Mesoscale eddies in the North Atlantic subtropical gyre: characteristics, vertical structure, and associated transport from satellite altimetry and Argo profile data.

The mean vertical structure and transport properties of mesoscale eddies in the subtropical North Atlantic are investigated by combining historical records of Argo temperature/salinity profiles and satellite sea level anomaly data in the framework of the eddy tracking technique. The area of interest, defined as (55-19 W, 18-32 N), delineates the interior of the subtropical gyre and is generally characterized by low level of eddy kinetic energy. The mean sea level anomaly amplitude of eddies, used in the composite, is about 3 cm. Despite being relatively weak at the sea surface, the eddy signal is found to penetrate to at least 1200 m depth, which is clearly seen in the eddy-induced salinity anomalies at the depth of the Mediterranean outflow water. The analysis also reveals that the eddy vertical structure is strongly affected by the background stratification, leading to the variability in the eddy structure across the gyre. A common feature of all the eddy composites, reconstructed in different parts of the gyre, is the phase shift between the eddy temperature/salinity and velocity anomalies in the upper ~300 m layer, resulting in transient eddy transports of heat and salt. The main effect of the eddies is to transport the excess of heat and salt out of the gyre. As an application, a box model of the near-surface layer is used to evaluate the role of mesoscale eddies in maintaining a quasi-steady-state distribution of salinity at the North Atlantic subtropical salinity maximum. The model shows that mesoscale eddies are able to remove between 10 and 40% of the excess of salt out of the area of salinity maximum (47-27 W, 21-29 N) required to compensate for the local excess of evaporation over precipitation, the rest being provided by the mean advection, vertical mixing, and other processes.