Corrections to ocean surface forcing in the California Current System using 4D variational data assimilation

The option for surface forcing correction, recently developed in the 4D-variational (4DVAR) data assimilation systems of the Regional Ocean Model System (ROMS), is presented. Assimilation of remotely-sensed (satellite sea surface height anomaly and sea surface temperature) and in situ (from mechanical and expendable bathythermographs, Argo floats and CTD profiles) oceanic observations has been applied in a realistic, high resolution configuration of the California Current System (CCS) to sequentially correct model initial conditions and surface forcing, using the Incremental Strong constraint version of ROMS-4DVAR (ROMS-IS4DVAR). Results from both twin and real data experiments are presented where it is demonstrated that ROMS-IS4DVAR always reduces the difference between the model and the observations that are assimilated. However, without corrections to the surface forcing, the assimilation of surface data can degrade the temperature structure at depth. When using surface forcing adjustment in ROMS-IS4DVAR the system does not degrade the temperature structure at depth, because differences between the model and surface observations can be reduced through corrections to surface forcing rather than to temperature at depth. However, corrections to surface forcing can generate abnormal spatial and temporal variability in the structure of the wind stress or surface heat flux fields if not properly constrained. This behavior can be partially controlled via the choice of decorrelation length scales that are assumed for the forcing errors. Abnormal forcing corrections may also arise due to the effects of model error which are not accounted for in IS4DVAR. In particular, data assimilation tends to weaken the alongshore wind stress in an attempt to reduce the rate of coastal upwelling, which seems to be too strong due to other sources of error. However, corrections to wind stress and surface heat flux improve systematically the ocean state analyses. Trends in the correction of surface heat fluxes indicate that, given the ocean model used and its potential limitations, the heat flux data from the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) used to impose surface conditions in the model are generally too low except in spring-summer, in the upwelling region, where they are too high. Comparisons with independent data provide confidence in the resulting forecast ocean circulation on timescales ～14 days, with less than 1.5 °C, 0.3 psu, and 9 cm RMS error in temperature, salinity and sea surface height anomaly, respectively, compared to observations.     

Heat,volume and chemical fluxes from submarine venting: A synthesis of results from the Rainbow hydrothermal field, 36°N MAR

High-temperature hydrothermal activity occurs in all ocean basins and along ridge crests of all spreading rates. While it has long been recognized that the fluxes associated with such venting are large, precise quantification of their impact on ocean biogeochemistry has proved elusive. Here, we report a comprehensive study of heat, fluid and chemical fluxes from a single submarine hydrothermal field. To achieve this, we have exploited the integrating nature of the non-buoyant plume dispersing above the Rainbow hydrothermal field, a long-lived and tectonically hosted high-temperature vent site on the Mid-Atlantic Ridge. Our calculations yield heat and volume fluxes for high-temperature fluids exiting the seafloor of ～0.5 GW and 450 L s^?1, together with accompanying chemical fluxes, for Fe, Mn and CH₄ of ～10, ～1 and ～1 mol s^?1, respectively. Accompanying fluxes for 25 additional chemical species that are associated with Fe-rich plume particles have also been calculated as they are transported away from the Rainbow vent site before settling to the seabed. High-temperature venting has been found to recur at least once every ～100 km along all slow-spreading ridges investigated to-date, with half of all known sites on the Mid-Atlantic Ridge occurring as long-lived and tectonically hosted systems. If these patterns persist along all slow- and ultraslow-spreading ridges, high-temperature venting of the kind reported here could account for ～50% of the on-axis hydrothermal heat flux along ～30,000 km of the ～55,000 km global ridge crest.     

