ALBERT

Description: The results of offshore bottom-mounted ADCP measurements and wind records carried out from August to September 2003 in the coastal waters off Freidoonkenar Bay (FB) in the south Caspian Sea (CS) are examined in order to characterize the shelf motion, the steady current field and to determine the main driving forces of currents on the study area. Owing to closed basin and absence of the astronomical tide, the atmospheric forcing plays an important role in the flow field of the CS. The lasting regular sea breeze system is present almost throughout the year that performs motive force in diurnal and semi-diurnal bands similar to tides in other regions. In general, current field in the continental shelf could be separated into two distinguishable schemes, which in cross-shelf direction is dominated by high frequencies (1 cpd and higher frequencies), and in along-shelf orientation mostly proportional to lower frequencies in synoptic weather bands. Long-period wave currents, whose velocities are much greater than those of direct wind-induced currents, are dominating the current field in the continental shelf off FB. The propagation of the latter could be described in terms of shore-controlled waves that are remotely generated and travel across the shelf in the southern CS. It has also been shown that long term displacements in this area follow the classic cyclonic, circulation pattern in the southern CS.

Description: These days, many marine autonomous environment monitoring networks are set up in the world. These systems take advantage of existing superstructures such as offshore platforms, lightships, piers, breakwaters or are placed on specially designed buoys or underwater oceanographic structures. These systems commonly use various sensors to measure parameters such as dissolved oxygen, turbidity, conductivity, pH or fluorescence. Emphasis has to be put on the long term quality of measurements, yet sensors may face very short-term biofouling effects. Biofouling can disrupt the quality of the measurements, sometimes in less than a week. Many techniques to prevent biofouling on instrumentation are listed and studied by researchers and manufacturers. Very few of them are implemented on instruments and of those very few have been tested in situ on oceanographic sensors for deployment of at least one or two months. This paper presents a review of techniques used to protect against biofouling of in situ sensors and will give a short list and description of promising techniques.

Description: With recently improved instrumental accuracy, the change in the heat content of the oceans and the corresponding contribution to the change of the sea level can be determined from in situ measurements of temperature variation with depth. Nevertheless, it would be favourable if the same changes could be evaluated from just the sea surface temperatures because the record could then be extended into the past and projected into the future. We show here that the average change in the heat content of the oceans and the corresponding contribution to a change in the sea level can be evaluated from the past sea surface temperatures. The calculation is based on the time-dependent diffusion equation with constant upwelling velocity and has no adjustable parameters. In the steady-state limit it recovers the well-known profile of the potential temperature variation as a function of depth. The results are in good agreement with the estimates obtained from the in situ data, even though most of the warming occurs in the upper 1000 m. The method allows us to obtain relevant timescales and average temperature profiles. The evaluation of the thermosteric sea level change is extended back to the beginning of accurate sea surface temperature records in 1880. The changes in sea surface temperature from 1880 until the present time led to a thermosteric sea level rise of 3 cm and to a commitment for a future rise of 5 cm.

Description: The Patagonia continental shelf located off southeastern South America is bounded offshore by the Malvinas Current, which extends northward from northern Drake Passage (~55° S) to nearly 38° S. The transition between relatively warm-fresh shelf waters and Subantarctic Waters from the western boundary current is characterized by a thermohaline front extending nearly 2500 km. We use satellite derived sea surface temperature, and chlorophyll-a data combined with hydrographic and surface drifter data to document the intrusions of slope waters onto the continental shelf near 41° S. These intrusions create vertically coherent localized negative temperature and positive salinity anomalies extending onshore about 150 km from the shelf break. The region is associated with a center of action of the first mode of non-seasonal sea surface temperature variability and also relatively high chlorophyll-a variability, suggesting that the intrusions are important in promoting the local development of phytoplankton. The generation of slope water penetrations at this location may be triggered by the inshore excursion of the 100 m isobath, which appears to steer the Malvinas Current waters over the outer shelf.

Description: In a sensitivity experiment, an eddy-permitting ocean general circulation model is forced with freshwater fluxes from the Greenland Ice Sheet, averaged for the period 1991–2000. The fluxes are obtained with a mass balance model for the ice sheet, forced with the ERA-40 reanalysis dataset. The freshwater flux is distributed around Greenland as an additional term in prescribed runoff, representing seasonal melting of the ice sheet and a fixed year-round iceberg calving flux, for 8.5 model years. The impacts on regional hydrography and circulation are investigated by comparing the sensitivity experiment to a control experiment, without Greenland fluxes. By the end of the sensitivity experiment, the majority of additional fresh water has accumulated in Baffin Bay, and only a small fraction has reached the interior of the Labrador Sea, where winter mixed layer depth is sensitive to small changes in salinity. As a consequence, the impact on large-scale circulation is very slight. An indirect impact of strong freshening off the west coast of Greenland is a small anti-cyclonic circulation around Greenland which opposes the wind-driven cyclonic circulation and reduces net southward flow through the Canadian Archipelago by ~10%. Implications for the post-2000 acceleration of Greenland mass loss are discussed.

Description: Salinity determination in seawater has been carried out for almost 30 years using the 1978 Practical Salinity Standard. However, the numerical value of so-called practical salinity, computed from electrical conductivity, differs slightly from the true or absolute salinity, defined as the mass of dissolved solids per unit mass of seawater. The difference arises because more recent knowledge about the composition of seawater is not reflected in the definition of practical salinity, which was chosen to maintain historical continuity with previous measures, and because of spatial and temporal variations in the relative composition of seawater. Accounting for these variations in density calculations requires the calculation of a correction factor δSA, which is known to range from 0 to 0.03 g kg−1 in the world oceans. Here a mathematical model relating compositional perturbations to δSA is developed, by combining a chemical model for the composition of seawater with a mathematical model for predicting the conductivity of multi-component aqueous solutions. Model calculations generally agree with estimates of δSA based on fits to direct density measurements, and show that biogeochemical perturbations affect conductivity only weakly. However, small systematic differences between model and density-based estimates remain. These may arise for several reasons, including uncertainty about the biogeochemical processes involved in the increase in Total Alkalinity in the North Pacific, uncertainty in the carbon content of IAPSO standard seawater, and uncertainty about the haline contraction coefficient for the constituents involved in biogeochemical processes. This model may then be important in constraining these processes, as well as in future efforts to improve parameterizations for δSA.

Description: A spatio-temporal complexity (STC) measure which has been previously used to analyze data from terrestrial ecosystems is employed to analyse 21 years of remotely sensed sea-surface temperature (SST) data from the Philippines. STC on the Philippine wide SST showed the monsoonal variability of the Philippine waters but did not show significant differences between El Niño, La Niña and normal years. The spatial domain was subsequently divided into six thermal regions computed via clustering of temperature means. The STC values of each thermal region showed variations corresponding to the monsoonal shifts – as well as – to ENSO events. STC characterized environmental heterogeneity over space and time has the potential to define limits of bio-regions. The same approach can be utilized for many long-term remotely sensed data.

Description: A two dimensional scavenging-circulation model is used to investigate the patterns of sediment 231Pa/230Th generated by the Atlantic Meridional Overturning Circulation (AMOC) and further advance the application of this proxy for ocean paleocirculation studies. The scavenging parameters and the geometry of the overturning circulation cell have been chosen so that the model generates meridional sections of dissolved 230Th and 231Pa consistent with published water column profiles and an additional 12 previously unpublished profiles measured in the North and Equatorial Atlantic. The processes that generate the meridional sections of dissolved and particulate 230Th, dissolved and particulate 231Pa, dissolved and particulate 231Pa/230Th, and sediment 231Pa/230Th are discussed in detail. The results indicate that the relationship between sediment 231Pa/230Th at any given site and the overturning circulation is very complex. They clearly show that constraining past changes in the strength and geometry of the AMOC requires an extensive data set and they suggest strategies to maximize information from a limited number of samples.

Description: Data from 38 Argo profiling floats are used to describe the subsurface Mediterranean currents for the period 2003–2009. These floats were programmed to execute 5-day cycles, to drift at a neutral parking depth of 350 m and measure temperature and salinity profiles from either 700 or 2000 m up to the surface. At the end of each cycle the floats remained at the sea surface for about 6 h, enough time to be localised and transmit the data to the Argos satellite system. The Argos positions were used to determine the float surface and subsurface displacements. At the surface, the float motion was approximated by a linear displacement and inertial motion. Subsurface velocities estimates were used to investigate the Mediterranean circulation at 350 m, to compute the pseudo-Eulerian statistics and to study the influence of bathymetry on the subsurface currents. Maximum speeds, as large as 33 cm/s, were found northeast of the Balearic Islands (Western basin) and in the Ierapetra eddy (Eastern basin). Typical speeds in the main along-slope currents (Liguro-Provençal-Catalan, Algerian and Libyo-Egyptian Currents) were ~20 cm/s. In the best sampled regions, the pseudo-Eulerian statistics show typical subsurface circulation pathways which can be related to the motion of Levantine Intermediate Water. In general our results agree with the qualitative subsurface circulation schemes proposed in the literature, except in the southern Ionian where we found westward-flowing subsurface currents. Fluctuating currents appeared to be usually larger than the mean flow. Subsurface currents were found to be essentially parallel to the isobaths over most of the areas characterized by strong bathymetry gradients, in particular, in the vicinity of the continental slopes.

Description: The poleward ocean heat transports in the North Atlantic controlled by the Atlantic Meridional Overturning Circulation (AMOC), play a key role in regional climate. If the AMOC can be initialized in numerical models through ocean assimilation this may help improve the predictability of North Atlantic climate variability on timescales out to a few years. Here we make an initial step toward the development of an ocean assimilation system that can determine the AMOC to support climate predictions. A detailed comparison is presented of 1° and 1/4° resolution global model simulations with and without sequential data assimilation to the observations and transport estimates from the RAPID/MOCHA mooring array across 26.5° N in the Atlantic. Comparisons of modelled water properties with the observations from the merged RAPID boundary arrays demonstrate the ability of in situ data assimilation to accurately constrain the east-west density gradient between these mooring arrays. However, the presence of an unconstrained "western boundary wedge" between Abaco Island and the RAPID mooring site WB2 (16 km offshore) leads to the intensification of an erroneous southwards flow in this region when in situ data are assimilated. The result is an overly intense southward upper mid-ocean transport (0–1100 m) as compared to the estimates derived from the RAPID array. Correction of upper layer zonal density gradients is found to compensate mostly for a weak subtropical gyre circulation in the free model run (i.e. with no assimilation). Despite the important changes to the density structure and transports in the upper layer imposed by the assimilation, very little change is found in the amplitude and sub-seasonal variability of the AMOC. This shows that assimilation of upper layer density information projects mainly on the gyre circulation with little effect on the AMOC at 26° N due to the absence of corrections to density gradients below 2000 m (the maximum depth of Argo). The sensitivity to initial conditions was explored through two additional experiments using a climatological initial condition. These experiments showed that the weak bias in gyre intensity in the control simulation (without data assimilation) develops over the period of about 6 months, but does so independently from the overturning, with no change to the AMOC. However, differences in the properties and volume of North Atlantic Deep Water (NADW) persisted throughout the 3 year simulations resulting in a difference of 3 Sv in AMOC intensity. The persistence of these dense water anomalies and their influence on the AMOC is promising for the development of decadal forecasting capabilities. The results suggest that the deeper waters must be accurately reproduced in order to constrain the AMOC.

Description: The first turbulence profiler observations beneath land fast sea ice which is directly adjacent to an Antarctic ice shelf are described. The stratification in the 325 m deep water column consisted of a layer of supercooled water in the upper 40 m lying above a quasi-linearly stratified water column with a sharp step in density at mid-depth. Turbulent energy dissipation rates were on average 3×10−8 m2 s−3 with peak bin-averaged values reaching 4×10−7 m2 s−3. The local dissipation rate per unit area was estimated to be 10 mWm−2 on average with a peak of 50 mWm−2. These values are consistent with a moderate baroclinic response to the tides. The small-scale turbulent energetics lie on the boundary between isotropy and buoyancy-affected. This will likely influence the formation and aggregation of frazil ice crystals within the supercooled layer. An estimate of the vertical diffusivity of mass Kρ yields a coefficient of around 10−3 m2 s−1. Combining this estimate of Kρ with available observations of average and maximum currents suggests the layer of supercooled water can persist for a distance of ~20 km from the front of the McMurdo Ice Shelf.

