Anticyclonic mesoscale eddies are often observed in the Balearic Sea (BS) toward the end of summer and autumn. In some years, these eddies become strong and persistent, modifying the local water mass properties. In this study, we analyze two of the most significant recent long-lived anticyclonic eddies, occurring in 2010 and 2017, using data from a high-resolution circulation model, altimetry and satellite-borne sea surface temperature observations. These eddies lasted around 2 and 4 months, respectively, with a radius varying between 40 and 75 km. The generation and intensification mechanisms of these long-lived anticyclonic eddies are studied by means of (a) energy conversion terms associated with eddy-mean flow interaction and (b) model sensitivity tests. Results show that these eddies were formed and intensified through mixed barotropic and baroclinic instabilities. The former are produced under the action of intense northwesterly (NW) winds. The latter are related to the existence of an intense summer thermal front between the BS and the Gulf of Lion, and to northward inflows of relatively lower salinity waters. Both the wind events and the presence of the thermal front are necessary for the formation of the eddies. The intensification process varied between both events. While in 2010 it was driven by significant salinity gradients produced by northwards inflows, in 2017 it was produced by additional intense NW winds. Both long-lived anticyclonic eddies created long-lasting surface temperature anomalies up to 2.5°C, which have characteristics of local marine heatwaves.
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