World Ocean Atlas Research Articles

A global ocean reanalysis product in the China Ocean Reanalysis (CORA) project

The first version of a global ocean reanalysis over multiple decades (1979–2008) has been completed by the National Marine Data and Information Service within the China Ocean Reanalysis (CORA) project. The global ocean model employed is based upon the ocean general circulation model of the Massachusetts Institute of Technology. A sequential data assimilation scheme within the framework of 3D variational (3DVar) analysis, called multi-grid 3DVar, is implemented in 3D space for retrieving multiple-scale observational information. Assimilated oceanic observations include sea level anomalies (SLAs) from multi-altimeters, sea surface temperatures (SSTs) from remote sensing satellites, and in-situ temperature/salinity profiles. Evaluation showed that compared to the model simulation, the annual mean heat content of the global reanalysis is significantly approaching that of World Ocean Atlas 2009 (WOA09) data. The quality of the global temperature climatology was found to be comparable with the product of Simple Ocean Data Assimilation (SODA), and the major ENSO events were reconstructed. The global and Atlantic meridional overturning circulations showed some similarity as SODA, although significant differences were found to exist. The analysis of temperature and salinity in the current version has relatively larger errors at high latitudes and improvements are ongoing in an updated version. CORA was found to provide a simulation of the subsurface current in the equatorial Pacific with a correlation coefficient beyond about 0.6 compared with the Tropical Atmosphere Ocean (TAO) mooring data. The mean difference of SLAs between altimetry data and CORA was less than 0.1 m in most years.

A climatology-based quality control procedure for profiling float oxygen data

[1] Over 450 Argo profiling floats equipped with oxygen sensors have been deployed, but no quality control (QC) protocols have been adopted by the oceanographic community for use by Argo data centers. As a consequence, the growing float oxygen data set as a whole is not readily utilized for many types of biogeochemical studies. Here we present a simple procedure that can be used to correct first-order errors (offset and drift) in profiling float oxygen data by comparing float data to a monthly climatology (World Ocean Atlas 2009). Float specific correction terms for the entire array were calculated. This QC procedure was evaluated by (1) comparing the climatology-derived correction coefficients to those derived from discrete samples for 14 floats and (2) comparing correction coefficients for seven floats that had been calibrated twice prior to deployment (once in the factory and once in-house), with the second calibration ostensibly more accurate than the first. The corrections presented here constrain most float oxygen measurements to better than 3% at the surface.

Comparison of [formula omitted], [formula omitted] and LDI temperature proxies for reconstruction of south-east Australian ocean temperatures

Multiple organic proxies are used to reconstruct sea surface temperature (SST) but are rarely applied in concert. Furthermore, they have not been extensively calibrated for the Southern Hemisphere, particularly Australian waters. In an attempt to remedy this gap, we have used three organic temperature proxies: the alkenone unsaturation index (U37K′), the tetraether index from tetraethers consisting of 86 carbons (TEX86) and the novel long chain diol index (LDI) to reconstruct SST using seafloor sediments from the sediment/water interface from near the Australian southern and eastern coasts. The proxies revealed temperature offsets compared with World Ocean Atlas 2009 temperature values that are attributed to differences in seasonal biomarker flux and depth habitat of the source organisms. In the southeast Australian region, U37K′ and LDI provided better estimates of winter temperature at the surface than of any other season but overall were a robust measure of mean annual SST. TEX86H was consistent with previous data from other oceans; it estimated annual temperature best within the 75–100m water column. Comparison of the Australian results with the published global data sets confirm that TEX86H closely matched the correlation for annual temperature in the integrated 0–200m water depth profiles, whereas the other 2 proxies U37K′ and LDI showed different correlations. The results indicate that the combined application of the three independent biomarker proxies may provide useful surface and subsurface temperature information in the Australian region.