Total kinetic energy in four global eddying ocean circulation models and over 5000 current meter records

We compare the total kinetic energy (TKE) in four global eddying ocean circulation simulations with a global dataset of over 5000, quality controlled, moored current meter records. At individual mooring sites, there was considerable scatter between models and observations that was greater than estimated statistical uncertainty. Averaging over all current meter records in various depth ranges, all four models had mean TKE within a factor of two of observations above 3500 m, and within a factor of three below 3500 m. With the exception of observations between 20 and 100 m, the models tended to straddle the observations. However, individual models had clear biases. The free running (no data assimilation) model biases were largest below 2000 m. Idealized simulations revealed that the parameterized bottom boundary layer tidal currents were not likely the source of the problem, but that reducing quadratic bottom drag coefficient may improve the fit with deep observations. Data assimilation clearly improved the model-observation comparison, especially below 2000 m, despite assimilated data existing mostly above this depth and only south of 47 °N. Different diagnostics revealed different aspects of the comparison, though in general the models appeared to be in an eddying-regime with TKE that compared reasonably well with observations.     

A global heat and freshwater forcing dataset for ocean models

A global dataset based on the ECMWF Re-Analyses (ERA) is presented that can be used as surface boundary conditions for ocean models with sea-ice components. The definition of these conditions is based on bulk formulae. To study the mean ocean circulation, a mean annual cycle on a daily basis was constructed from ERA for all relevant parameters including wind stress. Continental runoff is considered by using information about the catchment areas of the rivers and about the main drainage basins. The bulk formulae were extended by using sea ice concentration.To estimate meridional heat transports (MHT) and to avoid any drift in ocean model simulations, the heat and fresh water budgets have been closed by applying an inverse procedure to fine-tune the fluxes towards observed transports. To improve the MHTs on the Southern Hemisphere the winds and the short wave radiation at southern higher latitudes should be corrected. Furthermore, tests were performed concerning short wave radiation which was increased in the tropics and decreased in the subsidence zones.The heat and fresh water fluxes are assessed by using a scheme of Macdonald and Wunsch based on hydrographic sections. The net heat fluxes of ERA and of the forcing dataset are consistent with the heat flux divergences and convergences estimated by this scheme except for parts of the South Atlantic and the Indian Ocean sector of the Southern Ocean where none of these datasets is consistent with these estimates. In the subtropical South Indian Ocean the forcing dataset is consistent with these estimates while ERA are not. The flux components of ERA and the forcing dataset were compared to several observational datasets (SRB, SOC, HOAPS, GPCP, and CMAP). For each component, at least one of these datasets (especially HOAPS) supports the effects of the inverse procedure and the bulk formulae almost globally with some regional exceptions: short wave radiation in the tropical oceans and the subtropical North Atlantic, latent heat flux at higher latitudes, and precipitation in the northern North Atlantic.Comparisons to the NCAR/NCEP Re-Analyses (NRA) (versions 1 and 2) and the ECHAM model in place of ERA lead to similar results. In the North Atlantic the net heat fluxes of the model based datasets approach the hydrographic estimate with increasing resolution. Applied to any ocean/sea-ice model and compared to ERA, the forcing dataset would induce only a relative small net sea-surface buoyancy loss.A comparison of the forcing dataset to measurements made using one buoy deployed in the western Pacific warm pool and five buoys deployed in the subduction region of the Northeast Atlantic shows that at the site of the first buoy the net heat fluxes of the forcing dataset are in poorer agreement than those of ERA. At the sites of two subduction buoys both datasets show the same level of agreement within the error bars specified. At the sites of the three remaining subduction buoys the forcing dataset shows a marginal improvement on ERA.     

Seasonal and interannual variability in particle fluxes of carbon,nitrogen and silicon from time series of sediment traps at Ocean Station P, 1982–1993: relationship to changes in subarctic primary productivity

