Meddies and Their Sea Surface Expressions: Observations and Theory

Abstract

The sea surface expressions of Mediterranean Water eddies, known as “meddies,” are observed in satellite data, and their main characteristics are measured. Satellite altimeter observations of surface expressions are detected over the meddies observed in situ using the MEDTRANS meddy dataset (1950–2013). In this study 209 observed meddy cores in the North Atlantic Ocean, selected over the period of the 22 years of sea surface height measurements with satellite altimetry (1993–2013), were analyzed. Results show relatively good agreement between the theoretical estimates of the meddy surface signals as reported by Bashmachnikov and Carton and the measured surface expressions. It was found that, on average, the theoretical results underestimate the measured sea surface elevations of the meddy surface expressions by a factor of 2. Although the variability of the measured expressions is reasonably well described by the combination of meddy core and the ocean background parameters of the theoretical expression, we cannot define a single individual parameter of the meddy core, which chiefly shapes the magnitude of the meddy surface signal. Interestingly, the overall distribution of characteristics of meddy surface expressions in the Atlantic shows that the sea level anomalies formed by meddies intensify westward, growing both in magnitude and radius. This opposes the expected theoretical decrease of meddy surface signals due to a known progressive decay of the meddy cores with distance from their generation region at the Iberian continental slope. This observed tendency is attributed to meddy interaction with the upper-ocean currents and other eddies (in particular in the region of the North Atlantic Current and Azores Current) that are not considered by the theory.