ALBERT

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

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

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

Description: The land-ice contribution to 21st century dynamic sea-level rise Ocean Science Discussions, 11, 123-169, 2014 Author(s): T. Howard, J. Ridley, A. K. Pardaens, R. T. W. L. Hurkmans, A. J. Payne, R. H. Giesen, J. A. Lowe, J. L. Bamber, T. L. Edwards, and J. Oerlemans Climate change has the potential to locally influence mean sea level through a number of processes including (but not limited to) thermal expansion of the oceans and enhanced land ice melt. These lead to departures from the global mean sea level change, due to spatial variations in the change of water density and transport, which are termed dynamic sea level changes. In this study we present regional patterns of sea-level change projected by a global coupled atmosphere–ocean climate model forced by projected ice-melt fluxes from three sources: the Antarctic ice sheet, the Greenland ice sheet and small glaciers and ice caps. The largest ice melt flux we consider is equivalent to almost 0.7 m of global sea level rise over the 21st century. Since the ice melt is not constant, the evolution of the dynamic sea level changes is analysed. We find that the dynamic sea level change associated with the ice melt is small, with the largest changes, occurring in the North Atlantic, contributing of order 3 cm above the global mean rise. Furthermore, the dynamic sea level change associated with the ice melt is similar regardless of whether the simulated ice fluxes are applied to a simulation with fixed or changing atmospheric CO 2 .

Description: Modelling Seasonal Circulation and Thermohaline Structure of the Caspian Sea Ocean Science Discussions, 11, 259-292, 2014 Author(s): M. Gunduz and E. Özsoy The wind and buoyancy driven seasonal circulation of the Caspian Sea is investigated for a better understanding of its basin-wide and meso-scale dynamics, mixing and transport. The model successfully reproduces the following basic elements of the circulation: the southward flowing current systems along the eastern and western coasts, the upwelling along the eastern coast, the cyclonic circulation in the Middle Caspian Sea (MCS) especially in winter, and the cyclonic and anticyclonic cells of circulation in the South Caspian Sea (SCS). The observed seasonal variability of sea level and sea surface temperature (SST) is well reproduced. Mesoscale structures, not always evident from hydrographic observations, are identified.

Description: Variability of water mass properties in the Strait of Sicily in summer period of 1998–2013 Ocean Science Discussions, 11, 811-837, 2014 Author(s): A. Bonanno, F. Placenti, G. Basilone, R. Mifsud, S. Genovese, B. Patti, M. Di Bitetto, S. Aronica, M. Barra, G. Giacalone, R. Ferreri, I. Fontana, G. Buscaino, G. Tranchida, E. M. Quinci, and S. Mazzola The Strait of Sicily plays a crucial role in determining the water mass exchanges and related properties between western and eastern Mediterranean. The presence of sills to the east and west of the Strait of Sicily and the complex seabed topography modulate the thermohaline circulation of the Mediterranean basin. An anti-estuarine circulation is mainly characterized, from a dynamic point of view, by a two-layer system: a surface layer composed of Atlantic Water (AW) flowing eastward, essentially dominated by mesoscale processes, and a subsurface layer composed of Levantine Intermediate Water (LIW) flowing in the opposite direction; the topography appears to play an important role. Furthermore, there are transition water masses with variable hydrological characteristics. The dataset here studied is a time series 16 years long (1998–2013), which highlights the high horizontal and vertical interannual variability affecting the study area. Strong temperature-salinity correlations, in the intermediate layer, for specific time intervals, could be linked to the reversal of sub-surface circulation in the Central Ionian Sea. Moreover, a long-term monitoring of the hydrographic properties of water masses across this strait allow the modelers to assess the performance of hydrological models of this area.

Description: Changes in extreme regional sea surface height due to an abrupt weakening of the Atlantic MOC Ocean Science Discussions, 11, 1213-1241, 2014 Author(s): S.-E. Brunnabend, H. A. Dijkstra, M. A. Kliphuis, B. van Werkhoven, E. Bal, F. Seinstra, J. Maassen, and M. van Meersbergen As an extreme scenario of dynamical sea level changes, regional sea surface height (SSH) changes that occur in the North Atlantic due to an abrupt weakening of the Atlantic Meridional Overturning Circulation (AMOC) are simulated. Two versions of the same ocean-only model are used to study the effect of ocean model resolution on these SSH changes: a high-resolution (HR) strongly eddying version and a low-resolution (LR) version in which the effect of eddies are parameterized. The weakening of the AMOC is induced in both model versions by applying strong freshwater perturbations around Greenland. A rapid decrease of the AMOC in the HR version induces much shorter return times of several specific regional and coastal extremes in North Atlantic SSH than in the LR version. This effect is caused by a change in main eddy pathways associated with a change in separation latitude of the Gulf Stream.

Description: The shallow meridional overturning circulation of the South China Sea Ocean Science Discussions, 11, 1191-1212, 2014 Author(s): N. Zhang, J. Lan, and F. Cui In this paper, the structure and formation mechanism of the annual-mean shallow meridional overturning circulation of the South China Sea (SCS) are investigated. A distinct clockwise overturning circulation is present above 400 m in the SCS on the climatological annual mean scale. The shallow meridional overturning circulation consists of downwelling in the northern SCS, a southward subsurface branch supplying upwelling in the southern SCS and a northward return flow of surface water. The formation mechanism is explored by studying causes of the branches constituting the meridional overturning circulation. The surface branch is driven by the annual mean zonal component of the wind stress which is predominantly westward. Another effect of the wind is Ekman pumping related subduction in the north hence the main source of downwelling there. The mixed layer depth reaches its maximum in winter and shoals in spring, which causes the thermocline to outcrop and ventilate. Part of the water mass from the bottom of the mixed layer subducts into the thermocline and flows southward along the isopycnals. The upwelling region is mainly along the Vietnam coast and in the open-ocean off it. In summer, the alongshore component of wind stress off Vietnam can cause coastal upwelling and the increase of alongshore wind off the coast can also cause great upwelling in the open-ocean off the Vietnam coast.

Description: Quantification of octacalcium phosphate, authigenic apatite and detrital apatite in coastal sediments using differential dissolution and standard addition Ocean Science Discussions, 11, 293-329, 2014 Author(s): J. F. Oxmann and L. Schwendenmann Knowledge of calcium phosphate (Ca-P) solubility is crucial for understanding temporal and spatial variations of phosphorus (P) concentrations in water bodies and sedimentary reservoirs. In-situ relationships between liquid and solid-phase levels cannot be fully explained by dissolved analytes alone and need to be verified by determination of particular sediment P species. Lack of quantification methods for these species limits the knowledge of the P cycle. To address this issue, we (i) optimized a specifically developed conversion-extraction (CONVEX) method for P species quantification using standard additions; and (ii) simultaneously determined solubilities of Ca-P standards by measuring their pH-dependent contents in the sediment matrix. Ca-P minerals including various carbonate fluorapatite (CFAP) specimens from different localities, fluorapatite (FAP), fish bone apatite, synthetic hydroxylapatite (HAP) and octacalcium phosphate (OCP) were characterized by XRD, Raman, FTIR and elemental analysis. Sediment samples were incubated with and without these reference minerals and then sequentially extracted to quantify Ca-P species by their differential dissolution at pH values between 3 and 8. The quantification of solid-phase phosphates at varying pH revealed solubilities in the following order: OCP 〉 HAP 〉 CFAP (4.5% CO 3 ) 〉 CFAP (3.4% CO 3 ) 〉 CFAP (2.2% CO 3 ) 〉 FAP. Thus, CFAP was less soluble in sediment than HAP, and CFAP solubility increased with carbonate content. Unspiked sediment analyses together with standard addition analyses indicated consistent differential dissolution of natural sediment species vs. added reference species and therefore verified the applicability of the CONVEX method in separately determining the most prevalent Ca-P minerals. We found surprisingly high OCP contents in the analyzed coastal sediments which supports the hypothesis of apatite formation by an OCP precursor.

Description: An automated gas exchange tank for determining gas transfer velocities in natural seawater samples Ocean Science Discussions, 11, 693-733, 2014 Author(s): K. Schneider-Zapp, M. E. Salter, and R. C. Upstill-Goddard In order to advance understanding of the role of seawater surfactants in the air–sea exchange of climatically active trace gases via suppression of the gas transfer velocity ( k w ), we constructed a fully automated, closed air-water gas exchange tank and coupled analytical system. The system allows water-side turbulence in the tank to be precisely controlled with an electronically operated baffle. Two coupled gas chromatographs and an integral equilibrator, connected to the tank in a continuous gas-tight system, allow temporal changes in the partial pressures of SF 6 , CH 4 and N 2 O to be measured simultaneously in the tank water and headspace at multiple turbulence settings, during a typical experimental run of 3.25 h. PC software developed by the authors controls all operations and data acquisition, enabling the optimisation of experimental conditions with high reproducibility. The use of three gases allows three independent estimates of k w for each turbulence setting; these values are subsequently normalised to a constant Schmidt number for direct comparison. The normalised k w estimates show close agreement. Repeated experiments with MilliQ water demonstrate a typical measurement accuracy of 4% for k w . Experiments with natural seawater show that the system clearly resolves the effects on k w of spatial and temporal trends in natural surfactant activity. The system is an effective tool with which to probe the relationships between k w , surfactant activity and biogeochemical indices of primary productivity, and should assist in providing valuable new insights into the air–sea gas exchange process.

Description: Assessment of the ECCO2 reanalysis on the representation of Antarctic Bottom Water properties Ocean Science Discussions, 11, 1023-1091, 2014 Author(s): M. Azaneu, R. Kerr, and M. M. Mata We analyzed the ability of the Estimating the Circulation and Climate of the Ocean – Phase II (ECCO2) reanalysis to represent the hydrographic properties and variability of the Antarctic Bottom Water (AABW) in the Southern Ocean. We used a twenty-year observational database to perform comparisons of hydrographic properties and reanalysis data for the same time period (1992–2011). In addition, we evaluated four case studies based on current meter data and the AABW volume transport estimates previously reported in the literature. The main Southern Ocean oceanographic features, as well as the characteristic shape of the regional potential temperature–salinity (θ– S ) diagrams, are adequately represented by the reanalysis. However, the opening of an oceanic polynya in the Weddell Sea Sector, which has been clearly visible since 2005, contributed to an unrealistic representation of the hydrographic properties of the Southern Ocean primarily after 2004. In this sense, our analyses focused on the period that was identified as more reliable (1992–2004). In general, the reanalysis data showed surface waters that were warmer, saltier, and denser than observations, which may have resulted from the absence of Ice Shelf Water and from the overestimation of sea ice concentrations that limit oceanic heat loss during austral winters. Intermediate waters were generally colder, fresher, and denser than observations, whereas deep waters were warmer and less dense. These differences in deep water properties were partially a result of the inability to reproduce the densest AABW variety by reanalysis for most of the analyzed period and also because of the model's relatively coarse vertical resolution. Despite differences in absolute values, the upper AABW limit (γ n ≥ 28.27 kg m −3 ) and AABW occupied area were well represented in the WOCE repeat sections SR2 and SR4 for the studied periods. In section WOCE SR3, however, the estimates from the differences were not as well correlated, and the AABW layer thickness was underrepresented. The case studies showed a good representation of the AABW volume export and current velocity variability in the most important region of dense water export (i.e., the Weddell Sea). The exception is the AABW volume transport near the Kerguelen Plateau, in which the rugged local bathymetry and the relatively coarse model resolution hampered a fair representation of the transport variability by the reanalysis. Despite the consistency in terms of variability, absolute volume transport, and velocity, estimates were underrepresented in all cases. Moreover, the reanalysis was capable of reproducing the general variability pattern and trends of the AABW hydrographic properties reported by previous studies. Therefore, the ECCO2 data from the 1992–2004 period was considered adequate for investigating the circulation of the AABW and variability of the hydrographic properties, whereas data from the latter period (2005–2011) must be given careful attention.

Description: Tidal forcing, energetics, and mixing near the Yermak Plateau Ocean Science Discussions, 11, 2245-2287, 2014 Author(s): I. Fer, M. Müller, and A. K. Peterson The Yermak Plateau, located northwest of Svalbard in Fram Strait, is the final passage for the inflow of warm Atlantic Water into the Arctic Ocean. The region is characterized by the largest barotropic tidal velocities in the Arctic Ocean. Internal waves generated in response to the tidal flow over this topographic feature locally contribute to mixing and remove heat from the Atlantic Water. Here, we investigate the tidal forcing, barotropic-baroclinic energy conversion rates, and dissipation rates in the region using observations of oceanic currents, hydrography and microstructure collected on the southern flanks of the plateau in summer 2007, together with results from a global high resolution ocean circulation and tide model simulation. The energetics (depth-integrated conversion rates, baroclinic energy fluxes and dissipation rates) show large spatial variability over the plateau and are dominated by the K 1 and M 2 constituents. The volume-integrated conversion rate over the region enclosing the topographic feature is approximately 1 GW and accounts for about 50% of the M 2 and approximately all of the K 1 conversion in a larger domain covering the entire Fram Strait extended to the North Pole. Despite the substantial energy conversion into internal tides, a negligible fraction propagates out of the YP region, implying large local dissipation rates. An approximate local conversion – dissipation balance is found over shallows and also in the deep part of the sloping flanks. The baroclinic energy radiated away from the upper slope is dissipated over the deeper isobaths. From observations below the surface mixed layer, we infer upper and lower bounds on the total dissipation rate of about 0.5 and 1 GW, where about 0.4 GW can be attributed to the contribution of hot spots. The domain-integrated dissipation from the model is close to the upper bound of the observed dissipation, and implies that almost entire dissipation in the region can be attributed to the dissipation of baroclinic tidal energy.

Description: A wind-driven nonseasonal barotropic fluctuation of the Canadian Inland Seas Ocean Science Discussions, 11, 2337-2365, 2014 Author(s): C. G. Piecuch and R. M. Ponte A wind-driven, spatially coherent mode of nonseasonal, depth-independent variability in the Canadian Inland Seas (i.e., the collective of Hudson Bay, James Bay, and Foxe Basin) is identified based on Gravity Recovery and Climate Experiment (GRACE) retrievals, a tide-gauge record, and a barotropic model over 2003–2013. This dominant mode of nonseasonal variability is partly related to the North Atlantic Oscillation and is associated with net flows into and out of the Canadian Inland Seas; the anomalous inflows and outflows, which are reflected in mean sea level and bottom pressure changes, are driven by wind stress anomalies over Hudson Strait, possibly related to wind setup, as well as over the northern North Atlantic Ocean, potentially mediated by various wave mechanisms. The mode is also associated with mass redistribution within the Canadian Inland Seas, reflecting linear response to local wind stress variations under the combined influences of rotation, gravity, and variable bottom topography. Results exemplify the usefulness of GRACE for studying regional ocean circulation and climate.

Description: Wind increase above warm Agulhas Current eddies Ocean Science Discussions, 11, 2367-2389, 2014 Author(s): M. Rouault, P. Verley, and B. Backeberg Sea surface temperature estimated from the Advanced Microwave Scanning Radiometer E onboard the Aqua satellite and altimetry derived sea level anomalies are used south of the Agulhas Current to identify warm mesoscale eddies presenting a distinct SST perturbation superior to 1 °C to the surrounding ocean. The analysis of 2500 instantaneous charts of equivalent stability neutral wind speed estimates from the SeaWinds scatterometer onboard the QuikScat satellite collocated with sea surface temperature and sea level anomaly show stronger wind speed above warm eddies than surrounding water at all wind directions in about 800 of the 2500 cases. For those cases where the wind is stronger above warm eddies, we do not find any relationship between the increase in surface wind speed and the sea surface temperature perturbation. Sea surface temperature perturbations that we consider range from 1 to 5.5 °C. Sizes of eddies range from 100 to 250 km diameter. Mean background wind speed is about 11 m s −1 with a mean increase above the eddy of 2 m s −1 . Wind speed increase of 4 to 7 m s −1 above warm eddies is not uncommon.

Description: Technical Note: How long can seawater oxygen samples be stored before titration? Ocean Science Discussions, 11, 2447-2459, 2014 Author(s): M. Lankhorst, G. Chavez, S. H. Nam, and U. Send The concentration of dissolved oxygen in seawater is routinely measured using a standardized titration method that involves analysis shortly after the water sample has been collected. However, none of the existing procedural documents are specific about how soon after collection the titration has to be done. Here, we report on a small number of samples where duplicates were collected and one batch was titrated within days after collection, while the other batch was stored for several weeks before titration. In addition, for a subset of the samples a third batch was taken that was stored like the others but with a particular chemical already added before storage. Comparison between the batches confirms that there is no significant difference between the ones that were stored and the ones that were analyzed sooner, indicating that a month-long storage period is acceptable. The implication of this is that such oxygen samples do not necessarily have to be analyzed while still on the ship; instead, it is possible to transport them ashore for analysis there.