On the time‐mean state of ocean models and the properties of long range acoustic propagation

Receptions on three vertical hydrophone arrays from basin‐scale acoustic transmissions in the North Pacific during 1996 and 1998 are used to test the time‐mean sound‐speed properties of the World Ocean Atlas 2005 (WOA05), of an eddying unconstrained simulation of the Parallel Ocean Program (POP), and of three data‐constrained solutions provided by the estimating the circulation and climate of the ocean (ECCO) project: a solution based on an approximate Kalman filter from the Jet Propulsion Laboratory (ECCO‐JPL), a solution based on the adjoint method from the Massachusetts Institute of Technology (ECCO‐MIT), and an eddying solution based on a Green's function approach from ECCO, Phase II (ECCO2). Predictions for arrival patterns using annual average WOA05 fields match observations to within small travel time offsets (0.3–1.0 s). Predictions for arrival patterns from the models differ substantially from the measured arrival patterns, from the WOA05 climatology, and from each other, both in terms of travel time and in the structure of the arrival patterns. The acoustic arrival patterns are sensitive to the vertical gradients of sound speed that govern acoustic propagation. Basin‐scale acoustic transmissions, therefore, provide stringent tests of the vertical temperature structure of ocean state estimates. This structure ultimately influences the mixing between the surface waters and the ocean interior. The relatively good agreement of the acoustic data with the more recent ECCO solutions indicates that numerical ocean models have reached a level of accuracy where the acoustic data can provide useful additional constraints for ocean state estimation.

Performances of seven datasets in presenting the upper ocean heat content in the South China Sea

In this study, the upper ocean heat content (OHC) variations in the South China Sea (SCS) during 1993-2006 were investigated by examining ocean temperatures in seven datasets, including World Ocean Atlas 2009 (WOA09) (climatology), Ishii datasets, Ocean General Circulation Model for the Earth Simulator (OFES), Simple Ocean Data Assimilation system (SODA), Global Ocean Data Assimilation System (GODAS), China Oceanic ReAnalysis system (CORA), and an ocean reanalysis dataset for the joining area of Asia and Indian-Pacific Ocean (AIPO1.0). Among these datasets, two were independent of any numerical model, four relied on data assimilation, and one was generated without any data assimilation. The annual cycles revealed by the seven datasets were similar, but the interannual variations were different. Vertical structures of temperatures along the 18A degrees N, 12.75A degrees N, and 120A degrees E sections were compared with data collected during open cruises in 1998 and 2005-08. The results indicated that Ishii, OFES, CORA, and AIPO1.0 were more consistent with the observations. Through systematic comparisons, we found that each dataset had its own shortcomings and advantages in presenting the upper OHC in the SCS.

An operational biogeochemical model of the North-East Atlantic: Model description and skill assessment

This paper presents a high resolution operational biogeochemical model of the North-East Atlantic that encompasses part of the continental shelf and adjacent deep sea and includes all of Ireland's territorial waters. The setup of the model is described, followed by its skill assessment in reproducing chlorophyll and nitrate spatio-temporal variability. Part of the model skill assessment concerns the evaluation of its usefulness in a decision-making process and is based on the application of a binary discrimination analysis. The model is one-way nested within a 1/12° Mercator Ocean PSY2V4R2 operational model that provides physical forcing at the lateral open boundaries. Nitrate fields are also proscribed at the open boundaries; the World Ocean Atlas 2009 monthly climatologies are used in the upper 500m and at greater depths a formula that relates nitrate concentration to temperature and latitude is applied in the model. The model represents the intra-annual variability of surface chlorophyll and nitrate concentrations at monthly time scales across key oceanographic regions reasonably well; deficiencies are identified in some regions along with possible causes. The model can reproduce important characteristic bio-physicochemical features e.g. the frontal dynamics and upwelling off southwest Ireland and the properties of different water masses in the Rockall Trough. The model is deemed suitable for operational purposes, with a high probability to make correct positive and negative decisions. Operational since 2011, the output is publicly available via a dedicated THREDDS server.

