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Climatologies of the mixed layer depth (MLD) have been provided using several definitions based on temperature/density thresholds or hybrid approaches. The upper ocean pycnocline (UOP) that sits below the mixed layer base remains poorly characterized, though this transition layer is an ubiquitous feature of the ocean surface layer. Available hydrographic profiles provide near-global coverage of the world’s ocean and are used to build a seasonal climatology of UOP properties – intensity, depth, thickness – to characterize the spatial and seasonal variations of upper ocean stratification. The largest stratification values O(10−3s−2) are found in the intertropical band, where seasonal variations of the UOP are also very small. The deepest (> 200 m) and least stratified O(10−6s−2) UOPs are found in winter along the Antarctic Circumpolar Current (ACC) and at high latitudes of the North Atlantic. The UOP thickness has a median value of 23 m with limited seasonal and spatial variations; only a few regions have UOP thicknesses exceeding 35 m. The UOP properties allow the characterization of the upper ocean restratification that generally occurs in early spring and is generally associated with large variability. Depending on the region, this restratification may happen gradually as around the Rockall plateau or abruptly as in the Kuroshio Extension. The UOP is also likely to merge intermittently with the permanent pycnocline in winter. The upper edge of the UOP is eventually close to MLD estimates, except in a few notable regions such as in the Pacific Warm Pool where barrier layers are important, and during wintertime at high latitudes of the North Pacific.

The warming of the surface ocean is expected to increase the stratification of the upper water column. This would decrease the efficiency of the wind-induced mixing, reducing the nutrient supply to the euphotic layer and the productivity of the oceans. Climatic projections show that the Mediterranean Sea will experience a strong warming and salting along the twenty first century. Nevertheless, very few works have found and quantified changes in the water column stratification of the Western Mediterranean. In this work, we obtain time series of Mixed Layer Depth (MLD) along the Spanish Mediterranean waters and the Gulf of Cádiz, using periodic CTD profiles collected under the umbrella of the Ocean Observing system of the Instituto Español de Oceanografía (IEO-CSIC). The length of the time series analyzed is variable, depending on the geographical area, but in some cases these time series extend from the beginning of the 1990s decade. Our results show that at present, no statistically significant changes can be detected. These results are confirmed by the analysis of MLD time series obtained from Argo profilers. Some of the meteorological factors that could affect the water column stratification (wind intensity and precipitation rates) did not experience significant changes for the 1990-2021 period, neither were observed long-term changes in the chlorophyll concentration. The hypothesis proposed to explain this lack of trends, is that the salinity increase of the surface waters has compensated for the warming, and consequently, the density of the upper layer of the Western Mediterranean (WMED) has remained constant. As the wind intensity has not experienced significant trends, the stratification of the Spanish Mediterranean waters and those of the Gulf of Cádiz would have not been affected. Nevertheless, we do not discard that our results are a consequence of the short length of the available time series and the large variance of the variables analyzed, evidencing the importance of the maintenance of the ocean monitoring programs., SI

This work is a contribution to CSIC PTI Teledetect.-- 23 pages, 11 figures.-- Data Availability Statement: HYCOM reanalysis outputs (GLBu0.08/expt_19.1) analyzed in this study can be found at hycom.org/data/glbu0pt08/expt-19pt1, Satellite altimeters provide quasi-global measurements of sea surface height, and from those the vertically integrated geostrophic velocity can be directly estimated, but not its vertical structure. This study discusses whether the mesoscale (30–400 km) dynamics of three regions in the South Atlantic can be described by the surface quasi-geostrophic (SQG) theory, both at the surface and in depth, using outputs from an ocean general circulation model. At these scales, the model surface eddy kinetic energy (EKE) spectra show slopes close to k−5/3 (k−3) in winter (summer), characterizing the SQG and quasi-geostrophic (QG) turbulence regimes. We use surface density and temperature to (a) reconstruct the stream function under the SQG theory, (b) assess its capability of reproducing mesoscale motions, and (c) identify the main parameters that improve such reconstruction. For mixed layers shallower than 100 m, the changes in the mixed-layer depth contributes nine times more to the surface SQG reconstruction than the EKE, indicating the strong connection between the quality of the reconstruction and the seasonality of the mixed layer. To further explore the reconstruction vertical extension, we add the barotropic and first baroclinic QG modes to the surface solution. The SQG solutions reproduce the model density and geostrophic velocities in winter, whereas in summer, the interior QG modes prevail. Together, these solutions can improve surface correlations (>0.98) and can depict spatial patterns of mesoscale structures in both the horizontal and vertical domains. Improved spatial resolution from upcoming altimeter missions poses a motivating scenario to extend our findings into future observational studies, We would like to acknowledge the financial support of Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPq (130215/2018-0) and Fundação de Amparo à Pesquisa do Estado de São Paulo—FAPESP (2019/02968-9, 2019/13830-8, and 2017/09659-6). This work was supported in part by the Spanish R&D project L-BAND (ESP2017-89463-C3-1-R), which is funded by MCIN/AEI/10.13039/501100011033 and “ERDF A way of making Europe,” and project INTERACT (PID2020-114623RB-C31), which is funded by MCIN/AEI/10.13039/501100011033. We also acknowledge the financial support from the Spanish government through the “Severo Ochoa Centre of Excellence” accreditation (CEX2019-000928-S). [...] This work supports the SWOT-Brésil project, in preparation for the future SWOT wide-swath altimetry mission. SWOT-Brésil project is financed by CNES/INSU via the French TOSCA programme

