Mechanisms of the southward translation of meddies

Abstract

⋅ current-induced pseudo-topographic β effect, ⋅ nonlinear effect of secondary eddies. Meddies are warm and saline anticyclonic eddies found at the mid-depth of the Mediterranean Outflow in the Eastern North Atlantic. They are observed to move almost southward at average speeds as high as 1.3 ± 0.2 cm s -1 . This paper examines a mechanism which accelerates this curious translation to a reasonable speed when combined with nonlinearity and a suitable vertical eddy structure. This mechanism is the pseudo-topographic β effect due to the westward decrease in the thickness of the mid-layer induced by the surface southward mean current. The linear dispersion relation including this effect is shown to account for a quarter of the surface southward mean current or a fifth of the observed southward translation of meddies. Three-layer quasi-geostrophic experiments reveal that the surface southward mean current certainly enhances the southward translation velocity of meddies to a speed in agreement with observation, if the meddy has a current structure of plausible intensity and vertical coherence. In the light of the current-induced pseudo-topographic β effect, previous hypotheses are also re-examined through dynamic arguments together with numerical experiments; they are suggested to have some difficulty either in dynamics or in correspondence with observation. Thus the nonlinear effect due to secondary eddies combined with the current-induced pseudo-topographic β effect is the most likely mechanism responsible for the rapid southward translation of meddies among those examined in the paper. Mechanisms of the Southward Translation of Meddies

Baroclinic multipole formation from heton interaction

“…Hetons can also form from the instability of coastal currents McWilliams 2001, Chérubin et al 1997). Observations of Mediterranean water eddies south and west of the Iberian Peninsula show that deep anticyclones (meddies) can be associated with cyclones above or below them, thus forming warm or cold hetons (Chérubin et al 1997, Takahashi and Masuda 1998, Paillet et al 2002. Observations and models have also shown that hetons can originate in the baroclinic instability of convective columns (Legg and Marshall 1993).…”