Description: Reconstructing bottom water temperatures from measurements of temperature and thermal diffusivity in marine sediments Ocean Science Discussions, 11, 2391-2422, 2014 Author(s): F. Miesner, A. Lechleiter, and C. Müller Temperature fields in marine sediments are studied for various purposes. Often, the target of research is the steady state heat flow as a (possible) source of energy but there are also studies attempting to reconstruct bottom water temperature variations to understand more about climate history. The bottom water temperature propagates into the sediment to different depths, depending on the amplitude and period of the deviation. The steady state heat flow can only be determined when the bottom water temperature is constant while the bottom water temperature history can only be reconstructed when the deviation has an amplitude large enough or the measurements are taken in great depths. In this work, the aim is to reconstruct recent bottom water temperature history such as the last two years. To this end, measurements to depths of up to 6 m shall be adequate and amplitudes smaller than 1 K should be reconstructable. First, a commonly used forward model is introduced and analyzed: knowing the bottom water temperature deviation in the last years and the thermal properties of the sediments, the forward model gives the sediment temperature field. Next, an inversion operator and two common inversion schemes are introduced. The analysis of the inversion operator and both algorithms is kept short, but sources for further reading are given. The algorithms are then tested for artificial data with different noise levels and for two example data sets, one from the German North Sea and one from the Davis Strait. Both algorithms show good and stable results for artificial data. The achieved results for measured data have low variances and match to the observed oceanographic settings. Lastly, the desired and obtained accuracy are discussed. For artificial data, the presented method yields satisfying results. However, for measured data the interpretation of the results is more difficult as the exact form of the bottom water deviation is not known. Nevertheless, the presented inversion method seems rather promising due to its accuracy and stability for artificial data. Continuing to work on the development of more sophisticated models for the bottom water temperature, we hope to cover more different oceanographic settings in the future.

Description: Seasonal variability of subsurface high salinity water in the northern South China Sea and its relationship with the northwestern Pacific currents Ocean Science Discussions, 11, 2423-2446, 2014 Author(s): A. Wang, Y. Du, W. Zhuang, and Y. Qi The North Pacific Tropical Water (NPTW), characterized by the subsurface high salinity (〉 34.68 PSU), is observed in the South China Sea (SCS) and often used as an indicator of the water intrusion from the northwestern Pacific into the SCS. Based on the assimilation product from a global high-resolution Hybrid Coordinate Ocean Model (HYCOM), this study investigates the seasonal variability of subsurface high salinity water (SHSW) in the northern SCS and the influence from the northwestern Pacific. Results show that there exists obvious seasonal variability in the SHSW at about 100–200 m depth. It extends as far west as 111° E in the northern SCS, reaching its volume maximum (minimum) in January (May). Further analysis shows that the seasonal change of the high salinity water is strongly affected by the seasonal variability of large-scale circulations in the low-latitude northwestern Pacific. The changes of high salinity water volume are highly correlated with the shift of the North Equatorial Current (NEC) bifurcation latitude (NECBL), which reaches the northernmost in December and the southernmost in May. Due to the large-scale wind changes in the Pacific, the Luzon Strait transport weakens (strengthens) when the NECBL shifts to the south (north) during summer (winter), which results in the reduced (enhanced) SHSW intrusion from the northwestern Pacific into the northern SCS. The velocity and salinity distribution in the Luzon Strait show that the intrusion of the SHSW mainly occurs at around 20–21.3° N.

Description: Circulation, eddies, oxygen and nutrient changes in the eastern tropical South Pacific Ocean Ocean Science Discussions, 11, 2205-2243, 2014 Author(s): R. Czeschel, L. Stramma, R. A. Weller, and T. Fischer A large, subsurface oxygen deficiency zone is located in the eastern tropical South Pacific Ocean (ETSP). The large-scale circulation in the eastern equatorial Pacific and off Peru in November/December 2012 shows the influence of the equatorial current system, the eastern boundary currents, and the northern reaches of the subtropical gyre. In November 2012 the Equatorial Undercurrent is centered at 250 m depth, deeper than in earlier observations. In December 2012 the equatorial water is transported southeastward near the shelf in the Peru-Chile Undercurrent with a mean transport of 1.6 Sv. In the oxygen minimum zone (OMZ) the flow is overlaid with strong eddy activity on the poleward side of the OMZ. Floats with parking depth at 400 m show fast westward flow in the mid-depth equatorial channel and sluggish flow in the OMZ. Floats with oxygen sensors clearly show the passage of eddies with oxygen anomalies. The long-term float observations in the upper ocean lead to a net community production estimate at about 18° S of up to 16.7 mmol C m −3 yr 1 extrapolated to an annual rate and 7.7 mmol C m −3 yr −1 for the time period below the mixed layer. Oxygen differences between repeated ship sections are influenced by the Interdecadal Pacific Oscillation, by the phase of El Niño, by seasonal changes, and by eddies and hence have to be interpreted with care. At and south of the equator the decrease in oxygen in the upper ocean since 1976 is related to an increase in nitrate, phosphate, and in part in silicate.

Description: Deep drivers of mesoscale circulation in the central Rockall Trough Ocean Science Discussions, 11, 2607-2646, 2014 Author(s): T. J. Sherwin, D. Alyenik, E. Dumont, and M. Inall Mesoscale variability in the central Rockall Trough between about 56 and 58° N has been investigated using a combination of ship-borne, underwater glider and gridded satellite altimeter measurements. Altimeter observations show that mesoscale features such as eddies and large scale circulation cells are ubiquitous phenomena. They have horizontal length scales of order 100 km with vertical scales of over 1000 m and are associated with mean current speeds (over the upper 1000 m) of 15 ± 7 cm s −1 . Monthly area averaged surface Eddy Kinetic Energy (EKE) has substantial inter-annual variability, which at times can dominate a mean seasonal signal that varies from a maximum in May (74 cm 2 s −2 ) to a minimum in October (52 cm 2 s −2 ) and has increased gradually since 1992 at about 1.1 cm 2 s −2 per year. A five month glider mission in the Trough showed that much of this energy comes from features that are located over 1000 m below the surface in the deep cold waters of the Trough (possibly from eddies associated the North Atlantic Current). The surface currents from altimeters had similar magnitude to the drift currents averaged over 1000 m from the glider in the stratified autumn, but were half the deep water speed during late winter. Although the mesoscale features move in an apparent random manner they may also be quasi-trapped by submarine topography such as seamounts. Occasionally anti-cyclonic and cyclonic cells combine to cause a coherent westward deflection of the European slope current that warms the Rockall side of the Trough. Such deflections contribute to the inter-annual variability in the observed temperature and salinity that are monitored in the upper 800 m of the Trough. By combining glider and altimeter measurements it is shown that altimeter measurements fail to observe a 15 cm s −1 northward flowing slope current on the eastern side and a small persistent southward current on the western side. There is much to be gained from the synergy between satellite altimetry and in situ glider observations both in the interpretation of their separate data sets and in aiding glider pilots to steer their vehicles through EKE active regions such as the north-east Atlantic.

Description: Assessment of an ensemble system that assimilates Jason-1/Envisat altimeter data in a probabilistic model of the North Atlantic ocean circulation Ocean Science Discussions, 11, 2647-2690, 2014 Author(s): G. Candille, J. M. Brankart, and P. Brasseur A realistic circulation model of the North Atlantic ocean at 1/4° resolution (NATL025 NEMO configuration) has been adapted to explicitly simulate model uncertainties. This is achieved by introducing stochastic perturbations in the equation of state to represent the effect of unresolved scales on the model dynamics. The main motivation for this work is to develop ensemble data assimilation methods, assimilating altimetric data from past missions JASON-1 and ENVISAT. The assimilation experiment is designed to better control the Gulf Stream circulation for years 2005/06, focusing on frontal regions which are predominantly affected by unresolved dynamical scales. An ensemble based on such stochastic perturbations is first produced and evaluated using along-track altimetry observations. The Incremental Analysis Update (IAU) scheme is applied in order to obtain an ensemble of continuous trajectories all over the 2005/06 assimilation period. These three elements – stochastic parameterization, ensemble simulation and 4-D observation operator – are then used together to perform a 4-D analysis of along-track altimetry over 10 day windows. Finally, the results of this experiment are objectively evaluated using the standard probabilistic approach developed for meteorological applications (Toth et al., 2003; Candille et al., 2007). The results show that the free ensemble – before starting the assimilation process – correctly reproduces the statistical variability over the Gulf Stream area: the system is then pretty reliable but not informative (null probabilistic resolution). Updating the free ensemble with altimetric data leads to a better reliability with an information gain around 30% (for 10 day forecasts of the SSH variable). Diagnoses on fully independent data (i.e. data that are not assimilated, like temperature and salinity profiles) provide more contrasted results when the free and updated ensembles are compared.

Description: Using empirical orthogonal functions derived from remote sensing reflectance for the prediction of concentrations of phytoplankton pigments Ocean Science Discussions, 11, 2073-2117, 2014 Author(s): A. Bracher, M. H. Taylor, B. Taylor, T. Dinter, R. Röttgers, and F. Steinmetz The composition and abundance of algal pigments provide information on characteristics of a phytoplankton community in respect to its photoacclimation, overall biomass, and taxonomic composition. Particularly, these pigments play a major role in photoprotection and in the light-driven part of photosynthesis. Most phytoplankton pigments can be measured by High Performance Liquid Chromatography (HPLC) techniques to filtered water samples. This method, like others when water samples have to be analysed in the laboratory, is time consuming and therefore only a limited number of data points can be obtained. In order to receive information on phytoplankton pigment composition with a higher temporal and spatial resolution, we have developed a method to assess pigment concentrations from continuous optical measurements. The method applies an Empirical Orthogonal Function (EOF) analysis to remote sensing reflectance data derived from ship-based hyper-spectral underwater radiometric and from multispectral satellite data (using the MERIS Polymer product developed by Steinmetz et al., 2011) measured in the Eastern Tropical Atlantic. Subsequently we developed statistically linear models with measured (collocated) pigment concentrations as the response variable and EOF loadings as predictor variables. The model results, show that surface concentrations of a suite of pigments and pigment groups can be well predicted from the ship-based reflectance measurements, even when only a multi-spectral resolution is chosen (i.e. eight bands similar to those used by MERIS). Based on the MERIS reflectance data, concentrations of total and monovinyl chlorophyll a and the groups of photoprotective and photosynthetic carotenoids can be predicted with high quality. The fitted statistical model constructed on the satellite reflectance data as input was applied to one month of MERIS Polymer data to predict the concentration of those pigment groups for the whole Eastern Tropical Atlantic area. Bootstrapping explorations of cross-validation error indicate that the method can produce reliable predictions with relatively small data sets (e.g., 〈 50 collocated values of reflectance and pigment concentration). The method allows for the derivation of time series from continuous reflectance data of various pigment groups at various regions, which can be used to study variability and change of phytoplankton composition and photo-physiology.

Description: Global representation of tropical cyclone-induced ocean thermal changes using Argo data – Part 1: Methods and results Ocean Science Discussions, 11, 2831-2878, 2014 Author(s): L. Cheng, J. Zhu, and R. L. Sriver Argo floats are used to examine tropical cyclone (TC)-induced ocean thermal changes on the global scale by comparing temperature profiles before and after TC passage. We present a footprint method that analyzes cross-track thermal responses along all storm tracks during the period 2004–2012. We combine the results into composite representations of the vertical structure of the average thermal response for two different categories: tropical storms/depressions (TS/TD) and hurricanes. The two footprint composites are functions of three variables: cross-track distance, water depth and time relative to TC passage. We find that this footprint strategy captures the major features of the upper-ocean thermal response to TCs on time scales up to 20 days when compared against previous case study results using in situ measurements. Further, TC effects are distinguishable from background sampling variability, but the significance of this result depends on differences in regional oceanic conditions and the intensity of the TC events. On the global scale, results indicate that hurricanes induce strong upwelling near the storm center, along with downwelling away from the storm, during the first 3 days after storm passage. We also find significant subsurface warming between 30 and 200 m depth for both hurricanes and TS/TDs. On average, the subsurface ocean response persists along storm tracks for up to 20 days down to 200 (400) m depth for TS/TD (Hurricanes), exhibiting peak warming of 0.4 °C at 60 m for hurricanes and 0.2 °C at 35 m for TS/TD. The footprint method shows a weak cooling response between 200 and 400 m, which is significant for Hurricanes but not for TS/TD.

Description: A robust method for removal of glint effects from satellite ocean colour imagery Ocean Science Discussions, 11, 2791-2829, 2014 Author(s): R. K. Singh and P. Shanmugam Removal of the glint effects from satellite imagery for accurate retrieval of water-leaving radiances is a complicated problem since its contribution in the measured signal is dependent on many factors such as viewing geometry, sun elevation and azimuth, illumination conditions, wind speed and direction, and the water refractive index. To simplify the situation, existing glint correction models describe the extent of the glint-contaminated region and its contribution to the radiance essentially as a function of the wind speed and sea surface slope that often lead to a tremendous loss of information with a considerable scientific and financial impact. Even with the glint-tilting capability of modern sensors, glint contamination is severe on the satellite-derived ocean colour products in the equatorial and sub-tropical regions. To rescue a significant portion of data presently discarded as "glint contaminated" and improving the accuracy of water-leaving radiances in the glint contaminated regions, we developed a glint correction algorithm which is dependent only on the satellite derived Rayleigh Corrected Radiance and absorption by clear waters. The new algorithm is capable of achieving meaningful retrievals of ocean radiances from the glint-contaminated pixels unless saturated by strong glint in any of the wavebands. It takes into consideration the combination of the background absorption of radiance by water and the spectral glint function, to accurately minimize the glint contamination effects and produce robust ocean colour products. The new algorithm is implemented along with an aerosol correction method and its performance is demonstrated for many MODIS-Aqua images over the Arabian Sea, one of the regions that are heavily affected by sunglint due to their geographical location. The results with and without sunglint correction are compared indicating major improvements in the derived products with sunglint correction. When compared to the results of an existing model in the SeaDAS processing system, the new algorithm has the best performance in terms of yielding physically realistic water-leaving radiance spectra and improving the accuracy of the ocean colour products. Validation of MODIS-Aqua derived water-leaving radiances with in-situ data also corroborates the above results. Unlike the standard models, the new algorithm performs well in variable illumination and wind conditions and does not require any auxiliary data besides the Rayleigh-corrected radiance itself. Exploitation of signals observed by sensors looking within regions affected by bright white sunglint is possible with the present algorithm when the requirement of a stable response over a wide dynamical range for these sensors is fulfilled.

Description: Mean circulation in the coastal ocean off northeastern North America from a regional-scale ocean model Ocean Science Discussions, 11, 2755-2790, 2014 Author(s): K. Chen and R. He A regional-scale ocean model was used to hindcast the coastal circulation over the Middle Atlantic Bight (MAB) and Gulf of Maine (GOM) from 2004 to 2013. The model was nested inside a data assimilative global ocean model that provided initial and open boundary conditions. Realistic atmospheric forcing, tides and observed river runoff were also used to drive the model. Hindcast solutions were compared against observations, which included coastal sea levels, satellite altimetry sea surface height, temperature and salinity time series in the GOM, glider transects in the MAB, and observed mean depth-averaged velocities by Lentz (2008a). Good agreements with observations suggest that the hindcast model is capable of capturing the major circulation variability in the MAB and GOM. Time- and space-continuous hindcast fields were used to depict the mean circulation, along- and cross-shelf transport and the associated momentum balances. The hindcast confirms the presence of the equatorward mean shelf circulation, which varies from 2.33 Sv at Scotian Shelf to 0.22 Sv near Cape Hatteras. Using the 200 m isobath as the shelf/slope boundary, the mean cross-shelf transport calculations indicate that the shelfbreak segments off the Gulf of Maine (including the southern flank of Georges Bank and the Northeast Channel) and Cape Hatteras are the major sites for shelf water export. The momentum analysis reveals that the along-shelf sea level difference from Nova Scotia to Cape Hatteras is about 0.36 m. The nonlinear advection, stress, and horizontal viscosity terms all contribute to the ageostrophic circulation in the along-isobath direction, whereas the nonlinear advection plays a dominant role in determining the ageostrophic current in the cross-isobath direction.

Description: A Monte Carlo simulation of multivariate general Pareto distribution and its application Ocean Science Discussions, 11, 2733-2753, 2014 Author(s): L. Yao, W. Dongxiao, Z. Zhenwei, H. Weihong, and S. Hui This paper presents a multivariate general Pareto distribution (MGPD) method and builds a method for solving MGPD through the use of a Monte Carlo simulation for marine environmental extreme-value parameters. The simulation method has proven to be feasible in the analysis of the joint probability of wave height and its concomitant wind from a hydrological station in the South China Sea (SCS). The MGPD is the natural distribution of the multivariate peaks-over-threshold (MPOT) sampling method, and is based on the extreme-value theory. The existing dependence functions can be used in the MGPD, so it may describe more variables which have different dependence relationships. The MGPD method improves the efficiency of the extremes in raw data. For the wave and the concomitant wind from a period of 23 years (1960–1982), the number of the wave and wind selected is averaged to 19 per year. For the joint conditional probability of the MGPD, the relative error is rather small in the Monte Carlo simulation method.