Description: Four global ocean/sea-ice simulations driven by the same realistic 46-year daily atmospheric forcing were performed within the DRAKKAR project at 2°, 1°, ½° and ¼° resolutions. Model sea-level anomalies are collocated over the period 1993–2004 onto the AVISO SLA dataset. These five collocated SLA datasets are then filtered and quantitatively compared over various time and space scales regarding three characteristics: SLA standard deviations, spatial correlations between SLA variability maps, and temporal correlations between observed and simulated band-passed filtered local SLA timeseries. Beyond the 2°–1° transition whose benefits are quite moderate, further increases in resolution and associated changes in subgrid scale parameterizations simultaneously induce (i) strong increases in SLA standard deviations, (ii) strong improvements in the spatial distribution of SLA variability, and (iii) slight decreases in temporal correlations between observed and simulation SLA timeseries. These 3 effects are not only clear on mesoscale (14–180 days) and quasi-annual (5–18 months) fluctuations, but also on the slower (interannual), large-scale variability ultimately involved in ocean-atmosphere coupled processes. Most SLA characteristics are monotonically affected by successive resolution increases, but irregularly and with a strong dependance on frequency and latitude. Benefits of enhanced resolution are maximum in the ½°–¼° transition, in the 14–180 day range, and within eddy-active mid- and high-latitude regions. They are particularly clear in the Southern Ocean where mesoscale eddies probably sustain a substantial intrinsic interannual variability.

Description: Sensitivity of the Barents Sea to variation in ocean heat transport and surface fluxes is explored using a 1-D column model. Mean monthly ocean transport and atmospheric forcing are synthesised and force model results that reproduce the observed winter convection and surface warming and freshening well. Model results are compared to existing estimates of the ocean to air heat fluxes and horizontally averaged profiles for the southern and northern Barents Sea. Our results indicate that the ~70 TW of heat transported to the Barents Sea by ocean currents is lost in the southern Barents Sea as latent, sensible, and long wave radiation, each contributing 23–39 TW to the total heat loss. Solar radiation adds 26 TW in the south, as there is no significant ice production. The northern Barents Sea, the major part of the area, receives little ocean heat transport. This leads to a mixed layer at the freezing point during winter and significant ice production. There is little net surface heat loss in the north, the balance is achieved by long wave loss removing most of the solar heating, and the model also suggests a positive sensible heat gain. During the last decade the Barents Sea has experienced an atmospheric warming and an increased ocean heat transport. Despite large changes the Barents Sea heat loss remains robust, the temperature adjusts, and the yearly cycle remains. Decreasing the ocean heat transport below 50 TW starts a transition towards Arctic conditions. The heat loss in the Barents Sea depend on the effective area for cooling, and an increased heat transport probably leads to a spreading of warm water further north.

Description: We present a revisited 2-degree resolution global ocean climatology of monthly mean Barrier Layer Thickness (BLT) first proposed by de Boyer Montégut et al. (2007a). In addition to using an extended data set, we present a modified computation method in order to take into account the observed permeability of Barrier Layers (BL). We name permeability the fact that barrier layers can, in some areas, be very patchy regarding the space and time scales that are considered. This patchiness may have important consequences for the climatic impact of BLs. Differences between the two computation methods are weak for robust BLs that are formed by large-scale processes. The former approach can however largely underestimate the thickness of short and/or localized barrier layers. This is especially the case for the ones formed by mesoscale mechanisms (under the ITCZ for example and along western boundary currents) and patchy barrier layers detected equatorward of the sea surface salinity subtropical maxima. Complete characterisation of regional BL dynamics therefore requires the description of BL robustness through the determination of both BLT and BL permeability.

Description: Data assimilation methods often use an ensemble to represent the background error covariance. Two approaches are commonly used; a simple one with a static ensemble, or a more advanced one with a dynamic ensemble. The latter is often non-practical due to its high computational requirements. Some recent studies suggested using a hybrid covariance, which is a linear combination of the covariances represented by a static and a dynamic ensemble. Here, the use of the hybrid covariance is first extensively tested with a quasi-geostrophic model and with different analysis schemes, namely the Ensemble Kalman Filter (EnKF) and the Ensemble Square Root Filter (ESRF). The hybrid covariance ESRF (ESRF-OI) is more accurate and more stable than the hybrid covariance EnKF (EnKF-OI), but the overall conclusions are similar regardless of the analysis scheme used. The benefits of using the hybrid covariance are large compared to both the static and the dynamic methods with a small dynamic ensemble. The benefits over the dynamic methods become negligible, but remain, for large dynamic ensembles. The optimal value of the hybrid blending coefficient appears to decrease exponentially with the size of the dynamic ensemble. Finally, we consider a realistic application with the assimilation of altimetry data in a hybrid coordinate ocean model (HYCOM) for the Gulf of Mexico, during the shedding of Eddy Yankee (2006). A 10-member EnKF-OI is compared to a 10-member EnKF and a static method called the Ensemble Optimal Interpolation (EnOI). While 10 members seem insufficient for running the EnKF, the 10-member EnKF-OI reduces the forecast error compared to the EnOI, and improves the positions of the fronts.

Description: We examined Indian Ocean Subtropical Mode Water (IOSTMW) and described its characteristics using an isopycnally averaged three-dimensional hydrographic dataset. Through careful examination of the spatial distribution and water characteristics of the core in the layer of minimum vertical temperature gradient, we concluded that the IOSTMW exists as a robust structure in the western part of the Indian Ocean subtropical gyre in summer. The averaged IOSTMW properties during approximately 1960–2004 were 16.54±0.49°C, 35.51±0.04 psu, and 26.0±0.1 σθ. The IOSTMW distribution area was 27–38° S, 25–50° E.

Description: The spatial and temporal sea surface height energy distribution of the Northeast Pacific Ocean is described and discussed. Using an altimetric data set covering 15 years (1993–2007), the energy within the 3–9 month band is primarily located within 10° of the coast. In the Gulf of Alaska, this energy signal is on the shelf, while further south, west of the California/Oregon coast, the significant energy in this band is west of the shelf break. In both cases, it is primarily forced by the local wind. Within the 2–3 year band, the signal reflects energy generated by local changes to the wind stress from large atmospheric shifts indicated by the Pacific North American Index and by advective or propagating processes related to El Niño-Southern Oscillation. Over the two 4–6 year periods within this data set, the change is primarily due to the large scale shift in atmospheric systems north of about 30° N which also affect changes in current strengths. Based on the distribution of the energy signal and its variability, a set of three winter-time indexes are suggested to characterize the distinct differences in the SSH anomalies in these areas.

Description: Spectral characteristics of the oceanic boundary-layer response to wind stress forcing are assessed by comparing surface drifter observations from the Southern Ocean to a suite of idealized models that parameterize the vertical flux of horizontal momentum using a first-order turbulence closure scheme. The models vary in their representation of vertical viscosity and boundary conditions. Each is used to derive a theoretical transfer function for the spectral linear response of the ocean to wind stress. The transfer functions are evaluated using observational data. The ageostrophic component of near-surface velocity is computed by subtracting altimeter-derived geostrophic velocities from observed drifter velocities (nominally drogued to represent motions at 15-m depth.) Then the transfer function is computed to link these ageostrophic velocities to observed wind stresses. The traditional Ekman model, with infinite depth and constant vertical viscosity is among the worst of the models considered in this study. The model that most successfully describes the variability in the drifter data has a shallow layer of depth O(30–50 m), in which the viscosity is constant and O(100–1000 m2 s−1), with a no-slip bottom boundary condition. The second best model has a vertical viscosity with a surface value O(200 m2 s−1), which increases linearly with depth at a rate O(0.1–1 cm s−1) and a no-slip boundary condition at the base of the boundary layer of depth O(103m). The best model shows little latitudinal or seasonal variability, and there is no obvious link to wind stress or climatological mixed-layer depth. In contrast, in the second best model, the linear coefficient and the boundary layer depth seem to covary with wind stress. The depth of the boundary layer for this model is found to be unphysically large at some latitudes and seasons, possibly a consequence of the inability of Ekman models to remove energy from the system by other means than shear-induced dissipation. However, the Ekman depth scale appears to scale like the climatological mixed-layer depth.

Description: We consider the application of the Ensemble Kalman Filter (EnKF) to a coupled ocean ecosystem model (HYCOM-NORWECOM). Such models, especially the ecosystem models, are characterized by strongly non-linear interactions active in ocean blooms and present important limitations for the use of data assimilation methods based on linear statistical analysis. Besides the non-linearity of the model, one is confronted with physical/biological limitations, the analysis state having to be consistent with the model, especially with the constraints of positiveness of some variables. Furthermore the non-Gaussian distributions of the biogeochemical variables break an important assumption of the linear analysis, leading to a loss of optimality of the filter. We present an extension of the EnKF dealing with these limitations by introducing a non-linear change of variables (anamorphosis function) in order to execute the analysis step in a Gaussian space. We present also the initial results of the application of this non-Gaussian extension of the EnKF to the assimilation of simulated chlorophyll surface concentration data in a North Atlantic configuration of the HYCOM NORWECOM coupled model.

Description: The role of the penetration length scale of shortwave radiation into the surface ocean and its impact on tropical Pacific variability is investigated with a fully coupled ocean, atmosphere, land and ice model. Previous work has shown that removal of all ocean color results in a system that tends strongly towards an El Niño state. Results from a suite of surface chlorophyll perturbation experiments show that the mean state and variability of the tropical Pacific is highly sensitive to the concentration and distribution of ocean chlorophyll. Setting the near-oligotrophic regions to contain optically pure water warms the mean state and suppresses variability in the western tropical Pacific. Doing the same above the shadow zones of the tropical Pacific also warms the mean state but enhances the variability. It is shown that increasing penetration can both deepen the pycnocline (which tends to damp El Niño) while shifting the mean circulation so that the wind response to temperature changes is altered. Depending on what region is involved this change in the wind stress can either strengthen or weaken ENSO variability.

Description: New seawater density measurements were made as a function of temperature (0 to 90°C) and salinity (5 to 70). The measurements (N=230) from 0 to 40°C and Practical Salinity from 0 to 40 were found to be in good agreement (σ=0.0036) with the equation of state of seawater (Millero and Poisson, 1981) made on samples with a known chlorinity (Cl). These results indicate that the Practical Salinities (S) are in agreement to within ±0.003 with the values calculated from the Chlorinity, SCl=1.80655 Cl. The measurements from 25 to 90°C were used to extend the equation of state to high temperatures and salinities. All the relative densities (ρ−ρ0, where ρ0 is the density for pure water) were fitted to equations of the form (ρ−ρ0)/kg m−3 = ASA + BSA0.5 + CSA2 where A, B, and C are functions of temperature and SA (g kg−1) is the absolute salinity, SA=(35.16504/35)S. The fitted results from SA=0 to 50 and 0 to 40°C (N=247) gave standard errors of 0.0037 kg m−3. The fitted results from 25 to 90°C (N=271) gave standard errors of 0.0063 kg m−3 and all the results (N=501) from 0 to 90°C gave standard errors of 0.0062 kg m−3. The earlier density measurements used to determine the equation of state of seawater were combined to derive equations that are valid from 0 to 40°C and 0 to 90°C. The standard errors of these fits are, respectively, 0.0038 kg m−3 (N=713) and 0.0063 kg m−3 (N=962). These new measurements expand the equation of state of seawater to a wider range of temperature (0 to 90°C) and absolute salinity (0 to 70).

Description: The OceanSensors08 workshop held 31 March–4 April 2008 in Warnemünde, Germany, brought together an international group of marine scientists, sensor developers and technologists with a common interest in shaping the future of ocean sensing. In preparation for that meeting a series of review papers were commissioned, one of which was meant to cover Sensors for Ocean-omics. The "ocean-omics" topic was cast very broadly. The notion was to review use of genetic techniques for assessing presence and diversity of organisms, their genomic capacity and gene expression, and to provide a prospectus of how such methods could be applied in an autonomous capacity. I chose "ecogenomic sensor" as a descriptor to covey the essence of such a system – a device that integrates genetic level sensing with larger scale environmental characterization. But what exactly are ecogenomic sensors? A clear definition is lacking and conceptualizations of these devices far outweigh actual hardware that can be deployed in the ocean. This prospectus builds from that point. I advance a definition of "ecogenomic sensor" and outline the opportunities and challenges associated with developing such devices. Suggestions as to how this class of instrument may be further refined and applied are offered against the backdrop of the Autonomous Microbial Genosensor (AMG) and Environmental Sample Processor (ESP). Applications that center on detection of DNA and RNA are emphasized. The word "review" appears in the title at the request of the editors.

