Vortex cohérents profonds de longue durée dans l'océan Atlantique

Previous studies have extensively described the coherent vortices at the ocean surface and shallow sub-surface at global and regional scales (from satellites, in-situ measurements, and numerical models). Few studies have investigated the dynamics of the deep coherent vortices (DCVs) below the mixed layer depth. This study focuses on DCVs in the Atlantic Ocean using a high-resolution numerical model. Since the properties of the water asses move along isopycnals, the detection and tracking of DCVs are performed along three isopycnal surfaces: 27.60 kg/m3, 27.80 kg/m3, and 27.86 kg/m3 with depths of 250-1700 m, 1200-2800 m, and 1900-3800 m, respectively. The quantification of the physical characteristics of the DCVs (population, radius, Rossby number, polarity between cyclones and anticyclones, and propagation in space and time) in different parts of the Atlantic Ocean(Mediterranean Water vortices, meddies and Mid-Atlantic Ridge, MAR). The dynamics involved in the generation and destruction of the DCVs throughout their life cycle are analyzed.There is an asymmetry between cyclonic DCVs and anticyclonic DCVs, as they propagate poleward and equatorward, respectively, due to the beta-effect. Cyclonic DCVs tend to be smaller and shorter lived than anticyclonic DCVs, so anticyclones dominate in terms of energetic, large, long-lived, and long-distance DCVs. The results also show that anticyclonic meddies, as well as other DCVs, can cross MAR. The region of the ocean west of the MAR is also characterised by a large number of shielded vortices and the formation of a group of transient shielded multiplets. The DCVs contribute to the transport of characteristic properties of different water masses from their source origin to the distance offshore.This sheds new light on the understanding of the formation, life cycle, physical and dynamical properties of the ocean interior in the Atlantic Ocean for the long-lived DCVs.Des études antérieures ont largement décrit les tourbillons cohérents à la surface de l’océan et en sub-surface peu profonde, aux échelles globales et régionales (à partir de données satellitaires, de mesures in situ et de modèles numériques). Peu d’études ont examiné la dynamique des tourbillons cohérents profonds (deep coherent vortices - DCVs) sous la couche de mélange. Cette étude se concentre sur les DCVs dans l’océan Atlantique à l’aide d’un modèle numérique à haute résolution. Comme les propriétés des masses d’eau se déplacent le long d’isopycnes, la détection et le suivi des DCVs sont effectués le long de trois surfaces isopycnales : 27.60 kg/m3, 27.80 kg/m3, et 27.86 kg/m3, avec des profondeurs respectives de : 250-1700 m, 1200- 2800 m, et 1900-3800 m. La quantification des caractéristiques physiques des DCVs (population, rayon, nombre de Rossby, polarité entre cyclones et anticyclones et propagation dans l’espace et le temps) dans différentes zones de l’océan Atlantique (d’eau méditerranéenne, meddies et Mid-Atlantique Ridge, MAR). Les dynamiques impliquées dans la génération et la destruction des DCVs tout au long de leur cycle de vie sont analysées. Il existe une asymétrie entre les DCVs cycloniques et les DCVs anticycloniques, qui se propagent respectivement vers les pôles et vers l’équateur, en raison de l’effect Beta. Les DCVs cycloniques ont tendance à être plus petits et à durer moins longtemps que les DCVs anticycloniques, de sorte que les anticyclones dominent en termes d’énergie, de taille et de DCVs à longue durée de vie et se propageant à longues distances. Les résultats montrent également que les meddies anticycloniques, ainsi que d’autres DCVs, peuvent traverser la dorsale médioatlantique MAR. La région de l’océan située à l’ouest de la MAR est également caractérisée par un grand nombre de tourbillons protégés par un écran de vorticité, ainsi que par une formation d’un groupe de multiplets transitoires. Ce travail apporte un nouvel éclairage sur la formation, le cycle de vie, les propriétés physiques et dynamiques de l’océan Atlantique intérieur, pour les DCVs à longue durée de vie

Changes in Global Ocean Circulation due to Isopycnal Diffusion

We investigate changes in the ocean circulation due to the variation of isopycnal diffusivity (k(iso)) in a global non-eddy-resolving model. Although isopycnal diffusion is thought to have minor effects on interior density gradients, the model circulation shows a surprisingly large sensitivity to the changes: with increasing k(iso), the strength of the Atlantic residual overturning circulation (AMOC) and the Antarctic Circumpolar Current (ACC) transport weaken. At high latitudes, the isopycnal diffusion diffuses temperature and salinity upward and poleward, and at low latitudes downward close to the surface. Increasing isopycnal diffusivity increases the meridional isopycnal fluxes whose meridional gradient is equatorward, hence leading to a negative contribution to the flux divergence in the tracer equations and predominant cooling and freshening equatorward of 40 degrees. The effect on temperature overcompensates the countering effect of salinity diffusion, such that the meridional density differences decrease, along with which ACC and AMOC decrease. We diagnose the adjustment process to the new equilibrium with increased isopycnal diffusion to assess how the other terms in the tracer equations react to the increased k(iso). It reveals that around +/- 40 degrees latitude, the cooling induced by the increased isopycnal flux is only partly compensated by warming by advection, explaining the net cooling. Overall, the results emphasize the importance of isopycnal diffusion on ocean circulation and dynamics, and hence the necessity of its careful representation in models. SIGNIFICANCE STATEMENT: The effect of mixing by mesoscale eddies, represented as diffusion along surfaces of constant density in models, on the ocean circulation is not well understood. Here, we show that an increase in the eddy diffusivity in different setups of a global ocean model leads to a surprisingly large change of the ocean circulation. The strength of the Atlantic overturning circulation and the Antarctic Circumpolar Current decrease. We find that the interior ocean becomes cooler and fresher and that the temperature effect on density dominates over salinity, resulting in a decrease in the density gradients. Our results point out the importance of eddy diffusion on ocean circulation, and hence the necessity of its correct representation in ocean and climate models