The ocean surface boundary layer (OSBL) is one of the largest global regulators of climate. There are two definitions associated with its depth: the mixed layer depth (MLD) and the mixing layer depth (XLD). The former is estimated using vertical profiles of temperature and density, and the latter using measurements of the dissipation rate of turbulent kinetic energy. The relationship between the XLD and MLD has not been extensively studied in the past, since this requires specialized instruments which allow simultaneous measurements of the ocean state variables and turbulence across the full depth of the OSBL. The overarching objective of this thesis is to study the variability of the XLD and MLD under different conditions. To achieve this, different estimates of the MLD and XLD are applied to vertical profiles derived from an autonomous vertical profiler (ASIP) deployed in two different cruises in the North Atlantic. It was found that the MLD criteria differ between themselves when stratification is weaker and the transition between the mixed layer and the pycnocline is not well defined. On the contrary, the XLD criteria had a better agreement between themselves, and a new method was proposed to improve its estimation. Given the close link found between the XLD and the sources of ocean turbulence, a scaling for the XLD was tested dividing the ocean into different mixing regimes according to the ratio between the Monin-Obukhov length, the MLD, and the surface net heat fluxes. The results found using ASIP data were extrapolated to build a climatology of the upper mixing using reanalysis data from ERA5 and ARGO profiles during a period of 10 years (2009-2018). Significant differences between the MLD and the XLD climatology were found during this process.

Place: Oxford Publisher: Pergamon-Elsevier Science Ltd WOS:000556810400013; This work is part of the MADRidge Project special issue which aims to describe pelagic ecosystems in the vicinity of three prominent shallow seamounts in the South West Indian Ocean: one here named MAD-Ridge (240 m below the surface) plus Walters Shoal (18 m) on the Madagascar Ridge, and La Perouse (60 m) on the abyssal plain east of Madagascar. The three span latitudes 20 degrees S and 33 degrees S, some 1500 km. The study provides the background oceanography for the once-off, multidisciplinary snapshot cruise studies around the seamounts. As life on seamounts is determined by factors such as summit depth, proximity to the light layers of the ocean, and the ambient circulation, a first description of regional spatial-field climatologies (16-22 years) and monthly along-ridge gradients of surface wind (driving force), water column properties of sea surface temperature, mixed layer depth, chlorophyll-a and eddy kinetic energy, plus ocean currents is provided. Being relevant to many applications in the study domain, these properties in particular reveal contrasting environments along the Madagascar Ridge and between the three seamounts that should drive biological differences. Relative to the other two seamounts, MAD-Ridge is in the more extreme situation, being at the end of the East Madagascar Current, where it experiences sturdy, albeit variable, currents and the frequent passing of mesoscale eddies.

The seasonal and interannual variability of chlorophyll in the Gulf of Mexico open waters is studied using a three‐dimensional coupled physical‐biogeochemical model. A 5 years hindcast driven by realistic open‐boundary conditions, atmospheric forcings, and freshwater discharges from rivers is performed. The use of recent in situ observations allowed an in‐depth evaluation of the model nutrient and chlorophyll seasonal distributions, including the chlorophyll vertical structure. We find that different chlorophyll patterns of temporal variability coexist in the deep basin which thereby cannot be considered as a homogeneous region with respect to chlorophyll dynamics. A partitioning of the Gulf of Mexico open waters based on the winter chlorophyll concentration increase is then proposed. This partition is basically explained by the amount of nutrients injected into the euphotic layer which is highly constrained by the dynamic of the winter mixed layer. The seasonal and interannual variability appears to be affected by the variability of atmospheric fluxes and mesoscale dynamics (Loop Current eddies in particular). Finally, estimates of primary production in the deep basin are provided.