MAREDAT: towards a world atlas of MARine Ecosystem DATa

Abstract. We present a summary of biomass data for 11 plankton functional types (PFTs) plus phytoplankton pigment data, compiled as part of the MARine Ecosystem biomass DATa (MAREDAT) initiative. The goal of the MAREDAT initiative is to provide, in due course, global gridded data products with coverage of all planktic components of the global ocean ecosystem. This special issue is the first step towards achieving this. The PFTs presented here include picophytoplankton, diazotrophs, coccolithophores, Phaeocystis, diatoms, picoheterotrophs, microzooplankton, foraminifers, mesozooplankton, pteropods and macrozooplankton. All variables have been gridded onto a World Ocean Atlas (WOA) grid (1° × 1° × 33 vertical levels × monthly climatologies). The results show that abundance is much better constrained than their carbon content/elemental composition, and coastal seas and other high productivity regions have much better coverage than the much larger volumes where biomass is relatively low. The data show that (1) the global total heterotrophic biomass (2.0–4.6 Pg C) is at least as high as the total autotrophic biomass (0.5–2.4 Pg C excluding nanophytoplankton and autotrophic dinoflagellates); (2) the biomass of zooplankton calcifiers (0.03–0.67 Pg C) is substantially higher than that of coccolithophores (0.001–0.03 Pg C); (3) patchiness of biomass distribution increases with organism size; and (4) although zooplankton biomass measurements below 200 m are rare, the limited measurements available suggest that Bacteria and Archaea are not the only important heterotrophs in the deep sea. More data will be needed to characterise ocean ecosystem functioning and associated biogeochemistry in the Southern Hemisphere and below 200 m. Future efforts to understand marine ecosystem composition and functioning will be helped both by further archiving of historical data and future sampling at new locations. Microzooplankton database: doi:10.1594/PANGAEA.779970 All MAREDAT databases: http://www.pangaea.de/search?&q=maredat

Analysis of deep-layer and bottom circulations in the South China Sea based on eight quasi-global ocean model outputs

This study is a preliminary analysis of the South China Sea (SCS) deep circulations using eight quasi-global high-resolution ocean model outputs. The goal is to assess models’ ability to simulate these deep circulations. The analysis reveals that models’ deep temperatures are colder than the observations in the World Ocean Atlas, while most models’ deep salinity values are higher than the observations, indicating models’ deep water is generally colder and saltier than the reality. Moreover, there are long-term trends in both temperature and salinity simulations. The Luzon Strait transport below 1500 m is 0.36 Sv when averaged for all models, smaller compared with the observation, which is about 2.5 Sv. Four assimilated models and one unassimilated (OCCAM) display that the Luzon deep-layer overflow reaches its minimum in spring and its maximum in winter. The vertically integrated streamfunctions below 2400 m from these models show a deep cyclonic circulation in the SCS on a large scale, but the pattern is different from the diagnostic streamfunction from the U.S Navy Generalized Digital Environment Model (GDEM-Version 3.0, GDEMv3). The meridional overturning structure above 1000 m is similar in all models, but the spatial distribution and intensity below 1500 m are quite different from model to model. Moreover, the meridional overturning below 2400 m in these models is weaker than that of the GDEMv3, which indicates a deep vertical mixing process in these models is biased weak. Based on the above evaluation, this paper discusses the impacts of T/S initial value, topography, and mixing scheme on the SCS deep circulations, which may provide a reference for future model improvement.