Description: The upper ocean heat budget (0–300 m) of the North Atlantic from 20°–60° N is investigated using data from Argo profiling floats for 1999–2005 and the NCEP/NCAR and NOC surface flux datasets. Estimates of the different terms in the budget (heat storage, advection, diffusion and surface exchange) are obtained using the methodology developed by Hadfield et al. (2007). The method includes optimal interpolation of the individual profiles to produce gridded fields with error estimates at a 10×10 degree grid box resolution. Closure of the heat budget is obtained within the error estimates for some regions – particularly the eastern subtropical Atlantic – but not for those boxes that include the Gulf Stream. Over the whole range considered, closure is obtained for 13 (9) out of 20 boxes with the NOC (NCEP/NCAR) surface fluxes. The seasonal heat budget at 20°–30° N, 35°–25° W is considered in detail. Here, the NCEP based budget has an annual mean residual of -55±35 W m-2 compared with a NOC based value of -4±35 W m-2. For this box, the net heat divergence of 36 W m-2 (Ekman=-4 W m-2, geostrophic=11 W m-2, diffusion=29 W m-2) offsets the net heating of 32 W m-2 from the NOC surface heat fluxes. The results in this box are consistent with an earlier evaluation of the fluxes using measurements from research buoys in the subduction array which revealed biases in NCEP but good agreement of the buoy values with the NOC fields.

Description: A new error budget assessment of the global Mean Sea Level (MSL) determined by TOPEX/Poseidon and Jason-1 altimeter satellites between January 1993 and June 2008 is presented. We discuss all potential errors affecting the calculation of the global MSL rate. We also compare altimetry-based sea level with tide gauge measurements over the altimetric period. This allows us to provide a realistic error budget of the MSL rise measured by satellite altimetry. These new calculations highlight a reduction in the rate of sea level rise since 2005, by ~2 mm/yr. This represents a 60% reduction compared to the 3.3 mm/yr sea level rise (glacial isostatic adjustment correction applied) measured between 1993 and 2005. Since November 2005, MSL is accurately measured by a single satellite, Jason-1. However the error analysis performed here indicates that the recent reduction in MSL rate is real.

Description: Within the framework of the EGITTO/EGYPT program, the spatial structure and the temporal variability of the surface circulation in the eastern basin of the Mediterranean Sea was studied by means of satellite-tracked drifters. A total of 97 drifters drogued to 15-m nominal depth were released between September 2005 and March 2007, either regularly along ship-of-opportunity routes (Sicily Channel) or within specific structures during dedicated campaigns (Levantine sub-basin). After editing and low-pass filtering, the drifter trajectories were used to estimate pseudo-Eulerian statistics: mean current, variance ellipses, mean and eddy kinetic energies. A statistical analysis was also performed dividing the dataset in two extended seasons (winter and summer). This study completes previous ones in the Sicily Channel and in the Ionian. Several veins are evidenced, together with a seasonal variability inducing a reversal of the circulation in the southern part of the Ionian. However, in this latter area, data are too scarce and the dynamics too complex to achieve a circulation pattern yet. Eastward, the general circulation is described as a counterclockwise flow along the Libyo-Egyptian and Middle East slopes. Part of this flow is deflected toward the open sea by the anticyclones generated alongslope by the Libyo-Egyptian current (Libyan and Egyptian eddies), by the wind (Ierapetra) or by the topography (over the Eratosthenes Seamount and off Latakia). The entrainment of this flow around successive eddies (paddle-wheels effect) results in an open sea eastward transport of Atlantic water. When the Libyan eddies (anticyclonic) are close to the slope, the westward current is stronger than the mean eastward current and the circulation is temporally and locally reversed. The strong variability induced by the eddies and the meteorological conditions hinder evenly covering all the study area. Additional deployments are required to improve further our understanding of the circulation in this basin.

Description: The modal composition of the Central Water in the North Atlantic subtropical gyre is not clearly defined, as there are some uncertainties related to mode identification, as well as modes which are not well documented. This study shows that eastern North Atlantic Central Water (eastern NACW) in the subtropical gyre is composed of three modes: The North Atlantic Subpolar Mode Water (NASPMW σt=27.1 to 27.3), the Madeira Mode Water (MMW σt=26.4 to 26.6), and the mode water with a σt near 27.0, which is currently not well documented. We confirmed this mode based on the similarities found between it and the mode waters already reported. The similarities were determined from comparative analyses of the temperature/salinity standard curves and the gradients of the potential density anomalies of two concurrent data sets from two subtropical time-series stations (Bermuda Atlantic Time-series Study, BATS, in the west, and European Station for Time-series in the Ocean Canary Islands, ESTOC, in the east). In order to establish the outcropping regions, the corresponding pycnostads were determined using another climatologic data set (World Ocean Database, WOD2005). In this case, the pycnostads were located based on the presence of standard deviation minima from the average density anomalies. Finally, we confirmed that the pycnostads corresponded to the temperature values related to the modes by overlaying the characteristic modal isotherm of each of the modes in the geographic distribution of the pycnostads. Sea surface temperature data (SST) from the Ocean Pathfinder Program (OPP) were used to estimate the isotherms. The results showed a clear correspondence between the modal isotherms and the pycnostads, for both the modes that have already been documented and the mode confirmed in this study.

Description: Salinity is a quantity computed, in the actual state of the art, from conductivity ratio measurements, knowing temperature and pressure at the time of the measurement and using the Practical Salinity Scale algorithm of 1978 (PSS-78) which gives practical salinity values S. The uncertainty expected on PSS-78 values is ±0.002, but nothing has ever been detailed about the method to work out this uncertainty, and the sources of errors to include in this calculation. Following a guide edited by the Bureau International des Poids et Mesures (BIPM), this paper assess, by two independent methods, the uncertainties of salinity values obtained from a laboratory salinometer and Conductivity-Temperature-Depth (CTD) measurements after laboratory calibration of a conductivity cell. The results show that the part due to the PSS-78 relations fits is sometimes as much significant as the instruments one's. This is particularly the case with CTD measurements where correlations between the variables contribute to decrease largely the uncertainty on S, even when the expanded uncertainties on conductivity cells calibrations are largely up of 0.002 mS/cm. The relations given in this publication, and obtained with the normalized GUM method, allow a real analysis of the uncertainties sources and they can be used in a more general way, with instruments having different specifications.

Description: The generation of dipolar eddies within the Mediterranean Water (MW) layers, at the Portuguese continental slope, was observed using subsurface RAFOS floats. The surface expression of these mid-depth dipoles is here characterized with remote sensing data, namely with sea surface temperature, chlorophyll concentration and sea surface height measurements. Two cases are presented demonstrating the remote detection of these underwater structures in the Gulf of Cadiz during 1998 and 2001. The presence of subsurface MW dipoles in the Iberian coastal zone is shown to influence the development of coastal upwelling filaments. The surface circulation induced by the dipoles causes the upwelling filaments to extend offshorewards and thus to enhance the transport of physical, chemical and biological properties into the open ocean. A numerical model simulation of the ocean circulation around the Iberian Peninsula forced by heat and freshwater fluxes (computed using the NCEP reanalysis atmospheric state) and by the overflow of MW at the Strait of Gibraltar, corroborates the connection between the surface and the mid-depth flows. The high-resolution numerical experiment is used to help clarifying the occurrence of the MW dipoles surface expression and the impact of these dipoles on the eddy kinetic energy of the upper ocean and on the exchange of volume and salt between the shelf/slope and the open ocean.

Description: The seasonal variability of mixed-layer salinity (MLS) is examined in the Pacific Ocean between 20° S and 60° N using a variety of data sources. Significant seasonal cycles were found in 5 regions: 1) the western North Pacific, 2) the northeastern North Pacific and Alaska gyre, 3) the intertropical convergence zone (ITCZ) 4) an area of the central North Pacific north of the Hawaiian Islands, 5) the central south Pacific along 10–20° S. The phase and amplitude of the seasonal cycle were determined. Amplitudes range from 0.1 to 〉0.5. The largest amplitudes are in the tropical band and the western North Pacific. Maximum salinity is obtained in late (northern) winter in the western North Pacific, late winter and early spring in the northeastern North Pacific, early summer in the ITCZ area, late summer and early fall in the central North Pacific area and (northern) summer in the central South Pacific. Large areas of the North and Tropical Pacific have no significant seasonal variation in MLS. Seasonal variability of evaporation rate, precipitation rate and the difference between them (E-P) were calculated from the OAFlux and Global Precipitation Climatology Project datasets. Typical amplitudes of E-P are 0.1–1×10−4 kg m−2 s−1. The seasonal variability of E-P is largely dominated by variability in evaporation in the western North Pacific and precipitation elsewhere. The largest amplitudes are in areas along the edge of the western North Pacific and in the far eastern tropical Pacific around 10° N. Phases in these areas indicate maximum E-P in mid- to late winter in these areas of large amplitude. The closest correspondence between E-P and MLS is in the ITCZ. Some terms of the MLS balance were calculated, and found to have similar magnitudes, but very different distributions. Averaged over large areas in the western North Pacific, ITCZ, South Pacific and northern North Pacific, the seasonal cycle is a balance between changes in MLS, E-P, and entrainment, with advection playing a relatively minor role. This work highlights the potentially significant role of surface salinity in the hydrologic cycle and in subtropical mode water formation. It can also help to interpret measurements that will soon be available from the Aquarius and SMOS satellite missions.

Description: High-Frequency (HF) radars measure the ocean currents at various spatial and temporal scales. These include tidal currents, wind-driven circulation, density-driven circulation and Stokes drift. Sequential assimilation methods updating the model state have been proven successful to correct the density-driven currents by assimilation of observations such as sea surface height, sea surface temperature and in-situ profiles. However, the situation is different for tides in coastal models since these are not generated within the domain, but are rather propagated inside the domain through the boundary conditions. For improving the modeled tidal variability it is therefore not sufficient to update the model state via data assimilation without updating the boundary conditions. The optimization of boundary conditions to match observations inside the domain is traditionally achieved through variational assimilation methods. In this work we present an ensemble smoother to improve the tidal boundary values so that the model represents more closely the observed currents. To create an ensemble of dynamically realistic boundary conditions, a cost function is formulated which is directly related to the probability of each perturbation. This cost function ensures that the perturbations are spatially smooth and that the structure of the perturbations satisfies approximately the harmonic linearized shallow water equations. Based on those perturbations an ensemble simulation is carried out using the full three-dimension General Estuarine Ocean Model (GETM). Optimized boundary values are obtained using all observations within the assimilation period using the covariances of the ensemble simulation.

Description: A three dimensional numerical model namely, (Princeton Ocean Model) and some observational data are used to study the Persian Gulf outflow structure and its spreading pathways during two different time of the year, mid-winter and early summer. A few available observations show that the Persian Gulf outflow source water exhibits seasonal variations in temperature and salinity. The numerical model is set up by CTD measurements at its western boundary and monthly surface wind speed on the model domain from ICAODS data. The results show that the outflow originates from two branches at different depths in the Persian Gulf. The permanent branch that may exist during the whole year in deeper parts at about 40 m and originates from inner parts of the Persian Gulf and the other one is a seasonal branch that starts to form the vicinity of southern coast during winter months (February). Near the Strait of Hormuz the two branches are jointed together and form the main outflow source. Our findings reveal that during the winter the outflow boundary current detaches from the coast just at the Ras Al Hamra Cape, however for the summer the outflow seems to follow the coast even after this Cape, and appears to separate from the coast at the Ras Al Hadd Cape. This behavior is explained as follow: more saline outflow during February causes higher density and so sinking to deeper zone during the winter. Thus, it moves to deeper parts at about 500 m in contrast with that of May which is at about 300 m. During February at Ras Al Hamra Cape the deeper and stronger outflow is more affected by the steep topography slope leading to vortex stretching mechanism which causes it to meander as an "S" shape, while during May, weaker and shallower outflow is less influenced by bottom topography and so it continues along the boundary.

Description: We present 3-D images of ocean finestructure from a unique industry-collected 3-D multichannel seismic dataset from the Gulf of Mexico that includes expendable bathythermograpgh casts for both swaths. 2-D processing reveals strong laterally continuous reflectors throughout the upper ~800 m as well as a few weaker but still distinct reflectors as deep as ~1100 m. Two bright reflections are traced across the 225-m-wide swath to produce reflector surface images that show the 3-D structure of internal waves. We show that the orientation of internal wave crests can be obtained by calculating the orientations of contours of reflector relief. Preliminary 3-D processing further illustrates the potential of 3-D seismic data in interpreting images of oceanic features such as internal wave strains. This work demonstrates the viability of imaging oceanic finestructure in 3-D and shows that, beyond simply providing a way to see what oceanic finestructure looks like, quantitative information such as the spatial orientation of features like internal waves and solitons can be obtained from 3-D seismic images. We expect complete, optimized 3-D processing to improve both the signal to noise ratio and spatial resolution of our images resulting in increased options for analysis and interpretation.