The depth of the upper ocean mixed layer provides fundamental information on the amount of sea water that directly interacts with the atmosphere. Its space-time variability modulates water mass formation and carbon sequestration processes related to both the physical and biological pumps. These processes are particularly relevant in the Southern Ocean, where surface mixed layer depth estimates are generally obtained either as climatological fields derived from in situ observations or through numerical simulations. Here, we demonstrate that weekly observation-based reconstructions can be used to describe the variations of the mixed layer depth in the upper ocean over a range of space and time scales. We compare and validate four different products obtained by combining satellite measurements of the sea surface temperature, salinity and dynamic topography and in situ Argo profiles. We also compute an ensemble mean and use the corresponding spread to estimate mixed layer depth uncertainties and to identify the more reliable products. The analysis points out the advantage of synergistic approaches that include in input the sea surface salinity observations obtained through a multivariate optimal interpolation. Corresponding data allow to assess mixed layer depth seasonal and interannual variability. Specifically, the maximum correlations between mixed layer anomalies and the Southern Annular Mode are found at different time lags, related to distinct summer/winter responses in the Antarctic Intermediate Water and Sub-Antartic Mode Waters main formation areas.

The article of record as published may be found at http://dx.doi.org/10.1007/s10872-017-0418-0. Two distinct layers usually exist in the upper ocean. The rst has a near-zero vertical gradient in temperature (or density) from the surface and is called the iso-thermal layer (or mixed layer). Beneath that is a layer with a strong vertical gradient in temperature (or density), called the thermocline (or pycnocline). The isothermal layer depth (ILD) or mixed layer depth (MLD) for the same pro le var- ies depending on the method used to determine it. Also, whether they are subjective or objective, existing methods of determining the ILD do not estimate the thermocline (pycnocline) gradient. Here, we propose a new exponen- tial leap-forward gradient (ELG) method of determining the ILD that retains the strengths of subjective (simplicity) and objective (gradient change) methods and avoids their weaknesses (subjective methods are threshold-sensitive and objective methods are computationally intensive). This new method involves two steps: (1) the estimation of the ther- mocline gradient Gth for an individual temperature pro le, and (2) the computation of the vertical gradient by averag- ing over gradients using exponential leap-forward steps. Such averaging can lter out noise in the pro le data. Five existing methods of determining the ILD (difference, gra- dient, maximum curvature, maximum angle, and optimal linear tting methods) as well as the proposed ELG method were veri ed using global expendable bathythermograph (XBT) temperature and conductivity–temperature–depth (CTD) datasets. Among all the methods considered, the ELG method yielded the highest skill score and the lowest Shannon information entropy (i.e., the lowest uncertainty).

Purpose . Spatio-temporal variability of the mixed layer depth (MLD) in different regions of the Black Sea in 1985–2017, its relationship with large- and meso-scale dynamics and wind velocity are investigated. Metods and Results . The study is based on the hydrological data archive for 1985–2017 including the measurements of the ship expeditions, the Argo buoys and the moored buoy “Aqalog”. Seasonal and interannual variability of the mixed layer depth was determined using the density criterion ( dr = = 0,07kg/m 3 ) between the surface layers and the base of the upper mixed layer. Conclusions . In January – March, the large-scale and mesoscale dynamics significantly affects the mixed layer depth variability. Minimum monthly average values of the mixed layer depth in winter are observed in the mesoscale cyclonic eddies and in the center of the sea (20–30 m), the moderate values – on the periphery of the basin (40–45 m) and the maximum ones – in the mesoscale anticyclones (60–70 m). Several times the mixed layer depth values exceeding 150 m were detected in the downwelling areas of the basin. Analysis of the whole period (1985–2017) shows that the mixed layer density was never more than 1015 kg/m 3 . This isopycnal limits the maximum possible depth of the upper mixed layer. The impact of wind velocity on the spatial and temporal variability of the mixed layer thickness is the largest in spring and autumn when the seasonal thermocline is weak. It is less important in summer when solar heating stabilizes the upper layer, and in winter when the mixed layer depth is large. Rise of the mixed layer depth in summer is observed in recent years that is associated with rise of the wind speed in a warm period of a year.

We validated simulations of the Earth system model (ESM) EC-Earth-NEMO of present-day temperature, salinity, nutrient, and chlorophyll a profiles with in situ observations in the Northeast Atlantic Ocean (29&ndash, 63&ordm, N). Simulations with standard parametrization (run 1) and improved parametrization of vertical mixing (run 2) were compared. Run 1 showed shallower mixed layer depths (MLDs) in spring as compared to observations owing to lower salinities in the upper 200 m of the subpolar North Atlantic (&gt, 55&ordm, N). This coincided with a mismatch with observed timing and magnitude of the phytoplankton spring bloom. In contrast, the model performed well south of 55&ordm, N. Run 2 showed improved springtime MLD, phytoplankton dynamics, and nutrient distributions in the subpolar North Atlantic. Our study underlines the sensitivity of subpolar North Atlantic phytoplankton blooms to surface freshening, suggesting that future fresh-water inflow from Arctic and Greenland Ice sheet melting could significantly affect phytoplankton productivity. These findings contribute to the generic validation of the EC-Earth ESM and underline the need for rigorous validation of physics-biology links, in particular the sub polar North Atlantic where complex seasonal stratification/vertical mixing processes govern upper ocean phytoplankton productivity.