Description: Eddy characteristics in the South Indian Ocean as inferred from surface drifter Ocean Science Discussions, 11, 2879-2905, 2014 Author(s): Shaojun Zheng, Yan Du, Jiaxun Li, and Xuhua Cheng Using a geometric eddy identification method, cyclonic and anticyclonic eddies from submesoscale to mesoscale in the South Indian Ocean (SIO) have been statistically investigated based on 2082 surface drifters from 1979 to 2013. 19252 eddies are identified with 60% anticyclonic eddies. For the submesoscale eddies (radius r 〈 10 km), the ratio of cyclonic eddies (3183) to anticyclonic eddies (7182) is 1 to 2. In contrast, number of anticyclonic and cyclonic eddies with radius r ≥ 10 km is almost equal. Mesoscale and submesoscale eddies show different spatial distribution. Eddies with radius r ≥ 100 km mainly appear in a band along 25° S, in Mozambique Channel, and Agulhas Current, characterized by large eddy kinetic energy. The submesoscale anticyclonic eddies are densely distributed in the subtropical basin in the central SIO. The number of mesoscale eddies shows statistically significant seasonal variability, reaching a maximum in October and then minimum in February.

Description: Modelling origin and transport fate of waste materials on the south-eastern Adriatic coast (Croatia) Ocean Science Discussions, 11, 2939-2969, 2014 Author(s): M. Tudor and I. Janeković The south-eastern parts of the Adriatic Sea coastline were severely polluted by large amounts of accumulated waste material in the second half of November 2010. The waste, reported by major news agencies, accumulated dominantly during 21 November 2010 by favourable wind – ocean current transport system. In the study we analysed meteorological and oceanographic conditions that lead to the waste deposition using available in situ measurements, remote sensing data as well numerical models of the ocean and the atmosphere. The measured data reveal that an intensive rainfall event from 7 till 10 November 2010, over the parts of Montenegro and Albania, was followed by a substantial increase of the river water levels indicating flash floods that possibly splashed the waste material into a river and after to the Adriatic Sea. In order to test our hypothesis we set a number of numerical drifter experiments with trajectories initiated off the coast of Albania during the intensive rainfall events following their faith in space and time. One of the numerical drifter trajectory experiment resulted with drifters reached right position (south-eastern Adriatic coast) and time (exactly by the time the waste was observed) when initiated on 00:00 and 12:00 UTC of 10 November 2010 during the mentioned flash flood event.

Description: In situ autonomous optical radiometry measurements for satellite ocean color validation in the Western Black Sea Ocean Science Discussions, 11, 3003-3034, 2014 Author(s): G. Zibordi, F. Mélin, J.-F. Berthon, and M. Talone The accuracy of primary satellite ocean color data products from the Moderate Resolution Imaging Spectroradiometer on-board Aqua (MODIS-A) and the Visible/Infrared Imager/Radiometer Suite (VIIRS), is investigated in the Western Black Sea using in situ measurements from the Gloria site included in the Ocean Color component of the Aerosol Robotic Network (AERONET-OC). The analysis is also extended to an additional well-established AERONET-OC site in the northern Adriatic Sea characterized by optically complex coastal waters exhibiting similarities with those observed at the Gloria site. Results from the comparison of normalized-water leaving radiance L WN indicate biases of a few percent between satellite derived and in situ data at the center-wavelengths relevant for the determination of chlorophyll a concentration (443–547 nm, or equivalent). Remarkable is the consistency among the annual cycle determined with time series of satellite-derived and in situ L WN ratios at these center-wavelengths. Contrarily, the differences between in situ and satellite-derived L WN are pronounced at the blue (i.e., 412 nm) and red (i.e., 667 nm, or equivalent) center-wavelengths, suggesting difficulties in confidently applying satellite-derived radiometric data from these spectral regions for quantitative analysis in optically complex waters.

Description: Flow dynamics around downwelling submarine canyons Ocean Science Discussions, 11, 1301-1356, 2014 Author(s): J. M. Spurgin and S. E. Allen Flow dynamics around a downwelling submarine canyon were analysed with the Massachusetts Institute of Technology general circulation model. Blanes Canyon (Northwest Mediterranean) was used for topographic and initial forcing conditions. Fourteen scenarios were modelled with varying forcing conditions. Rossby number and Burger number were used to determine the significance of Coriolis acceleration and stratification (respectively) and their impacts on flow dynamics. A new non-dimensional parameter (χ) was introduced to determine the significance of vertical variations in stratification. Some simulations do see brief periods of upwards displacement of water during the 10 day model period, however, the presence of the submarine canyon is found to enhance downwards advection of density in all model scenarios. High Burger numbers lead to negative vorticity and a trapped anticyclonic eddy within the canyon, as well as an increased density anomaly. Low Burger numbers lead to positive vorticity, cyclonic circulation and weaker density anomalies. Vertical variations in stratification affect zonal jet placement. Under the same forcing conditions, the zonal jet is pushed offshore in more uniformly stratified domains. Offshore jet location generates upwards density advection away from the canyon, while onshore jets generate downwards density advection everywhere within the model domain. Increasing Rossby values across the canyon axis, as well as decreasing Burger values, increase negative vertical flux at shelf break depth (150 m). Increasing Rossby numbers lead to stronger downwards advection of a passive tracer (nitrate) as well as stronger vorticity within the canyon. Results from previous studies were explained within this new dynamic framework.

Description: Observing and modeling currents on the continental slope: assimilation of high frequency radar currents and hydrography profiles Ocean Science Discussions, 11, 1357-1390, 2014 Author(s): A. K. Sperrevik, K. H. Christensen, and J. Röhrs Assimilation of High Frequency (HF) radar current observations and CTD hydrography is performed with the 4D-Var analysis scheme implemented in the Regional Ocean Modeling System (ROMS). We consider both an idealized case, with a baroclinic slope current in a periodic channel, and a realistic case for the coast of Vesterålen in Northern Norway. In the realistic case the results of the data assimilation are compared with independent data from acoustic profilers and surface drifters. Best results are obtained when background error correlation scales are small (10 km or less) and when the data assimilation window is short, i.e. about one day. Furthermore, we find that the impact of assimilating HF radar currents is generally larger than the impact of CTD hydrography, which implies that the amount of hydrographic data is insufficient to constrain the solution. Combining the HF radar currents with a few hydrographic profiles gives significantly better results, which demonstrates the importance of complementing surface observations with observations of the vertical structure of the ocean.

Description: A geographical and seasonal comparison of nitrogen uptake by phytoplankton in the Southern Ocean Ocean Science Discussions, 11, 1829-1869, 2014 Author(s): R. Philibert, H. Waldron, and D. Clark Primary production in the Southern Ocean has been shown to be regulated by light and nutrients (such as silicate and iron) availability. However, the impact of these factors vary seasonally and differ from region to region. The seasonal cycle of primary production in this region is not fully resolved over an annual scale due to the lack of winter in situ measurements. In this study, nitrate and ammonium uptake rates were measured using 15 N tracers during a winter cruise in July 2012 and a summer cruise in February/March 2013. In winter, nitrogen uptake rates were measured at 55% and 1% of the surface photosynthetically active radiation (sPAR). The summer uptake rates were measured at 4 light depths corresponding to 55, 30, 10 and 3% sPAR. The integrated nitrate uptake rates during the winter cruise ranged from 0.16–5.20 (average 1.14) mmol N m −2 d −1 while the ammonium uptake rates ranged from 0.6–32.8 (average 6.72) mmol N m −2 d −1 . During the summer cruise, the mean integrated nitrate uptake rate was 0.34 mmol N m −2 d −1 with a range between 0.16–0.65 mmol N m −2 d −1 . The integrated ammonium uptake rate averaged 5.61 mmol N m −2 d −1 and ranged from 1.44–11.28 mmol N m −2 d −1 . The factors controlling primary production in winter and summer were investigated. During the winter cruise, it was found the different nitrogen uptake regimes were not separated by fronts. Light (in terms of day length) and ammonium concentration had the most influence on the nitrogen uptake regime. In the summer, increases in the mixed layer depth (MLD) resulted in increased nitrogen uptake rates. This suggests that the increases in the MLD could be alleviating nutrient limitations experienced by the phytoplankton at the end of summer.

Description: Thermohaline properties in the Eastern Mediterranean in the last three decades: is the basin returning to the pre-EMT situation? Ocean Science Discussions, 11, 391-423, 2014 Author(s): V. Cardin, G. Civitarese, D. Hainbucher, M. Bensi, and A. Rubino We present temperature, salinity and oxygen data collected during the M84/3 and P414 cruises in April and June 2011 on a basin-wide scale to determine the ongoing oceanographic characteristics in the Eastern Mediterranean (EM). The east–west transect through the EM sampled during the M84/3 cruise together with data gained on previous cruises over the period 1987–2011 are analysed in terms of regional aspects of the evolution of water mass properties and heat and salt content variation. The present state of the EM basin is also evaluated in the context of the evolution of the Eastern Mediterranean Transient (EMT). From this analysis we can infer that the state of the basin is still far from achieving the pre-EMT conditions. Indeed, the 2011 oceanographic conditions of the deep layer of the central Ionian lie between the thermohaline characteristics of the EMT and the pre-EMT phase, indicating a possible slow return towards the latter. In addition, the thermohaline properties of the Adriatic Deep Water are still in line (warmer and saltier) as when it restarted to produce dense waters after the EMT. Special attention is given to the variability of thermohaline properties of the Levantine Intermediate Water and Adriatic Deep Water in three main areas: the Cretan, the central Levantine and the central Ionian Seas. Finally, this study evidences the relationships among the hydrological property distributions of the upper-layer in the Levantine basin and the circulation regime in the Ionian.

Description: Extreme winter 2012 in the Adriatic: an example of climatic effect on the BiOS rhythm Ocean Science Discussions, 11, 425-452, 2014 Author(s): M. Gačić, G. Civitarese, V. Kovačević, L. Ursella, M. Bensi, M. Menna, V. Cardin, P.-M. Poulain, S. Cosoli, G. Notarstefano, and C. Pizzi Adriatic and Ionian Seas are Mediterranean sub-basins linked through the Bimodal Oscillating System mechanism responsible for decadal reversals of the Ionian basin-wide circulation. Altimetric maps showed that the last cyclonic mode started in 2011 but unexpectedly in 2012 reversed to anticyclone. We related this "premature" inversion to extremely strong winter in 2012, which caused the formation of very dense Adriatic waters, flooding Ionian flanks in May and inverting the bottom pressure gradient. Using Lagrangian float measurements, the linear regression between the sea surface height and three isopycnal depths suggests that the southward deep-layer flow coincided with the surface northward geostrophic current and the anti-cyclonic circulation regime. Density variations at depth in the north-western Ionian revealed the arrival of Adriatic dense waters in May and maximum density in September. Comparison between the sea level height in the north-western Ionian and in the basin centre showed that in coincidence with the arrival of the newly formed Adriatic dense waters the sea level lowered in the north-western flank inverting the surface pressure gradient. Toward the end of 2012, the density gradient between the basin flanks and its centre went to zero, coinciding with the weakening of the anticyclonic circulation and eventually with its return to the cyclonic pattern. Thus, the premature and transient reversal of Ionian surface circulation originated from the extremely harsh winter in the Adriatic, resulting in the formation and spreading of highly dense bottom waters. The present study highlights the remarkable sensitiveness of the Adriatic–Ionian BiOS to climatic forcing.

Description: Qualified temperature, salinity and dissolved oxygen climatologies in a changing Adriatic Sea Ocean Science Discussions, 11, 331-390, 2014 Author(s): M. Lipizer, E. Partescano, A. Rabitti, A. Giorgetti, and A. Crise An updated climatology, based on a comprehensive dataset (1911–2009) of temperature, salinity and dissolved oxygen, has been produced for the whole Adriatic Sea with the Variational Inverse Method using the DIVA software. Climatological maps were produced at 26 levels and validated with Ordinary Cross Validation and with real vs. synthetic Temperature–Salinity diagram intercomparison. The concept of Climatology–Observation Misfit (COM) has been introduced as an estimate of the physical variability associated with the climatological structures. In order to verify the temporal stability of the climatology, long-term variability has been investigated in the Mid Adriatic and the South Adriatic Pits, regarded as the most suitable records of possible long-term changes. Compared with previous climatologies, this study reveals a surface temperature rise (up to 2 °C), a clear deep dissolved oxygen minimum in the South Adriatic Gyre and a bottom summer oxygen minimum in the North Adriatic. Below 100 m all properties profoundly differ between the Middle and the South Adriatic. The South Adriatic Pit clearly shows the remote effects of the Eastern Mediterranean Transient, while no effect is observed in Middle Adriatic Pits. The deepest part of the South Adriatic seems now to be significantly saltier (+0.18 since the period 1911–1914, with an increase of +0.018 decade −1 since the late 1940s) and warmer (+0.54 °C since 1911–1914), even though a long-term temperature trend could not be statistically demonstrated. Conversely, the Middle Adriatic Pits present a long-term increase in apparent oxygen utilisation (+0.77 mL L −1 since 1911–1914, with a constant increase of +0.2 mL L −1 decade −1 after the 1970s).

Description: Computation of a new Mean Dynamic Topography for the Mediterranean Sea from model outputs, altimeter measurements and oceanographic in-situ data Ocean Science Discussions, 11, 655-692, 2014 Author(s): M.-H. Rio, A. Pascual, P.-M. Poulain, M. Menna, B. Barceló, and J. Tintoré The accurate knowledge of the ocean Mean Dynamic Topography (MDT) is a crucial issue for a number of oceanographic applications and in some areas of the Mediterranean Sea, important limitations have been found pointing to the need of an upgrade. We present a new Mean Dynamic Topography (MDT) that was computed for the Mediterranean Sea. It takes profit of improvements made possible by the use of extended datasets and refined processing. The updated dataset spans the 1993–2012 period and consists of: drifter velocities, altimetry data, hydrological profiles and model data. The methodology is similar to the previous MDT Rio et al. (2007). However, in Rio et al. (2007) no hydrological profiles had been taken into account. This has required the development of dedicated processing. A number of sensitivity studies have been carried out to obtain the most accurate MDT as possible. The main results from these sensitivity studies are the following: moderate impact to the choice of correlation scales but almost negligible sensitivity to the choice of the first guess (model solution). A systematic external validation to independent data has been made to evaluate the performance of the new MDT. Compared to previous version, SMDT-MED-2014 features shorter scales structures, which results in an altimeter velocity variance closer to the observed velocity variance and, at the same time, gives better Taylor skills.

Description: Coastal sea level response to the tropical cyclonic forcing in the north Indian Ocean Ocean Science Discussions, 11, 575-611, 2014 Author(s): P. Mehra, S. Mohan, P. Vethamony, K. Vijaykumar, T. M. Balakrishnan Nair, Y. Agarvadekar, K. Jyoti, K. Sudheesh, R. Luis, S. Lobo, and B. Harmalkar The study examines the observed storm-generated sea-level variation due to deep depression (Event-E1) in the Arabian Sea from 26 November–1 December 2011 and a cyclonic storm "THANE" (Event-E2) over the Bay of Bengal during 25–31 December 2011. The sea-level and surface meteorological measurements collected during these extreme events exhibit strong synoptic disturbances leading to storm surge up to 43 cm on the west coast and 29 cm on the east coast of India due to E1 and E2. E1 generated sea level oscillations at the measuring stations on the west coast (Ratnagiri, Verem and Karwar) and east coast (Mandapam and Tuticorin) of India with significant energy bands centered at periods of 92, 43 and 23 min. The surge dome has a duration of 92.6, 84.5 and 74.8 h at Ratnagiri, Verem and Karwar, respectively. However, on the east coast, the sea level oscillations during Thane were similar to those during calm period except for more energy bands centred at periods of ~ 100, 42 and 24 min at Gopalpur, Gangavarm and Kakinada, respectively. Multi-linear regression analysis shows that the local surface meteorological data (daily-mean wind and atmospheric pressure) is able to account for ~ 57% and ~ 70% of daily-mean sea-level variability along the east and west coast of India. The remaining part of variability observed in the sea level may be attributed to local coastal currents and remote forcing.