Salinity drift of global Argo profiles and recent halosteric sea level variation

A salinity sensor drift in the Array for Real-time Geostrophic Oceanography (Argo) float has been identified. In the present study the salinity calibration results for global Argo floats indicate that an upper 900dbar averaged-positive salinity drift appears during the floats' operating period from January 2001 to December 2010. These offsets are shown to cause an uncertainty in the estimate of the steric height anomaly of up to 17±5%. Based on a multi-grid, three-dimensional, variational method we construct monthly gridded salinity fields using post-calibration Argo salinity profiles for the period 2003–2010. Monthly time series of halosteric height (HSH) for both the global and regional oceans are calculated using gridded salinity fields over the entire 8years. It is shown that there is a global HSH pattern with a 0.05±0.03mm/year trend of increasing HSH anomaly relative to the World Ocean Atlas (WOA) 2001 in the upper 900dbar. Most regions of the North Atlantic Ocean show predominately inter-annual and decadal fluctuations in the HSH anomaly over the observational period. These fluctuations are associated with the Atlantic Oscillation (AO) and/or the Atlantic Multi-decadal Oscillation (AMO). For the Pacific Ocean, inter-annual variability of the HSH anomaly in the Equatorial Pacific Ocean is modulated mainly by the El Niño/Southern Oscillation (ENSO) phenomenon while in the subtropical Northern Pacific Ocean, the wet, cold phase of the Pacific Decadal Oscillation (PDO) modulates the variability of the HSH anomaly, contributing to a positive anomaly observed over the 8years. For the Indian Ocean, both ENSO and the Indian Ocean Dipole (IOD) influence the inter-annual variability of the HSH anomaly in the Equatorial Indian Ocean. In addition, HSH trends of sub-basin regions are also investigated. We find the most pronounced positive trend occurred in the tropical and subtropical regions of the Southern Indian Ocean with HSH increasing by 0.55±0.08mm/year, while the most remarkable negative trend occurred in the tropical and subtropical regions of the North Atlantic with HSH decreasing by −0.44±0.04mm/year.

Biogeographical patterns of phytoplankton community size structure in the oceans

AbstractAimDevelop a biogeographical classification of phytoplankton size distributions for the Atlantic Ocean and predict the global phytoplankton size composition based on prevailing environmental conditions.LocationAtlantic Ocean and Global OceanMethodsUsing phytoplankton size composition data, nutrient concentrations (nitrite+nitrate, phosphate, and silicate), irradiance, temperature and zooplankton abundances of the Atlantic Meridional Transect programme, we derived and tested an environmental classification method of phytoplankton size distribution with a k‐means clustering technique. We then used principal component and Dirichlet multivariate regression analyses to disentangle the relative influence of different environmental conditions on the phytoplankton size composition. Subsequently, we evaluated different probabilisitic models and selected the most parsimonious one to estimate the global phytoplankton size distributions in the world oceans based on global climatology data of the World Ocean Atlas 2009.ResultsBased only on prevailing environmental conditions and without a priori knowledge concerning, for example, the position of oceanic fronts, the primary productivity, the distribution of organisms or any geographical information, our classification method captures the size structures of phytoplankton communities across the Atlantic. We find a strong influence of temperature and nitrite+nitrate concentration on the prevalence of the different size classes, and we present evidence that both factors may act independently on structuring phytoplankton communities. While at low nitrite+nitrate concentrations temperature has a major structuring impact, at high nitrite+nitrate concentrations its influence is reduced. Finally, we show that the global distribution of phytoplankton community size structure can be predicted by a probabilistic model based only on temperature and nitrite+nitrate.Main conclusionThe global distribution of phytoplankton community size structure can be predicted with good approximation using a parsimonious probabilistic model forced by only temperature and nitrite+nitrate data.

The southern Caribbean upwelling system: Sea surface temperature, wind forcing and chlorophyll concentration patterns

Sixteen years of sea surface temperature (SST, 1994–2009) were used to characterize the southern Caribbean upwelling system. This system extends from 61–75.5°W and 10–12.5°N, with 21 upwelling foci clustered in seven groups differentiated by their SST cycles. Two of those groups had the strongest coastal upwelling: the ‘eastern area’ (63–65°W) and the ‘western area’ (70–73°W). The literature reports that the eastern and western upwelling areas hold 78% and 18% of the small pelagic biomass within the upwelling system, respectively. We looked into variations of the upwelling dynamics in those areas using seasonal cycles of satellite SST, chlorophyll-a (Chl) and sea-wind, as well as climatological hydrographic data from the World Ocean Atlas. Comparing their annual averages, the eastern area featured the lowest SST (25.24°C) and the highest Chl (1.65mgm−3); it has moderate wind intensity (6.12ms−1) and shallower 22°C isotherm (85m). The western area had stronger winds (8.23ms−1) but deeper 22°C isotherm (115m), slightly higher SST (25.53°C) and moderate Chl (1.15mgm−3). The upwelling in the eastern area was more prolonged than in the western area (SST <26°C during 8.5 and 6.9 months, respectively). According to the ‘optimal environmental window’ theory, small clupeoid recruitment is a dome-shaped function of the upwelling intensity, turbulence and SST, with an optimum wind speed around 5–6ms−1. The eastern upwelling area wind speed is close to this optimum value. The western upwelling area shows much higher wind speed that causes high level of turbulence and strong offshore transport that could hinder small pelagics recruitment in that area.