WOS:000502584900001; International audience; To assess the eastern Atlantic tropical mixed layer depth (MLD) at 4 degrees W in the Gulf of Guinea, water temperature and density profiles from over five hundred historical observational Conductivity - Temperature - Depth (CTD) data were used. These data, obtained from key oceanographic survey databases covering 60 years (1956 to 2016) were used for a numerical and visual determination of the MLD. The numerical approach consists of the use of the algorithms of three methods; while the visual estimation of the MLD were made on both temperature and density profiles. The numerical approaches were evaluated by comparing their results on the determination of tropical MLD with those of the visual inspections by means of statistical and graphical analysis in order to determine the most suitable method for the determination of MLD in the study area. Our results show that the Boyer Montegut density threshold method (potential density) with the constant criterion Delta sigma = 0.03 kg.m(-3) and a reference depth of ten meters is the most appropriate for determining the MLD in the Gulf of Guinea regardless the season. However, in the situation where only temperature profiles are available it is advisable to estimate MLD using the Lorbacher curvative method.

Located in the mid-latitudes, the Caspian Sea is the largest enclosed basin in the world. A fully-coupled atmosphere-ocean-wave model of the Caspian Sea at high resolution (8 km) for a period of three years is presented. After validating each component of the modelling platform, the wave state of the Caspian Sea is studied. Results show very different wave regimes between the three different basins, a strong seasonality and an almost swell-free state. It is shown here that waves modify the horizontal eddy viscosity and vertical heat diffusion. However, due to a reasonably weak annual wave state, these effects are restricted to the upper-ocean layer (< 30 m) except during the most severe events (100 m). Three main experiments are conducted: 1) the ROMS ocean model forced by atmospheric reanalysis (CFSR), 2) ROMS coupled with the atmospheric model WRF and 3) the impact of wave-induced processes. The seasonality of the Caspian Sea is accurately captured in each experiment which highlights a rapid warming of the sea surface temperature (SST) in spring while the mixed layer depths (MLD) become very rapidly shallow (shifting from over 100 m to 15 m in two months). Contrarily, a gentle cooling of the SST accompanied with a deepening of the MLD is modelled during autumn and winter. The results also show a significant improvement of the model skill in the representation of the dynamics when ROMS is coupled to WRF. Finally, as ocean surface waves imply feedback at the interface atmosphere-ocean through the transfer of momentum, mass and heat, we investigate their potential effects on the Caspian Sea dynamics. Results are mixed and show a reasonably weak impact of wave-induced processes. While waves have a negligible effect during the winter as wave-induced mixing is confined to the MLD, the summer global SST are less accurately modelled due to the enhancement of mixing in shallow MLDs. However the SST bias, temperature at a subsurface location are improved.

In-situ measurement of physical characteristics of seawater (temperature, salinity, and density) collected with a single fire module CTD (Conductivity, Temperature, Depth) in the northern Bay of Bengal (NBoB), as a part of the Indian Ocean, are given in this article. Due to the scanty of data in the NBoB, such in-situ measurements will certainly play a crucial role in understanding regional oceanic processes. The presented data were collected onboard during two cruises covering 12 stations with high resolution (0.2 m depth) up to 50-meter contour depth from 20.5778 N, 92.3578 E to 21.287 N, 89.696 E. We present raw CTD data as ASCII (.dat) and Comma Separated Value (.csv), and processed CTD data as netCDF (.nc) format in this article. Additionally, the calculated Mixed Layer Depth that was derived from the netCDF (.nc) file was included in the tabular format. This processed in-situ data would be most useful as a baseline for further studies in coastal and offshore physicochemical properties in the NBoB.