Description: Heat loss from the Atlantic water layer in the St. Anna Trough (northern Kara Sea): causes and consequences Ocean Science Discussions, 11, 543-573, 2014 Author(s): I. A. Dmitrenko, S. A. Kirillov, N. Serra, N. V. Koldunov, V. V. Ivanov, U. Schauer, I. V. Polyakov, D. Barber, M. Janout, V. S. Lien, M. Makhotin, and Y. Aksenov A distinct, subsurface density front along the eastern St. Anna Trough in the northern Kara Sea is inferred from hydrographic observations in 1996 and 2008–2010. Direct velocity measurements show a persistent northward subsurface current (~ 20 cm s −1 ) along the St. Anna Trough eastern flank. This sheared flow, carrying the outflow from the Barents and Kara Seas to the Arctic Ocean, is also evident from shipboard observations as well as from geostrophic velocities and numerical model simulations. Although no clear evidence for the occurrence of shear instabilities could be obtained, we speculate that the enhanced vertical mixing along the St. Anna Trough eastern flank promoted by a vertical velocity shear favors the upward heat loss from the intermediate warm Atlantic water layer. The associated upward heat flux is inferred to 50–100 W m −2 using hydrographic data and model simulations. The zone of lowered sea ice thickness and concentration essentially marks the Atlantic water pathway in the St. Anna Trough and adjacent Nansen Basin continental margin from both sea-ice remote sensing observations and model simulations. In fact, the seaice shows a consistently delayed freeze-up onset during fall and a reduction in the seaice thickness during winter. This is consistent with our results on the enhanced Atlantic water heat loss along the Atlantic water pathway in the St. Anna Trough. 1 1 Dedicated to the memory of our colleague Klaus Hochheim who tragically lost his life in the Arctic expedition in September 2013

Description: Is coccolithophore distribution in the Mediterranean Sea related to seawater carbonate chemistry? Ocean Science Discussions, 11, 613-653, 2014 Author(s): A. M. Oviedo, P. Ziveri, M. Álvarez, and T. Tanhua The Mediterranean Sea is considered a "hot-spot" for climate change, being characterized by oligotrophic to ultra-oligotrophic waters and rapidly changing carbonate chemistry. Coccolithophores are considered a dominant phytoplankton group in these waters. As a marine calcifying organism they are expected to respond to the ongoing changes in seawater CO 2 systems parameters. However, very few studies have covered the entire Mediterranean physiochemical gradients from the Strait of Gibraltar to the Eastern Mediterranean Levantine Basin. We provide here an updated state of knowledge of the coccolithophore distribution in the Mediterranean Sea and relate this to a broad set of in situ measured environmental variables. Samples were taken during the Meteor (M84/3) oceanographic cruise in April 2011, between 0–100 m water depth from 28 stations. Total diatom, dinoflagellate and silicoflagellate cell concentrations are also presented. Our results highlight the importance of seawater carbonate chemistry, especially CO 3 2− , in unraveling the distribution of heterococcolithophores, the most abundant coccolithophore life phase. Holo- and hetero-coccolithophores respond differently to environmental factors. For instance, changes in heterococcolithophore assemblages were best linked to the combination of [CO 3 2− ], pH, and salinity (ρ = 0.57) although salinity might be not functionally related to coccolithophore assemblage distribution. Holococcolithophores, on the other hand, were preferentially distributed and showed higher species diversity in oligotrophic areas (Best fit, ρ = 0.32 for nutrients), thriving in nutrient depleted waters. Clustering of heterococcolithophores revealed three groups of species sharing more than 65% similarities. These clusters could be assigned to the eastern and western basins, and deeper layers (below 50 m), respectively. In addition, the species Gephyrocapsa oceanica, G. muellerae and Emiliania huxleyi morphotype B/C are spatially distributed together and trace the influx of Atlantic waters into the Mediterranean Sea. The results of the present work emphasize the importance of considering holo- and hetero-coccolithophores separately when analyzing changes in species assemblages and diversity. Our findings clearly show that coccolithophores are a dominant phytoplankton group in the entire Mediterranean Sea; they have life stages that are expected to respond differently to the variability in seawater carbonate chemistry and nutrient concentrations.

Description: On the glacial and inter-glacial thermohaline circulation and the associated transports of heat and freshwater Ocean Science Discussions, 11, 979-1022, 2014 Author(s): M. Ballarotta, S. Falahat, L. Brodeau, and K. Döös The change of the thermohaline circulation (THC) between the Last Glacial Maximum (LGM, ≈ 21 kyr ago) and the present day climate are explored using an Ocean General Circulation Model and stream functions projected in various coordinates. Compared to the present day period, the LGM circulation is reorganised in the Atlantic Ocean, in the Southern Ocean and particularly in the abyssal ocean, mainly due to the different haline stratification. Due to stronger wind stress, the LGM tropical circulation is more vigorous than under modern conditions. Consequently, the maximum tropical transport of heat is slightly larger during the LGM. In the North Atlantic basin, the large sea-ice extent during the LGM constrains the Gulf Stream to propagate in a more zonal direction, reducing the transport of heat towards high latitudes and reorganising the freshwater transport. The LGM circulation is represented as a large intrusion of saline Antarctic Bottom Water into the Northern Hemisphere basins. As a result, the North Atlantic Deep Water is shallower in the LGM simulation. The stream functions in latitude-salinity coordinates and thermohaline coordinates point out the different haline regimes between the glacial and interglacial period, as well as a LGM Conveyor Belt circulation largely driven by enhanced salinity contrast between the Atlantic and the Pacific basin. The thermohaline structure in the LGM simulation is the result of an abyssal circulation that lifts and deviates the Conveyor Belt cell from the area of maximum volumetric distribution, resulting in a ventilated upper layer above a deep stagnant layer, and an Atlantic circulation more isolated from the Pacific. An estimation of the turnover times reveal a deep circulation almost sluggish during the LGM, and a Conveyor Belt cell more vigorous due to the combination of stronger wind stress and shortened circulation route.

Description: Modelling survival and connectivity of Mnemiopsis leidyi in the southern North Sea and Scheldt estuaries Ocean Science Discussions, 11, 1561-1611, 2014 Author(s): J. van der Molen, J. van Beek, S. Augustine, L. Vansteenbrugge, L. van Walraven, V. Langenberg, H. W. van der Veer, K. Hostens, S. Pitois, and J. Robbens Three different models were applied to study the reproduction, survival and dispersal of Mnemiopsis leidyi in the Scheldt estuaries and the southern North Sea: a high-resolution particle tracking model with passive particles, a low resolution particle tracking model with a reproduction model coupled to a biogeochemical model, and a dynamic energy budget (DEB) model. The results of the models, each with its strengths and weaknesses, suggest the following conceptual situation: (i) the estuaries possess enough retention capability to keep an overwintering population, and enough exchange with coastal waters of the North Sea to seed offshore populations; (ii) M. leidyi can survive in the North Sea, and be transported over considerable distances, thus facilitating connectivity between coastal embayments; (iii) under current climatic conditions, M. leidyi may not be able to reproduce in large numbers in coastal and offshore waters of the North Sea, but this may change with global warming – however this result is subject to substantial uncertainty. Further quantitative observational work is needed on the effects of temperature, salinity and food availability on reproduction and on mortality at different life stages to improve models such as used here.

Description: Measuring air–sea gas exchange velocities in a large scale annular wind-wave tank Ocean Science Discussions, 11, 1643-1689, 2014 Author(s): E. Mesarchaki, C. Kräuter, K. E. Krall, M. Bopp, F. Helleis, J. Williams, and B. Jähne In this study we present gas exchange measurements conducted in a large scale wind-wave tank. Fourteen chemical species spanning a wide range of solubility (dimensionless solubility, α = 0.4 to 5470) and diffusivity (Schmidt number in water, Sc w = 594 to 1194) were examined under various turbulent ( u 10 = 0.8 to 15 m s −1 conditions. Additional experiments were performed under different surfactant modulated (two different concentration levels of Triton X-100) surface states. This paper details the complete methodology, experimental procedure and instrumentation used to derive the total transfer velocity for all examined tracers. The results presented here demonstrate the efficacy of the proposed method, and the derived gas exchange velocities are shown to be comparable to previous investigations. The gas transfer behaviour is exemplified by contrasting two species at the two solubility extremes, namely nitrous oxide (N 2 O) and methanol (CH 3 OH). Interestingly, a strong transfer velocity reduction (up to a factor of three) was observed for N 2 O under a surfactant covered water surface. In contrast, the surfactant affected CH 3 OH, the high solubility tracer only weakly.

Description: Argo data assimilation into HYCOM with an EnOI method in the Atlantic Ocean Ocean Science Discussions, 11, 1733-1781, 2014 Author(s): D. Mignac, C. A. S. Tanajura, A. N. Santana, L. N. Lima, and J. Xie An ocean data assimilation system to assimilate Argo temperature ( T ) and salinity ( S ) profiles into HYCOM was constructed, implemented and evaluated for the first time in the Atlantic Ocean (78° S to 50° N and 98° W to 20° E). The system is based on the Ensemble Optimal Interpolation (EnOI) algorithm proposed by Xie and Zhu (2010), especially made to deal with the hybrid nature of HYCOM vertical coordinate system with multiple steps. The Argo T/S profiles were projected to the model vertical space to create pseudo-observed layer thicknesses (Δ p obs ) which correspond to the model target densities. The first step was to assimilate Δ p obs considering the sub-state vector composed by the model layer thickness (Δ p ) and the baroclinic velocity components. After that, T and S were assimilated separately. At last, T was diagnosed below the mixed layer to preserve the density of the model isopycnal layers. Five experiments were performed from 1 January 2010 until 31 December 2012: a control run without assimilation, and four assimilation runs considering different vertical localizations of T , S and Δ p . The assimilation experiments were able to significantly improve the thermohaline structure produced by the control run. They reduced the RMSD of T ( S ) calculated with respect to Argo independent data in 34.11% (43.56%) in comparison to the control run. In some regions, such as the west North Atlantic, substantial corrections in the 20 °C isotherm depth and the upper ocean heat content towards climatological states were achieved. The runs with vertical localization of Δ p showed positive impacts in the correction of the thermohaline structure and reduced the RMSD of T ( S ) from 0.993 °C (0.149 psu) to 0.905 °C (0.138 psu) for the whole domain with respect to the other assimilation runs.

Description: Technical Note: Watershed strategy for oceanic mesoscale eddy splitting Ocean Science Discussions, 11, 1719-1732, 2014 Author(s): Q. Y. Li and L. Sun To identify oceanic mononuclear mesoscale eddies, a threshold-free splitting method was developed based on the watershed. Because oceanic eddies are similar to plateaus and basins in the map of the sea level anomaly (SLA) data, the natural divisions of the basins are the watersheds between them. The splitting algorithm is based on identifying these watersheds by finding the path of steepest descent. Compared to previous splitting methods, the proposed splitting algorithm has some advantages. First, there are no artificial parameters. Second, the algorithm is robust; the splitting strategy is independent of the algorithm and procedure and automatically guarantees that the split mononuclear eddies are simply-connected pixel sets. Third, the new method is very fast, and the time complexity is O(N) , where N is the number of multinuclear eddy pixels; each pixel is scanned only once for splitting, regardless of how many extremes there are. Fourth, the algorithm is independent of parameters; the strategy can potentially be applied to any possible physical parameters (e.g., SLA, geostrophic potential vorticity, Okubo–Weiss parameter, etc.). Besides, the present strategy can also be applied to automatic identification of troughs and ridges from weather charts. Because this general method can be applied to a variety of eddy parameter fields, we denoted it the Universal Splitting Technology for Circulations (USTC) method.

Description: One plausible reason for the change in ENSO characteristics in the 2000s Ocean Science Discussions, 11, 943-978, 2014 Author(s): V. N. Stepanov It is well known that El Niño Southern Oscillation (ENSO) causes floods, droughts in different regions of the Earth and the collapse of fisheries in the tropical Pacific, 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 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: Multi-scale optimal interpolation: application to DINEOF analysis spiced with a local optimal interpolation Ocean Science Discussions, 11, 895-941, 2014 Author(s): J.-M. Beckers, A. Barth, I. Tomazic, and A. Alvera-Azcárate We present a method in which the optimal interpolation of multi-scale processes can be untangled into a succession of simpler interpolations. First, we prove how the optimal analysis of a superposition of two processes can be obtained by different mathematical formulations involving iterations and analysis focusing on a single process. From the different mathematical equivalent formulations we then select the most efficient ones by analyzing the behavior of the different possibilities in a simple and well controlled test case. The clear guidelines deduced from this experiment are then applied in a real situation in which we combine large-scale analysis of hourly SEVIRI satellite images using DINEOF with a local optimal interpolation using a Gaussian covariance. It is shown that the optimal combination indeed provides the best reconstruction and can therefore be exploited to extract the maximum amount of useful information from the original data.

Description: Sensitivity of phytoplankton distributions to vertical mixing along a North Atlantic transect Ocean Science Discussions, 11, 839-893, 2014 Author(s): L. Hahn-Woernle, H. A. Dijkstra, and H. J. Van der Woerd Using in situ data of upper ocean vertical mixing profiles along a transect in the North Atlantic and an idealised phytoplankton growth model (PGM), we study the sensitivity of the surface phytoplankton concentration to vertical mixing distributions. Optical parameters in the PGM are calibrated with observations of light attenuation. The calibration of the biological parameters in the PGM is carried out at three different referent stations with observed vertical profiles of chlorophyll a (Chl a ) and nutrient concentration. Vertical mixing profiles at all other stations are next used at the reference stations to study the sensitivity of modelled phytoplankton distributions to vertical mixing. We find that shifts in vertical mixing are able to induce a transition from an upper chlorophyll maximum to a deep one and vice-versa. Furthermore, a clear correlation between the surface phytoplankton concentration and the mixing induced nutrient flux is found in nutrient limited regions. This may open up the possibility to extract characteristics of vertical mixing from satellite ocean colour data using data-assimilation methods.

Description: Characterisation and quantification of regional diurnal SST cycles from SEVIRI Ocean Science Discussions, 11, 1093-1128, 2014 Author(s): I. Karagali and J. L. Høyer Hourly SST fields from the geostationary Spinning Enhanced Visible and Infrared Imager (SEVIRI) offer a unique opportunity for the characterisation and quantification of the diurnal cycle of SST in the Atlantic Ocean, the Mediterranean Sea and the Northern European Shelf seas. Six years of SST fields from the SEVIRI dataset are validated against the polar orbiting Advanced Along Track Scanning Radiometer (AATSR) archive to identify biases in the SEVIRI data. Identification of the diurnal signal requires a night-time SST field representative of foundation temperatures, i.e. well-mixed conditions and free of any diurnal signal. Such fields are generated from the SEVIRI archive and are validated against pre-dawn SEVIRI SSTs and night-time SSTs from drifting buoys. The overall SEVIRI–AATSR bias is −0.07 K, and the standard deviation is 0.51 K, based on more than 53 × 10 6 match-ups. The different methodologies tested for the foundation temperature fields reveal variability introduced by averaging night-time SSTs over many days compared to single-day, pre-dawn values. Diurnal warming is most pronounced in the Mediterranean and Baltic Seas while smallest diurnal signals are found in the Tropics. Longer diurnal warming duration is identified in the high latitudes compared to the Tropics. The mean diurnal signal of monthly mean SST can be up to 0.5° in specific regions.

Description: Effects of bottom topography on dynamics of river discharges in tidal regions: case study of twin plumes in Taiwan Strait Ocean Science Discussions, 11, 1149-1189, 2014 Author(s): K. A. Korotenko, A. A. Osadchiev, P. O. Zavialov, R.-C. Kao, and C.-F. Ding The Princeton Ocean Model is used to investigate the intratidal variability of currents and turbulent mixing and their impact on the characteristics and evolution of the plumes of two neighboring rivers, the Zhuoshui River and the Wu River, at the central eastern coast of Taiwan Strait. The two estuaries are located close to each other and their conditions are similar in many respects, and yet the two plumes exhibit significantly different behavior. We explain this through differences of the bottom topography in the areas adjacent to the two river mouths. The Zhuoshui River runs into a shallow area that is permanently exposed to strong tidal mixing, while the Wu River mouth is located in a deeper, stratified area outside the region of intense mixing. This destruction of the plume by tidal mixing is confirmed by the results of numerical modeling with POM. The spatial and temporal variability of turbulent kinetic energy and its production rate in the study region, as well as the horizontal diffusivity, are analyzed with the emphasis given to the dependence of the turbulence parameters on the bottom topography on the one hand and their influence on the river plumes on the other. Further, we use a Lagrangian particle tracking model in combination with POM to investigate the effect of the tidal wetting-and-drying (WAD) of land taking place near the Zhuoshui estuary, and demonstrate that WAD leads to significant reduction of the plume extent and surface salinity deficit near the river mouth. We use observational data from a short field campaign in the study area to tune and validate the model experiments.

Description: Evaluation of the eastern equatorial Pacific SST seasonal cycle in CMIP5 models Ocean Science Discussions, 11, 1129-1147, 2014 Author(s): Z. Song, H. Liu, L. Zhang, F. Qiao, and C. Wang The annual cycle of sea surface temperature (SST) in the eastern equatorial Pacific (EEP) with the largest amplitude in the tropical oceans is poorly represented in the coupled general circulation models (CGCMs) of the Coupled Model Intercomparison Project phase 3 (CMIP3). In this study, 18 models from CMIP5 projects are evaluated in simulating the annual cycle in the EEP. Fourteen models are able to simulate the annual cycle, and four still show erroneous information in the simulation, which suggests that the performances of CGCMs have been improved. The results of multi-model ensemble (MME) mean show that CMIP5 CGCMs can capture the annual cycle signal in the EEP with correlation coefficients up to 0.9. For amplitude simulations, EEP region 1 (EP1) near the eastern coast shows weaker results than observations due to the large warm SST bias from the southeastern tropical Pacific in the boreal autumn. In EEP region 2 (EP2) near the central equatorial Pacific, the simulated amplitudes are nearly the same as the observations because of the presence of a quasi-constant cold bias associated with poor cold tongue climatology simulation in the CGCMs. To improve CGCMs in the simulation of a realistic SST seasonal cycle, local and remote climatology SST biases that exist in both CMIP3 and CMIP5 CGCMs must be resolved at least for the simulation in the central equatorial Pacific and the southeastern tropical Pacific.

