmosphere-ocean heat exchange. Totten ice shelf melting is high when the nearby Dalton polynya interannual strength is below average, and vice versa. Melting of the Dalton ice shelf is primarily controlled by the strength of warm water intrusions across the Dalton Rise and into the ice shelf cavity. During periods of strong westwards coastal current flow, Dalton melt water flows directly under the Totten ice shelf further reducing melting. This is the first such modelling study of this region, providing a valuable framework for directing future observational and modelling efforts.

Description: An outstanding problem in biogeochemical modelling of the ocean is that many of the key processes occur intermittently at small scales, such as the sub-mesoscale, that are not well represented in global ocean models. As an example, state-of-the-art models give values of primary production approximately two orders of magnitude lower than those observed in the ocean's oligotrophic gyres, which cover a third of the Earth's surface. This is partly due to their failure to resolve sub-mesoscale phenomena, which play a significant role in nutrient supply. Simply increasing the resolution of the models may be an inefficient computational solution to this problem. An approach based on recent advances in adaptive mesh computational techniques may offer an alternative. Here the first steps in such an approach are described, using the example of a~simple vertical column (quasi 1-D) ocean biogeochemical model. We present a novel method of simulating ocean biogeochemical behaviour on a vertically adaptive computational mesh, where the mesh changes in response to the biogeochemical and physical state of the system throughout the simulation. We show that the model reproduces the general physical and biological behaviour at three ocean stations (India, Papa and Bermuda) as compared to a high-resolution fixed mesh simulation and to observations. The simulations capture both the seasonal and inter-annual variations. The use of an adaptive mesh does not increase the computational error, but reduces the number of mesh elements by a factor of 2–3, so reducing computational overhead. We then show the potential of this method in two case studies where we change the metric used to determine the varying mesh sizes in order to capture the dynamics of chlorophyll at Bermuda and sinking detritus at Papa. We therefore demonstrate adaptive meshes may provide a~suitable numerical technique for simulating seasonal or transient biogeochemical behaviour at high spatial resolution whilst minimising computational cost.

Description: In this paper, the variability of the Antarctic Circumpolar Current system produced by purely intrinsic nonlinear oceanic mechanisms is studied through a sigma-coordinate ocean model, implemented in a large portion of the Southern Ocean at an eddy-permitting resolution under steady surface heat and momentum fluxes. The mean transport through Drake Passage and the structure of the main Antarctic Circumpolar Current fronts are well reproduced by the model. Intrinsic variability is found to be particularly intense in the Subantarctic Front and in the Argentine Basin, on which further analysis is focused. The low-frequency variability at interannual time scales is related to bimodal behavior of the Zapiola Anticyclone, with transitions between a strong and collapsed anticyclonic circulation in substantial agreement with altimeter observations. Variability on smaller time scales shows clear evidence of topographic Rossby-mode propagation along the eastern and southern flanks of the Zapiola Rise and of mesoscale eddies, also in agreement with satellite altimeter observations. The analysis of the relationship between the low- and high-frequency variability suggests possible mechanisms of mutual interaction.

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

Description: The resonances of Hudson Bay, Foxe Basin and Hudson Strait are investigated using a~linear shallow water numerical model. The region is of particular interest because it is the most important region of the world ocean for dissipating tidal energy. The model shows that the semi-diurnal tides of the region are dominated by four nearby overlapping resonances. It shows that these not only affect Ungava Bay, a region of extreme tidal range, but they also extend far into Foxe Basin and Hudson Bay and appear to be affected by the geometry of those regions. The results also indicate that it is the four resonances acting together which make the region such an important area for dissipating tidal energy.