An extended time series of particle fluxes at 3800 m was recorded using automated sediment traps moored at Ocean Station Papa (OSP, 50°N, 145°W) in the northeast Pacific Ocean for more than a decade (1982–1993). Time-series observations at 200 and 1000 m, and short-term measurements using surface-tethered free-drifting sediment traps also were made intermittently. We present data for fluxes of total mass (dry weight), particulate organic carbon (POC), particulate organic nitrogen (PON), biogenic Si (BSi), and particulate inorganic carbon (PIC) in calcium carbonate. Mean monthly fluxes at 3800 m showed distinct seasonality with an annual minimum during winter months (December–March), and maximum during summer and fall (April–November). Fluxes of total mass, POC, PIC and BSi showed 4-, 10-, 7- and 5-fold increases between extreme months, respectively. Mean monthly fluxes of PIC often showed two plateaus, one in May–August dominated by <63 μm particles and one in October–November, which was mainly >63 μm particles. Dominant components of the mass flux throughout the year were CaCO₃ and opal in equal amounts. The mean annual fluxes at 3800 m were 32±9 g dry weight g m⁻² yr⁻¹, 1.1±0.5 g POC m⁻² yr⁻¹, 0.15±0.07 g PON m⁻² yr⁻¹, 5.9±2.0 g BSi m⁻² yr⁻¹ and 1.7±0.6 g PIC m⁻² yr⁻¹. These biogenic fluxes clearly decreased with depth, and increased during “warm” years (1983 and 1987) of the El Niño, Southern Oscillation cycle (ENSO). Enhancement of annual mass flux rates to 3800 m was 49% in 1983 and 36% in 1987 above the decadal average, and was especially rich in biogenic Si. Biological events allowed estimates of sinking rates of detritus that range from 175 to 300 m d⁻¹, and demonstrate that, during periods of high productivity, particles sink quickly to deep ocean with less loss of organic components. Average POC flux into the deep ocean approximated the “canonical” 1% of the surface primary production.     

Radiolarian fluxes from the southern Bay of Bengal: sediment trap results

A study of radiolarian fluxes collected during 1991–93 from time-series sediment traps deployed at 1071 and 3010 m water depth in the southern Bay of Bengal (SBBT) yielded 40 species/groups of radiolarians. Among the order Polycystina, the species of sub-order Spumellaria were by far the most abundant (∼95%) followed by sub-order Nassellaria (5%). This is contrary to reports from the Atlantic and Pacific Oceans and is attributed to the prevailing hyposaline condition resulting from the monsoonal rainfall. Higher radiolarian fluxes occurred during March–May, when moderate salinity and a high sea surface temperature (SST) regime prevailed at the trap site. R-mode cluster analysis of the radiolarian flux data revealed three assemblages represented by the cooler (A) and warmer (C) surface dwelling fauna (0–50 m) dominated by spumellarians, and a deeper dwelling (B) sub-surface fauna (50–100 m) associated with deep dwelling (>100 m) nassellarian species. Spongaster tetras tetras, a surface water radiolarian species, exhibited its preference for high SST and moderate salinity conditions during the pre-monsoon season (March–May). Radiolarian fluxes responded to seasonal changes in SST and salinity variations due to the monsoonal precipitation, and the freshwater runoff from the Indian rivers causing a hyposaline condition in the Bay of Bengal. Results imply that the radiolarian assemblages in the down core data may reveal the monsoonal history in the geological past.     

Sensitivity of near-surface Tropical Instability Waves to submonthly wind forcing in the tropical Atlantic

This study demonstrates the sensitivity of the near-surface properties in the tropical Atlantic Ocean to the high-frequency of the winds in numerical simulations. At intra-seasonal timescales (2–50 days), two distinct period ranges dominate the variability in the upper ocean: periods between 2 and 20 days, which are essentially wind-forced and periods between 20 and 50 days, due mostly to Tropical Instability Waves (TIWs). Using a numerical model forced by different wind fields, it is shown that the characteristics of the intra-seasonal variability in the ocean surface mixed-layer are strongly dependent on the wind forcing. Submonthly winds are shown to force large variability in the upper ocean that can strikingly decrease the amplitude of the TIWs in the mixed-layer and their imprint on the horizontal distribution of sea surface temperatures. Wind products containing too much energy at submonthly periods thus prevent wind-forced simulations from reproducing a realistic surface signature of TIWs, when compared to satellite observations of sea surface temperature. In addition, submonthly wind variability may be responsible for part of the observed interannual variability of the TIW signature in the temperature. The impact of submonthly winds is strongest in the mixed-layer: beneath the mixed-layer, all simulations show similar characteristics of the TIWs.     