The year to year variability of surface mixing in the Bay of Bengal (BoB) is examined with the help of an Ocean Dynamic-Thermodynamic Model (ODTM) and observational data. The model embeds a conventional nonlinear primitive equation based reduced gravity model (RGM) with N active layers overlying a 1/2 quiescent layer. The model is coupled to a high-resolution mixed layer model (MLM) to capture the physics of the mixed layer. The MLM incorporates a level-2 turbulence closure mixing scheme based on Mellor and Yamada (1982). As a result of the coupling, our model calculates the net hydrostatic pressure as a sum of those due to the thickness of RGM layers with constant densities and due to the dynamic topography of MLM levels with variable densities. At each time step the coupling between RGM and MLM is achieved as follows: (i) a layer-averaged tendency of momentum and tracers (i.e., temperature and salinity) of the MLM is updated to the RGM, (ii) momentum and tracers of the MLM get advected by large-scale horizontal dynamics of the RGM, and (iii) horizontal layer divergence of the RGM aides for the vertical advection of the MLM. By virtue of this coupling, the model faithfully reproduces the seasonal cycle of temperature, salinity, sea surface height, thermocline, mixed and barrier layer thickness of the tropical Indian Ocean. A simulation with Coordinated Ocean-Ice Reference Experiment (CORE) forcing for 15 years (from 1995 to 2009) is used as a test case to study the interannual variability (IAV) of the mixed layer depth and barrier layer thickness (BL) in BoB. The dominant modes of IAV in the BoB mixing are governed by the correspondingly varying surface momentum, heat, and fresh water fluxes with very little contribution from entrainment of heat and/or salt at the base of the mixed layer. Further, these fluxes are controlled by El Nino-Southern Oscillation (ENSO) variability with very little influence from Indian Ocean Dipole-Zonal Mode (IODZM). The BL IAV is predominantly controlled by precipitation forcing from ENSO. A stability analysis revealed that the turbulent kinetic energy (TKE) and the stability function (S-H) are negatively correlated when ENSO is the dominant forcing. Such a correlation between TKE and S-H is expected since unstable stratification conditions exist during positive ENSO, where the kinetic energy production is enhanced by the unstable buoyancy forcing leading to an increased TKE. During the negative phase of ENSO, stably stratified conditions exist, where the kinetic energy production is offset by the stable buoyancy force, reducing the TKE. This is indicative of high (low) turbulent kinetic energy production, low (high) flux Richardson number based stability function (S-H), and low (high) dominance of buoyancy-driven mixing during positive (negative) phases of ENSO. The results highlight that the counteracting influence of TKE and S-H is a plausible reason for relatively weaker amplitude of IAV of BoB mixing compared to its normal seasonal cycle. Plain Language Summary A dynamic-thermodynamic reduced gravity ocean model embedded with a high-resolution vertical mixed layer model is introduced. Further the model is utilized to study the Bay of Bengal mixing and variability. The year to year variabilities of Bay of Bengal mixing and barrier layer are predominantly controlled by ENSO. Turbulent kinetic energy (TKE) and stability of water column dissociate and mitigate the variability of Bay of Bengal mixing when ENSO is a controlling factor. The study has implications for fine tuning the state of the art general circulation models and climate models in order to represent a robust spatiotemporal variability of surface mixing in the Bay of Bengal.

From the eleven-year data record at the Southern Adriatic Observatory (E2M3A), we study for the first time in the Adriatic Sea the strength of the backscatter signal from an Acoustic Doppler Current Profiler (ADCP) in relation to zooplankton vertical movement. The signal represents well the general behaviour of the zooplankton to sink at dawn and to emerge at the sunset. The modulation of the signal along the year is linked in particular to the different environmental conditions and the presence of different zooplankton groups. The correlation between the backscatter signal and distinct zooplankton groups (copepods, euphausiids, ostracods, appendicularians, salps) shows that in different seasons and environmental conditions, different groups are responsible for the strength of the signal. Furthermore, the backscatter signal, interpreted in terms of the qualitative composition of the zooplankton community, provides important information on the behaviour and habits of zooplankton in the southern Adriatic. Poorly known aspects of zooplankton behaviour are described in more detail, such as: recovery of zooplankton vertical distribution after mixing events, vertical migration in conditions of increased food availability at depth due to vertical mixing, modulation of vertical migration in dependence of the lunar phase and vertical migration in relation to deep chlorophyll maximum position. In addition, the present study emphasizes the possible influence of gelatinous zooplankton at the open sea on high backscatter values during the warmer part of the year.

Global climate models were used to assess changes in the mean, variability and extreme sea surface temperatures (SSTs) in northern oceans with a focus on large marine ecosystems (LMEs) adjacent to North America, Europe, and the Arctic Ocean. Results were obtained from 26 models in the Community Model Intercomparison Project Phase 5 (CMIP5) archive and 30 simulations from the National Center for Atmospheric Research Large Ensemble Community Project (CESM-LENS). All of the simulations used the observed greenhouse gas concentrations for 1976–2005 and the RCP8.5 “business as usual” scenario for greenhouse gases through the remainder of the 21st century. In general, differences between models are substantially larger than among the simulations in the CESM-LENS, indicating that the SST changes are more strongly affected by model formulation than internal climate variability. The annual SST trends over 1976–2099 in the 18 LMEs examined here are all positive ranging from 0.05 to 0.5°C decade–1. SST changes by the end of the 21st century are primarily due to a positive shift in the mean with only modest changes in the variability in most LMEs, resulting in a substantial increase in warm extremes and decrease in cold extremes. The shift in the mean is so large that in many regions SSTs during 2070–2099 will always be warmer than the warmest year during 1976–2005. The SST trends are generally stronger in summer than in winter, as greenhouse gas heating is integrated over a much shallower climatological mixed layer depth in summer than in winter, which amplifies the seasonal cycle of SST over the 21st century. In the Arctic, the mean SST and its variability increases substantially during summer, when it is ice free, but not during winter when a thin layer of ice reforms and SSTs remain near the freezing point.

