Nicolas Grima

Both deepwater formation and the obduction processes converting dense deep water to lighter surface water are the engine for the global meridional overturning circulation (MOC). Their spatio‐temporal variations effectively modify the ocean circulation and related carbon cycle, which affects climate evolution throughout geological time. Using early‐Eocene bathymetry and enhanced atmospheric CO₂ concentration, the IPSL‐CM5A2 climate model has simulated a well‐ventilated Southern Ocean associated with a strong anticlockwise MOC. To trace the fates of these paleo‐Antarctic Bottom Water (paleo‐AABW), we conducted Lagrangian analyses using these IPSL‐CM5A2 model results and tracking virtual particles released at the lower limb of the MOC, defined as an initial section at 60°S below 1,900 m depth. Diagnostic analysis of these particles trajectories reveals that most paleo‐AABW circulates back to the Southern Ocean through either the initial section (43%) or the section above (31%), the remaining (>25%) crossing the base of the mixed layer mostly in tropical regions (up to half). The majority of water parcels ending in the mixed layer experience negative density transformations, intensified in the upper 500 m and mostly occurring in tropical upwelling regions, with a spatial pattern consistent with the wind‐driven Ekman pumping, largely determined by the Eocene wind stress and continental geometry. In the same way as present‐day North Atlantic Deep Water upwells in the Southern Ocean, our results suggest that the strong tropical and equatorial upwelling during the Eocene provides an efficient pathway from the abyss to the surface, but at much higher temperature, with potential implications for the oceanic carbon cycle.

We study the dispersion and convergence of marine floating material by surface currents from a model reanalysis that represents explicitly mesoscale eddy variability. Lagrangian experiments about the long‐term evolution (29 years) of an initially homogeneous concentration of particles are performed at global scale with horizontal current at one fourth degree resolution and refreshed daily over the 1985–2013 period. Results not only confirm and document the five known sites of surface convergence at the scale of individual oceanic basins but also reveal a convergent pathway connecting the South Indian subtropical region with the convergence zone of the South Pacific through the Great Australian Bight, the Tasman Sea, and the southwest Pacific Ocean. This “superconvergent” pathway at the ocean surface is robust and permanent over a distance longer than 8,000 km. The current variability is crucial to sustain this pathway.

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.

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.