Causes of deep-water variations: Reply to comment by E. Fahrbach,M. Hoppema,G. Rohardt,M. Schröder and A. Wisotzki

Vertically integrated heat content from the Weddell Sea along 69 °S in vicinity of the Greenwich meridian from the 1977–2001 period is presented. This demonstrates that the east-west variability in the area is lower than the natural variability. The sensitivity on the choice of lower boundary for the heat content integral is discussed in relation to upwelling from the deeper water masses. The cooling around Maud Rise during the 1990's on the order of 20 W/m² remains significant, and may be explained by increased surface heat fluxes during winter months with lower than normal sea ice concentration.     

Ocean current estimation using a Multi-Model Ensemble Kalman Filter during the Grand Lagrangian Deployment experiment (GLAD)

In the summer and fall of 2012, during the GLAD experiment in the Gulf of Mexico, the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) used several ocean models to assist the deployment of more than 300 surface drifters. The Navy Coastal Ocean Model (NCOM) at 1 km and 3 km resolutions, the US Navy operational NCOM at 3 km resolution (AMSEAS), and two versions of the Hybrid Coordinates Ocean Model (HYCOM) set at 4 km were running daily and delivering 72-h range forecasts. They all assimilated remote sensing and local profile data but they were not assimilating the drifter’s observations. This work presents a non-intrusive methodology named Multi-Model Ensemble Kalman Filter that allows assimilating the local drifter data into such a set of models, to produce improved ocean currents forecasts. The filter is to be used when several modeling systems or ensembles are available and/or observations are not entirely handled by the operational data assimilation process. It allows using generic in situ measurements over short time windows to improve the predictability of local ocean dynamics and associated high-resolution parameters of interest for which a forward model exists (e.g. oil spill plumes). Results can be used for operational applications or to derive enhanced background fields for other data assimilation systems, thus providing an expedite method to non-intrusively assimilate local observations of variables with complex operators. Results for the GLAD experiment show the method can improve water velocity predictions along the observed drifter trajectories, hence enhancing the skills of the models to predict individual trajectories.     

A global wave parameter database for geophysical applications. Part 1: Wave-current–turbulence interaction parameters for the open ocean based on traditional parameterizations

Ocean surface mixing and drift are influenced by the mixed layer depth, buoyancy fluxes and currents below the mixed layer. Drift and mixing are also functions of the surface Stokes drift U_ss, volume Stokes transport T_S, a wave breaking height scale H_swg, and the flux of energy from waves to ocean turbulence Φ_oc. Here we describe a global database of these parameters, estimated from a well-validated numerical wave model, that uses traditional forms of the wave generation and dissipation parameterizations, and covers the years 2003–2007. Compared to previous studies, the present work has the advantage of being consistent with the known physical processes that regulate the wave field and the air–sea fluxes, and also consistent with a very large number of in situ and satellite observations of wave parameters. Consequently, some of our estimates differ significantly from previous estimates. In particular, we find that the mean global integral of Φ_oc is 68 TW, and the yearly mean value of T_S is typically 10–30% of the Ekman transport, except in well-defined regions where it can reach 60%. We also have refined our previous estimates of U_ss by using a better treatment of the high frequency part of the wave spectrum. In the open ocean, U_ss ≃ 0.013U₁₀, where U₁₀ is the wind speed at 10 m height.     

Diagnosis of vertical velocities with the QG omega equation: an examination of the errors due to sampling strategy

Vertical motion at the mesoscale plays a key role in ocean circulation, ocean-atmosphere interaction, and hence climate. It is not yet possible to make direct Eulerian measurements of vertical velocities less than 1000 m day⁻¹. However, by assuming quasi-geostrophic (QG) balance, vertical velocities O (10 m day⁻¹) can be diagnosed from the geostrophic velocity field and suitable boundary conditions. Significant errors in the accuracy of this diagnosis arise from the necessary compromise between spatial resolution and synopticity of a hydrographic survey. This problem has been addressed by sampling the output of a numerical ocean model to simulate typical oceanographic surveys of mesoscale fronts. The balance between the number of observations and the synopticity of observations affects the apparent flow and in particular the diagnosed vertical motion. A combination of effects can typically lead to errors of 85% in the estimation of net vertical heat flux. An analytical two-layer model is used to understand components of this error and indicate the key parameters for the design of mesoscale sampling.     