Description: The Norwegian Atlantic Current (NwAC) is the main part of Atlantic water (AW) towards the Arctic. Fluxes of volume, heat and freshwater in the NwAC are estimated for the period 1992–2007 by combining data from a repeated hydrographic transect in the Norwegian Sea with a recently available data set of absolute topography. The analysis shows a two-branch structure of the NwAC in the section, and the calculated absolute velocities are basically in accordance with independent current measurements. Compared with previous estimated fluxes of volume and heat the estimates in the eastern branch (3.7 Sv/118 TW, Sv=106 m3 s−1, TW=1012 W) are comparable, the estimates for the western branch (1.4 Sv/39 TW) are lower, partly because of a region with recirculation that previously has been neglected. The total fluxes are lower than the upstream fluxes, but several processes, which are addressed, make the upstream-downstream comparison problematic. The heat flux has positive trend of 4.7 TW yr−1 and the freshwater flux has negative trend of −2.4 m Sv yr−1. The trends are results of both increased volume flux (0.13 Sv yr−1) and warmer and saltier inflowing AW. The wind stress curl, spatially averaged over the Norwegian Basin, is on inter-annual scale correlated with both the total fluxes and the occupied area of AW in the section. The latter is influenced by upstream changes in the North Atlantic subpolar gyre on longer time-scale. A first exploitation of the results suggests that increased Atlantic inflow leads to decreased upper ocean stability and delayed phytoplankton spring bloom in the Norwegian Sea, one year later.

Description: Present studies of mixed layer sub-mesoscales rely primarily on high resolution numerical simulations. Only few of these studies have attempted to parameterize the ensuing buoyancy submesoscale fluxes in terms of the resolved fields so that they can be used in OGCMs (ocean circulation models) that do not resolve sub-mesoscales. In reality, OGCMs used in climate studies include a carbon-cycle which also requires the flux of a passive tracer. The goal of this work is to derive and assess a parameterization of the submesoscale vertical flux of an arbitrary tracer in terms of the resolved fields. The parameterization is obtained by first solving the dynamic equations governing the velocity and tracer fields that describe sub-mesoscales and then constructing second-order moments such as the tracer fluxes. A key ingredient of the present approach is the modeling of the non-linear terms that enter the dynamic equations of the velocity and tracer fields, a problem that we discuss in two Appendices. The derivation of the sub-mesoscale tracer vertical flux is analytical and can be followed in detail since no additional information is required. The external forcing includes both baroclinic instabilities and wind stresses. We compare the model results with data from sub-mesoscale resolving simulations available in the literature which are of two kinds, one with no wind (baroclinic instabilities only) and the other with both baroclinic instabilities and wind. In both cases, the model results reproduce the simulation data satisfactorily.

Description: A 1-D biogeochemical/physical model of marine systems has been applied to study the oxygen cycle in four stations of the different sub-basins of the Baltic Sea, namely, in Gotland Deep, Bornholm, Arkona and Fladen. The model consists of biogeochemical model of Neumann et al. (2002) coupled with the 1-D General Ocean Turbulence Model (GOTM). The model has been forced with meteorological data from the ECMWF reanalysis project for the period 1998–2003, producing a 6-year hindcast validated with datasets from the Baltic Environmental Database (BED) for the same period. The vertical profiles of temperature and salinity are relaxed towards both profiles provided by 3-D simulations of General Estuarine Turbulent Model (GETM) and observed profiles from BED. Modifications in the parameterisation of the air/sea oxygen fluxes have led to significant improvement of the model results in the surface and intermediate water levels. The largest mismatch with observation is found in simulating the oxygen dynamics in the Baltic Sea bottom waters. The model results demonstrate the good capability of the model to predict the time-evolution of the physical and biogeochemical variables at all different stations. Comparative analysis of the modelled oxygen concentrations with respect to the observation data is performed to distinguish the relative importance of several factors on the seasonal, interannual and long-term variations of oxygen. It is found that the natural physical factors, like the magnitude of the vertical turbulent mixing, wind speed, the variation in temperature and salinity field are the major factors controlling the oxygen dynamics in the Baltic Sea. The influence of limiting nutrients is less pronounced, at least under the nutrient flux parameterisation assumed in the model.

Description: We examine the behaviour of the El Niño – Southern Oscillation (ENSO) in an ensemble of global climate model simulations with perturbations to parameters in the atmosphere and ocean components respectively. The influence of the uncertainty in these parametrisations on ENSO are investigated systematically. The ensemble exhibits a range of different ENSO behaviour in terms of the amplitude and spatial structure of the SST variability. The nature of the individual feedbacks that operate within the ENSO system are diagnosed using an Intermediate Complexity Model (ICM), which has been used previously to examine the diverse ENSO behaviour of the CMIP3 multi-model ensemble. Unlike in that case, the ENSO in these perturbed physics experiments is not principally controlled by variations in the mean climate state. Rather the parameter perturbations influence the ENSO characteristics by modifying the coupling feedbacks within the cycle. The associated feedbacks that contribute most to the ensemble variations are the response of SST to local wind variability and damping, followed by the response of SST to thermocline anomalies and the response of the zonal wind stress to those SST anomalies. Atmospheric noise amplitudes and oceanic processes play a relatively minor role.

Description: The analysis of the heat and salinity contents have been made for the Northern Atlantic for the decade between January 1999 and December 2008. This analysis is based on the Argo profiling data for the upper 2000 m. Basin-averaged values of anomaly of heat (AHC) and anomaly of salinity content (ASC) are robust and stable. The AHC and ASC demonstrate positive trends in the last decade in the upper 2000 m of the North Atlantic. The linear trend of AHC and ASC are (126.43±18.52)×1020 J, and (47.07±6.90)×1013 kg, respectively. Both trends are significant at 95% level of significance. The main contribution to the positive trend of AHC/ASC comes from the northern parts of the basin.

Description: Very accurate thermodynamic potential functions are available for fluid water, ice, seawater and humid air covering wide ranges of temperature and pressure conditions. They permit the consistent computation of all equilibrium properties as, for example, required for coupled atmosphere–ocean models or the analysis of observational or experimental data. With the exception of humid air, these potential functions are already formulated as international standards released by the International Association for the Properties of Water and Steam (IAPWS), and have been adopted in 2009 for oceanography by IOC/UNESCO. In this paper, we derive a collection of formulas for important quantities expressed in terms of the thermodynamic potentials, valid for typical phase transitions and composite systems of humid air and water/ice/seawater. Particular attention is given to equilibria between seawater and humid air, referred to as ''sea air'' here. In a related initiative, these formulas will soon be implemented in a source-code library for easy practical use. The library is primarily aimed at oceanographic applications but will be relevant to air-sea interaction and meteorology as well. The formulas provided are valid for any consistent set of suitable thermodynamic potential functions. Here we adopt potential functions from previous publications in which they are constructed from theoretical laws and empirical data; they are briefly summarized in the appendix. The formulas make use of the full accuracy of these thermodynamic potentials, without additional approximations or empirical coefficients. They are expressed in the temperature scale ITS-90 and the 2008 Reference-Composition Salinity Scale.

Description: A three-dimensional primitive equation model including sea ice thermodynamics and air-sea interaction is used to study seasonal circulation and water mass variability in the Caspian Sea under the influence of realistic mass, momentum and heat fluxes. River discharges, precipitation, radiation and wind stress are seasonally specified in the model, based on available data sets. The evaporation rate, sensible and latent heat fluxes at the sea surface are computed interactively through an atmospheric boundary layer sub-model, using the ECMWF-ERA15 re-analysis atmospheric data and model generated sea surface temperature. The model successfully simulates sea-level changes and baroclinic circulation/mixing features with forcing specified for a selected year. The results suggest that the seasonal cycle of wind stress is crucial in producing basin circulation. Seasonal cycle of sea surface currents presents three types: cyclonic gyres in December–January; Eckman south-, south-westward drift in February–July embedded by western and eastern southward coastal currents and transition type in August–November. Western and eastern northward sub-surface coastal currents being a result of coastal local dynamics at the same time play an important role in meridional redistribution of water masses. An important part of the work is the simulation of sea surface topography, yielding verifiable results in terms of sea level. Model successfully reproduces sea level variability for four coastal points, where the observed data are available. Analyses of heat and water budgets confirm climatologic estimates of heat and moisture fluxes at the sea surface. Experiments performed with variations in external forcing suggest a sensitive response of the circulation and the water budget to atmospheric and river forcing.

Description: The brackish water of the Baltic Sea is a mixture of ocean water from the Atlantic/North Sea with fresh water from various rivers draining a large area of lowlands and mountain ranges. The evaporation-precipitation balance results in an additional but minor excess of fresh water. The rivers carry different loads of salts washed out of the ground, in particular calcium carbonate, which cause a composition anomaly of the salt dissolved in the Baltic Sea in comparison to Standard Seawater. Directly measured seawater density shows a related anomaly when compared to the density computed from the equation of state as a function of Practical Salinity, temperature and pressure. Samples collected from different regions of the Baltic Sea during 2006–2009 were analysed for their density anomaly. The results obtained for the river load deviate significantly from similar measurements carried out forty years ago; the reasons for this decadal variability are not yet fully understood. An empirical formula is derived which estimates Absolute from Practical Salinity of Baltic Sea water, to be used in conjunction with the new Thermodynamic Equation of Seawater 2010 (TEOS-10), endorsed by IOC/UNESCO in June 2009 as the substitute for the 1980 International Equation of State, EOS-80. Our routine measurements of the samples were accompanied by studies of additional selected properties which are reported here: conductivity, density, chloride, bromide and sulphate content, total CO2 and alkalinity.

Description: The transformation of Agulhas eddies near the continental slope of South Africa and their subsequent self-propagation are analyzed in both observational data and numerical simulations. Self-propagation results from a net dipole moment of a generalized heton structure consisting of a surface-intensified anticyclonic eddy and deep cyclonic pattern. Such Agulhas vortical structures can form near the retroflection region and further north along the western coast of South Africa. We analyze nonlinear topographic wave generation, vortex deformations, and filament production as an important part in water mass exchange. Self-propagating structures provide a conduit for exchange between the deep ocean and shelf regions in the Benguela upwelling system.

Description: The study is a statistical analysis of sea states timeseries derived using the wave model WAM forced by the ERA-40 dataset in selected areas near the Italian coasts. For the period 1 January 1958 to 31 December 1999 the analysis yields: (i) the existence of a negative trend in the annual- and winter-averaged sea state heights; (ii) the existence of a turning-point in late 70's in the annual-averaged trend of sea state heights at a site in the Northern Adriatic Sea; (iii) the overall absence of a significant trend in the annual-averaged mean durations of sea states over thresholds; (iv) the assessment of the extreme values on a time-scale of thousand years. The analysis uses two methods to obtain samples of extremes from the independent sea states: the r-largest annual maxima and the peak-over-threshold. The two methods show statistical differences in retrieving the return values and more generally in describing the significant wave field. The study shows the existence of decadal negative trends in the significant wave heights and by this it conveys useful information on the wave climatology of the Italian seas during the second half of the 20th century.

Description: The relationship between the gyre-scale circulation of the North Atlantic, represented by air pressure near to the centre of the sub-tropical gyre, and sea level measured at the eastern boundary of the ocean has been investigated using records commencing in the middle of the 18th century. These time series are twice as long as those employed in an earlier study of this relationship. Near-continuous values of annual mean sea level and mean high water from Brest, and air pressure fields for the eastern North Atlantic derived from terrestrial instrumental pressure records and ship logbook information, have been used to demonstrate that sea level on the eastern boundary does indeed appear to be related to air pressure at the centre of the gyre, confirming the earlier conclusions but on much longer timescales. This relationship can explain at least part of the century timescale accelerations in European sea level records obtained from tide gauge and saltmarsh data. This finding has important implications for interpretation of the observed sea level rise and acceleration on the European Atlantic coast, suggesting that redistribution of water could play an important role instead of (or as well as) change in ocean volume.

Description: Four cruises between 2002 and 2006 sampled physical and chemical tracers in the southern Irminger Sea during the period of weak convection in the North Atlantic Subpolar Gyre. The upper Labrador Sea Water (uLSW) shows complex and time variable patterns reflecting different formation sites: Irminger Sea, South Greenland and Labrador Sea.

Description: A profiling float equipped with a fluorometer, a dissolved oxygen sensor, and temperature and salinity sensors was deployed in the Subtropical Mode Water (STMW) formation region. It acquired quasi-Lagrangian, 5-day-interval time-series records from March to July 2006. The time-series distribution of chlorophyll a showed a sustained and sizable deep chlorophyll maximum at 50–80 m, just above the upper boundary of the STMW, throughout early summer (May–July). Vertically integrated chlorophyll values during this period consistently ranged from 15–30 mg m−2, indicating sustained primary production and a continuous supply of nutrients ranging from 10–30 mgN m−2 day−1. The time-series data showed no appreciable sporadic events of nutrient supply. Instead, our results support the recently measured large vertical diffusivity values (~5×10−4 m2 s

Description: This paper presents a review of applications of electrochemical methods in ocean sensing. It follows the white paper presented at the OceanSensors08 workshop held at the Leibniz-Institut für Ostseeforschung, Warnemünde, Germany, from 31 March to 4 April 2008. The principles of electrochemical techniques are briefly recalled and described. For each technique, relevant electrochemical sensors are discussed; known successful deployments of electrochemical sensors are recalled; challenges experienced when taking sensors from the research lab to the field are raised; future trends in development and applications are proposed and assessed for their potential for oceanographic applications; where possible technological readiness levels are estimated. The document is supported with references drawn from both the electrochemical and oceanographic literature.