Alkenone and tetraether lipids reflect different seasonal seawater temperatures in the coastal northern South China Sea

We examined the lipid-based temperature proxies U37K′ and TEX86H in five short sediment cores from the inner shelf of the northern South China Sea and related them to local sea water temperature from the World Ocean Atlas (WOA) database. The U37K′-based temperature values are consistent with euphotic zone temperature in spring and summer, when the southeast monsoon prevails and vertical stratification occurs in the water column, whereas TEX86H-based temperature values reflect sea surface temperature in winter when the northeast monsoon prevails and vertical mixing occurs. The different temperature signals between the TEX86H and U37K′ proxies could be a result of differences in the growing season between the source organisms, with alkenone-producing haptophyte algae blooming in the warmer and nutrient-rich season with higher fluvial influx, and tetraether-producing Thaumarchaeota blooming in the cooler and oligotrophic season.

Evaluation of the carbon cycle components in the Norwegian Earth System Model (NorESM)

Abstract. The recently developed Norwegian Earth System Model (NorESM) is employed for simulations contributing to the CMIP5 (Coupled Model Intercomparison Project phase 5) experiments and the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC-AR5). In this manuscript, we focus on evaluating the ocean and land carbon cycle components of the NorESM, based on the preindustrial control and historical simulations. Many of the observed large scale ocean biogeochemical features are reproduced satisfactorily by the NorESM. When compared to the climatological estimates from the World Ocean Atlas (WOA), the model simulated temperature, salinity, oxygen, and phosphate distributions agree reasonably well in both the surface layer and deep water structure. However, the model simulates a relatively strong overturning circulation strength that leads to noticeable model-data bias, especially within the North Atlantic Deep Water (NADW). This strong overturning circulation slightly distorts the structure of the biogeochemical tracers at depth. Advancements in simulating the oceanic mixed layer depth with respect to the previous generation model particularly improve the surface tracer distribution as well as the upper ocean biogeochemical processes, particularly in the Southern Ocean. Consequently, near-surface ocean processes such as biological production and air–sea gas exchange, are in good agreement with climatological observations. The NorESM adopts the same terrestrial model as the Community Earth System Model (CESM1). It reproduces the general pattern of land-vegetation gross primary productivity (GPP) when compared to the observationally based values derived from the FLUXNET network of eddy covariance towers. While the model simulates well the vegetation carbon pool, the soil carbon pool is smaller by a factor of three relative to the observational based estimates. The simulated annual mean terrestrial GPP and total respiration are slightly larger than observed, but the difference between the global GPP and respiration is comparable. Model-data bias in GPP is mainly simulated in the tropics (overestimation) and in high latitudes (underestimation). Within the NorESM framework, both the ocean and terrestrial carbon cycle models simulate a steady increase in carbon uptake from the preindustrial period to the present-day. The land carbon uptake is noticeably smaller than the observations, which is attributed to the strong nitrogen limitation formulated by the land model.