Description: The seasonal thermocline in shelf-seas represents an important biogeophysical barrier to the vertical flux of nutrients into the photic zone. Episodic weakening of this barrier plays an important role in sustaining the sub-surface chlorophyll maximum in summer and hence impacts the carbon draw-down in the seasonally-stratified zones of the shelf seas. Here we present estimates of the rate of turbulent kinetic energy dissipation inferred from microstructure shear probes and compare them with dissipation rates inferred from a standard conductivity-temperature-depth instrument and from a fast thermistor (Thorpe Scale methodology) at a site in the seasonally-stratified Irish Sea. All methods show strong dissipation rates in response to tidal stresses near the bed (order 10−2 Wm−3) with qualitatively similar temporal and spatial patterns. In the interior of the water column, however, only the microstructure shear probe estimates resolve the mixing in the region of the thermocline.

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

Description: 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: The estuarine behavior of organic carbon (OC) and trace elements (TE) was studied for the largest European sub-Arctic river, which is the Severnaya Dvina; this river is a deltaic estuary covered in ice during several hydrological seasons: summer (July 2010, 2012) and winter (March 2009) baseflow, and the November–December 2011 ice-free period. Colloidal forms of OC and TE were assessed using three pore size cutoff (1, 10, and 50 kDa) using an in-situ dialysis procedure. Conventionally dissolved (〈 0.22 μm) fractions demonstrated clear conservative behavior for Li, B, Na, Mg, K, Ca, Sr, Mo, Rb, Cs, and U during the mixing of freshwater with the White Sea; a significant (up to a factor of 10) concentration increase occurs with increases in salinity. Si and OC also displayed conservative behavior but with a pronounced decrease of concentration seawards. Rather conservative behavior, but with much smaller changes in concentration (variation within ±30%) over a full range of salinities, was observed for Ti, Ni, Cr, As, Co, Cu, Ga, Y, and heavy REE. Strong non-conservative behavior with coagulation/removal at low salinities (〈 5‰) was exhibited by Fe, Al, Zr, Hf, and light REE. Finally, certain divalent metals exhibited non-conservative behavior with a concentration gain at low (~2–5‰, Ba, Mn) or intermediate (~10–15‰, Ba, Zn, Pb, Cd) salinities, which is most likely linked to TE desorption from suspended matter or sediment outflux. The most important result of this study is the elucidation of the behavior of the "truly" dissolved low molecular weight LMW〈 1 kDa fraction containing Fe, OC, and a number of insoluble elements. The concentration of the LMW fraction either remains constant or increases its relative contribution to the overall dissolved (〈 0.22 μm) pool as the salinity increases. Similarly, the relative proportion of colloidal (1 kDa–0.22 μm) pool for the OC and insoluble TE bound to ferric colloids systematically decreased seaward, with the largest decrease occurring at low (〈 5‰) salinities. Overall, the observed decrease of the colloidal fraction may be related to the coagulation of organo-ferric colloids at the beginning of the mixing zone and therefore the replacement of the HMW1 kDa–0.22 μm portion by the LMW〈 1 kDa fraction. These patterns are highly reproducible across different sampling seasons, suggesting significant enrichment of the mixing zone by the most labile (and potentially bioavailable) fraction of the OC, Fe and insoluble TE. The size fractionation of the colloidal material during estuarine mixing reflects a number of inorganic and biological processes, the relative contribution of which to element speciation varies depending on the hydrological stage and time of year. In particular, LMW〈 1 kDa ligand production in the surface horizons of the mixing zone may be linked to heterotrophic mineralization of allochthonous DOM and/or photodestruction. Given the relatively low concentration of particulate vs. dissolved load of most trace elements, desorption from the river suspended material was less pronounced than in other rivers in the world. As a result, the majority of dissolved components exhibited either a conservative (OC and related elements such as divalent metals) or non-conservative, coagulation-controlled (Fe, Al, and insoluble TE associated with organo-ferric colloids) behavior. The climate warming in high latitudes is likely to intensify the production of LMW〈 1 kDa organic ligands and the associated TE; therefore, the delivery of potentially bioavailable trace metal micronutrients from the land to the ocean may increase.