Description: In this article, we describe the first operational implementation of the GHER hydrodynamic model. This happened onboard the research vessel "Alliance", in the context of the Turkish Straits System 2008 campaign, which aimed at the real-time characterization of the Marmara Sea and (south-western) Black Sea. The model performed badly at first, mainly because of poor initial conditions. Hence, as the model includes a reduced-rank extended Kalman filter assimilation scheme, after a hindcast where sea surface temperature and temperature and salinity profiles were assimilated, the model yielded realistic forecasts. Furthermore, the time required to run a one-day simulation (about 5 min of simulation, or 10 min with pre-processing and data transfers included) is very limited and thus operational use of the model is possible.

Description: DINEOF (Data Interpolating Empirical Orthogonal Functions) is an EOF-based technique for the reconstruction of missing data in geophysical fields, such as those produced by clouds in sea surface temperature satellite images. A technique to reduce spurious time variability in DINEOF reconstructions is presented. The reconstruction of these images within a long time series using DINEOF can lead to large discontinuities in the reconstruction. Filtering the temporal covariance matrix allows to reduce this spurious variability and therefore more realistic reconstructions are obtained. The approach is tested in a three years sea surface temperature data set over the Black Sea. The effect of the filter in the temporal EOFs is presented, as well as some examples of the improvement achieved with the filtering in the SST reconstruction, both compared to the DINEOF approach without filtering.

Description: The extending record of ocean colour derived information, an important asset for the study of marine ecosystems and biogeochemistry, presently relies on individual satellite missions launched by several space agencies with differences in sensor design, calibration strategies and algorithms. In this study we present an extensive comparative analysis of standard products obtained from operational global ocean colour sensors (SeaWiFS, MERIS, MODIS-Aqua, MODIS-Terra), on both global and regional scales. The analysis is based on monthly mean chlorophyll-a (Chl-a) surface concentration between 2002 and 2009. Based on global statistics, the Chl-a records appear relatively consistent. The root mean square (RMS) difference Δ between (log-transformed) Chl-a from SeaWiFS and MODIS Aqua amounts to 0.137, with a bias of 0.074 (SeaWiFS Chl-a higher). The difference between these two products and MERIS Chl-a is approximately 0.15. Restricting the analysis to 2007 only, Δ between MODIS Aqua and Terra is 0.142. This global convergence is significantly modulated regionally. Statistics for biogeographic provinces representing a partition of the global ocean, show Δ values varying between 0.08 and 0.3. High latitude regions, as well as coastal and shelf provinces are generally the areas with the largest differences. Moreover, RMS differences and biases are modulated in time, with a coefficient of variation of Δ varying between 10% and 40%, with clear seasonal patterns in some provinces. The comparison of the province-averaged time series obtained from the various satellite products also shows a level of agreement that is geographically variable. Overall, the Chl-a SeaWiFS and MODIS Aqua series appear to have similar levels of variance and display high correlation coefficients, an agreement likely favoured by the common elements shared by the two missions. These results are degraded if the MERIS series is compared to either SeaWiFS or MODIS Aqua. An important outcome of the study is that the results of the inter-comparison analysis are variable with time and location, and therefore globally averaged statistics are not necessarily applicable on a seasonal or regional basis.

Description: Data from two Voluntary Observing Ship (VOS) (MS Trans Carrier and MV Nuka Arctica), acquired along one zonal and one meridional transect (2005–2007) augmented with data subsets from ten cruises (1987–2005) were used to investigate the spatio-temporal variations of the CO2 fugacity in seawater (fCO2sw) in the North Sea at seasonal and inter-annual time scales. The observed seasonal fCO2sw variations were related to variations in sea surface temperature (SST), biology plus mixing, and air-sea CO2 exchange. Over the study period, the seasonal amplitude in fCO2sw induced by SST changes was 0.4–0.75 times those resulting from variations in biology plus mixing. Along the meridional transect, fCO2sw normally decreased northwards (−12 μatm per degree latitude), but the gradient disappeared/reversed during spring as a consequence of an enhanced seasonal amplitude of fCO2sw in southern parts of the North Sea. Along the zonal transect, a weak gradient (−0.8 μatm per degree longitude) was observed in the mean annual fCO2sw. Annually and averaged over the study area, surface waters of the North Sea were CO2 undersaturated and thus a sink of atmospheric CO2 throughout the year. However, during summer, surface waters in the region 55.5–54.5° N were CO2 supersaturated and, hence, a source for atmospheric CO2. Comparison of fCO2sw data acquired within two 1°×1° regions in the northern and southern North Sea during different years (1987, 2001, 2002, and 2005–2007) revealed large interannual variations, especially during spring and summer when year-to-year fCO2sw differences (≈160–200 μatm) approached seasonal changes (≈200–250 μatm). The springtime variations resulted from changes in magnitude and timing of the phytoplankton bloom, whereas changes in SST, wind speed, and total alkalinity may have contributed to the summertime interannual fCO2sw differences. The lowest interannual variation (10–50 μatm) was observed during fall and early winter. The comparison with data reported in October 1967 suggests that the fCO2sw growth rate in the central North Sea is similar to that in the atmosphere.

Description: A compact, low-cost atmospheric pressure, chemical ionization mass spectrometer ("mini-CIMS") has been developed for continuous underway shipboard measurements of dimethylsulfide (DMS) in seawater. The instrument was used to analyze DMS in air equilibrated with flowing seawater across a porous Teflon membrane equilibrator. The equilibrated gas stream was diluted with air containing an isotopically-labeled internal standard. DMS is ionized at atmospheric pressure via proton transfer from water vapor, then declustered, mass filtered via quadrupole mass spectrometry, and detected with an electron multiplier. The instrument described here is based on a low-cost residual gas analyzer (Stanford Research Systems), which has been modified for use as a chemical ionization mass spectrometer. The mini-CIMS has a gas phase detection limit of 170 ppt DMS for a 1 min averaging time, which is roughly equivalent to a seawater DMS concentration of 0.1 nM DMS at 20°C. The mini-CIMS has the sensitivity, selectivity, and time response required for underway measurements of surface ocean DMS over the full range of oceanographic conditions. The simple, robust design and relatively low cost of the instrument are intended to facilitate use in process studies and surveys, with potential for long-term deployment on research vessels, ships of opportunity, and large buoys.

Description: Mesoscale iron enrichment experiments have revealed that additional iron affects the phytoplankton productivity and carbon cycle. However, the role of initial size of fertilized patch in determining the patch evolution is poorly quantified due to the limited time of research vessels at sea. Using a three-dimensional ocean circulation model, we simulated different sizes of inert tracer patches that were only regulated by physical circulation and diffusion. Model results showed that during the first few days since release of inert tracer, the calculated dilution rate was found to be a linear function with time, which was sensitive to the initial patch size with steeper slope for smaller size patch. After the initial phase of rapid decay, the relationship between dilution rate and time became an exponential function, which was also size dependent. Therefore, larger initial size patches can usually last longer and ultimately affect biogeochemical processes much stronger than smaller patches.

Description: Several biogeochemical data have been collected in the last 10 years of Italian activity in Antarctica (ABIOCLEAR, ROSSMIZE, BIOSESO-I/II). A comprehensive 1-D biogeochemical model was implemented as a tool to link observations with processes and to investigate the mechanisms that regulate the flux of biogenic material through the water column. The model is ideally located at station B (175° E–74° S) and was set up to reproduce the seasonal cycle of phytoplankton and organic matter fluxes as forced by the dominant water column physics over the period 1990–2001. Austral spring-summer bloom conditions are assessed by comparing simulated nutrient drawdown, primary production rates, bacterial respiration and biomass with the available observations. The simulated biogenic fluxes of carbon, nitrogen and silica have been compared with the fluxes derived from sediment traps data. The model reproduces the observed magnitude of the biogenic fluxes, especially those found in the bottom sediment trap, but the peaks are markedly delayed in time. Sensitivity experiments have shown that the characterization of detritus, the choice of the sinking velocity and the degradation rates are crucial for the timing and magnitude of the vertical fluxes. An increase of velocity leads to a shift towards observation but also to an overestimation of the deposition flux which can be counteracted by higher bacterial remineralization rates. Model results suggest that the timing of the observed fluxes depends first and foremost on the timing of surface production and on a combination of size-distribution and quality of the autochtonous biogenic material. It is hypothesized that the bottom sediment trap collects material originated from the rapid sinking of freshly-produced particles and also from the previous year's production period.

Description: Cross shelf-break exchange is limited by the tendency of geostrophic flow to follow bathymetric contours, not cross them. However, small scale topography, such as canyons, can reduce the local lengthscale of the flow and increase the local Rossby number. These higher Rossby numbers mean the flow is no longer purely geostrophic and significant cross-isobath flow can occur. This cross-isobath flow includes both upwelling and downwelling due to wind-driven shelf currents and the strong cascading flows of dense shelf-water into the ocean. Tidal currents usually run primarily parallel to the shelf-break topography. Canyons cut across these flows and thus are often regions of generation of strong baroclinic tides and internal waves. Canyons can also focus internal waves. Both processes lead to greatly elevated levels of mixing. Thus, through both advection and mixing processes, canyons can enhance Deep Ocean Shelf Exchange. Here we review the state of the science describing the dynamics of the flows and suggest further areas of research, particularly into quantifying fluxes of nutrients and carbon as well as heat and salt through canyons.

Description: The increasing need for continuous monitoring of the world oceans has stimulated the development of a range of autonomous sampling platforms. One novel addition to these approaches is a small, relatively inexpensive data-relaying device that can be deployed on marine mammals to provide vertical oceanographic profiles throughout the upper 2000 m of the water column. When an animal dives, the CTD-Satellite Relay Data Logger (CTD-SRDL) records vertical profiles of temperature, conductivity and pressure. Data are compressed once the animal returns to the surface where it is located by, and relays data to, the Argos satellite system. The technical challenges met in the design of the CTD-SRDL are the maximising of energy efficiency by minimising size, whilst simultaneously maintaining the reliability of an instrument that cannot be recovered and is required to survive its lifetime attached to a marine mammal. The CTD-SRDLs record temperature and salinity with an accuracy of better than 0.005°C and 0.02 respectively. However, due to the limited availability of reference data for post-processing, data are often associated with slightly higher errors. The potential to collect large numbers of profiles cost-effectively makes data collection using CTD-SRDL technology particularly beneficial in regions where traditional oceanographic measurements are scarce. Depending on the CTD-SRDL configuration, it is possible to sample and transmit hydrographic profiles on a daily basis, providing valuable and often unique information for a real-time ocean observing system.

Description: In biogeochemical models coupled to ocean circulation models, vertical mixing is an important physical process which governs the nutrient supply and the plankton residence in the euphotic layer. However, mixing is often poorly represented in numerical simulations because of approximate parameterizations of sub-grid scale turbulence, wind forcing errors and other mis-represented processes such as restratification by mesoscale eddies. Getting a sufficient knowledge of the nature and structure of these error sources is necessary to implement appropriate data assimilation methods and to evaluate their controllability by a given observation system. In this paper, Monte Carlo simulations are conducted to study mixing errors induced by approximate wind forcings in a three-dimensional coupled physical-biogeochemical model of the North Atlantic with a 1/4° horizontal resolution. An ensemble forecast involving 200 members is performed during the 1998 spring bloom, by prescribing realistic wind perturbations to generate mixing errors. It is shown that the biogeochemical response can be rather complex because of nonlinearities and threshold effects in the coupled model. In particular, the response of the surface phytoplankton depends on the region of interest and is particularly sensitive to the local stratification. We examine the robustness of the statistical relationships computed between the various physical and biogeochemical variables, and we show that significant information on the ecosystem can be obtained from observations of chlorophyll concentration or sea surface temperature. In order to improve the analysis step of sequential assimilation schemes, we propose to perform a simple nonlinear change of variables that operates separately on each state variable, by mapping their ensemble percentiles on the Gaussian percentiles. It is shown that this method is able to substantially reduce the estimation error with respect to the linear estimates computed by the Kalman filter.

