Nicolas Grima

An eddy-permitting model of the Atlantic circulation: Evaluating open boundary conditions

As part of the French CLIPPER project, an eddy permitting model of the Atlantic circulation has been run for 22 years. The domain has open boundaries at Drake passage and at 30°E, from Africa to Antarctica. The simulated mean circulation, as well as the eddy activity, is satisfactory for a 1/3° model resolution, and the meridional heat transport at 30°S is within the range estimated from observations. We use the “mixed” open boundary algorithm of Barnier et al. [1998], which has both a radiation condition and a relaxation to climatology. The climatological boundary forcing strongly constrains the solution in the whole domain. The model heat balance adjusts through the surface (heat flux retroaction term) more than the open boundaries. The radiation phase velocities calculated within the algorithm are analyzed. This shows, quite surprisingly, that both the eastern and western boundaries have a similar behavior, regardless of the preferred directions for advection (mainly eastward) and wave propagation (mainly westward). Our results confirm that open boundary algorithms behave differently according to the dynamics of the region considered. The passive boundary condition that Penduff et al. [2000] applied successfully in the north eastern Atlantic does not work in the present South Atlantic model. We emphasize the need for a careful prescription of the climatology at the open boundary, for which a new approach based on synoptic sections is implemented.

Estimates of the mortality and the duration of the trans-Atlantic migration of European eel Anguilla anguilla leptocephali using a particle tracking model

Using Lagrangian simulations, based on circulation models over three different hydroclimatic periods in the last 45 years in the North Atlantic Ocean, the trans-Atlantic migration of the European eel Anguilla anguilla leptocephali was simulated via the passive drift of particles released in the spawning area. Three different behaviours were modelled: drifting at fixed depth, undergoing a vertical migration or choosing the fastest currents. Simulations included mortality hypotheses to estimate a realistic mean migration duration and relative survival of A. anguilla larvae. The mean migration duration was estimated as 21 months and the mortality rate as 3.8 per year, i.e. < 0.2% of A. anguilla larvae may typically survive the trans-Atlantic migration.

Environmental characteristics of Agulhas rings affect interocean plankton transport

Agulhas rings provide the principal route for ocean waters to circulate from the Indo-Pacific to the Atlantic basin. Their influence on global ocean circulation is well known, but their role in plankton transport is largely unexplored. We show that, although the coarse taxonomic structure of plankton communities is continuous across the Agulhas choke point, South Atlantic plankton diversity is altered compared with Indian Ocean source populations. Modeling and in situ sampling of a young Agulhas ring indicate that strong vertical mixing drives complex nitrogen cycling, shaping community metabolism and biogeochemical signatures as the ring and associated plankton transit westward. The peculiar local environment inside Agulhas rings may provide a selective mechanism contributing to the limited dispersal of Indian Ocean plankton populations into the Atlantic.

On the origins of water masses exported along both sides of Greenland: A Lagrangian model analysis

The origin of the water masses exported from the Arctic to the North Atlantic along both sides of Greenland is investigated using an original numerical method. A quantitative Lagrangian analysis is applied to the monthly climatological 3-D output of a global ocean/sea ice high-resolution model. It allows quantification of the different branches of the export to the North Atlantic, as well as related timescales and water mass transformations. In the model, the outflow through Davis Strait consists in equal parts of Pacific and Atlantic water, whilst the export through Fram Strait consists almost fully of Atlantic water (contrary to observations). Pacific water is transferred quickly (O(10 years)) to the North Atlantic through the Beaufort Gyre, where gradual warming and salinification occur. Atlantic water exiting in the surface layer along both sides of Greenland remains about 10 years in the Arctic Basin and undergoes cooling and significant freshening. Below the surface water, Atlantic water exiting through the intermediate and deep layers in Fram Strait follows different pathways in the Arctic, with trajectories being subject to topography constraints. The travel time depends strongly on the pathway (from 1 to 1000 years). The intermediate outflow consists mainly of water entering the Arctic at Fram Strait, while half the deep outflow is composed of water from the Barents Sea. We find that the Barents Sea Branch, which contributes to both the outflows at Fram and Davis straits, is almost fully transformed after a year due to heat exchanges with the very cold atmosphere (in the Barents Sea).

Sensitivity of an oceanic general circulation model forced by satellite wind stress fields