Temporal variations in the deep-water colonization rates of small benthic foraminifera: the results of an experiment on Cross Seamount

Modern benthic foraminifera are important remineralizers of organic matter and a link between surface-ocean production and life in the deep sea. Assemblages of benthic foraminifera are preserved in the fossil record, providing clues to paleoceanographic conditions. These clues can be fully interpreted only when our knowledge of the biology and ecology of modern species is more complete. To study factors influencing foraminiferal colonization rates, artificial substrates were placed on Cross Seamount (18°40′N, 158°17′W) for 1 to 42 months between 1989 and 1994. The colonization rates of benthic foraminifera onto different substrates (five mineral types were used) at four water depths (800, 975, 1285 and 2000 m) were determined. Both calcareous and agglutinated foraminifera inhabited these artificial substrates at different rates. Many of the agglutinated forms colonized at a uniform rate through time. The colonization rates of other foraminifera, primarily calcareous forms, were not constant through time, nor could the variability be attributed to the controlled variables (water depth and substrate composition). Instead, these temporal variations in colonization rate corresponded with shifts in surface ocean conditions and export fluxes in the central Pacific. Other ecological observations are also presented, including size- and spatial-distributions.     

Eighteen Degree Water formation within the Gulf Stream during CLIMODE

Analysis of wintertime CLIMODE data for 2007 indicates that a substantial portion of new Eighteen Degree Water (EDW) is likely ventilated within the eastward flowing Gulf Stream (GS) between 67°W and 52°W longitudes, possibly exceeding that formed elsewhere in the northern Sargasso Sea. Use of some global air–sea interaction data sets applied to the study region for Feb/Mar of 2007 indicate that this winter may have been anomalously energetic in air–sea exchange compared to the mean of the prior 19 yr. The largest heat and freshwater fluxes found directly over the meandering warm core of the Gulf Stream are capable of removing most of the subtropical heat anomaly of the GS, but cross-frontal fluxes of salinity are required to account for the observed regional salinity structure. An isopycnal diffusivity of ∼100 m² s⁻¹ is inferred from the salinity balance. This mixing would also account for the observation that EDW formed in the GS is slightly fresher than that formed in northern Sargasso Sea. The lateral flux of heat across the GS north wall also acts to cool the resulting EDW water, but the heat balance for EDW production is largely determined from GS advection and air–sea fluxes, in contrast to salinity. Based on oxygen saturation data, we estimate that 1.8–3.0 Sv-yr of new EDW is formed in the GS for the winter of 2007. EDW originating from the GS is generated in a separate location from where it is accumulated in the northern Sargasso Sea. This manner of EDW formation will produce unique characteristics of EDW found in the northern Sargasso Sea: ones that differ in T/S properties from that formed south of the GS under the more traditional 1D, cooling-driven convection process.     

High resolution profiles of vertical particulate organic matter export off Cape Blanc,Mauritania: Degradation processes and ballasting effects

Vertical carbon fluxes between the surface and 2500 m depth were estimated from in situ profiles of particle size distributions and abundances me/asured off Cape Blanc (Mauritania) related to deep ocean sediment traps. Vertical mass fluxes off Cape Blanc were significantly higher than recent global estimates in the open ocean. The aggregates off Cape Blanc contained high amounts of ballast material due to the presence of coccoliths and fine-grained dust from the Sahara desert, leading to a dominance of small and fast-settling aggregates. The largest changes in vertical fluxes were observed in the surface waters (<250 m), and, thus, showing this site to be the most important zone for aggregate formation and degradation. The degradation length scale (L), i.e. the fractional degradation of aggregates per meter settled, was estimated from vertical fluxes derived from the particle size distribution through the water column. This was compared with fractional remineralization rate of aggregates per meter settled derived from direct ship-board measurements of sinking velocity and small-scale O₂ fluxes to aggregates measured by micro-sensors. Microbial respiration by attached bacteria alone could not explain the degradation of organic matter in the upper ocean. Instead, flux feeding from zooplankton organisms was indicated as the dominant degradation process of aggregated carbon in the surface ocean. Below the surface ocean, microbes became more important for the degradation as zooplankton was rare at these depths.     