Description: Operational prediction of the marine environment is recognised as a fundamental research issue for Europe. We present a pre-operational implementation of a biogeochemical model for pelagic waters of the Mediterranean Sea, as developed within the framework of the MERSEA-IP European project. The OPATM-BFM coupled model is the core of a fully automatic system that weekly delivers analysis and forecast maps for the Mediterranean Sea biogeochemistry. The system in the present configuration has been working since April 2007 with successful execution of the fully automatic operational chain in the 87% of the cases, and in the remaining cases the runs were successfully accomplished after operator intervention. A description of the system developed and a comparison of the model results with satellite data are also presented, with Spearman correlation on surface chlorophyll temporal evolution equal to 0.71. Future studies will be addressed to the implementations of a data assimilation scheme for the biogeochemical compartment in order to increase the skill of the model performances.

Description: The relation between the Agulhas Current retroflection location and the magnitude of Agulhas leakage, the transport of water from the Indian to the Atlantic Ocean, is investigated in a high-resolution numerical ocean model. Sudden eastward retreats of the Agulhas Current retroflection loop are linearly related to the shedding of Agulhas rings, where larger retreats generate larger rings. Using numerical Lagrangian floats a 37 year time series of the magnitude of Agulhas leakage in the model is constructed. The time series exhibits large amounts of variability, both on weekly and annual time scales. A linear relation is found between the magnitude of Agulhas leakage and the location of the Agulhas Current retroflection, both binned to three month averages. In the relation, a more westward location of the Agulhas Current retroflection corresponds to an increased transport from the Indian Ocean to the Atlantic Ocean. When this relation is used in a linear regression and applied to almost 20 years of altimetry data, it yields a best estimate of the mean magnitude of Agulhas leakage of 13.2 Sv. The early retroflection of 2000, when Agulhas leakage was probably halved, can be identified using the regression.

Description: Three independent studies of carbon export and sequestration in the southern Benguela upwelling system are presented. They were undertaken by Waldron (upwelling index), Monteiro (discrete upwelling centres – gate hypothesis model) and Swart (cross-shelf advection in bottom nelpheloid layers). The annual estimates were, 3.9×1013, 0.72×1013 and 8.6×1011 gC respectively. The lowest estimate was derived from a consideration of low frequency lateral carbon export in the bottom nepheloid layer and was thought likely to be an under-estimate. Taking into account high frequency episodic events, intermediate nepheloid layers and along isopycnal export of DOC at surface and intermediate depths was thought likely to result in a substantial upward revision. The remaining two estimates were considered to be an upper and lower estimate of carbon export and sequestration due to factors inherent in the methodologies. The upper estimate presents a two-dimensional system, integrated alongshore; the lower estimate sums a series of upwelling centres in order to obtain a system flux. The former is therefore a uniform extrapolation along the coast while the latter omits upwelling between the upwelling centres.

Description: In the context of stand alone ocean models, the atmospheric forcing is generally computed using atmospheric parameters that are derived from atmospheric reanalysis data and/or satellite products. With such a forcing, the sea surface temperature that is simulated by the ocean model is usually significantly less accurate than the synoptic maps that can be obtained from the satellite observations. This not only penalizes the realism of the ocean long-term simulations, but also the accuracy of the reanalyses or the usefulness of the short-term operational forecasts (which are key GODAE and MERSEA objectives). In order to improve the situation, partly resulting from inaccuracies in the atmospheric forcing parameters, the purpose of this paper is to investigate a way of further adjusting the state of the atmosphere (within appropriate error bars), so that an explicit ocean model can produce a sea surface temperature that better fits the available observations. This is done by performing idealized assimilation experiments in which Mercator-Ocean reanalysis data are considered as a reference simulation describing the true state of the ocean. Synthetic observation datasets for sea surface temperature and salinity are extracted from the reanalysis to be assimilated in a low resolution global ocean model. The results of these experiments show that it is possible to compute piecewise constant parameter corrections, with predefined amplitude limitations, so that long-term free model simulations become much closer to the reanalysis data, with misfit variance typically divided by a factor 3. These results are obtained by applying a Monte Carlo method to simulate the joint parameter/state prior probability distribution. A truncated Gaussian assumption is used to avoid the most extreme and non-physical parameter corrections. The general lesson of our experiments is indeed that a careful specification of the prior information on the parameters and on their associated uncertainties is a key element in the computation of realistic parameter estimates, especially if the system is affected by other potential sources of model errors.

Description: A new numerical general circulation ocean model for the Mediterranean Sea has been implemented nested within an Atlantic general circulation model within the framework of the Marine Environment and Security for the European Area project (MERSEA, Desaubies, 2006). A 4-year twin experiment was carried out from January 2004 to December 2007 with two different models to evaluate the impact on the Mediterranean Sea circulation of open lateral boundary conditions in the Atlantic Ocean. One model considers a closed lateral boundary in a large Atlantic box and the other is nested in the same box in a global ocean circulation model. Impact was observed comparing the two simulations with independent observations: ARGO for temperature and salinity profiles and tide gauges and along-track satellite observations for the sea surface height. The improvement in the nested Atlantic-Mediterranean model with respect to the closed one is particularly evident in the salinity characteristics of the Modified Atlantic Water and in the Mediterranean sea level seasonal variability.

Description: We review mechanisms and studies of exchange between the north-east Atlantic and the adjacent shelf sea. Mechanisms include: well-developed summer upwelling and associated filaments off Portugal and north-west Spain giving exchange O(3 m2/s per unit length of shelf); prevailing westerly winds further north driving exchange O(1 m2/s); poleward flow along most of the upper slope with associated secondary circulation O(1 m2/s); meanders and eddies in this poleward flow; eddies shed from slope waters into the Bay of Biscay; local exchanges at shelf spurs and depressions or canyons (e.g. dense-water cascading of order 1 m2/s). Tidal transports are larger; their reversal every six hours makes exchange largely ineffective except where internal tides are large and non-linear, as in the Celtic Sea where solitons carry water with exchange O(1 m2/s). These various physical exchanges amount to an estimated 2–3 m2/s per unit length of shelf, between ocean and shelf; a numerical model estimate is comparable: 2.5×106 m3/s onto and off the shelf from Brittany to Norway. Mixing controls the seasonal thermocline, affecting primary production and hence fluxes and fate of organic matter. Specifically, CO2 take-up by primary production, settling below the thermocline before respiration, and then off-shelf transport, make an effective shelf-sea "pump" (for CO2 from the atmosphere to the deep ocean). However, knowledge of biogeochemical fluxes is generally sparse; there is scope for more measurements, model validation and estimates from models.

Description: While the concept of oceanic heat transport – or rather heat transport divergence – is known since long, it is sometimes applied inaccurately. Often temperature transports are computed across sections with unbalanced volume flow which then depend entirely on the choice of the temperature scale. The consequences of such arbitrariness are demonstrated with a simple calculation exercise for the passages to the Arctic Ocean. To circumvent the arising difficulties for the Fram Strait as an example we propose a stream tube concept to define a net zero volume flow section which can, with coarse assumptions, be used to determine oceanic heat transport by the Atlantic water flow. Weaknesses of this approach and consequences for observational strategies are discussed.

Description: The height of storm surges is extremely important for a low-lying country like the Netherlands. By law, part of the coastal defence system has to withstand a water level that on average occurs only once every 10 000 years. The question then arises whether and how climate change affects the heights of extreme storm surges. Published research points to only small changes. However, due to the limited amount of data available results are usually limited to relatively frequent extremes like the annual 99%-ile. We here report on results from a 17-member ensemble of North Sea water levels spaning the period 1950–2100. It was created by forcing a surge model of the North Sea with meteorological output from a state-of-the-art global climate model which has been driven by greenhouse gas emissions following the SRES A1b scenario. The large ensemble size enables us to calculate 10 000 year return water levels with a low statistical uncertainty. We find no statistically significant change in the 10 000 year return values of surge heights along the Dutch during the 21st century. Also a higher sea level resulting from global warming does not impact the height of the storm surges. As a side effect of our simulations we also obtain results on the interplay between surge and tide.

Description: Daily timeseries of the meridional overturning circulation (MOC) estimated from the UK/US RAPID/MOCHA array at 26.5° N in the Atlantic are used to evaluate the MOC as simulated in two global circulation models: (i) an 8-member ensemble of the coupled climate model ECHAM5/MPI-OM, and (ii) the ECCO-GODAE state estimate. In ECHAM5/MPI-OM, we find that the observed and simulated MOC have a similar variability and time-mean within the 99 percent confidence interval. In ECCO-GODAE, we find that the observed and simulated MOC show a significant correlation within the 99 percent confidence interval. To investigate the contribution of the different transport components, the MOC is decomposed into Florida Current, Ekman and upper mid-ocean geostrophic transports. In both models, the mid-ocean transport is closely approximated by the residual of the MOC minus Florida Current and Ekman transports. As the models conserve volume by definition, future comparisons of the RAPID/MOCHA mid-ocean transport should be done against the residual transport in the models. The similarity in the variance and the correlation between the RAPID/MOCHA, and respectively ECHAM5/MPI-OM and ECCO-GODAE MOC estimates at 26.5° N is encouraging in the context of estimating (natural) variability in climate simulations and its use in climate change signal-to-noise detection analyses. Enhanced confidence in simulated hydrographic and transport variability will require longer observational time series.

Description: This paper aims to review the availability and application of sensors for observing marine ecosystem status. It gives a broad overview of important ecosystem variables to be investigated, such as biogeochemical cycles, primary and secondary production, species distribution, animal movements, habitats and pollutants. Some relevant legislative drivers are listed, as they provide one context in which ecosystem studies are undertaken. In addition to literature cited within the text the paper contains some useful web links to assist the reader in making an informed instrument choice, as the authors feel that the topic is so broad, it is impossible to discuss all relevant systems or to provide appropriate detail for those discussed. This is therefore an introduction to how and why ecosystem status is currently observed, what variables are quantified, from what platforms, using remote sensing or in-situ measurements, and gives examples of useful sensor based tools. Starting with those presently available, to those under development and also highlighting sensors not yet realised but desirable for future studies.

Description: We present the late summer distribution and transports of freshwater components in the upper western part of the Fram Strait during 1998, 2004 and 2005. Hydrographic data and and water δ18O values are analyzed to distinguish Atlantic Water, ice-melt (IMW) and freshwater removal from ice formation (IFW), and Meteoric Water (precipitation and riverine sources; MW). Concentrations of these water masses are combined with volume transport estimates from an inverse model. The average liquid freshwater transport relative to a reference salinity of 34.92, was 2500 km3/yr or 80 mSv southward, which is at the upper end of values reported in the literature. Our results indicate that not only the region of the continental slope but also parts of the East Greenland Shelf are important for freshwater transports. The average transports of MW and IFW were 160 mSv (5000 km3/yr) and 90 mSv (2800 km3/yr) southward, respectively. The southward transport of MW was higher in 2005 than in 1998, but was compensated by a higher IFW transport. These differences in transports were associated with stronger southward velocities and the absence of northward velocities over the continental slope and the eastern East Greenland Shelf in 2005. A simulation using the North Atlantic-Arctic Ocean Sea Ice Model (NAOSIM) shows that the high transport of MW in the Fram Strait in 2005 is in agreement with the temporary storage of river water on the Siberian shelf in the mid-1990s, which reached the north of Greenland in 2003. Our results indicate that IFW follows the same pathways as MW before reaching the Fram Strait.

Description: High resolution hydrographic observations of temperature and salinity were used to analyze the formation and distribution of isothermal (ZT), mixed (ZM) and barrier layers (BL) in a section of the southwestern Atlantic (0°30' N–14°00' S; 31°24'–41°48' W), adjacent to the northeastern Brazilian coast. Analyzed data consisted of 279 CTD casts acquired during two cruises under the Brazilian REVIZEE Program, one in late austral winter (August–October 1995) and another in austral summer (January–April 1997). Results indicated that the intrusion of subtropical Salinity Maximum Waters (SMW) brought by the South Equatorial Current (SEC) from the subtropical region into the western tropical Atlantic boundary is the major process contributing to the seasonal BL formation. During late austral winter, BL 5–90 m thickness (BLT) (median=15 m) was observed, but BLT〉30 m was restricted to latitudes higher than 8° S as a result of a combination of deep isothermal layers (ZT≥90 m) and shallow mixed layers, where the latter was created by the intrusion of salty waters between 8–12.3° S. During austral summer, shallow isothermal and mixed layers prevailed and the BL formation was clearly driven by establishing a salt-induced pycnocline inside an isothermal layer. Observed BLT was less variable (5–70 m) and thicker (median=35 m). BLT≥30 m was observed not only in the southernmost part of the study area, as verified during late winter, but in the latitude range 2° S–14° S, where near surface salty waters were transported westward by the SEC flow.