Variability in the chlorophyll a concentration (Chl) in relation to fluctuations in the mixed layer (ML) was investigated together with turbidity (Tur) in the Kuroshio–Oyashio Extension region, using profiling floats. A particular focus was the validity of two hypotheses concerning the spring bloom: the critical depth hypothesis (CDH) and the recently proposed alternative, the disturbance-recovery hypothesis (DRH). During the period from winter to early spring, Chl and Tur integrated over the photosynthetically active layer (PL; defined as the greatest depth of the ML and the euphotic layer) increased with increasing PL depth (PLD), indicating an increase in the phytoplankton biomass. This result is partly consistent with the DRH in that the observed increase in biomass was not explained by an increase in production. Instead, it was more likely attributable to a reduction in the loss rate. However, theoretical analyses revealed that grazer dilution alone could not cause this increase in biomass because such an increase in the ML in the real ocean (as opposed to a dilution experiment within a bottle) would cause a reduction in the mean light intensity. Despite the loss-controlled fluctuation in biomass during the period of low light, a production-driven fluctuation in biomass was also revealed. This occurred when the light intensity was elevated, particularly after late spring, and was consistent with the CDH. Thus, the present study suggests that both the production-driven and loss-driven hypotheses are responsible for the dynamics of the phytoplankton dynamics from winter to spring in the Kuroshio–Oyashio Extension region.

International audience; Using Argo floats, we show that a major deep convective activity occurred simultaneously in the Labrador Sea (LAB), south of Cape Farewell (SCF), and the Irminger Sea (IRM) during winter 2014-2015. Convection was driven by exceptional heat loss to the atmosphere (up to 50% higher than the climatological mean). This is the first observation of deep convection over such a widespread area. Mixed layer depths exceptionally reached 1700 m in SCF and 1400 m in IRM. The deep thermocline density gradient limited the mixed layer deepening in the Labrador Sea to 1800 m. Potential densities of deep waters were similar in the three basins (27.73-27.74 kg m-3) but warmer by 0.3°C and saltier by 0.04 in IRM than in LAB and SCF, meaning that each basin formed locally its own deep water. The cold anomaly that developed recently in the North Atlantic Ocean favored and was enhanced by this exceptional convection.

International audience; The vertical thermohaline and biogeochemical structures of the upper layer (0 - 200 m) were studied in the Gulf of Tadjourah using high-resolution hydrographic data collected in July-August 2013, September 2013 and February 2014. During summer, the superficial layer consisted of the mixed layer (ML) extending to a depth of about 20 - 30 m followed by the thermocline located between 30 and 50 m depth. The ML was thicker in the west and the southeast where the thermal gradient and chlorophyll a concentrations were particularly high. During September, this stratification persisted but the ML became warmer and saltier and the thermocline moved slightly deeper. In February, the ML extended to about 120 m, and the thermocline was less pronounced. A comparison of the directly measured currents to the wind induced Ekman current and to geostrophic velocity profiles revealed that the thermohaline and the biogeochemical features in summer were related to the southwest monsoon (SWM). The SWM drives surface water from the Gulf of Tadjourah to the Gulf of Aden and thus induces westward intrusion of the high salinity thermocline water from the Gulf of Aden; this near surface flow mixes surface waters in the extreme west of the Gulf of Tajourah. In contrast, the northeast monsoon (NEM), predominant in winter, brings cold water toward the Gulf of Tadjourah and thickens the ML through convective mixing. Our study shows that the SWM plays a crucial role in the stratification of the water column during summer but bathymetry influences its effects. The bowl-shape of the basin and its elongated slope in the west enhance the upwelling in this area where negative sea surface temperature anomalies and high chlorophyll a concentrations were observed.