Northwestern Weddell Sea deep outflow into the Scotia Sea during the austral summers of 2000 and 2001 estimated by inverse methods

The northward outflow of cold, dense water from the Weddell Sea into the world ocean basins plays a key role in balancing the global heat budget. We estimate the geostrophic flow patterns in the northwestern Weddell Sea using box inverse methods applied to quasi-synoptic hydrographic data collected during the Brazilian DOVETAIL 2000 and 2001 austral summer cruises. The analysis is focused on the variations of the deep Weddell Sea outflow into the Scotia Sea within boxes that bound the main deep gaps over the South Scotia Ridge. To determine the geostrophic volume transports in each box, mass, salt, and heat are conserved within neutral density layers that are not in contact with the atmosphere. Implementing the inverse model and using property anomaly equations weighted by the flow estimate uncertainty our results are consistent with those reported in the literature. A bottom triangle extrapolation method is introduced, which improves the estimated property fluxes through hydrographic sections. In the austral summer of 2000 the transports of Weddell Sea Deep Water (WSDW) through the Philip Passage, Orkney Passage, and southwestern Bruce Passage are 0.01±0.01, 1.15±0.33, and 1.03±0.23 Sv (1 Sv=10⁶ m³ s⁻¹, >0 is northward), respectively. After extrapolation within bottom triangles these transports increase to 0.12±0.03, 3.48±1.81, and 1.20±2.16 Sv. Analysis of the hydrographic data reveal distinct oceanographic conditions over the Philip Passage region, with evidence of mesoscale meanders, warmer and saltier Warm Deep Water (WDW) and colder WSDW observed in 2001 than in 2000. Despite these differences the WSDW transport does not present a significant variation between 2000 and 2001. The WSDW transports through the Philip Passage in 2001 are 0.012±0.001 and 0.113±0.001 Sv after extrapolation within bottom triangles. The circulation derived from the inversion in the austral summer of 2001 suggests a sharp weakening of the barotropic cyclonic flow in the Powell Basin, which may be due to northerly and northeasterly winds associated with an atmospheric low-pressure center located west of the Antarctic Peninsula. We suggest that similar variations in atmospheric forcing may explain changes in the intensity of the cyclonic flow observed in the northwestern Weddell Sea and Powell Basin.     

Structure of potential upwelling areas in the Philippines

The three-dimensional structure of two potential mesoscale upwelling areas that are located in the external waters of the Philippine archipelago (i.e. northwest of Luzon and east of Mindanao) were constructed by analysis of historical data. A unique characteristic of both upwelling sites is that they can be identified by their anomalously cold subsurface temperatures rather than sea surface temperature distributions. As such, they cannot be observed in sea surface temperature fields derived by satellite imagery. The data used in the analysis were obtained from the National Oceanographic Data Center hydrographic database. Objective analysis was performed to produce monthly temperature fields at several standard depths within the upper 500 m of the region 0–30°N and 100–140°E with a horizontal grid resolution of 0.5°. The extent and timing of these upwelling areas are described. A review of existing hypothesis on the mechanisms for their evolution and seasonal modulation are presented. The change in heat content during the upwelling season is greater than 300 W m⁻² in both areas. Based on the excursion of isotherms, vertical velocities of 83 cm day⁻¹ and 26 cm day⁻¹ were obtained for upwelling northwest of Luzon and east of Mindanao, respectively.     

The diffusive component of particulate organic carbon export in the North Atlantic estimated from SeaWiFS ocean color

The diffusive component of the particulate organic carbon (POC) export from the ocean's surface layer has been estimated using a combination of the mixed layer model and SeaWiFS ocean color data. The calculations were carried out for several example sites located in the North Atlantic over a 10-year time period (1998–2007). Satellite estimates of surface POC derived from ocean color were applied as an input to the model driven by local surface heat and momentum fluxes. For each year of the examined period, the diffusive POC flux was estimated at a 200 m depth. The highest flux is generally observed in the spring and fall seasons, when surface waters are weakly stratified. In addition, the model results demonstrate significant interannual and geographical variability of the flux. The highest diffusive POC flux occurs in the northern North Atlantic and the lowest in the subtropical region. The interannual variability of the diffusive POC flux is associated with mixed layer dynamics and underscores the importance of atmospheric forcing for POC export from the surface layer to the ocean's interior.     