Description: A reanalysis of the North Atlantic spring bloom in 2007 was produced using the real-time analyses from the TOPAZ (Towards an Operational Prediction system for the North Atlantic European coastal Zones) North Atlantic and Arctic forecasting system. The TOPAZ system uses a hybrid coordinate general circulation ocean model and assimilates physical observations: sea surface anomalies, sea surface temperatures, and sea-ice concentrations using the Ensemble Kalman Filter. This ocean model was coupled to an ecosystem model, NORWECOM (Norwegian Ecological Model System), and the TOPAZ-NORWECOM coupled model was run throughout the spring and summer of 2007. The ecosystem model was run online, restarting from analyzed physical fields (result after data assimilation) every 7 days. Biological variables were not assimilated in the model. The forecast was compared to remotely sensed chlorophyll and in-situ data. The impact of physical data assimilation on the ecosystem model was determined by comparing the results to those from a model without assimilation of physical data. The regions of focus are the North Atlantic and the Arctic Ocean. The results show that the model reproduces a realistic annual cycle, but the chlorophyll concentrations tend to be too low during winter and spring and too high during summer. Surface nutrients on the other hand are generally too low throughout the year. Assimilation of physical variables does not affect the results from the ecosystem model significantly. The differences between the weekly mean values of chlorophyll are normally within 5–10% during the summer months, and the maximum difference of ~20% occurs in the Arctic, also during summer. Special attention was paid to the nutrient input from the North Atlantic to the Nordic Seas and the impact of ice-assimilation on the ecosystem. The ice-assimilation increased the phytoplankton concentration: because there was less ice in the assimilation run, this increased both the mixing of nutrients during winter and the area where production could occur during summer.

Description: A method is presented to create an ensemble of perturbations that satisfies linear dynamical constraints. A cost function is formulated defining the probability of each perturbation. It is shown that the perturbations created with this approach take the land-sea mask into account in a similar way as variational analysis techniques. The impact of the land-sea mask is illustrated with an idealized configuration of a barrier island. Perturbations with a spatially variable correlation length can be also created by this approach. The method is applied to a realistic configuration of the West Florida Shelf to create perturbations of the M2 tidal parameters for elevation and depth-averaged currents. The perturbations are weakly constrained to satisfy the linear shallow-water equations. Despite that the constraint is derived from an idealized assumption, it is shown that this approach is applicable to a non-linear and baroclinic model. The amplitude of spurious transient motions created by constrained perturbations of initial and boundary conditions is significantly lower compared to perturbing the variables independently or to using only the momentum equation to compute the velocity perturbations from the elevation.

Description: In the Gulf of Guinea, intraseasonal variability is large at the equator and along the coast. Current data on the continental slope near 7.5° S show very energetic biweekly oscillations at 1300 m depth. A high resolution numerical model demonstrates that this deep variability is forced by equatorial winds, through the generation of equatorial Yanai waves that propagate eastward and at depth, and then poleward as coastal-trapped waves upon reaching the coast of Africa. Intraseasonal variability is intensified along the coast, especially in the 500–1500 m depth range, with the largest intensification in the 10–20 day period range. The structure of kinetic energy is well explained at first order by a linear model with six baroclinic modes. Along the equator, eastward intensification of energy and bottom intensification are in qualitative agreement with vertically propagating Yanai waves, although the signal is clearly influenced by the details of the bathymetry. Along the coast, vertical modes 3 to 5 are important close to the equator, and the signal is dominated by lower modes farther south. Additional current meter data on the continental slope near 3° N display an energy profile in the 10–20 day period band that is strikingly different from the one at 7.5° S, with surface intensification rather than bottom intensification and a secondary maximum near 800 m. The model reproduces these features and explains them: the surface intensification in the north is due to the regional wind forcing, and the north-south dissymetry of the deep signal is due to the presence of the zonal African coast near 5° N. A 4 years time series at 7.5° S displays intermittencies of the 10–20 day signal near the bottom. This intermittency is not correlated with fluctuations of the equatorial winds and does not seem to be a simple linear response to the wind forcing.

Description: A 4th order advection scheme is applied in a nested eddy-resolving Hybrid Coordinate Ocean Model (HYCOM) of the greater Agulhas Current system for the purpose of testing advanced numerics as a means for improving the model simulation for eventual operational implementation. Model validation techniques comparing sea surface height variations, sea level skewness and variogram analyses to satellite altimetry measurements quantify that generally the 4th order advection scheme improves the realism of the model simulation. The most striking improvement over the standard 2nd order momentum advection scheme, is that the Southern Agulhas Current is simulated as a well-defined meandering current, rather than a train of successive eddies. A better vertical structure and stronger poleward transports in the Agulhas Current core contribute toward a better southwestward penetration of the current, and its temperature field, implying a stronger Indo-Atlantic inter-ocean exchange. It is found that the transport, and hence this exchange, is sensitive to the occurrences of mesoscale features originating upstream in the Mozambique Channel and Southern East Madagascar Current, and that the improved HYCOM simulation is well suited for further studies of these inter-actions.

Description: Highly accurate and precise measurements of marine carbon components are required in the study of the marine carbon cycle, particularly when investigating the causes for its variability from seasonal to interannual timescales. This is especially true in the investigation of the consequences of anthropogenic influences. The analysis of any component requires elaborate instrumentation, most of which is currently used onboard ships, either in manual mode or autonomous mode. Technological developments result in more and more instruments that have long-term reliability so that they can be deployed on surface moorings and buoys, whilst the great technological and operational challenges mean that only few sensors have been developed that can be used for sub-surface in situ measurements on floats, robots, or gliders. There is a special need for autonomous instruments and sensors that are able to measure a combination of different components, in order to increase the spatial and temporal coverage of marine carbon data. This paper describes analytical techniques used for the detection of the marine dissolved carbon components, both inorganic and organic: the fugacity of CO2, total dissolved inorganic carbon, pH, alkalinity, and dissolved organic carbon. By pointing out advantages, disadvantages, and challenges of the techniques employed in the analysis of each component, we aim to aid non-carbon marine scientists, sensor developers and technologists, in the decision where to tackle the challenges of further development.

Description: Using the ECHAM5/MPI-OM model, we study the relation between the variations in the Atlantic meridional overturning circulation (AMOC) and both the Pacific sea surface temperature (SST) and the El Niño-Southern Oscillation (ENSO) amplitude. In a 17-member 20C3M/SRES-A1b ensemble for 1950–2100 the Pacific response to AMOC variations on different time scales and amplitudes is considered. The Pacific response to AMOC variations associated with the Atlantic Multidecadal Oscillation (AMO) is very small. In a 5-member hosing ensemble where the AMOC collapses due to a large freshwater anomaly, the Pacific SST response is large and in agreement with previous work. Our results show that the modelled connection between AMOC and ENSO depends very strongly on the frequency and/or the amplitude of the AMOC variations. Interannual AMOC variations, decadal AMOC variations and an AMOC collapse lead to with entirely different responses in the Pacific Ocean.

Description: (p, ρ, T) data of standard seawater with practical salinity S≈35 (corresponding to an absolute salinity SA≈35.16504 g/kg) measured at T=(273.14 to 468.06) K and pressures up to p=140 MPa are reported with an estimated experimental relative combined standard uncertainty of 0.006% in density. The measurements were made with a newly constructed vibration-tube densimeter. The system was calibrated using double-distilled water, methanol and aqueous NaCl solutions. An empirical correlation for the density of standard seawater has been developed as a function of pressure and temperature. This equation of state was used to calculate other volumetric properties such as isothermal compressibility, isobaric thermal expansibility, differences in isobaric and isochoric heat capacities, thermal pressure coefficient, internal pressure and secant bulk modulus. The results can be used to extend the present equation of state of seawater to higher temperature as a function of pressure.

Description: At the present time, little is known about how broad salinity and temperature ranges are for seawater thermodynamic models that are functions of absolute salinity (SA), temperature (T) and pressure (P). Such models rely on fixed compositional ratios of the major components (e.g. Na/Cl, Mg/Cl, Ca/Cl, SO4/Cl, etc.). As seawater evaporates or freezes, solid phases (e.g. CaCO3(s) or CaSO42H2O(s)) will eventually precipitate. This will change the compositional ratios, and these salinity models will no longer be applicable. A future complicating factor is the lowering of seawater pH as the atmospheric concentrations of CO2 increase. A geochemical model (FREZCHEM) was used to quantify the SA-T boundaries at P=0.1 MPa and the range of these boundaries for future atmospheric CO2 increases. An omega supersaturation model for CaCO3 minerals based on homogeneous nucleation was extended from 25–40°C to 3°C. CaCO3 minerals were the boundary defining minerals (first to precipitate) between 3°C (at SA=104 g kg-1 and 40°C (at SA=66 g kg-1. At 2.82°C, calcite(CaCO3) transitioned to ikaite(CaCO36H2O) as the dominant boundary defining mineral for colder temperatures, which culminated in a low temperature boundary of −4.93°C. Increasing atmospheric CO2 from 385 μatm (in Year 2008) to 550 μatm (in Year 2100) would increase the SA and t boundaries as much as 11 g kg−1 and 0.66°C, respectively. The model-calculated calcite-ikaite transition temperature of 2.82°C is in excellent agreement with ikaite formation in natural environments that occurs at temperatures of 3°C or lower. Furthermore, these results provide a quantitative theoretical explanation (FREZCHEM model calculations) for why ikaite is the solid phase CaCO3 mineral that precipitates during seawater freezing.

Description: To date, density and other thermodynamic properties of seawater have been calculated from Practical Salinity, S P. It is more accurate however to use Absolute Salinity, S A (the mass fraction of dissolved material in seawater). Absolute Salinity S A can be expressed in terms of Practical Salinity S P as S A=(35.165 04 g kg-1/35)S P+δ S A(φ, λ, p) where δ S A is the Absolute Salinity Anomaly as a function of longitude φ, latitude λ and pressure. When a seawater sample has standard composition (i.e. the ratios of the constituents of sea salt are the same as those of surface water of the North Atlantic), the Absolute Salinity Anomaly is zero. When seawater is not of standard composition, the Absolute Salinity Anomaly needs to be estimated; this anomaly is as large as 0.025 g kg−1 in the northernmost North Pacific. Here we provide an algorithm for estimating Absolute Salinity Anomaly for any location (φ, λ, p) in the world ocean. To develop this algorithm we use the Absolute Salinity Anomaly that is found by comparing the density calculated from Practical Salinity to the density measured in the laboratory. These estimates of Absolute Salinity Anomaly however are limited to the number of available observations (namely 811). To expand our data set we take advantage of approximate relationships between Absolute Salinity Anomaly and silicate concentrations (which are available globally). We approximate the laboratory-determined values of δ S A of the 811 seawater samples as a series of simple functions of the silicate concentration of the seawater sample and latitude; one function for each ocean basin. We use these basin-specific correlations and a digital atlas of silicate in the world ocean to deduce the Absolute Salinity Anomaly globally and this is stored as an atlas, δ S A (φ, λ, p). This atlas can be interpolated to the latitude, longitude and pressure of a seawater sample to estimate its Absolute Salinity Anomaly. For the 811 samples studied, ignoring the Absolute Salinity Anomaly results in a standard error in S A of 0.0107 g kg-1. Using our algorithm for δ S A reduces the error to 0.0048 g kg−1, reducing the mean square error by a factor of five. The number of sea water samples used to develop the correlation relationship is limited, and we hope that the algorithm and error can be improved as further data becomes available.

Description: Nowadays Earth observation satellites provide information about many relevant variables of the ocean-climate system, such as temperature, moisture, aerosols, etc. However, to retrieve the velocity field, which is the most relevant dynamical variable, is still a technological challenge, specially in the case of oceans. New processing techniques, emerged from the theory of turbulent flows, have come to assist us in this task. In this paper, we show that multifractal techniques applied to new Sea Surface Temperature satellite products opens the way to build maps of ocean currents with unprecedented accuracy. With the application of singularity analysis, we show that global ocean circulation patterns can be retrieved in a daily basis. We compare these results with high-quality altimetry-derived geostrophic velocities, finding a quite good correspondence of the observed patterns both qualitatively and quantitatively. The implications of this findings from the perspective both of theory and of operational applications are discussed.

Description: The entrainment of bouyant ambient water into the overflow plume of Denmark Strait and the associated downstream warming of the plume are estimated using time series of currents and temperature from moored instrumentation and classical hydrographic data. Warming rates are highest (0.4–0.5 K/100 km) within the first 200 km of the sill, and decrease to 0.05–0.1 K/100 km further downstream. Stirring by mesoscale eddies cause lateral heatfluxes that explain the 0.1 K/100 km warming, but in the first 200 km from the sill also vertical diapycnal fluxes, probably caused by breaking internal waves, must contribute to the entrainment.