6 pages, 5 figures, The solar radiation dose in the oceanic upper mixed layer (SRD) has recently been identified as the main climatic force driving global dimethylsulfide (DMS) dynamics and seasonality. Because DMS is suggested to exert a cooling effect on the earth radiative budget through its involvement in the formation and optical properties of tropospheric clouds over the ocean, a positive relationship between DMS and the SRD supports the occurrence of a negative feedback between the oceanic biosphere and climate, as postulated 20 years ago. Such a natural feedback might partly counteract anthropogenic global warming through a shoaling of the mixed layer depth (MLD) and a consequent increase of the SRD and DMS concentrations and emission. By applying two globally derived DMS diagnostic models to global fields of MLD and chlorophyll simulated with an Ocean General Circulation Model coupled to a biogeochemistry model for a 50% increase of atmospheric CO2 and an unperturbed control run, we have estimated the response of the DMS-producing pelagic ocean to global warming. Our results show a net global increase in surface DMS concentrations, especially in summer. This increase, however, is so weak (globally 1.2%) that it can hardly be relevant as compared with the radiative forcing of the increase of greenhouse gases. This contrasts with the seasonal variability of DMS (1000-2000% summer-to-winter ratio). We suggest that the >plankton-DMS-clouds-earth albedo feedback> hypothesis is less strong a long-term thermostatic system than a seasonal mechanism that contributes to regulate the solar radiation doses reaching the earth's biosphere. Copyright © 2007 National Academy of Sciences, This work was supported by the Spanish Ministry of Education and Science through AMIGOS Project Contract REN2001-3462/CLI (to R.S.) and MIMOSA Project Contract CTM2005-06513 (to R.S.), and a Ph.D. studentship (to S.M.V.)

International audience; Pop‐up archival tags (PAT) provide summary and high‐resolution time series data at predefined temporal intervals. The limited battery capabilities of PATs often restrict the transmission success and thus temporal coverage of both data products. While summary data are usually less affected by this problem, as a result of its lower size, it might be less informative. We here investigate the accuracy and feasibility of using temperature at depth summary data provided by PATs to describe encountered oceanographic conditions. Interpolated temperature at depth summary data was found to provide accurate estimates of three major thermal water column structure indicators: thermocline depth, stratification and ocean heat content. Such indicators are useful for the interpretation of the tagged animal's horizontal and vertical behaviour. The accuracy of these indicators was found to be particularly sensitive to the number of data points available in the first 100 m, which in turn depends on the vertical behaviour of the tagged animal. Based on our results, we recommend the use of temperature at depth summary data as opposed to temperature time series data for PAT studies; doing so during the tag programming will help to maximize the amount of transmitted time series data for other key data types such as light levels and depth.

Current spatial interpolation products may be biased by uneven distribution of measurements in time. This manuscript presents a detrending method that recognizes and eliminates this bias. The method estimates temporal trend components in addition to the spatial structure and has been implemented within the Data Interpolating Variational Analysis (DIVA) analysis tool. The assets of this new detrending method are illustrated by producing monthly and annual climatologies of two vertical properties of the Black Sea while recognizing their seasonal and interannual variabilities: the mixed layer depth and the cold content of its cold intermediate layer (CIL). The temporal trends, given as by-products of the method, are used to analyze the seasonal and interannual variability of these variables over the past decades (1955-2011). In particular, the CIL interannual variability is related to the cumulated winter air temperature anomalies, explaining 88 % of its variation. © 2014 Springer-Verlag Berlin Heidelberg., DIVA has received funding from the European Union Seventh Framework Program (FP7/2007–2013) under grant agreement no. 283607, SeaDataNet 2, and from project EMODNET (MARE/2008/03 - Lot 3 Chemistry - SI2.531432) from the Directorate-General for Maritime Affairs and Fisheries. This research was supported by the SESAME Project (EC contract no. GOCE036949) and the PERSEUS Project (EC grant agreement no. 287600)

International audience; Phytoplankton phenology is primarily affected by physical forcing. However, its quantification is far from being completely understood. Among the physical forcing factors, the mixed layer depth (MLD) is considered to have the strongest impact on phytoplankton dynamics, and consequently, on their phenology. The role of MLD variations in shaping the phytoplankton phenology was explored in the Mediterranean Sea, a basin displaying contrasting phenological regimes. A database of MLD estimations was merged with ocean color chlorophyll concentrations ([Chl]SAT) to generate concomitant annual MLD and [Chl]SAT cycles. Several indices were calculated to quantitatively analyze these cycles. The relevance of indices summarizing the temporal difference between main characteristics of MLD and [Chl]SAT cycles was emphasized. As previously observed, two dominant phenological regimes coexist in the Mediterranean Sea. The first is marked by a typical spring bloom, as in temperate regions. The second displays a low seasonality and an absence of an intense [Chl]SAT peak as in subtropical areas. The MLD is shown to play a key role in determining the dominant phenological regime in a given area. Results also show that regions having low seasonality display concomitant MLD and [Chl]SAT maxima, whereas [Chl]SAT peaks are generally observed 30 days after MLD peaks in regions with strongest seasonality. Over the whole basin, [Chl]SAT increase starts 1 month after the initiation of MLD deepening. Finally, after examining the impact of MLD on light and nutrient availability for phytoplankton, mechanisms were proposed to explain the time lags between MLD and [Chl]SAT increase and MLD and [Chl]SAT maxima.