Description: Water level oscillations in Monterey Bay and Harbor Ocean Science Discussions, 11, 2569-2606, 2014 Author(s): J. Park, W. Sweet, and R. Heitsenrether Seiches are normal modes of water bodies responding to geophysical forcings with potential to significantly impact ecology and maritime operations. Analysis of high-frequency (1 Hz) water level data in Monterey California identifies Harbor modes between 10 and 120 s that are attributed with specific geographic features. It found that modal amplitude modulation arises from cross-modal interaction and that offshore wave energy is a primary driver of these modes. Synchronous coupling between modes is observed to significantly impact dynamic water levels. At lower frequencies between 15 and 60 min modes are independent of offshore wave energy, yet are continuously present. This is unexpected since seiches normally dissipate after cessation of the driving force, indicating an unknown forcing. Spectral and kinematic estimates of these low frequency oscillations supports the idea that a persistent anticyclonic mesoscale gyre adjacent to the Bay is a potential mode driver, while discounting other sources.

Description: Exploring the isopycnal mixing and helium-heat paradoxes in a suite of Earth System Models Ocean Science Discussions, 11, 2533-2567, 2014 Author(s): A. Gnanadesikan, R. Abernathey, and M.-A. Pradal This paper uses a suite of Earth System models which simulate the distribution of He isotopes and radiocarbon to examine two paradoxes in Earth science. The helium-heat paradox refers to the fact that helium emissions to the deep ocean are far lower than would be expected given the rate of geothermal heating, since both are thought to be the result of radioactive decay in the earth's interior. The isopycnal mixing paradox comes from the fact that many theoretical parameterizations of the isopycnal mixing coefficient A Redi that link it to baroclinic instability project it to be small (of order a few hundred m 2 s −1 ) in the ocean interior away from boundary currents. However, direct observations using tracers and floats (largely in the upper ocean) suggest that values of this coefficient are an order of magnitude higher. Because helium isotopes equilibrate rapidly with the atmosphere, but radiocarbon equilibrates slowly, it might be thought that resolving the isopycnal mixing paradox in favor of the higher observational estimates of A Redi might also solve the helium paradox. In this paper we show that this is not the case. In a suite of models with different spatially constant and spatially varying values of A Redi the distribution of radiocarbon and helium isotopes is sensitive to the value of A Redi . However, away from strong helium sources in the Southeast Pacific, the relationship between the two is not sensitive, indicating that large-scale advection is the limiting process for removing helium and radiocarbon from the deep ocean. The helium isotopes, in turn, suggest a higher value of A Redi in the deep ocean than is seen in theoretical parameterizations based on baroclinic growth rates. We argue that a key part of resolving the isopycnal mixing paradox is to abandon the idea that A Redi has a direct relationship to local baroclinic instability and to the so called "thickness" mixing coefficient A GM .

Description: Friction and mixing effects on potential vorticity for bottom current crossing a marine strait: an application to the Sicily Channel (central Mediterranean Sea) Ocean Science Discussions, 11, 2495-2532, 2014 Author(s): F. Falcini and E. Salusti We discuss here the evolution of vorticity and potential vorticity (PV) for a bottom current crossing a marine channel in shallow-water approximation, focusing on the effect of friction and mixing. We argue that bottom current vorticity is prone to significant sign changes and oscillations due to topographic effects when, in particular, the current flows over the sill of a channel. These vorticity variations are, however, modulated by frictional effects due to seafloor roughness and morphology. Such behavior is also reflected in the PV spatial evolution, which shows an abrupt peak around the sill region. Our theoretical findings are discussed by means of in situ hydrographic data related to the Eastern Mediterranean Deep Water, i.e., a dense, bottom water vein that flows northwestward, along the Sicily Channel (Mediterranean Sea). Indeed, the narrow sill of this channel implies that friction and entrainment need to be considered. Small tidal effects in the Sicily Channel allow for a steady theoretical approach. Our diagnoses on vorticity and PV allow us to obtain general insights about the effect of mixing and friction on the pathway and internal structure of bottom-trapped currents flowing through channels and straits, and to discuss spatial variability of the frictional coefficient. Our approach significantly differs from other PV-constant approaches previously used in studying the dynamics of bottom currents flowing through rotating channels.

Description: Transient tracer applications in the Southern Ocean Ocean Science Discussions, 11, 2289-2335, 2014 Author(s): T. Stöven, T. Tanhua, and M. Hoppema Transient tracers can be used to constrain the Inverse-Gaussian transit time distribution (IG-TTD) and thus provide information about ocean ventilation. Individual transient tracers have different time and application ranges which are defined by their atmospheric history (chronological transient tracers) or their decay rate (radioactive transient tracers). The classification ranges from tracers for highly ventilated water masses, e.g. sulfur hexafluoride (SF 6 ), the decay of Tritium (δ 3 H) and to some extent also dichlorodifluoromethane (CFC-12) to tracers for less ventilated deep ocean basins, e.g. CFC-12, Argon-39 ( 39 Ar) and radiocarbon ( 14 C). The IG-TTD can be empirically constrained by using transient tracer couples with sufficiently different input functions. Each tracer couple has specific characteristics which influence the application limit of the IG-TTD. Here we provide an overview of commonly used transient tracer couples and their validity areas within the IG-TTD by using the concept of tracer age differences (TAD). New measured CFC-12 and SF 6 data from a section along 10° E in the Southern Ocean in 2012 are presented. These are combined with a similar data set of 1998 along 6° E in the Southern Ocean as well as with 39 Ar data from the early 1980s in the western Atlantic Ocean and the Weddell Sea for investigating the application limit of the IG-TTD and to analyze changes in ventilation in the Southern Ocean. We found that the IG-TTD can be constrained south to 46° S which corresponds to the Subantarctic Front (SAF) denoting the application limit. The constrained IG-TTD north of the SAF shows a slight increase in mean ages between 1998 and 2012 in the upper 1200 m between 42–46° S. The absence of SF 6 inhibits ventilation analyses below this depth. The time lag analysis between the 1998 and 2012 data shows an increase in ventilation down to 1000 m and a steady ventilation between 2000 m-bottom south of the SAF between 51–55° S.

Description: Modelling of underwater light fields in turbid and eutrophic waters: application and validation with experimental data Ocean Science Discussions, 11, 2119-2172, 2014 Author(s): B. Sundarabalan and P. Shanmugam A reliable radiative transfer model is an essential and indispensable tool for understanding of the radiative transfer processes in homogenous and layered waters, analyzing measurements made by radiance sensors and developing remote sensing algorithms to derive meaningful physical quantities and biogeochemical variables in turbid and productive coastal waters. Existing radiative transfer models have been designed to be applicable to either homogenous waters or inhomogeneous waters. To overcome such constraints associated with these models, this study presents a radiative transfer model that treats a homogenous layer as a diffuse part and an inhomogeneous layer as a direct part in the water column and combines these two parts appropriately in order to generate more reliable underwater light field data such as upwelling radiance ( L u ), downwelling irradiance ( E d ) and upwelling irradiance ( E u ). The diffuse model assumes the inherent optical properties (IOPs) to be vertically continuous and the light fields to exponentially decrease with the depth, whereas the direct part considers the water column to be vertically inhomogeneous (layer-by-layer phenomena) with the vertically varying phase function. The surface and bottom boundary conditions, source function due to chlorophyll and solar incident geometry are also included in the present RT model. The performance of this model is assessed in a variety of waters (clear, turbid and eutrophic) using the measured radiometric data. The present model shows an advantage in terms of producing accurate L u , E d and E u profiles (in spatial domain) in different waters determined by both homogenous and inhomogeneous conditions. The feasibility of predicting these underwater light fields based on the remotely estimated IOP data is also examined using the present RT model. For this application, vertical profiles of the water constituents and IOPs are estimated by empirical models based on our in-situ data. The present RT model generates L u , E d and E u spectra closely consistent with the measured data. These results lead to a conclusion that the present RT model is a viable alternative to existing RT models and has an important implication for remote sensing of optically complex waters.

Description: Phytoplankton blooms on the western shelf of Tasmania: evidence of a highly productive ecosystem Ocean Science Discussions, 11, 2173-2204, 2014 Author(s): J. Kämpf Analyses of 〉10 years of satellite-derived ocean-color data reveal the existence of a highly productive ecosystem on the west Tasmanian shelf. A closer event-based analysis indicates that the nutrient supply for this system has two different dynamical origins: (a) wind-driven coastal upwelling and (b) river plumes. During austral summer months, the west Tasmanian shelf forms a previously unknown upwelling center of the "Great South Australian Coastal Upwelling System", presumably injecting nutrient-rich water into western Bass Strait. Surprisingly, river discharges render the study region productive during other seasons of the year, except when nutrient-poor water of the South Australian Current reaches the region. Overall, the west Tasmanian shelf is more phytoplankton-productive than the long-known coastal upwelling along the Bonney Coast. The existence of phytoplankton blooms during the off-upwelling-season may explain the wintertime spawing aggregations of the blue grenadier ( Macruronus novaezelandiae ) and the associated regionally high abundance of Australian fur seals ( Arctocephalus pusillus doriferus ).

Description: A century of sea level data and the UK's 2013/14 storm surges: an assessment of extremes and clustering using the Newlyn tide gauge record Ocean Science Discussions, 11, 1995-2028, 2014 Author(s): M. P. Wadey, I. D. Haigh, and J. M. Brown For the UK's longest and most complete sea level record (Newlyn), we assess extreme high water events and their temporal clustering; prompted by the 2013/2014 winter of flooding and storms. These are set into context against this almost 100 yr record. We define annual periods for which storm activity, tides and sea levels can be compared on a year-by-year basis. Amongst the storms and high tides which affected Newlyn the recent winter produced the largest recorded high water (3 February 2014) and five others above a 1 in 1 yr return period. The large magnitude of tide and mean sea level, and the close inter-event spacings (of large return period high waters), suggests that the 2013/2014 high water "season" may be considered the most extreme on record. However, storm and sea level events may be classified in different ways. For example in the context of sea level rise (which we calculate linearly as 1.81 ± 0.1 mm yr −1 from 1915 to 2014), a lower probability combination of surge and tide occurred on 29 January 1948, whilst 1995/1996 storm surge season saw the most high waters of ≥ 1 in 1 yr return period. We provide a basic categorisation of five types of high water cluster, ranging from consecutive tidal cycles to multiple years. The assessment is extended to other UK sites (with shorter sea level records and different tide-surge characteristics), which suggests 2013/2014 was extreme, although further work should assess clustering mechanisms and flood system "memory".

Description: Improved sea level record over the satellite altimetry era (1993–2010) from the Climate Change Initiative Project Ocean Science Discussions, 11, 2029-2071, 2014 Author(s): M. Ablain, A. Cazenave, G. Larnicol, M. Balmaseda, P. Cipollini, Y. Faugère, M. J. Fernandes, O. Henry, J. A. Johannessen, P. Knudsen, O. Andersen, J. Legeais, B. Meyssignac, M. Picot, M. Roca, S. Rudenko, M. G. Scharffenberg, D. Stammer, G. Timms, and J. Benveniste Sea level is one of the 50 Essential Climate Variables (ECVs) listed by the Global Climate Observing System (GCOS) in climate change monitoring. In the last two decades, sea level has been routinely measured from space using satellite altimetry techniques. In order to address a number of important scientific questions such as: "Is sea level rise accelerating?", "Can we close the sea level budget?", "What are the causes of the regional and interannual variability?", "Can we already detect the anthropogenic forcing signature and separate it from the internal/natural climate variability?", and "What are the coastal impacts of sea level rise?", the accuracy of altimetry-based sea level records at global and regional scales needs to be significantly improved. For example, the global mean and regional sea level trend uncertainty should become better than 0.3 and 0.5 mm year −1 , respectively (currently of 0.6 and 1–2 mm year −1 ). Similarly, interannual global mean sea level variations (currently uncertain to 2–3 mm) need to be monitored with better accuracy. In this paper, we present various respective data improvements achieved within the European Space Agency (ESA) Climate Change Initiative (ESA CCI) project on "Sea Level" during its first phase (2010–2013), using multi-mission satellite altimetry data over the 1993–2010 time span. In a first step, using a new processing system with dedicated algorithms and adapted data processing strategies, an improved set of sea level products has been produced. The main improvements include: reduction of orbit errors and wet/dry atmospheric correction errors, reduction of instrumental drifts and bias, inter-calibration biases, intercalibration between missions and combination of the different sea level data sets, and an improvement of the reference mean sea surface. We also present preliminary independent validations of the SL_cci products, based on tide gauges comparison and sea level budget closure approach, as well as comparisons with ocean re-analyses and climate model outputs.

Description: Roles of initial ocean surface and subsurface states on successfully predicting 2006–2007 El Niño Ocean Science Discussions, 11, 1543-1560, 2014 Author(s): F. Zheng and J. Zhu The 2006–2007 El Niño event, an unusually weak event, was predicted by most models only after the warming in the eastern Pacific had commenced. In this study, on the basis of an El Niño prediction system, roles of the initial ocean surface and subsurface states on predicting the 2006–2007 El Niño event are investigated to determine conditions favorable for predicting El Niño growth and are isolated in three sets of hindcast experiments. The hindcast is initialized through assimilation of only the sea surface temperature (SST) observations to optimize the initial surface condition (Assim_SST), only the sea level (SL) data to update the initial subsurface state (Assim_SL), or both the SST and SL data (Assim_SST + SL). Results highlight that the hindcasts with three different initial states all can successfully predict the 2006–2007 El Niño event one year in advance and that the Assim_SST + SL hindcast performs best. A comparison between the various sets of hindcast results further demonstrates that successful prediction is more significantly affected by the initial subsurface state than by the initial surface condition. The accurate initial surface state can easier trigger the prediction of the 2006–2007 El Niño, whereas a more reasonable initial subsurface state can contribute to improving the prediction in the growth of the warm event.

Description: Evaluation of wet troposphere path delays from atmospheric reanalyses and radiometers and their impact on the altimeter sea level Ocean Science Discussions, 11, 1613-1642, 2014 Author(s): J.-F. Legeais, M. Ablain, and S. Thao The assessment of long-term errors in altimeter sea level measurements is essential for studies related to the mean sea level (MSL) evolution. One of the main contributors to the long-term sea level uncertainties is the correction of the altimeter range from the wet troposphere path delay, which is provided by onboard microwave radiometers for the main altimeter missions. The wet troposphere correction (WTC) derived from the operational European Centre for Medium-Range Weather Forecast (ECMWF) atmospheric model is usually used as a reference for comparison with the radiometer WTC. However, due to several improvements of the processing, this model is not homogenous over the altimetry period (from 1993 onwards), preventing the detection of errors in the radiometer WTC, especially in the first altimetry decade. In this study, we determine the quality of WTC provided by the operational ECMWF atmospheric model in comparison with the fields derived from the ERA Interim (ECMWF) and the National Centers for Environmental Predictions/National Center for Atmospheric Research (NCEP/NCAR) reanalyses. Separating our analyses on several temporal and spatial scales, we demonstrate that ERA Interim provides the best modeled WTC for the altimeter sea level at climate scales. This allows us to better evaluate the radiometer WTC errors, especially for the first altimetry decade (1993–2002), and thus to improve the altimeter MSL error budget. This work also demonstrates the relevance of the feed-backs that the "altimetry" and "atmosphere" communities can bring to each other.

Description: Detailed temperature–salinity distribution in the Northeast Atlantic from ship and Argo vertical casts Ocean Science Discussions, 11, 1473-1517, 2014 Author(s): I. Bashmachnikov, F. Neves, A. Nascimento, J. Medeiros, I. Ambar, J. Dias, and X. Carton The present study defines new interpolation functions for hydrological data. These functions are applied to generate climatological maps of temperature–salinity distribution with 25 m depth interval and 30 km space interval (MEDTRANS data-set). The data underwent a rigorous data quality control, having passed several filtering procedures. The gridding was done on neutral density surfaces, which allows better representation of the relative intensity of thermohaline fronts for the same gridding radius. The gridding was done using multi-pass Barnes' Optimum Interpolation procedure with spatially variable size of the gridding window. The shape of the window accounted for topographic influence: the dominant along-slope direction of water mass transport. One of the new features was the use of a local ratio of topographic to planetary β-effects to define the shape of the window as a function of the relative importance of the topographic influence. The N/f ratio was used for simulation of the baroclinic decrease of the topographic influence on water mass transport with the distance from the bottom. The gridded fields are available at the web-site of the Center of Oceanography of the University of Lisbon ( http://co.fc.ul.pt/en/data ). The new MEDTRANS climatology gives more details to the distribution of water characteristics in the Subtropical Northeast Atlantic, in particular near the Iberian Peninsula. The geostrophic currents relative to the 1900 m reference level demonstrate the local circulation features, in good correspondence to the theory and to previous studies: the acceleration in the meanders of the Azores current; the cyclonic gyre in the Gulf of Cadiz; the splitting and separation of the Mediterranean Water outflow from the continental slope near the Gorringe and the Galicia banks. Those features are not reproduced by the alternative climatologies. Seasonal climatologies, computed for the warm (May–October) and cold (November–April) seasons, revealed stronger zonal extension of the upper ocean patterns during the warm season, as compared to the cold one. At the Iberian continental slope, the seasonality manifested itself in more saline and denser lower core of the Mediterranean Water during the warm season as compared to the cold season.