Satellite wind and wind stress fields at the sea surface, derived from the scatterometers on European Remote Sensing satellites 1 and 2 (ERS-1 and ERS-2) are used to drive the ocean general circulation model (OGCM) “OPA” in the tropical oceans. The results of the impact of ERS winds are discussed in terms of the resulting thermocline, current structures, and sea level anomalies. Their adequacy is evaluated on the one hand by comparison with simulations forced by the Arpege-Climat model and on the other hand by comparison with measurements of the Tropical Atmosphere-Ocean (TAO) buoy network and of the TOPEX/Poseidon altimeter. Regarding annual mean values, the thermal and current responses of the OGCM forced by ERS winds are in good agreement with the TAO buoy observations, especially in the central and eastern Pacific Ocean. In these regions the South Equatorial Current, the Equatorial Undercurrent, and the thermocline features simulated by the OGCM forced by scatterometer wind fields are described. The impact of the ERS-1 winds is particularly significant to the description of the main oceanic variability. Compared to the TAO buoy observations, the high-frequency (a few weeks) and the low-frequency of the thermocline and zonal current variations are described. The correlation coefficients between the time series of the thermocline simulated by ERS winds and that observed by the TAO buoy network are highly significant; their mean value is 0.73, over the whole basin width, while it is 0.58 between Arpege model simulation and buoy observations. At the equator the time series of the zonal current simulated by the ERS winds, at three locations (110°W, 140°W, and 165°E) and at two depths, are compared to the TAO current meter and acoustic Doppler current profiler (ADCP) measurements. The mean value of the significant correlation coefficients computed with the in situ measurements is 0.72 for ERS, while it is 0.51 for the Arpege-Climat model. Thus ERS wind fields through the OGCM generate more realistic current variations than those obtained with Arpege climate winds, and they are particularly efficient in capturing abrupt changes (“wind bursts”) which may be important regarding ocean dynamics.

Comparative study of potential transfer of natural and anthropogenic cadmium to plankton communities in the North-West African upwelling

A Lagrangian approach based on a physical-biogeochemical modeling was used to compare the potential transfer of cadmium (Cd) from natural and anthropogenic sources to plankton communities (Cd-uptake) in the North-West African upwelling. In this region, coastal upwelling was estimated to be the main natural source of Cd while the most significant anthropogenic source for marine ecosystem is provided by phosphate industry. In our model experiment, Cd-uptake (natural or anthropogenic) in the North-West African upwelling is the result of an interplay between the Cd dispersion (by advection processes) and the simulated biological productivity. In the Moroccan waters, advection processes limit the residence time of water masses resulting in a low natural Cd-uptake by plankton communities while anthropogenic Cd-uptake is high. As expected, the situation is reversed in the Senegalo-Mauritanian upwelling where natural Cd-uptake is higher than anthropogenic Cd-uptake. Based upon an estimate of Cd sources, our modeling study shows, unexpectedly, that the anthropogenic signal of potential Cd-bioaccumulation in the Moroccan upwelling is of the same order of magnitude as the natural signal mainly present in the Senegalo-Mauritanian upwelling region. A comparison with observed Cd levels in mollusk and fishes, which shows overall agreement with our simulations, is confirming our estimates.

Origin of fine‐scale wind stress curl structures in the Benguela and Canary upwelling systems

Numerous studies have shown the primary importance of wind stress curl in coastal upwelling dynamics. The main goal of this new analysis is to describe the QuikSCAT surface wind stress curl at various scales in the Benguela and Canary upwelling systems. The dominant spatial pattern is characterized by cyclonic curl near continental boundaries and anticyclonic curl offshore, in association with equatorward alongshore (upwelling favorable) wind stress. At a smaller scale, we demonstrate the sensitivity of the QuikSCAT wind stress curl to coastal processes related to sea surface temperature (SST) mesoscale fluctuations by presenting a linear relationship between the curl and crosswind SST gradients. Despite the spatial and temporal sensitivity of the underlying thermal coupling coefficient, a local analysis of the fraction of the curl ascribed to SST variability shows that SST is a main driver of the wind stress curl variability and magnitude over the upwelling extension zone (∼100–300 km from the coast) in both the Canary and Benguela systems. Closer to the shore, the curl patterns derived from QuikSCAT observations are only loosely related to SST-wind interactions. As a working hypothesis, they can also be associated with the coastline geometry and orographic effects that are likely to play an important role in local cooling processes.

Plankton dynamics in a cyclonic eddy in the Southern California Current System

The California Current System is an eastern boundary upwelling system (EBUS) with high biological production along the coast. Oligotrophic offshore waters create cross-shore gradients of biological and physical properties, which are affected by intense mesoscale eddy activity. The influence of eddies on ecosystem dynamics in EBUS is still in debate. To elucidate the mechanisms that influence the dynamics of ecosystems trapped in eddies, and the relative contribution of horizontal and vertical advection in determining local production, we analyze a particular cyclonic eddy using Lagrangian particle-tracking analyses of numerical Eulerian. The eddy formed in a coastal upwelling system; coastal waters trapped in the eddy enabled it to leave the upwelling region with high concentrations of plankton and nutrients. The ecosystem was initially driven mainly by recycling of biological material. As the eddy moved offshore, production in its core was enhanced compared to eddy exterior waters through Ekman pumping of nitrate from below the euphotic zone; this Ekman pumping was particularly effective due to the shallow nitracline in the eddy compared to eddy exterior waters. Both eddy trapping and Ekman pumping helped to isolate and maintain the ecosystem productivity in the eddy core. This study shows the importance of cyclonic eddies for biological production in EBUS: they contribute both to the redistribution of the coastal upwelling ecosystem and are local regions of enhanced new production. Together, these processes impact cross-shore gradients of important biological properties.