Toward a multivariate reanalysis of the North Atlantic Ocean biogeochemistry during 1998–2006 based on the assimilation of SeaWiFS chlorophyll data

Abstract. Today, the routine assimilation of satellite data into operational models of ocean circulation is mature enough to enable the production of global reanalyses describing the ocean circulation variability during the past decades. The expansion of the "reanalysis" concept from ocean physics to biogeochemistry is a timely challenge that motivates the present study. The objective of this paper is to investigate the potential benefits of assimilating satellite-estimated chlorophyll data into a basin-scale three-dimensional coupled physical–biogeochemical model of the North Atlantic. The aim is on the one hand to improve forecasts of ocean biogeochemical properties and on the other hand to define a methodology for producing data-driven climatologies based on coupled physical–biogeochemical modeling. A simplified variant of the Kalman filter is used to assimilate ocean color data during a 9-year period. In this frame, two experiments are carried out, with and without anamorphic transformations of the state vector variables. Data assimilation efficiency is assessed with respect to the assimilated data set, nitrate of the World Ocean Atlas database and a derived climatology. Along the simulation period, the non-linear assimilation scheme clearly improves the surface analysis and forecast chlorophyll concentrations, especially in the North Atlantic bloom region. Nitrate concentration forecasts are also improved thanks to the assimilation of ocean color data while this improvement is limited to the upper layer of the water column, in agreement with recent related literature. This feature is explained by the weak correlation taken into account by the assimilation between surface phytoplankton and nitrate concentrations deeper than 50 meters. The assessment of the non-linear assimilation experiments indicates that the proposed methodology provides the skeleton of an assimilative system suitable for reanalyzing the ocean biogeochemistry based on ocean color data.

Areas of the global major river plumes

River plumes are the regions where the most intense river-sea-land interaction occurs, and they are characterized by complex material transport and biogeochemical processes. However, due to their highly dynamic nature, global river plume areas have not yet been determined for use in synthetic studies of global oceanography. Based on global climatological monthly averaged salinity data from the NOAA World Ocean Atlas 2009 (WOA09), and monthly averaged salinity contour maps of the East and South China Seas from the Chinese Marine Atlas, we extract the monthly plume areas of major global rivers using a geographic information system (GIS) technique. Only areas with salinities that are three salinity units lower than the average salinity in each ocean are counted. This conservative estimate shows that the minimum and maximum monthly values of the total plume area of the world’s 19 largest rivers are 1.72×106 km2 in May and 5.38×106 km2 in August. The annual mean area of these river plumes (3.72×106 km2) takes up approximately 14.2% of the total continental shelves areaworldwide (26.15×106 km2). This paper also presents river plume areas for different oceans and latitude zones, and analyzes seasonal variations of the plume areas and their relationships with river discharge. These statistics describing the major global river plume areas can now provide the basic data for the various flux calculations in the marginal seas, and therefore will be of useful for many oceanographic studies.

Сезонная и межгодовая изменчивость переносов геострофических течений в Южном океане по данным контактных и спутниковых измерений

On the basis of climatic array World Ocean Atlas-2009 and sea level anomalies altimetry data for 1993–2010 the seasonal and interannual variability of geostrophic current’s transport in the South Ocean is investigated. The connection of geostrophic transport’s spatial-temporal variability with indexes of atmospheric circulation Southern Oscillation and Antarctic Oscillation is analyzed.

Comparison of mixed layer depth and barrier layer thickness for the Indian Ocean using two different climatologies

The variability of mixed layer depth (MLD) and barrier layer thickness (BLT) has profound implications on energy exchange processes at the air-sea interface. More important is the role of MLD and BLT in the genesis and intensification of weather systems. The physical and chemical changes that take place within these layers have significance on biological productivity of the oceans. In this study, the monthly evolution of MLD and BLT for Indian Ocean was compared using the state-of-art world ocean atlas (WOA) and a recently developed comprehensive ocean atlas [referred to as new climatology (NC)]. The study area comprises the geographical boundaries encompassing 30°N to 60°S and 30°E to 120°E. Qualitative skill assessment of these variables demonstrates that NC is in good agreement with recently reported observational and modelling studies. This brings out the fact that MLD and BLT climatology derived from NC is better than that of WOA.