Description: Nutrient enrichment experiments were carried out in the Central Indian Ocean during the Chinese First Around-the world Research Cruise, adding nitrate, phosphate, or a mixture of both of them to surface seawater. The concentration of nitrate, nitrite, ammonia, and phosphate were analyzed spectrophotometrically, the chlorophyll-a concentration with fluorescence analysis, and the temperature variation during the experiment recorded. Addition of nitrate resulted in rapid growth of phytoplankton concomitant with depletion of nitrate in the water samples. No apparent variation occurred in chlorophyll-a concentration when phosphate was added. Combining nitrate and phosphate proved to be best to promote phytoplankton bloom, and nitrate was depleted prior to phosphate. After nitrate was consumed, a substantial amount of phytoplankton survived on the supplied phosphate. No correlation was found between the nitrate to phosphate ratio and chlorophyll-a or phytoplankton growth rate. We also found no correlation between water temperature and chlorophyll-a or phytoplankton growth rate. We conclude that neither nitrate to phosphate ratio nor water temperature control the growth of phytoplankton.

Description: In this article, we describe physical parameters structures and different water masses using CTD measurements in southwestern part of the Caspian Sea (CS) in adjacent to Anzali Port (AP). CTD profilings were conducted along a transect perpendicular to the coastline over 13 stations from the coast down to 720 m in winter 2008. According to the results the continental shelf waters are located in surface mixed layer. Surface mixed layer extends itself down to almost 100 m in outer areas of the continental shelf with a weak seasonal thermocline layer between 80 to 140 m. Freshwaters inflow of local rivers is clearly seen outside continental shelf at the surface layers. Investigating the dissolved oxygen reveals that winter convection is traceable down to 500 m in the lateral waters over the shelf break. Among the deeper stations that are located in continental rise and abyssal plain, 350 m seems to be threshold for penetration of seasonal changes; therefore deeper waters tend to be impermeable against seasonal variances. Despite to the small variations, stability is positive in most region of the study area and temperature plays an important role in static stability and in triggering the lateral mixing. In view of both temperature-salinity and temperature-oxygen distributions in the southwestern part of the CS, three different water masses are separable in cold phase. Snapshot observation of physical properties in the early winter 2008, to some extent revealed that a mixing was triggered at least in the lateral waters of the study area.

Description: We study the contribution of eastern-boundary density variations to sub-seasonal and seasonal anomalies of the strength and vertical structure of the Atlantic Meridional Overturning Circulation (AMOC) at 26.5° N, by means of the RAPID/MOCHA mooring array between April 2004 and October 2007. The major density anomalies are found in the upper 500 m, and they are often coherent down to 1400 m. The densities have 13-day fluctuations that are apparent down to 3500 m. The two strategies for measuring eastern-boundary density – a tall offshore mooring (EB1) and an array of moorings on the continental slope (EBH) – show little correspondence in terms of amplitude, vertical structure, and frequency distribution of the resulting basin-wide integrated transport fluctuations, implying that there are significant transport contributions between EB1 and EBH. Contrary to the original planning, measurements from EB1 cannot serve as backup or replacement for EBH: density needs to be measured directly at the continental slope to compute the full-basin density gradient. Fluctuations in density at EBH generate transport variability of 2 Sv rms in the AMOC, while the overall AMOC variability is 4.9 Sv rms. There is a pronounced deep-reaching seasonal cycle in density at the eastern boundary, which is apparent between 100 m and 1400 m, with maximum positive anomalies in spring and maximum negative anomalies in autumn. These changes drive anomalous southward upper mid-ocean flow in spring, implying maximum reduction of the AMOC, and vice-versa in autumn. The amplitude of the seasonal cycle of the AMOC arising from the eastern-boundary densities is 5.2 Sv peak-to-peak, dominating the 7.0 Sv peak-to-peak seasonal cycle of the total AMOC. Our analysis suggests that the seasonal cycle in density may be forced by the strong near-coastal seasonal cycle in wind stress curl.

Description: There is an international commitment to develop a comprehensive, coordinated, and sustained ocean observation system. However, a foundation for any observing, monitoring, or research effort is effective and reliable in situ sensor technologies that accurately measure key environmental parameters. Ultimately, the data used for modeling efforts, management decisions, and rapid responses to ocean hazards are only as good as the instruments that collect them. There is also a compelling need to develop and incorporate new or novel technologies to improve all aspects of existing observing systems and meet various emerging challenges. Assessment of Sensor Performance was a cross-cutting issues session at the international OceanSensor08 workshop in Warnemünde, Germany. The discussions were focused on how best to classify and validate the instruments required for effective and reliable ocean observations and research. The following is a summary of the discussions and conclusions drawn from this workshop, which specifically addresses the characterization of sensor systems, technology readiness levels, verification of sensor performance, and quality management of sensor systems.

Description: The ocean thermal field is often represented in hurricane-ocean interaction by a metric termed the upper Ocean Heat Content (OHC), the vertical integral of ocean temperature in excess of 26°C. High values of OHC have proven useful for identifying ocean regions that are especially favorable for hurricane intensification. Nevertheless, it is argued here that a more direct and robust metric of the ocean thermal field may be afforded by a vertical average of temperature, in one version from the surface to 100 m, a typical depth of vertical mixing by a mature hurricane. OHC and the depth-averaged temperature, dubbed T100, are well correlated over the deep open ocean in the high range of OHC, OHC≥75 kJ cm−2. They are poorly correlated in the low range of OHC, ≤50 kJ cm−2, in part because OHC is degenerate when evaluated on cool ocean temperatures ≤26°C. OHC and T100 can be qualitatively different also over shallow continental shelves: OHC will generally indicate comparatively low values regardless of the ocean temperature, while T100 will take on high values over a shelf that is warm and upwelling neutral or negative, since there will be little cool water that could be mixed into the surface layer. Some limited evidence is that continental shelves may be regions of comparatively small sea surface cooling during a hurricane passage, but more research is clearly required on this important issue.

Description: The Rapid instrument array across the Atlantic Ocean along 26° N provides unprecedented monitoring of the basin-scale circulation. A unique feature of the Rapid array is the combination of full-depth moorings with instruments measuring temperature, salinity, pressure time series at many depths with co-located bottom pressure measurements so that dynamic pressure can be measured from surface to bottom. Bottom pressure measurements show a zonally uniform rise (and fall) of bottom pressure of 0.015 dbar on a 5 to 10 day time scale, suggesting that the Atlantic basin is filling and draining on a short time scale. After removing the zonally uniform bottom pressure fluctuations, bottom pressure variations at 4000 m depth against the western boundary compensate instantaneously for baroclinic fluctuations in the strength and structure of the deep western boundary current so there is no basin-scale mass imbalance resulting from variations in the deep western boundary current. After removing the mass compensating bottom pressure, residual bottom pressure fluctuations at the western boundary just east of the Bahamas balance variations in Gulf Stream transport. Again the compensation appears to be especially confined close to the western boundary. Thus, fluctuations in either Gulf Stream or deep western boundary current transports are compensated in a depth independent (barotropic) manner very close to the continental slope off the Bahamas. In contrast, compensation for variations in wind-driven surface Ekman transport appears to involve fluctuations in both western basin and eastern basin bottom pressures, though the bottom pressure difference fluctuations appear to be a factor of 3 too large, perhaps due to an inability to resolve small bottom pressure fluctuations after removal of larger zonal average, baroclinic, and Gulf Stream pressure components. For 4 tall moorings where time series dynamic height (geostrophic pressure) profiles can be estimated from sea surface to ocean bottom and bottom pressure can be added, there is no general correlation between surface dynamic height and bottom pressure.

Description: A motion of an individual ice field in the Arctic Ocean was monitored at the Russian research station North Pole 35 established on the ice pack in 2008. The ice field velocity (V) was found to be correlated with wind velocity (v) in main features, such as the positions of maxima and minima of V and v. However, the fine structure of the V-variation cannot be explained by the wind forcing only. There were periods of time when the field drift was highly affected by either the tidal activity or the interactions of ice sheets between each other. These data were put in comparison with the "waiting times" statistics that is with the distributions of lengths of time intervals between subsequent important local accelerations of the ice field. These distributions were measured in several time windows differing in the average wind velocity and/or the mechanical state of the ice pack. The distribution functions N(〉τ) (N is the number of successive events of accelerations separated by the time interval that exceeds τ) constructed in different time windows demonstrated fractal or multifractal nature in the consolidated ice pack but were truly random when the ice field drifted in the highly fragmented sea ice. The latter result evidences the existence of a relationship between the long-range mechanical interactions in the pack and long-term memory (time scaling behavior) of the sea ice motion.

Description: This paper is a review which briefly describes a selection of acoustic observation techniques and certain aspects of underwater video technology suitable for observations in an underwater environment. The review is divided into two sections, one for each subject, where each section concludes with a discussion of the current challenges within the respective fields. The acoustic section of the review covers bathymetric and geometrical measurements, imaging sonars, subsurface penetrating profilers, positioning methods, acoustic underwater communication and sensor networks, and water speed measurements. The section ends by considering temperature measurements by ocean acoustic tomography and passive acoustic monitoring. The underwater video section initially deals with questions of acquisition including underwater visibility, the type of platform, and video formats, image sensors and specialized cameras. This is followed by notes on processing techniques including mosaicking, stereo video, structured light, recording and transmission, image enhancement techniques and ends with a short discussion of underwater holographic cameras.

Description: Marine environments are influenced by a wide diversity of anthropogenic and natural substances and organisms that may have adverse effects on human health and ecosystems. Real-time measurements of pollutants, toxins, and pathogens across a range of spatial scales are required to adequately monitor these hazards, manage the consequences, and to understand the processes governing their magnitude and distribution. Significant technological advancements have been made in recent years for the detection and analysis of such marine hazards. In particular, sensors deployed on a variety of mobile and fixed-point observing platforms provide a valuable means to assess hazards. In this review, we present state-of-the-art of sensor technology for the detection of harmful substances and organisms in the ocean. Sensors are classified by their adaptability to various platforms, addressing large, intermediate, or small areal scales. Current gaps and future demands are identified with an indication of the urgent need for new sensors to detect marine hazards at all scales in autonomous real-time mode. Progress in sensor technology is expected to depend on the development of small-scale sensor technologies with a high sensitivity and specificity towards target analytes or organisms. However, deployable systems must comply with platform requirements as these interconnect the three areal scales. Future developments will include the integration of existing methods into complex and operational sensing systems for a comprehensive strategy for long-term monitoring. The combination of sensor techniques on all scales will remain crucial for the demand of large spatial and temporal coverage.

Description: The Kuroshio flows along the edges of the marginal East Asian seas such as the South China Sea (SCS) and East China Sea (ECS). Exchanges of materials and energy between the Kuroshio and the marginal seas partly control the environments of the marginal seas. In particular, saline water from the Kuroshio maintains certain salinity in the shelf water in the ECS. Nutrients from the subsurface of the Kuroshio may influence primary production on the shelf. We summarize how the Kuroshio comes into contact with the shelf water or marginal seas, describing phenomena related to the exchange between the Kuroshio and the ECS along with the SCS, using reports in the literature along with original data. The Kuroshio tends to intrude into the SCS as a loop current around the Luzon Strait. The Kuroshio intrusion into the shelf region of the ECS has distinct seasonal variation and the Taiwan Warm Current plays a significant role in the determination of water properties in the outer shelf associated with the Kuroshio intrusion. We then examine physical processes related to the interaction between the Kuroshio and shelf water. Interaction between the Kuroshio and the bottom topography is an important process in the control of the exchange around the shelf break. Vertical mixing and frontal eddies are also important factors that control the water exchange and formation of water masses in the outer shelf. Wind stress plays a significant role in the exchange with a rather event-like manner. To determine the source of the water masses, chemical tracers could be powerful tools and it is suggested that a significant part of the shelf water consists of Kuroshio intermediate water.

Description: There exist two central measures of turbulent diffusive mixing in turbulent stratified fluids, which are both caused by molecular diffusion: 1) the dissipation rate D(APE) of available potential energy APE; 2) the rate of change Wr,mixing of background gravitational potential energy GPEr. So far, these two quantities have often been regarded as representing the same kind of energy conversion, i.e., the irreversible conversion of APE into GPEr, owing to the well known result that D(APE)≈Wr,mixing in a Boussinesq fluid with a linear equation of state. Here, this idea is challenged by showing that while D(APE) remains largely unaffected by a nonlinear equation of state, Wr,mixing is in contrast strongly affected by the latter. This result is rationalized by using the recent results of Tailleux (2008), which argues that D(APE) represents the dissipation of APE into one particular subcomponent of internal energy called the "dead" internal energy IE0, whereas Wr,mixing represents the conversion between a different subcomponent of internal energy – called the "exergy" IEexergy – and GPEr. It follows that the concept of mixing efficiency, which represents the fraction of the stirring mechanical energy ultimately dissipated by molecular diffusion is related to D(APE), not Wr,mixing, which ensures that it should be largely unaffected by the nonlinear character of the equation of state, and therefore correctly described in the context of a Boussinesq fluid with a linear equation of state. The variations of GPEr, on the other hand, are sensitive to the linear or nonlinear character of the equation of state.