Abstract
Abstract. Meddies, intra-thermocline eddies of Mediterranean water, can often be detected 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. While pushing its way through the water column, a meddy raises isopycnals above. As a consequence of potential vorticity conservation, negative relative vorticity is generated in the upper layer. During the initial period of meddy acceleration after meddy formation or after a stagnation stage, a cyclonic signal is also generated at the sea-surface, but mostly the anticyclonic surface signal follows the meddy. Based on geostrophy and potential vorticity balance, we present theoretical estimates of the intensity of the surface signature. It appears to be 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. Using climatic distribution of the stratification intensity, we claim that the southernmost limit for detection in altimetry of small meddies (with radii on the order of 10–15 km) should lie in the subtropics (35–45° N), while large meddies (with radii of 25–30 km) could be detected as far south as the northern tropics (25–35° N). Those results agree with observations.
Highlights
Generated by the destabilization of the Mediterranean outflow along the Iberian Peninsula and at surrounding banks, meddies (Mediterranean Water Eddies) drift across the Northeastern Atlantic, sometimes reaching the Mid-Atlantic Ridge without major change in their dynamical properties (Richardson et al, 2000; Serra and Ambar, 2002).Meddies, warm and salty intrathermocline eddies, are isolated from the surrounding waters by strong potential vorticity gradients and show low horizontal and vertical diffusivities (Hebert, 1988; Martin et al, 2001)
In-situ (Table 1) and altimetric (Table 2) observations clearly indicate that most of the registered meddies showed a vertical alignment with an anticyclonic eddy at the sea surface
The related sea-surface elevation and relative vorticity anomaly peaked near the meddy centre and reached 5 to 15 cm and −0.05 to −0.15f, respectively
Summary
Generated by the destabilization of the Mediterranean outflow along the Iberian Peninsula and at surrounding banks, meddies (Mediterranean Water Eddies) drift across the Northeastern Atlantic, sometimes reaching the Mid-Atlantic Ridge without major change in their dynamical properties (Richardson et al, 2000; Serra and Ambar, 2002). “Beta gyres” in a meddy may be formed by planetary, baroclinic or topographic beta effects. The geostrophic azimuthal velocities near the sea-surface are around 70 % of those of the meddy core, varying from 30 to 100 % (Bashmachnikov et al, 2009a). These strong surface signals, and the stability of meddies, allow their possible tracking with altimetry (Armi et al, 1988; Stammer et al, 1991; Pingree and Le Cann, 1993b; Bower et al, 1997; Richardson et al, 2000). In this paper we develop criteria to determine where meddies with given characteristics can be observed at the sea-surface
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