Surface signature of Mediterranean water eddies in the North-East Atlantic: effect of the upper ocean stratification

Abstract

Abstract. Meddies, intra-thermocline eddies of Mediterranean water, are often visible at the sea surface as positive sea-level anomalies. Here we study the surface signature of several meddies tracked with RAFOS floats and AVISO altimetry. Then, theoretical estimates of the surface signature of a meddy are derived, based on geostrophy and potential vorticity balance. The intensity of the surface signature is proportional to the meddy core radius and to the Coriolis parameter, and inversely proportional to the core depth and buoyancy frequency. This indicates that surface signature of a meddy may be strongly reduced by the upper ocean stratification. Estimates suggest that the southernmost limit for detection in altimetry of small meddies (with radii on the order of 15 km) should lie in the northern subtropics, while large meddies (with radii of 25–30 km) could be detected as far south as the northern tropics. During the initial period of meddy acceleration after meddy formation or a stagnation stage, a cyclonic signal also is generated at the sea-surface, but mostly the anticyclonic surface signal follows the meddy.