Regional variations in the fluxes of foraminifera carbonate,coccolithophorid carbonate and biogenic opal in the northern Indian Ocean

Mass fluxes of diatom opal, planktonic foraminifera carbonate and coccolithophorid carbonate were measured with time-series sediment traps at six sites in the Arabian Sea, Bay of Bengal and Equatorial Indian Ocean (EIOT). The above fluxes were related to regional variations in salinity, temperature and nutrient distribution. Annual fluxes of diatom opal range between 3 and 28 g m⁻² yr⁻¹, while planktonic foraminifera carbonate fluxes range between 6 and 23 g m⁻² yr⁻¹ and coccolithophorid carbonate fluxes range between 4 and 24 g m⁻² yr⁻¹. Annual planktonic foraminifera carbonate to coccolithophorid carbonate ratios range between 0.8 and 2.2 and coccolithophorid carbonate to diatom opal ratios range between 0.5 and 3.3.In the western Arabian Sea, coccolithophorids are the major contributors to biogenic flux during periods of low nutrient concentrations. Coccolithophorid carbonate fluxes decrease and planktonic foraminiferal carbonate and diatom opal fluxes increase when nutrient-rich upwelled waters are advected over the trap site. In the oligotropic eastern Arabian Sea, coccolithophorid carbonate fluxes are high throughout the year. Planktonic foraminiferal carbonate fluxes are the major contributors to biogenic flux in the EIOT. In the northern and central Bay of Bengal, when surface salinity values drop sharply during the SW monsoon, there is a drastic reduction in planktonic foraminiferal carbonate fluxes, but coccolithophorid carbonate and diatom opal fluxes remain steady or continue to increase. Distinctly higher annual molar Si_bio/C_inorg (>1) and C_org/C_inorg (>1.5) ratios are observed in the northern and central Bay of Bengal mainly due to lower foraminiferal carbonate production as a result of sharp salinity variations. We can thus infer that the enhanced freshwater supply from rivers should increase oceanic CO₂ uptake. Its silicate supply favours the production of diatoms while the salinity drop produces conditions unfavourable for most planktonic foraminifera species.     

Atmospheric iron deposition and sea-surface dissolved iron concentrations in the eastern Atlantic Ocean

Atmospheric iron and underway sea-surface dissolved (<0.2 μm) iron (DFe) concentrations were investigated along a north–south transect in the eastern Atlantic Ocean (27°N/16°W–19°S/5°E). Fe concentrations in aerosols and dry deposition fluxes of soluble Fe were at least two orders of magnitude higher in the Saharan dust plume than at the equator or at the extreme south of the transect. A weaker source of atmospheric Fe was also observed in the South Atlantic, possibly originating in southern Africa via the north-easterly outflow of the Angolan plume. Estimations of total atmospheric deposition fluxes (dry plus wet) of soluble Fe suggested that wet deposition dominated in the intertropical convergence zone, due to the very high amount of precipitation and to the fact that a substantial part of Fe was delivered in dissolved form. On the other hand, dry deposition dominated in the other regions of the transect (73–97%), where rainfall rates were much lower. Underway sea-surface DFe concentrations ranged 0.02–1.1 nM. Such low values (0.02 nM) are reported for the first time in the Atlantic Ocean and may be (co)-limiting for primary production. A significant correlation (Spearman's rho=0.862, p<0.01) was observed between mean DFe concentrations and total atmospheric deposition fluxes, confirming the importance of atmospheric deposition on the iron cycle in the Atlantic. Residence time of DFe in the surface waters relative to atmospheric inputs were estimated in the northern part of our study area (17±8 to 28±16 d). These values confirmed the rapid removal of Fe from the surface waters, possibly by colloidal aggregation.