Wind-Driven Cross-Equatorial Flow in the Indian Ocean

Abstract Meridional velocity, mass, and heat transport in the equatorial oceans are difficult to estimate because of the nonapplicability of the geostrophic balance. For this purpose a steady-state model is utilized in the equatorial Indian Ocean using NCEP wind stress and temperature and salinity data from the World Ocean Atlas 2005 (WOA05) and Argo. The results show a Somali Current flowing to the south during the winter monsoon carrying −11.5 ± 1.3 Sv (1 Sv ≡ 106 m3 s−1) and −12.3 ± 0.3 Sv from WOA05 and Argo, respectively. In the summer monsoon the Somali Current reverses to the north transporting 16.8 ± 1.2 Sv and 19.8 ± 0.6 Sv in the WOA05 and Argo results. Transitional periods are considered together and in consequence, there is not a clear Somali Current present in this period. Model results fit with in situ measurements made around the region, although Argo data results are quite more realistic than WOA05 data results.

Mass, nutrient and oxygen budgets for the northeastern Atlantic Ocean

Abstract. The northeast Atlantic is a key horizontal and vertical crossroads region for the meridional overturning circulation, but basic nutrient and oxygen fluxes are still poorly constrained by observations in the region. A surface to bottom northeast Atlantic Ocean budget for mass, nutrients (nitrate and phosphate) and oxygen is determined using an optimization method based on three surveys of the OVIDE transect (from Greenland to Portugal) completed with the World Ocean Atlas 2009. Budgets are derived for two communicating boxes representing the northeastern European basin (NEEB) and the Irminger Sea. For the NEEB (Irminger) box, it is found that 30% of the mass import (export) across the OVIDE section reach (originate from) the Nordic Seas, while 70% are redistributed between both boxes through the Reykjanes Ridge (9.3 ± 0.7 × 109 kg s−1). Net biological source/sink terms of nitrate point to both the Irminger and NEEB boxes as net organic matter production sites (consuming nitrate at a rate of –7.8 ± 6.5 kmol s−1 and –8.4 ± 6.6 kmol s−1, respectively). Using a standard Redfield ratio of C : N = 106 : 16, nitrate consumption rates indicate that about 40 TgC yr−1 of carbon is fixed by organic matter production between the OVIDE transect and the Greenland–Scotland Ridge. Nutrient fluxes also induce a net biological production of oxygen of 73 ± 60 kmol s−1 and 79 ± 62 kmol s−1 in the Irminger and NEEB boxes, which points to the region as being autotrophic. The abiotic air–sea oxygen flux leads to an oceanic oxygen uptake in the two regions (264 ± 66 kmol s−1 in the north and 443 ± 70 kmol s−1 in the south). The abiotic flux is partitioned into a mixing and a thermal component. It is found that the Irminger Sea oceanic oxygen uptake is driven by an air–sea heat flux cooling increasing the ocean surface oxygen solubility. Over the northeastern European basin the mixing component is about half the thermal flux, presumably because of the oxygen minimum in the subtropical thermocline.

Impact of tidal mixing with different scales of bottom roughness on the general circulation

The current study deals with a parameterization of diapycnal diffusivity in an ocean model. The parameterization estimates the diapycnal diffusivity depending on the location of tidal-related energy dissipation over rough topography. The scheme requires a bottom roughness map that can be chosen depending on the scales of topographic features. Here, we implement the parameterization on an ocean general circulation model, and we examine the sensitivity of the modeled circulations to different spatial scales of the modeled bottom roughness. We compare three simulations that include the tidal mixing scheme using bottom roughness calculated at three different ranges of spatial scales, with the largest scale varying up to 200 km. Three main results are discussed. First, the dependence of the topographic spectra with depth, characterized by an increase in spectral energy over short length scales in the deep ocean, influences the vertical profile of the diffusivity. Second, the changes in diffusivities lead to different equilibrium solutions in the Atlantic meridional overturning circulation and bottom circulation. In particular, the lower cell of the Atlantic overturning and the bottom water transport in the Pacific Ocean are stronger for stronger diffusivities at the corresponding basins and depths, and the strongest when using the small-scale roughness map. Third, a comparison of the density fields of the three simulations with the density field of World Ocean Atlas dataset, from which the models are initialized, shows that among the simulations with three different roughness maps, the one using small-scale bottom roughness map has the smallest density bias.