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

Description: We investigate the respective role of variations in subpolar deep water formation and Nordic Seas overflows for the decadal to multidecadal variability of the Atlantic meridional overturning circulation (AMOC). This is done by analysing long (order of 1000 yr) control simulations with five coupled climate models as well as sensitivity experiments performed with one of the models, in which we suppress the variability of either subpolar deep water formation or Nordic Seas overflows. For all models, the maximum influence of variations in subpolar deep water formation is found at about 45° N, while the maximum influence of variations in Nordic Seas overflows is rather found at 55° N to 60° N. Regarding the two overflow branches, the influence of variations in the Denmark Strait overflow is, for all models, substantially larger than that of variations in the overflow across the Iceland–Scotland–Ridge. The influence of variations in subpolar deep water formation is, on multi-model average, larger than that of variations in the Denmark Strait overflow. This is true both at 45° N, where the maximum standard deviation of decadal to multidecadal AMOC variability is located for all but one model, and at the more classical latitude of 30° N. At 30° N, variations in subpolar deep water formation and Denmark Strait overflow explain, on multi-model average, about half and one third respectively of the decadal to multidecadal AMOC variance.

Description: Drifter measurements and satellite altimetry data are merged to reconstruct the surface geostrophic circulation of the Black Sea in the period 1999–2009. This combined dataset is used to estimate pseudo-Eulerian velocity statistics for different time periods. Seasonal and interannual variability of currents and kinetic energy fields are described with particular attention to the mesoscale and sub-basin coastal eddies. The mean currents are generally stronger in winter and enhanced speeds are observed in the period 2002–2006. The most intense activity of sub-basin Batumi Eddy occurs in summer with greater speeds and dimensions in 2006 and 2008. The sub-basin Sevastopol Eddy is generated in spring from a meander of the Rim Current. Mesoscale eddies located along the Anatolia, Caucasus and Crimea coasts are permanent, quasi-permanent or intermittent features and can interact and merge with each other, showing higher values of kinetic energy.

Description: The Secchi depth and its relationships to other properties of the sea water in the Oslofjord–Skagerrak area have been investigated. White and black disks of different sizes have been applied, and the Secchi depth has been observed with the naked eye, through colour filters and with a water telescope. Spectral luminances and illuminances have been calculated from recordings of radiance and irradiance. A theoretical expression for the Secchi depth has been tested against field observations, and statistical relationships between Secchi depths and attenuation coefficients have been determined. Effects of size, colour filters, sun glitter and ship shadow have been quantified. The possibility to estimate quanta irradiance, chlorophyll a and total suspended material has also been studied.

Description: 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: The Mediterranean Sea has a fast overturning circulation and the deep water masses are well ventilated in comparison to the deep waters of the world ocean. Significant changes in the overturning circulation has been observed during the last few decades, the most prominent phenomena being the Eastern Mediterranean Transient (EMT) in the early 1990s and the Western Mediterranean Transit (WMT) near the mid of the decade following. During both of these events unusually large amounts of deep water were formed, and in the case of the EMT, the deep water formation area shifted from the Adriatic to the Aegean Sea. This variability is important to understand and to monitor, because ventilation is the main process to propagate surface perturbations, such as uptake of anthropogenic CO2, into the ocean interior. Here we synthesize a unique collection of transient tracer (CFC-12, SF6 and tritium) data from nine cruises conducted between 1987 and 2011 and use these data to determine temporal variability of Mediterranean ventilation. We also discuss biases and technical problems with transient tracer-based ages arising from their different input histories over time; particularly in the case of time-dependent ventilation. We observe a period of stagnation in the deep eastern (Levantine) basin after it was ventilated by the EMT so that the age of the deep water is increasing with time. In the Ionian Sea, on the other hand, we see evidence of increased ventilation after year 2001, indicating the restarted deep water formation in the Adriatic Sea. This is also reflected in the increasing age of the Cretan Sea deep water and decreasing age of Adriatic Sea deep water since the end of the 1980s. In the western Mediterranean deep basin we see the massive input of recently ventilated waters during the WMT. This signal is not yet apparent in the Tyrrhenian Sea, where the ventilation seems to be fairly constant since the EMT. Also the western Alboran Sea does not show any temporal trends in ventilation.