Description: Forecasting the mixed layer depth in the north east Atlantic: an ensemble approach, with uncertainties based on data from operational oceanic systems Ocean Science Discussions, 11, 1435-1472, 2014 Author(s): Y. Drillet, J. M. Lellouche, B. Levier, M. Drévillon, O. Le Galloudec, G. Reffray, C. Regnier, E. Greiner, and M. Clavier Operational systems operated by Mercator Océan provide daily ocean forecasts, and combining these forecasts we can produce ensemble forecast and uncertainty estimates. This study focuses on the mixed layer depth in the North East Atlantic near the Porcupine Abyssal Plain for May 2013. This period is of interest for several reasons: (1) four Mercator Océan operational systems provide daily forecasts at a horizontal resolution of 1/4°, 1/12° and 1/36° with different physics; (2) glider deployment under the OSMOSIS project provides observation of the changes in mixed layer depth; (3) the ocean stratifies in May, but mixing events induced by gale force wind are observed and forecasted by the systems. A statistical approach and forecast error quantification for each system and for the combined products are presented. Skill scores indicate that forecasts are in any case better than persistence, and temporal correlations between forecast and observations are greater than 0.8 even for the 4 day forecast. The impact of atmospheric forecast error, and for the wind field in particular, is also quantified in terms of the forecast time delay and the intensity of mixing or stratification events.

Description: Wave induced mixing and transport of buoyant particles: application to the Statfjord A oil spill Ocean Science Discussions, 11, 1265-1300, 2014 Author(s): M. Drivdal, G. Broström, and K. H. Christensen The modelling of wave-current and wave-turbulence interactions have received much attention in recent years. In this study the focus is on how these wave effects modify the transport of particles in the ocean. Here the particles are buoyant tracers that can represent oil droplets, plastic particles or plankton, for example fish eggs and larvae. Using the General Ocean Turbulence Model (GOTM), modified to take surface wave effects into account, we investigate how the increased mixing by wave breaking and Stokes shear production as well as the stronger veering by the Coriolis–Stokes force affect the drift of the particles. The energy and momentum fluxes as well as the Stokes drift depend on the directional wave spectrum that can be obtained from a wave model or from observations. As a first test the depth and velocity scales from the model are compared with analytical solutions based on a constant eddy viscosity (e.g. classical Ekman theory). Secondly the model is applied to a case where we investigate the oil drift after an offshore oil spill outside the western coast of Norway in 2007. During this accident the average net drift of oil was observed to be both slower and more deflected away from the wind direction than predicted by empirical models. With wind and wave forcing from the ERA Interim archive, it is shown that the wave effects are important for the resultant drift in this case, and has the potential to improve drift forecasting.

Description: Comparative heat and gas exchange measurements in the Heidelberg Aeolotron, a large annular wind-wave tank Ocean Science Discussions, 11, 1691-1718, 2014 Author(s): L. Nagel, K. E. Krall, and B. Jähne A comparative study of simultaneous heat and gas exchange measurements was performed in the large annular Heidelberg Air–Sea Interaction Facility, the Aeolotron, under homogeneous water surface conditions. The use of two gas tracers, N 2 O and C 2 HF 5 , resulted not only in gas transfer velocities, but also in the measurement of the Schmidt number exponent n with a precision of ± 0.025. The original controlled flux or active thermographic technique proposed by Jähne et al. (1989) was applied by heating a large patch at the water surface to measure heat transfer velocities. Heating a large patch, the active thermography technique is laterally homogeneous and problems of lateral transport effects are avoided. Using the measured Schmidt number exponents, the ratio of the scaled heat transfer velocities to the measured gas transfer velocities is 1.046 ± 0.040, a good agreement within the limits of experimental uncertainties. This indicates the possibility to scale heat transfer velocities measured by active thermography to gas transfer velocities, provided the Schmidt number exponent is known and that the heated patch is large enough to reach the thermal equilibrium.

Description: An optimised method for correcting quenched fluorescence yield Ocean Science Discussions, 11, 1243-1264, 2014 Author(s): L. Biermann, C. Guinet, M. Bester, A. Brierley, and L. Boehme Under high light intensity, phytoplankton protect their photosystems from bleaching through non-photochemical quenching processes. The consequence of this is suppression of fluorescence emission, which must be corrected when measuring in situ yield with fluorometers. Previously, this has been done using the limit of the mixed layer, assuming that phytoplankton are uniformly mixed from the surface to this depth. However, the assumption of homogeneity is not robust in oceanic regimes that support deep chlorophyll maxima. To account for these features, we correct from the limit of the euphotic zone, defined as the depth at which light is at ~1% of the surface value. This method was applied to fluorescence data collected by eleven animal-borne fluorometers deployed in the Southern Ocean over four austral summers. Six tags returned data showing evidence of deep chlorophyll features. Using the depth of the euphotic layer, quenching was corrected without masking subsurface fluorescence signals.

Description: Impact of a 30% reduction in Atlantic meridional overturning during 2009–2010 Ocean Science Discussions, 11, 789-810, 2014 Author(s): H. L. Bryden, B. A. King, G. D. McCarthy, and E. L. McDonagh The Atlantic meridional overturning circulation comprises warm upper waters flowing northward, becoming colder and denser until they form deep water in the Labrador and Nordic Seas that then returns southward through the North and South Atlantic. The ocean heat transport associated with this circulation is 1.3 PW, accounting for 25% of the maximum combined atmosphere–ocean heat transport necessary to balance the earth's radiation budget. We have been monitoring the circulation at 25° N since 2004. A 30% slowdown in the circulation for 15 months during 2009–2010 reduced northward ocean heat transport across 25° N by 0.4 PW and resulted in colder upper ocean waters north of 25° N and warmer waters south of 25° N. The spatial pattern of upper ocean temperature anomalies helped push the wintertime circulation 2010–2011 into record-low negative NAO conditions with accompanying severe winter conditions over northwestern Europe. The warmer temperatures south of 25° N contributed to the high intensity hurricane season in Summer 2010.

Description: Sea level trend and variability around the Peninsular Malaysia Ocean Science Discussions, 11, 1519-1541, 2014 Author(s): Q. H. Luu, P. Tkalich, and T. W. Tay Peninsular Malaysia is bounded from the west by Malacca Strait and the Andaman Sea both connected to the Indian Ocean, and from the east by South China Sea being largest marginal sea in the Pacific Basin. Resulting sea level along Peninsular Malaysia coast is assumed to be governed by various regional phenomena associated with the adjacent parts of the Indian and Pacific Oceans. At annual scale, sea level anomalies (SLAs) are generated by the Asian monsoon; interannual sea level variability is determined by the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD); while long-term sea level trend is related to global climate change. To quantify the relative impacts of these multi-scale phenomena on sea level trend and variability around the Peninsular Malaysia, long-term tide gauge record and satellite altimetry are used. During 1984–2011, relative sea level rise (SLR) rates in waters of Malacca Strait and eastern Peninsular Malaysia are found to be 2.4 ± 1.6 mm yr −1 and 2.7 ± 1.0 mm yr −1 , respectively. Allowing for corresponding vertical land movements (VLM; 0.8 ± 2.6 mm yr −1 and 0.9 ± 2.2 mm yr −1 ), their absolute SLR rates are 3.2 ± 4.2 mm yr −1 and 3.6 ± 3.2 mm yr −1 , respectively. For the common period 1993–2009, absolute SLR rates obtained from both tide gauge and satellite altimetry in Peninsular Malaysia are similar; and they are slightly higher than the global tendency. It further underlines that VLM should be taken into account to get better estimates of SLR observations. At interannual scale, ENSO affects sea level over the Malaysian coast in the range of ±5 cm with a very high correlation. Meanwhile, IOD modulates sea level anomalies mainly in the Malacca Strait in the range of ±2 cm with a high correlation coefficient. Interannual regional sea level drops are associated with El Niño events and positive phases of the IOD index; while the rises are correlated with La Niña episodes and the negative periods of the IOD index. Seasonally, SLAs are mainly monsoon-driven, in the order of 10–25 cm. Geographically, sea level responds differently to the monsoon: two cycles per year are observed in the Malacca Strait, presumably due to South Asian-Indian Monsoon; whereas single annual cycle is noted along east coast of Peninsular Malaysia, mostly due to East Asian-Western Pacific Monsoon. These results imply that a narrow topographic constriction in Singapore Strait may separate different modes of annual and interannual sea level variability along coastline of Peninsular Malaysia.

Description: Numerical modelling of sediment transport in the Adriatic Sea Ocean Science Discussions, 11, 1391-1433, 2014 Author(s): A. Guarnieri, A. J. Souza, N. Pinardi, and P. Traykovski A new sediment transport model, considering currents, tides and waves is presented for the Adriatic Sea basin. The simulations concentrate on the winter of 2002–2003 because of field data availability and interesting intermittent processes occurrence. A process oriented analysis is performed to investigate the impact that Sirocco and Bora wind regimes have on sediment transport. The comparisons of the simulations with the observed data show that the model is capable to capture the main dynamics of sediment transport along the Italian coasts and the sediment concentration within the water column. This latter can reach values up to several g L −1 , especially within the first centimetres above the bottom. The sediments are transported mainly southwards along the Italian coasts, consistently with the known literature results, except during Sirocco wind events, which can be responsible for reversing the coastal circulation in the northern area of the basin, and consequently the sediment transport. The resuspension of sediments is also related to the specific wave regimes induced by Bora and Sirocco, the former inducing resuspension events near the coasts while the latter causing a more diffused resuspension regime in the Northern Adriatic basin. Beside the realistic representation of short timescales resuspension/deposition events due to storms, the model was also used to investigate persistent erosion or deposition areas in the Adriatic Sea. Two main depocenters were identified: one, very pronounced, in the surroundings of the Po river delta, and another one a few kilometres off the coast in front of the Ancona promontory. A third region of accumulation, even if less intense, was found to be offshore the southernmost limit of the Gargano region. On the contrary the whole western coast within a distance of a few kilometres from the shore was found to be subject to prevailing erosion. The comparison with observed accumulation and erosion data shows that the model captures well the main depocenters in the domain and the erosion within the very coastal belt of the western side of the basin, but seems to be too erosive in a few areas, in particular those where the contribution of sediment inflow to the sea of some minor but intermittently important rivers is not considered in a realistic way as input to the model.

Description: Flow separation of intermediate water in the lees of sills off Taiwan from seismic observations Ocean Science Discussions, 11, 1871-1893, 2014 Author(s): Q. S. Tang, L. T. Sun, J. B. Li, and C. S. Liu Flow separation can occur when a stratified flow passes over an obstacle. Previous studies concerning flow separation are mostly observed in the shallow water regions, e.g., fjord, driven by tidal flows. In this study, a novel view of flow separations of the intermediate water over the Hengchun Ridge and Ryukyu Arc off Taiwan is firstly reported using seismic oceanography by reprocessing the seismic data from two cruises 97306 and EW9509 in 2001 and 1995, respectively. Both seismic images show that water masses are separated by internal interfaces detached from sill crests. Over the Hengchun Ridge, it was previously considered that the intermediate water flowed out of the South China Sea into the Hengchun Trough. Flow separation occurred at the lee-side of the ridge forming a continuous interface 〉 15 km between the intermediate and deep water at 900 m depth. This interface suggests that (1) mean flow of the intermediate water always flowed eastward and (2) no lee wave was generated or radiated even during a spring ebb tide. Near the East Taiwan Channel, the only major passage of the Kuroshio into the Okinawa Trough, flow separation occurred when the Kuroshio intermediate water flowed over the Ryukyu Arc at 500 m depth. The separation interface is nearly horizontal and extends 〉 25 km from the sill break to the trough. Parameter estimations and morphologic comparisons show that both separation cases are neither boundary-layer separation nor post-wave separation. Model data from OFES (Ocean General Circulation Model For the Earth Simulator) reveal dense pools existing downstream of the sill crests. We propose that a density-forced flow separation mechanism could be responsible for the observed flow separations and absence of lee waves in this study.

Description: Deriving a sea surface climatology of CO 2 fugacity in support of air–sea gas flux studies Ocean Science Discussions, 11, 1895-1948, 2014 Author(s): L. M. Goddijn-Murphy, D. K. Woolf, P. E. Land, J. D. Shutler, and C. Donlon Climatologies, or long-term averages, of essential climate variables are useful for evaluating models and providing a baseline for studying anomalies. The Surface Ocean Carbon Dioxide (CO 2 ) Atlas (SOCAT) has made millions of global underway sea surface measurements of CO 2 publicly available, all in a uniform format and presented as fugacity, f CO 2 . f CO 2 is highly sensitive to temperature and the measurements are only valid for the instantaneous sea surface temperature (SST) that is measured concurrent with the in-water CO 2 measurement. To create a climatology of f CO 2 data suitable for calculating air–sea CO 2 fluxes it is therefore desirable to calculate f CO 2 valid for climate quality SST. This paper presents a method for creating such a climatology. We recomputed SOCAT's f CO 2 values for their respective measurement month and year using climate quality SST data from satellite Earth observation and then extrapolated the resulting f CO 2 values to reference year 2010. The data were then spatially interpolated onto a 1° × 1° grid of the global oceans to produce 12 monthly f CO 2 distributions for 2010. The partial pressure of CO 2 ( p CO 2 ) is also provided for those who prefer to use p CO 2 . The CO 2 concentration difference between ocean and atmosphere is the thermodynamic driving force of the air–sea CO 2 flux, and hence the presented f CO 2 distributions can be used in air–sea gas flux calculations together with climatologies of other climate variables.

Description: Influence of frontal cyclones evolution on the 2009 (Ekman) and 2010 (Franklin) Loop Current Eddy detachment events Ocean Science Discussions, 11, 1949-1994, 2014 Author(s): Y. S. Androulidakis, V. H. Kourafalou, and M. Le Hénaff The anticyclonic Loop Current Eddy (LCE) shedding events are strongly associated with the evolution of Loop Current Frontal Eddies (LCFEs) over the eastern Gulf of Mexico (GoM). A numerical simulation, in tandem with in situ measurements and satellite data, was used to investigate the Loop Current (LC) evolution and the surrounding LCFEs formation, structure, growth and migration during the Eddy Ekman and Eddy Franklin shedding events in the summers of 2009 and 2010, respectively. During both events, Northern GoM LCFEs appeared vertically coherent to at least 1500 m in temperature observations. They propagated towards the base of the LC where, together with the migration of Campeche Bank eddies from south of the LC, contributed to its "necking down". Growth of Campeche Bank LCFEs involved in Eddy Franklin was partially attributed to Campeche Bank waters following upwelling events. Slope processes associated with such upwelling include offshore exports of high positive vorticity that may trigger cyclone formation and growth. The advection and growth of LCFEs, originating from the northern and southern GoM, and their interaction with the LC over the LCE detachment area favor shedding conditions and may lead to the final separation of the LCE.

Description: Consequences of artificial deepwater ventilation in the Bornholm Basin for oxygen conditions, cod reproduction and benthic biomass – a model study Ocean Science Discussions, 11, 1783-1827, 2014 Author(s): A. Stigebrandt, R. Rosenberg, L. Råman Vinnå, and M. Ödalen We develop and use a circulation model to estimate hydrographical and ecological changes in the isolated basin water of the Bornholm Basin. By pumping well oxygenated so-called winter water, residing beneath the level of the summer thermocline, to the greatest depth of the basin, where it is forced to mix with the resident water, the rate of density reduction should increase and thereby the frequency of intrusions of new oxygen-rich deepwater. We show that pumping 1000 m 3 s −1 should increase the rates of water exchange and oxygen supply by 2.5 and 3 times, respectively. The CRV (Cod Reproduction Volume), the volume of water in the isolated basin meeting the requirements for successful cod reproduction ( S 〉 11, O 2 〉 2 mL L −1 ), should every year be greater than 54 km 3 , which is an immense improvement since it in certain years is currently much less. Anoxic bottoms should no longer occur in the basin and hypoxic events will become rare. This should permit extensive colonization of fauna on the earlier periodically anoxic bottoms. Increased biomass of benthic fauna should also mean increased food supply to economically valuable demersal fish like cod and flatfish. In addition, the bioturbation activity and re-oxygenation of the sediments should lead to increased phosphorus retention by the sediments.

Description: An observed 20 yr time-series of Agulhas leakage Ocean Science Discussions, 11, 171-196, 2014 Author(s): D. Le Bars, J. V. Durgadoo, H. A. Dijkstra, A. Biastoch, and W. P. M. De Ruijter We provide a time series of Agulhas leakage anomalies over the last twenty years from satellite altimetry. Until now, measuring the interannual variability of Indo-Atlantic exchange has been the major barrier in the investigation of the dynamics and large scale impact of Agulhas leakage. We compute the difference of transport between Agulhas Current and Agulhas Return Current which allows to deduce Agulhas leakage. The main difficulty is to separate the Agulhas Return Current from the southern limb of the subtropical "supergyre" south of Africa. For this purpose, an algorithm that uses absolute dynamic topography data is developed. The algorithm is applied to a state-of-the-art ocean model. The comparison with a Lagrangian method to measure the leakage allows to validate the new method. An important result is that it is possible to measure Agulhas leakage in this model using the velocity field along a section that crosses both the Agulhas Current and the Agulhas Return Current. In the model a good correlation is found between measuring leakage using the full depth velocities and using only the surface geostrophic velocities. This allows us to extend the method to along-track absolute dynamic topography from satellites. It is shown that the accuracy of the mean dynamic topography does not allow to determine the mean leakage but that leakage anomalies can be accurately computed.