In early May 2012, South Africa will take delivery of a new polar research ship, the SA Agulhas II, representing a significant investment of R1.6 billion in polar infrastructure to further strengthen South Africa’s presence in the polar region, particularly in support of its stewardship of the Southern Ocean and Antarctic Treaty obligations. This investment follows closely on recent DST–CSIR infrastructure investments in the Centre for High Performance Computing, used to run global ocean–climate models, an ocean robotics observational capability and five other new research facilities. Together, these investments offer opportunities to enhance South Africa’s advanced numerical and technological capacity as well as its impact on Southern Hemisphere polar climate and ecosystem science by using its geographical advantage. We are planning the first scientific programme that will capitalise on these major infrastructure investments: the Southern Ocean Seasonal Cycle Experiment (SOSCEx). The Southern Ocean is arguably the main source of medium-term uncertainty in terms of the effectiveness of global CO 2 mitigation plans. The reason for this is that the Southern Ocean plays both an important role in the uptake of anthropogenic CO 2 (50% of all ocean uptake) as well as in the very large (90 Gt/Cy) natural CO 2 exchange between the oceans and the atmosphere. The Southern Ocean is the only region where deep-ocean CO 2 reservoirs (38 000 Gt C) exchange directly with the smaller atmospheric reservoir (700 Gt C). Moreover, although 85% of all ocean productivity is supported by nutrients derived from the Southern Ocean, little is known about the sensitivity of these carbon and nutrient fluxes to climate change driven adjustments or – most importantly – at what scales these links couple. One of the important gaps in the reliable prediction of the response of the Southern Ocean carbon cycle to climate change is its sensitivity to seasonal, subseasonal forcings (in time) and mesoscales (in space). The Southern Ocean Carbon and Climate Observatory (SOCCO), a CSIR-led consortium, is planning SOSCEx, which will be a new type of large-scale experiment. SOSCEx reflects a shift from the historical focus on shipbased descriptive Southern Ocean oceanography and living resource conservation, to systemscale dynamics studies spanning much greater time and space scales. Our limited grasp on these climate and climate-feedback sensitivities are linked to key knowledge gaps on the scales that link climate to carbon. Moreover, existing global models show only weak agreement in terms of the seasonal cycle of the upper-ocean in both physical and biogeochemical indicators such as primary productivity (Lenton A, Tilbrook B 2011, personal communication, Aug 01). The seasonal cycle is one of the strongest modes of variability in the primary productivity and the carbon cycle of the Southern Ocean. Additionally it reflects the coupling between climate forcing and important ecosystem responses, such as productivity. 1

The Arabian Sea is unique due to the extremes in atmospheric forcing that lead to the semi-annual seasonal changes. The reversing winds of summer and winter monsoon induce the variation in the characteristics of mixed layer depth. The importance of mixed layer depth is recognized in studying the biological productivity in the ocean. In this paper variability of mixed layer depth in the north Arabian Sea have been discussed. The study is based on the data collected under North Arabian Sea Environment and Ecosystem Research (NASEER) program. The results of the study indicate that there is a significant variation in the mixed layer depth from summer to winter monsoon as well as coast to offshore.

Buoy data provided clear evidence of mixed layer deepening and an internal wave caused by Hurricane Eloise, September 1975. Logarithmic temperature profiles below an isothermal mixed layer were assumed and used to model thermocline oscillation and heat budget calculation as influenced by Eloise over a 21-day period. Results show that prior to the arrival of Eloise at the buoy, the average mixed layer depth was about 33m. As the winds increased due to hurricane approach, the mixed layer deepened steadily to about 42m before upwelling to approximately 22m. The thermocline then underwent three distinctly large oscillations of inertial periodicity, while the mixed layer continued to deepen. The post-storm average mixed layer depth was about 52m. Values of mixed layer depth were concluded to be accurate to within 2m. Vertical velocities, calculated first by assuming zero horizontal temperature advection in the material derivative equation and second by finding the mass transport necessary to balance the heat budget, show that in the upper 500m of the water column downward vertical motion of lm/hr or less prevailed during storm approach, followed by upward vertical velocity as great as 5.35m/hr during the 12 hr immediately following hurricane passage followed by downward vertical velocity during the large thermocline oscillations. http://archive.org/details/responseofuppero1094518260 Lieutenant Commander, United States Navy Approved for public release; distribution is unlimited.