Description: 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: Ventilation is the prime pathway for ocean surface perturbations, such as temperature anomalies, to be relayed to the ocean interior. It is also the conduit for gas exchange between atmosphere and ocean and thus the mechanism whereby, for instance, the interior ocean is oxygenated and enriched in anthropogenic carbon. The ventilation of the Mediterranean Sea is fast in comparison to the world ocean and has large temporal variability, so that quantification of Mediterranean Sea ventilation rates is challenging and very relevant for Mediterranean oceanography and biogeochemistry. Here we present transient tracer data from a field-campaign in April 2011 that sampled a unique suite of transient tracers (SF6, CFC-12, tritium and 3He) in all major basins of the Mediterranean. We apply the Transit Time Distribution (TTD) model to the data which then constrain the mean age, the ratio of the advective/diffusive transport mechanism, and the presence, or not, of more than one significant (for ventilation) water mass. We find that the eastern part of the Eastern Mediterranean can be reasonable described with a one dimensional Inverse Gaussian (1IG) TTD, and thus constrained with two independent tracers. The ventilation of the Ionian Sea and the Western Mediterranean can only be constrained by a multidimensional TTD. We approximate the ventilation with a two-dimensional Inverse Gaussian (2IG) TTD for these areas and demonstrate one way of constraining a 2IG-TTD from the available transient tracer data. The deep water in the Ionian Sea has higher mean ages than the deep water of the Levantine Basin despite higher transient tracer concentrations. This is partly due to the deep water of Adriatic origin having more diffusive properties in the transport and formation, i.e. a high ratio of diffusion over advection, compared to the deep water of Aegean Sea origin that still dominates the deep Levantine Basin deep water after the Eastern Mediterranean Transient (EMT) in the early 1990s. We also show that the deep Western Mediterranean has approximately 40% contribution of recently ventilated deep water from the Western Mediterranean Transition (WMT) event of the mid-2000s. The deep water has higher transient tracer concentrations than the mid-depth water, but the mean age is similar.

Description: This paper provides an extensive vertical and longitudinal description of the CO2 system variables (Total Alkalinity – TA, dissolved inorganic carbon – DIC and pH) along an East-West transect and across the Sardinia–Sicily passage in the Mediterranean Sea (MedSea) from two oceanographic cruises conducted in 2011 measuring CO2 variables in a coordinated fashion, the RV Meteor M84/3 and the RV Urania EuroFleets 11, respectively. The over-determined CO2 system allowed performing the first internal consistency analysis for the particularly warm, high salinity and alkalinity MedSea waters. This basin is considered a "laboratory basin" suffering dramatic changes in its oceanographic and biogeochemical conditions derived from natural and anthropogenic forces. Despite this, little is known about the CO2 system variability in the whole basin. This work aims to be a benchmark for future studies about the CO2 system space-time variability in the MedSea. In this sense we provide full-depth and length CO2 distributions across the MedSea, and property – property plots showing in each sub-basin post-Eastern Mediterranean Transient (EMT) situation with regard to TA, DIC and pH.

Description: In the present study, the classical description of diffusive convection is updated to interpret the instability of diffusive interfaces and the dynamical evolution of the bottom layer in the deep Arctic Ocean. In the new consideration of convective instability, both the background salinity stratification and rotation are involved. The critical Rayleigh number of diffusive convection is found to vary from 103 to 1011 in the deep Arctic Ocean as well as in other oceans and lakes. In such a wide range of conditions, the interface-induced thermal Rayleigh number is indicated to be consistent with the critical Rayleigh number of diffusive convection. In most regions, background salinity stratification is found to be the main hindrance to the occurrence of convecting layers. With the new parameterization, it is predicted that the maximum thickness of the bottom layer is 1051 m, which is close to the observed value of 929 m. And the evolution time of the bottom layer is predicted to be of the same order as that based on 14C isolation age estimation.