Description: Residual circulation and fresh-water transport in the Dutch Wadden Sea: a numerical modeling study Ocean Science Discussions, 11, 197-257, 2014 Author(s): M. Duran-Matute, T. Gerkema, G. J. de Boer, J. J. Nauw, and U. Gräwe The Dutch Wadden Sea is a region of intertidal flats located between the chain of Wadden Islands and the Dutch mainland. We present numerical model results on the tidal prisms and residual flows through the tidal inlets and across one of the main watersheds. The model also provides insight into the pathways of fresh water originating from the two sluices at the Afsluitdijk (enclosure dike) through the use of passive tracers. All these results are obtained from three-dimensional numerical simulations carried out with the General Estuarine Transport Model (GETM), at a horizontal resolution of 200 m and with terrain-following vertical coordinates with 30 layers. We concentrate on the years 2009–2010, for which we impose meterological forcing, fresh-water discharge, and boundary conditions for tidal forcing and storm surges. Results from the model show an excellent agreement with various observational data sets for sea surface height, temperature, salinity, and transport through the Texel Inlet. The simulations show that although tides are responsible for a characteristic pattern of residual transport through the inlets, the wind imposes a large variability on its magnitude and can even invert its direction during strong southwesterly winds. Even though these events are sporadic, they play an important role in the flushing of the Dutch Wadden Sea, as they strongly diminish the flushing time of fresh water. In addition, wind can force a residual transport across the Terschelling watershed of the same order, if not larger, than through any of the main tidal inlets, despite the fact that the tidal prism associated with the watershed is much smaller than any of those of the inlets. For the pathways of fresh water, the same watershed turns out to be more important than was previously thought, while the opposite holds for the Vlie Inlet.

Description: Possible signals of poleward surface ocean heat transport, of Arctic basal ice melt, and of the twentieth century solar maximum in the 1904–2012 Isle of Man daily timeseries Ocean Science Discussions, 11, 47-122, 2014 Author(s): J. B. Matthews and J. B. R. Matthews This is the second of two papers on observational timeseries of top of ocean heat capture. The first reports hourly and daily meridional central tropical Pacific top 3 m timeseries showing high Southern Hemisphere evaporation (2.67 m yr −1 ) and Northern Hemisphere trapped heat (12 MJ m −2 day −1 ). We suggested that wind drift/geostrophic stratified gyre circulation transported warm water to the Arctic and led to three phases of Arctic basal ice melt and fluxes of brackish nutrient-rich waters to north Atlantic on centennial timescales. Here we examine daily top metre 1904–2012 timeseries at Isle of Man west coast ~54° N for evidence of tropical and polar surface waters. We compare these to Central England (CET) daily land-air temperatures and to Arctic floating ice heat content and extent. We find three phases of ocean surface heating consistent with basal icemelt buffering greenhouse gas warming until a regime shift post-1986 led to the modern surface temperature rise of ~1 °C in 20 yr. Three phases were: warming +0.018 °C yr −1 from 1904–1939, slight cooling −0.002 °C yr −1 1940–86 and strong warming +0.037 °C yr −1 1986–2012. For the same periods CET land-air showed: warming +0.015 °C yr −1 , slight cooling −0.004 °C yr −1 , about half SST warming at +0.018 °C yr −1 . The mid-century cooling and a 1959/1963 hot/cold event is consistent with sunspot/solar radiation maximum 1923–2008 leading to record volumes of Arctic ice meltwater and runoff that peaked in 1962/3 British Isles extreme cold winter. The warming Arctic resulted in wind regime and surface water regime shifts post 1986. This coincides with the onset of rapid Arctic annual ice melt. Continued heat imbalance is likely to lead to tidewater glacier basal icemelt and future sealevel rise after remaining relatively stable over 4000 yr. Our work needs confirmation by further fieldwork concentrating on the dynamics and thermodynamics of ocean top 3 m that controls the 93 % anthropogenic global warming in the oceans. This may be done most cost-effectively through focussed multidisciplinary scientific research adaptively managed and funded.

Description: Chemical and physical transformations of mercury in the ocean: a review Ocean Science Discussions, 11, 1-45, 2014 Author(s): N. Batrakova, O. Travnikov, and O. Rozovskaya Mercury is well known as a dangerous neurotoxin enriched in the environment by human activities. It disperses over the globe, cycling between different environmental media. The ocean plays an important role in the global mercury cycle, acting both as a dispersion medium and as an exposure pathway. In this paper, we review the current knowledge on the major physical and chemical transformations of mercury in the ocean. This review describes the mechanisms and provides a compilation of available rate constants for the major processes in seawater, including oxidation and reduction reactions under light and dark conditions, biotic and abiotic methylation/demethylation, and adsorption by particles. In perspective, these data could be useful for the development of transport models describing processes undergone by mercury in the ocean and in air–seawater exchange.

Description: Weighing the ocean with bottom-pressure sensors: robustness of the ocean mass annual cycle estimate Ocean Science Discussions, 11, 453-496, 2014 Author(s): Joanne Williams, C. W. Hughes, M. E. Tamisiea, and S. D. P. Williams We use ocean bottom pressure measurements from 17 tropical sites to determine the annual cycle of ocean mass. We show that such a calculation is robust, and use three methods to estimate errors in the mass determination. Our final best estimate, using data from the best sites and two ocean models, is that the annual cycle has an amplitude of 0.85 mbar (equivalent to 8.4 mm of sea level, or 3100 Gt of water), with a 95% chance of lying within the range 0.61–1.17 mbar. The time of the peak in ocean mass is 10 October, with 95% chance of occuring between 21 September and 25 October. The simultaneous fitting of annual ocean mass also improves the fitting of bottom pressure instrument drift.

Description: Assessment of the structure and variability of Weddell Sea water masses in distinct ocean reanalysis products Ocean Science Discussions, 11, 497-542, 2014 Author(s): T. S. Dotto, R. Kerr, M. M. Mata, M. Azaneu, and I. Wainer We assessed and evaluated the performance of five ocean reanalysis in reproducing essential hydrographic properties and their associated temporal variability for the Weddell Sea, Antarctica. The products used in this assessment were ECMWF ORAS4, CFSR, MyOcean UR025.4, ECCO2 and SODA. The present study focuses on the Weddell Sea deep layer, which is composed of the following three main water masses: Warm Deep Water (WDW), Weddell Sea Deep Water (WSDW) and Weddell Sea Bottom Water (WSBW). Moreover, all the ocean reanalysis products analyzed showed limited capabilities in representing the surface water masses in the Weddell Sea. The MyOcean UR025.4 product provided the most accurate representation of the structure of the Weddell Sea water masses when compared to observations. The CFSR and ECCO2 products were not able to represent the WSBW throughout the simulation period. The expected WDW warming was only reproduced by the SODA product, while the ECCO2 product was able to represent the WSDW's hydrographic properties trends. All of these ocean reanalysis systems were able to represent the decrease in the WSBW's density. Our results also showed that a simple increase in horizontal resolution does not necessarily imply better representation of the deep layers. Rather, it is needed to observe the physics involved in each model and their parameterizations because the Southern Ocean suffers from the lack of in situ data, and it is biased by summer observations. The choice of the reanalysis product should be made carefully, taking into account the performance, the parameters of interest, and the type of physical processes to be evaluated.

Description: The Mediterranean is getting saltier Ocean Science Discussions, 11, 735-752, 2014 Author(s): M. Borghini, H. Bryden, K. Schroeder, S. Sparnocchia, and A. Vetrano The deep waters of the Mediterranean Sea have been getting saltier and warmer for at least the past 40 yr at rates of about 0.015 and 0.04 °C per decade. Here we show that two processes contribute to these increases in temperature and salinity. On interannual time scales, deep water formation events in severe winters transmit increasingly salty intermediate waters into the deep water. The second process is a steady downward flux of heat and salt through the halocline-thermocline that connects the Levantine Intermediate Water with the deep water. We illustrate these two processes with observations from repeat surveys of the western Mediterranean basin we have made over the past 10 yr.

Description: Dynamics of turbulent western boundary currents at low latitude in a shallow water model Ocean Science Discussions, 11, 753-788, 2014 Author(s): C. Q. C. Akuetevi and A. Wirth The dynamics of low latitude turbulent western boundary currents, subject to two different types of idealized wind forcing, Monsoon Wind and Trade Wind, is considered using numerical results from integrations of a reduced gravity shallow-water model. For viscosity values of 1000 m 2 s −1 and above, the boundary layer dynamics compares well to the analytical solutions of the Munk-layer and the inertial-layer, derived from quasigeostrophic theory. Modifications due to variations in the layer thickness (vortex stretching) are only important close to the boundary. When the viscosity is reduced the boundary layer becomes turbulent and coherent structures in form of anticyclonic eddies, bursts (violent detachments of the viscous sub-layer) and dipoles appear. Three distinct boundary layers emerge, the viscous sub-layer, the advective boundary layer and the extended boundary layer. The first is characterized by a dominant vorticity balance between the viscous transport and the advective transport of vorticity. The second by a balance between the advection of planetary vorticity and the advective transport of relative vorticity. The extended boundary layer is the area to which turbulent motion from the boundary extends. The scaling of the three boundary layer thicknesses with viscosity is evaluated. A pragmatic approach to determine the eddy viscosity diagnostically for coarse resolution numerical models is proposed.

Description: Technical Note: A fully automated purge and trap-GC-MS system for quantification of volatile organic compound (VOC) fluxes between the ocean and atmosphere Ocean Science Discussions, 11, 2979-3002, 2014 Author(s): S. J. Andrews, S. C. Hackenberg, and L. J. Carpenter The oceans are a key source of a number of atmospherically important volatile gases. The accurate and robust determination of trace gases in seawater is a significant analytical challenge, requiring reproducible and ideally automated sample handling, a high efficiency of seawater–air transfer, removal of water vapour from the sample stream, and high sensitivity and selectivity of the analysis. Here we describe a system that was developed for the fully automated analysis of dissolved very short-lived halogenated species (VSLS) sampled from an under-way seawater supply. The system can also be used for semi-automated batch sampling from Niskin bottles filled during CTD (Conductivity, Temperature, Depth) profiles. The essential components comprise of a bespoke, automated purge and trap (AutoP & T) unit coupled to a commercial thermal desorption and gas chromatograph–mass spectrometer (TD-GC-MS). The AutoP & T system has completed five research cruises, from the tropics to the poles, and collected over 2500 oceanic samples to date. It is able to quantify 〉25 species over a boiling point range of 34–180 °C with Henry's Law coefficients of 0.018 and greater (CH 2 I 2 , k H cc dimensionless gas/aqueous) and has been used to measure organic sulfurs, hydrocarbons, halocarbons and terpenes. In the east tropical Pacific, the high sensitivity and sampling frequency provided new information regarding the distribution of VSLS, including novel measurements of a photolytically driven diurnal cycle of CH 2 I 2 within the surface ocean water.

Description: Technical Note: Remote sensing of sea surface salinity using the propagation of low-frequency navigation signals Ocean Science Discussions, 11, 2971-2978, 2014 Author(s): I. Astin and Y. Feng This paper introduces a potential method for the remote sensing of sea surface salinity (SSS) using measured propagation delay of low-frequency Loran-C signals transmitted over an all-seawater path between the Sylt station in Germany and an integrated Loran-C/GPS receiver located in Harwich, UK. The overall delay variations in Loran-C surface waves along the path may be explained by changes in sea surface properties (especially the temperature and salinity), as well as atmospheric dynamics that determine the refractive index of the atmosphere. After removing the atmospheric and sea surface temperature (SST) effects, the residual delay revealed a temporal variation similar to that of SSS data obtained by the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) satellite.

Description: A wind-driven, spatially coherent mode of nonseasonal, depth-independent variability in the Canadian Inland Seas (i.e., the collective of Hudson Bay, James Bay, and Foxe Basin) is identified based on Gravity Recovery and Climate Experiment (GRACE) retrievals, a tide-gauge record, and a barotropic model over 2003–2013. This dominant mode of nonseasonal variability is partly related to the North Atlantic Oscillation and is associated with net flows into and out of the Canadian Inland Seas; the anomalous inflows and outflows, which are reflected in mean sea level and bottom pressure changes, are driven by wind stress anomalies over Hudson Strait, possibly related to wind setup, as well as over the northern North Atlantic Ocean, potentially mediated by various wave mechanisms. The mode is also associated with mass redistribution within the Canadian Inland Seas, reflecting linear response to local wind stress variations under the combined influences of rotation, gravity, and variable bottom topography. Results exemplify the usefulness of GRACE for studying regional ocean circulation and climate.

Description: A reliable radiative transfer model is an essential and indispensable tool for understanding of the radiative transfer processes in homogenous and layered waters, analyzing measurements made by radiance sensors and developing remote sensing algorithms to derive meaningful physical quantities and biogeochemical variables in turbid and productive coastal waters. Existing radiative transfer models have been designed to be applicable to either homogenous waters or inhomogeneous waters. To overcome such constraints associated with these models, this study presents a radiative transfer model that treats a homogenous layer as a diffuse part and an inhomogeneous layer as a direct part in the water column and combines these two parts appropriately in order to generate more reliable underwater light field data such as upwelling radiance (Lu), downwelling irradiance (Ed) and upwelling irradiance (Eu). The diffuse model assumes the inherent optical properties (IOPs) to be vertically continuous and the light fields to exponentially decrease with the depth, whereas the direct part considers the water column to be vertically inhomogeneous (layer-by-layer phenomena) with the vertically varying phase function. The surface and bottom boundary conditions, source function due to chlorophyll and solar incident geometry are also included in the present RT model. The performance of this model is assessed in a variety of waters (clear, turbid and eutrophic) using the measured radiometric data. The present model shows an advantage in terms of producing accurate Lu, Ed and Eu profiles (in spatial domain) in different waters determined by both homogenous and inhomogeneous conditions. The feasibility of predicting these underwater light fields based on the remotely estimated IOP data is also examined using the present RT model. For this application, vertical profiles of the water constituents and IOPs are estimated by empirical models based on our in-situ data. The present RT model generates Lu, Ed and Eu spectra closely consistent with the measured data. These results lead to a conclusion that the present RT model is a viable alternative to existing RT models and has an important implication for remote sensing of optically complex waters.

Description: Flow separation can occur when a stratified flow passes over an obstacle. Previous studies concerning flow separation are mostly observed in the shallow water regions, e.g., fjord, driven by tidal flows. In this study, a novel view of flow separations of the intermediate water over the Hengchun Ridge and Ryukyu Arc off Taiwan is firstly reported using seismic oceanography by reprocessing the seismic data from two cruises 97306 and EW9509 in 2001 and 1995, respectively. Both seismic images show that water masses are separated by internal interfaces detached from sill crests. Over the Hengchun Ridge, it was previously considered that the intermediate water flowed out of the South China Sea into the Hengchun Trough. Flow separation occurred at the lee-side of the ridge forming a continuous interface 〉 15 km between the intermediate and deep water at 900 m depth. This interface suggests that (1) mean flow of the intermediate water always flowed eastward and (2) no lee wave was generated or radiated even during a spring ebb tide. Near the East Taiwan Channel, the only major passage of the Kuroshio into the Okinawa Trough, flow separation occurred when the Kuroshio intermediate water flowed over the Ryukyu Arc at 500 m depth. The separation interface is nearly horizontal and extends 〉 25 km from the sill break to the trough. Parameter estimations and morphologic comparisons show that both separation cases are neither boundary-layer separation nor post-wave separation. Model data from OFES (Ocean General Circulation Model For the Earth Simulator) reveal dense pools existing downstream of the sill crests. We propose that a density-forced flow separation mechanism could be responsible for the observed flow separations and absence of lee waves in this study.

Description: Sea level is one of the 50 Essential Climate Variables (ECVs) listed by the Global Climate Observing System (GCOS) in climate change monitoring. In the last two decades, sea level has been routinely measured from space using satellite altimetry techniques. In order to address a number of important scientific questions such as: "Is sea level rise accelerating?", "Can we close the sea level budget?", "What are the causes of the regional and interannual variability?", "Can we already detect the anthropogenic forcing signature and separate it from the internal/natural climate variability?", and "What are the coastal impacts of sea level rise?", the accuracy of altimetry-based sea level records at global and regional scales needs to be significantly improved. For example, the global mean and regional sea level trend uncertainty should become better than 0.3 and 0.5 mm year−1, respectively (currently of 0.6 and 1–2 mm year−1). Similarly, interannual global mean sea level variations (currently uncertain to 2–3 mm) need to be monitored with better accuracy. In this paper, we present various respective data improvements achieved within the European Space Agency (ESA) Climate Change Initiative (ESA CCI) project on "Sea Level" during its first phase (2010–2013), using multi-mission satellite altimetry data over the 1993–2010 time span. In a first step, using a new processing system with dedicated algorithms and adapted data processing strategies, an improved set of sea level products has been produced. The main improvements include: reduction of orbit errors and wet/dry atmospheric correction errors, reduction of instrumental drifts and bias, inter-calibration biases, intercalibration between missions and combination of the different sea level data sets, and an improvement of the reference mean sea surface. We also present preliminary independent validations of the SL_cci products, based on tide gauges comparison and sea level budget closure approach, as well as comparisons with ocean re-analyses and climate model outputs.

Description: The oceans are a key source of a number of atmospherically important volatile gases. The accurate and robust determination of trace gases in seawater is a significant analytical challenge, requiring reproducible and ideally automated sample handling, a high efficiency of seawater–air transfer, removal of water vapour from the sample stream, and high sensitivity and selectivity of the analysis. Here we describe a system that was developed for the fully automated analysis of dissolved very short-lived halogenated species (VSLS) sampled from an under-way seawater supply. The system can also be used for semi-automated batch sampling from Niskin bottles filled during CTD (Conductivity, Temperature, Depth) profiles. The essential components comprise of a bespoke, automated purge and trap (AutoP & T) unit coupled to a commercial thermal desorption and gas chromatograph–mass spectrometer (TD-GC-MS). The AutoP & T system has completed five research cruises, from the tropics to the poles, and collected over 2500 oceanic samples to date. It is able to quantify 〉25 species over a boiling point range of 34–180 °C with Henry's Law coefficients of 0.018 and greater (CH2I2, kHcc dimensionless gas/aqueous) and has been used to measure organic sulfurs, hydrocarbons, halocarbons and terpenes. In the east tropical Pacific, the high sensitivity and sampling frequency provided new information regarding the distribution of VSLS, including novel measurements of a photolytically driven diurnal cycle of CH2I2 within the surface ocean water.

Description: This paper introduces a potential method for the remote sensing of sea surface salinity (SSS) using measured propagation delay of low-frequency Loran-C signals transmitted over an all-seawater path between the Sylt station in Germany and an integrated Loran-C/GPS receiver located in Harwich, UK. The overall delay variations in Loran-C surface waves along the path may be explained by changes in sea surface properties (especially the temperature and salinity), as well as atmospheric dynamics that determine the refractive index of the atmosphere. After removing the atmospheric and sea surface temperature (SST) effects, the residual delay revealed a temporal variation similar to that of SSS data obtained by the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) satellite.

Description: Using a geometric eddy identification method, cyclonic and anticyclonic eddies from submesoscale to mesoscale in the South Indian Ocean (SIO) have been statistically investigated based on 2082 surface drifters from 1979 to 2013. 19252 eddies are identified with 60% anticyclonic eddies. For the submesoscale eddies (radius r 〈 10 km), the ratio of cyclonic eddies (3183) to anticyclonic eddies (7182) is 1 to 2. In contrast, number of anticyclonic and cyclonic eddies with radius r ≥ 10 km is almost equal. Mesoscale and submesoscale eddies show different spatial distribution. Eddies with radius r ≥ 100 km mainly appear in a band along 25° S, in Mozambique Channel, and Agulhas Current, characterized by large eddy kinetic energy. The submesoscale anticyclonic eddies are densely distributed in the subtropical basin in the central SIO. The number of mesoscale eddies shows statistically significant seasonal variability, reaching a maximum in October and then minimum in February.

Description: The south-eastern parts of the Adriatic Sea coastline were severely polluted by large amounts of accumulated waste material in the second half of November 2010. The waste, reported by major news agencies, accumulated dominantly during 21 November 2010 by favourable wind – ocean current transport system. In the study we analysed meteorological and oceanographic conditions that lead to the waste deposition using available in situ measurements, remote sensing data as well numerical models of the ocean and the atmosphere. The measured data reveal that an intensive rainfall event from 7 till 10 November 2010, over the parts of Montenegro and Albania, was followed by a substantial increase of the river water levels indicating flash floods that possibly splashed the waste material into a river and after to the Adriatic Sea. In order to test our hypothesis we set a number of numerical drifter experiments with trajectories initiated off the coast of Albania during the intensive rainfall events following their faith in space and time. One of the numerical drifter trajectory experiment resulted with drifters reached right position (south-eastern Adriatic coast) and time (exactly by the time the waste was observed) when initiated on 00:00 and 12:00 UTC of 10 November 2010 during the mentioned flash flood event.

Description: Removal of the glint effects from satellite imagery for accurate retrieval of water-leaving radiances is a complicated problem since its contribution in the measured signal is dependent on many factors such as viewing geometry, sun elevation and azimuth, illumination conditions, wind speed and direction, and the water refractive index. To simplify the situation, existing glint correction models describe the extent of the glint-contaminated region and its contribution to the radiance essentially as a function of the wind speed and sea surface slope that often lead to a tremendous loss of information with a considerable scientific and financial impact. Even with the glint-tilting capability of modern sensors, glint contamination is severe on the satellite-derived ocean colour products in the equatorial and sub-tropical regions. To rescue a significant portion of data presently discarded as "glint contaminated" and improving the accuracy of water-leaving radiances in the glint contaminated regions, we developed a glint correction algorithm which is dependent only on the satellite derived Rayleigh Corrected Radiance and absorption by clear waters. The new algorithm is capable of achieving meaningful retrievals of ocean radiances from the glint-contaminated pixels unless saturated by strong glint in any of the wavebands. It takes into consideration the combination of the background absorption of radiance by water and the spectral glint function, to accurately minimize the glint contamination effects and produce robust ocean colour products. The new algorithm is implemented along with an aerosol correction method and its performance is demonstrated for many MODIS-Aqua images over the Arabian Sea, one of the regions that are heavily affected by sunglint due to their geographical location. The results with and without sunglint correction are compared indicating major improvements in the derived products with sunglint correction. When compared to the results of an existing model in the SeaDAS processing system, the new algorithm has the best performance in terms of yielding physically realistic water-leaving radiance spectra and improving the accuracy of the ocean colour products. Validation of MODIS-Aqua derived water-leaving radiances with in-situ data also corroborates the above results. Unlike the standard models, the new algorithm performs well in variable illumination and wind conditions and does not require any auxiliary data besides the Rayleigh-corrected radiance itself. Exploitation of signals observed by sensors looking within regions affected by bright white sunglint is possible with the present algorithm when the requirement of a stable response over a wide dynamical range for these sensors is fulfilled.

Description: This paper presents a multivariate general Pareto distribution (MGPD) method and builds a method for solving MGPD through the use of a Monte Carlo simulation for marine environmental extreme-value parameters. The simulation method has proven to be feasible in the analysis of the joint probability of wave height and its concomitant wind from a hydrological station in the South China Sea (SCS). The MGPD is the natural distribution of the multivariate peaks-over-threshold (MPOT) sampling method, and is based on the extreme-value theory. The existing dependence functions can be used in the MGPD, so it may describe more variables which have different dependence relationships. The MGPD method improves the efficiency of the extremes in raw data. For the wave and the concomitant wind from a period of 23 years (1960–1982), the number of the wave and wind selected is averaged to 19 per year. For the joint conditional probability of the MGPD, the relative error is rather small in the Monte Carlo simulation method.

Description: The accuracy of primary satellite ocean color data products from the Moderate Resolution Imaging Spectroradiometer on-board Aqua (MODIS-A) and the Visible/Infrared Imager/Radiometer Suite (VIIRS), is investigated in the Western Black Sea using in situ measurements from the Gloria site included in the Ocean Color component of the Aerosol Robotic Network (AERONET-OC). The analysis is also extended to an additional well-established AERONET-OC site in the northern Adriatic Sea characterized by optically complex coastal waters exhibiting similarities with those observed at the Gloria site. Results from the comparison of normalized-water leaving radiance LWN indicate biases of a few percent between satellite derived and in situ data at the center-wavelengths relevant for the determination of chlorophyll a concentration (443–547 nm, or equivalent). Remarkable is the consistency among the annual cycle determined with time series of satellite-derived and in situ LWN ratios at these center-wavelengths. Contrarily, the differences between in situ and satellite-derived LWN are pronounced at the blue (i.e., 412 nm) and red (i.e., 667 nm, or equivalent) center-wavelengths, suggesting difficulties in confidently applying satellite-derived radiometric data from these spectral regions for quantitative analysis in optically complex waters.

Description: A realistic circulation model of the North Atlantic ocean at 1/4° resolution (NATL025 NEMO configuration) has been adapted to explicitly simulate model uncertainties. This is achieved by introducing stochastic perturbations in the equation of state to represent the effect of unresolved scales on the model dynamics. The main motivation for this work is to develop ensemble data assimilation methods, assimilating altimetric data from past missions JASON-1 and ENVISAT. The assimilation experiment is designed to better control the Gulf Stream circulation for years 2005/06, focusing on frontal regions which are predominantly affected by unresolved dynamical scales. An ensemble based on such stochastic perturbations is first produced and evaluated using along-track altimetry observations. The Incremental Analysis Update (IAU) scheme is applied in order to obtain an ensemble of continuous trajectories all over the 2005/06 assimilation period. These three elements – stochastic parameterization, ensemble simulation and 4-D observation operator – are then used together to perform a 4-D analysis of along-track altimetry over 10 day windows. Finally, the results of this experiment are objectively evaluated using the standard probabilistic approach developed for meteorological applications (Toth et al., 2003; Candille et al., 2007). The results show that the free ensemble – before starting the assimilation process – correctly reproduces the statistical variability over the Gulf Stream area: the system is then pretty reliable but not informative (null probabilistic resolution). Updating the free ensemble with altimetric data leads to a better reliability with an information gain around 30% (for 10 day forecasts of the SSH variable). Diagnoses on fully independent data (i.e. data that are not assimilated, like temperature and salinity profiles) provide more contrasted results when the free and updated ensembles are compared.

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

Description: Mesoscale variability in the central Rockall Trough between about 56 and 58° N has been investigated using a combination of ship-borne, underwater glider and gridded satellite altimeter measurements. Altimeter observations show that mesoscale features such as eddies and large scale circulation cells are ubiquitous phenomena. They have horizontal length scales of order 100 km with vertical scales of over 1000 m and are associated with mean current speeds (over the upper 1000 m) of 15 ± 7 cm s−1. Monthly area averaged surface Eddy Kinetic Energy (EKE) has substantial inter-annual variability, which at times can dominate a mean seasonal signal that varies from a maximum in May (74 cm2 s−2) to a minimum in October (52 cm2 s−2) and has increased gradually since 1992 at about 1.1 cm2 s−2 per year. A five month glider mission in the Trough showed that much of this energy comes from features that are located over 1000 m below the surface in the deep cold waters of the Trough (possibly from eddies associated the North Atlantic Current). The surface currents from altimeters had similar magnitude to the drift currents averaged over 1000 m from the glider in the stratified autumn, but were half the deep water speed during late winter. Although the mesoscale features move in an apparent random manner they may also be quasi-trapped by submarine topography such as seamounts. Occasionally anti-cyclonic and cyclonic cells combine to cause a coherent westward deflection of the European slope current that warms the Rockall side of the Trough. Such deflections contribute to the inter-annual variability in the observed temperature and salinity that are monitored in the upper 800 m of the Trough. By combining glider and altimeter measurements it is shown that altimeter measurements fail to observe a 15 cm s−1 northward flowing slope current on the eastern side and a small persistent southward current on the western side. There is much to be gained from the synergy between satellite altimetry and in situ glider observations both in the interpretation of their separate data sets and in aiding glider pilots to steer their vehicles through EKE active regions such as the north-east Atlantic.

Description: This paper uses a suite of Earth System models which simulate the distribution of He isotopes and radiocarbon to examine two paradoxes in Earth science. The helium-heat paradox refers to the fact that helium emissions to the deep ocean are far lower than would be expected given the rate of geothermal heating, since both are thought to be the result of radioactive decay in the earth's interior. The isopycnal mixing paradox comes from the fact that many theoretical parameterizations of the isopycnal mixing coefficient ARedi that link it to baroclinic instability project it to be small (of order a few hundred m2 s−1) in the ocean interior away from boundary currents. However, direct observations using tracers and floats (largely in the upper ocean) suggest that values of this coefficient are an order of magnitude higher. Because helium isotopes equilibrate rapidly with the atmosphere, but radiocarbon equilibrates slowly, it might be thought that resolving the isopycnal mixing paradox in favor of the higher observational estimates of ARedi might also solve the helium paradox. In this paper we show that this is not the case. In a suite of models with different spatially constant and spatially varying values of ARedi the distribution of radiocarbon and helium isotopes is sensitive to the value of ARedi. However, away from strong helium sources in the Southeast Pacific, the relationship between the two is not sensitive, indicating that large-scale advection is the limiting process for removing helium and radiocarbon from the deep ocean. The helium isotopes, in turn, suggest a higher value of ARedi in the deep ocean than is seen in theoretical parameterizations based on baroclinic growth rates. We argue that a key part of resolving the isopycnal mixing paradox is to abandon the idea that ARedi has a direct relationship to local baroclinic instability and to the so called "thickness" mixing coefficient AGM.

Description: Seiches are normal modes of water bodies responding to geophysical forcings with potential to significantly impact ecology and maritime operations. Analysis of high-frequency (1 Hz) water level data in Monterey California identifies Harbor modes between 10 and 120 s that are attributed with specific geographic features. It found that modal amplitude modulation arises from cross-modal interaction and that offshore wave energy is a primary driver of these modes. Synchronous coupling between modes is observed to significantly impact dynamic water levels. At lower frequencies between 15 and 60 min modes are independent of offshore wave energy, yet are continuously present. This is unexpected since seiches normally dissipate after cessation of the driving force, indicating an unknown forcing. Spectral and kinematic estimates of these low frequency oscillations supports the idea that a persistent anticyclonic mesoscale gyre adjacent to the Bay is a potential mode driver, while discounting other sources.

Description: We discuss here the evolution of vorticity and potential vorticity (PV) for a bottom current crossing a marine channel in shallow-water approximation, focusing on the effect of friction and mixing. We argue that bottom current vorticity is prone to significant sign changes and oscillations due to topographic effects when, in particular, the current flows over the sill of a channel. These vorticity variations are, however, modulated by frictional effects due to seafloor roughness and morphology. Such behavior is also reflected in the PV spatial evolution, which shows an abrupt peak around the sill region. Our theoretical findings are discussed by means of in situ hydrographic data related to the Eastern Mediterranean Deep Water, i.e., a dense, bottom water vein that flows northwestward, along the Sicily Channel (Mediterranean Sea). Indeed, the narrow sill of this channel implies that friction and entrainment need to be considered. Small tidal effects in the Sicily Channel allow for a steady theoretical approach. Our diagnoses on vorticity and PV allow us to obtain general insights about the effect of mixing and friction on the pathway and internal structure of bottom-trapped currents flowing through channels and straits, and to discuss spatial variability of the frictional coefficient. Our approach significantly differs from other PV-constant approaches previously used in studying the dynamics of bottom currents flowing through rotating channels.

Description: A regional-scale ocean model was used to hindcast the coastal circulation over the Middle Atlantic Bight (MAB) and Gulf of Maine (GOM) from 2004 to 2013. The model was nested inside a data assimilative global ocean model that provided initial and open boundary conditions. Realistic atmospheric forcing, tides and observed river runoff were also used to drive the model. Hindcast solutions were compared against observations, which included coastal sea levels, satellite altimetry sea surface height, temperature and salinity time series in the GOM, glider transects in the MAB, and observed mean depth-averaged velocities by Lentz (2008a). Good agreements with observations suggest that the hindcast model is capable of capturing the major circulation variability in the MAB and GOM. Time- and space-continuous hindcast fields were used to depict the mean circulation, along- and cross-shelf transport and the associated momentum balances. The hindcast confirms the presence of the equatorward mean shelf circulation, which varies from 2.33 Sv at Scotian Shelf to 0.22 Sv near Cape Hatteras. Using the 200 m isobath as the shelf/slope boundary, the mean cross-shelf transport calculations indicate that the shelfbreak segments off the Gulf of Maine (including the southern flank of Georges Bank and the Northeast Channel) and Cape Hatteras are the major sites for shelf water export. The momentum analysis reveals that the along-shelf sea level difference from Nova Scotia to Cape Hatteras is about 0.36 m. The nonlinear advection, stress, and horizontal viscosity terms all contribute to the ageostrophic circulation in the along-isobath direction, whereas the nonlinear advection plays a dominant role in determining the ageostrophic current in the cross-isobath direction.

Description: Argo floats are used to examine tropical cyclone (TC)-induced ocean thermal changes on the global scale by comparing temperature profiles before and after TC passage. We present a footprint method that analyzes cross-track thermal responses along all storm tracks during the period 2004–2012. We combine the results into composite representations of the vertical structure of the average thermal response for two different categories: tropical storms/depressions (TS/TD) and hurricanes. The two footprint composites are functions of three variables: cross-track distance, water depth and time relative to TC passage. We find that this footprint strategy captures the major features of the upper-ocean thermal response to TCs on time scales up to 20 days when compared against previous case study results using in situ measurements. Further, TC effects are distinguishable from background sampling variability, but the significance of this result depends on differences in regional oceanic conditions and the intensity of the TC events. On the global scale, results indicate that hurricanes induce strong upwelling near the storm center, along with downwelling away from the storm, during the first 3 days after storm passage. We also find significant subsurface warming between 30 and 200 m depth for both hurricanes and TS/TDs. On average, the subsurface ocean response persists along storm tracks for up to 20 days down to 200 (400) m depth for TS/TD (Hurricanes), exhibiting peak warming of 0.4 °C at 60 m for hurricanes and 0.2 °C at 35 m for TS/TD. The footprint method shows a weak cooling response between 200 and 400 m, which is significant for Hurricanes but not for TS/TD.