Description: Three years of 300 kHz ADCP data collected in the central Ligurian Sea are analyzed to investigate the variability of the zooplankton biomass and the Diel Vertical Migrations (DVM) in the upper thermocline. After a pre-processing aimed at avoiding the slant range attenuation, hourly volume backscattering strength time series are obtained. Despite the lack of concurrent net samples collection, different migration patterns are identified and their temporal variability examined by means of time-frequency analysis. The effect of changes in the environmental condition is also investigated. Highest zooplankton biomasses are observed in April–May just after the peak of surface primary production in March–April. The main migration pattern points to a "nocturnal" migration with zooplankton organisms occurring deeper in the water column during the day and shallower at night. Also twilight migration is highlighted during this study. The largest migrations are recorded in November–December, corresponding to lowest backscattering strength values and are likely attributable to larger and more active organisms (i.e. euphausiids and mesopelagic fish). The results suggest further applications of the historical ADCP time series available.

Description: A new system for continuous, highly-resolved oceanic and atmospheric measurements of N2O, CO and CO2 is described. The system is based upon off-axis integrated cavity output spectroscopy (OA-ICOS) and a non-dispersive infrared analyzer (NDIR) both coupled to a Weiss-type equilibrator. Performance of the combined setup was evaluated by testing its precision, accuracy, long-term stability, linearity and response time. Furthermore, the setup was tested during two oceanographic campaigns in the equatorial Atlantic Ocean in order to explore its potential for autonomous deployment onboard voluntary observing ships (VOS). Improved equilibrator response times for N2O (2.5 min) and CO (45 min) were achieved in comparison to response times from similar chamber designs used by previous studies. High stability of the OA-ICOS analyzer was demonstrated by low optimal integration times of 2 and 4 min for N2O and CO respectively, as well as detection limits of 〈 40 ppt and precision better than 0.3 ppb Hz−1/2. Results from a direct comparison of the method presented here and well-established discrete methods for oceanic N2O and CO2 measurements showed very good consistency. The favorable agreement between underway atmospheric N2O, CO and CO2 measurements and monthly means at Ascension Island (7.96° S 14.4° W) further suggests a reliable operation of the underway setup in the field. The potential of the system as an improved platform for measurements of trace gases was explored by using continuous N2O and CO2 data to characterize the development of the seasonal equatorial upwelling in the Atlantic Ocean during two R/V Maria S. Merian cruises. A similar record of high-resolution CO measurements was simultaneously obtained offering for the first time the possibility of a comprehensive view on the distribution and emissions of these climate relevant gases on the area. The relatively simple underway N2O/CO/CO2 setup is suitable for long-term deployment on board of research and commercial vessels although potential sources of drift such as cavity temperature and further technical improvements towards automation still need to be addressed.

Description: The launch of the US/French mission Topex/Poseidon (T/P) (CNES/NASA) in August 1992 was the start of a revolution in oceanography. For the first time, a very precise altimeter system optimized for large scale sea level and ocean circulation observations was flying. T/P alone could not observe the mesoscale circulation. In the 1990s, the ESA satellites ERS-1/2 were flying simultaneously with T/P. Together with my CLS colleagues, we demonstrated that we could use T/P as a reference mission for ERS-1/2 and bring the ERS-1/2 data to an accuracy level comparable to T/P. Near real time high resolution global sea level anomaly maps were then derived. These maps have been operationally produced as part of the SSALTO/DUACS system for the last 15 yr. They are now widely used by the oceanographic community and have contributed to a much better understanding and recognition of the role and importance of mesoscale dynamics. Altimetry needs to be complemented with global in situ observations. In the end of the 90s, a major international initiative was launched to develop Argo, the global array of profiling floats. This has been an outstanding success. Argo floats now provide the most important in situ observations to monitor and understand the role of the ocean on the earth climate and for operational oceanography. This is a second revolution in oceanography. The unique capability of satellite altimetry to observe the global ocean in near real time at high resolution and the development of Argo were essential to the development of global operational oceanography, the third revolution in oceanography. The Global Ocean Data Assimilation Experiment (GODAE) was instrumental in the development of the required capabilities. This paper provides an historical perspective on the development of these three revolutions in oceanography which are very much interlinked. This is not an exhaustive review and I will mainly focus on the contributions we made together with many colleagues and friends.