What can we learn from eddy-induced signatures on sea surface temperature ?

Anticyclonic (cyclonic) eddies are commonly considered to have a warm (cold) signature on the sea surface temperature (SST). However several recent studies revealed the existence of a non-negligible fraction of "inverse" eddy-induced SST anomalies : cold-core anticyclones and warm-core cyclones. Using remote sensing and in situ observations in the Mediterranean sea over 3 years (2016-2018), we built an eddy core surface temperature index and showed that these inverse SST signatures have a seasonal distribution, scarce in winter but very common and even predominant in early summer. Warm-core cyclones and cold-core anticyclones proportion gets a maximum of 70% of the signatures in May and June, with a quick rise in coincidence with spring restratification and mixed layer depth (MLD) shallowing.

To understand further the physical processes we used a simple 1D vertical model of a water column forced by a seasonal surface temperature flux. It is a known observation that MLD is deeper (shallower) inside anticyclones (cyclones), and we tested if this difference of vertical structure alone was sufficient to reproduce eddy-induced SST signature inversion during spring restratification. This proved not to be enough, and it is only by taking into account a differential diapycnal eddy mixing - increased in anticyclones and reduced in cyclones - that we reproduce correctly, in agreement with the observations, the eddy surface temperature inversion. Furthermore, idealized 3D numerical simulations (so far for an anticyclone) at sufficiently high resolution were able to reproduce the shift from a winter warm-core eddy to a summer cold-core eddy, and they revealed a dependence on the wind forcing strength and frequency in the magnitude of the eddy-induced SST signature.

This simple 1D model tends to show that vertical mixing modulation by mesoscale eddies might be a key mechanism explaining inverse eddy SST signatures. It also suggests beyond that these signatures could represent an integrated signal of both temperature and momentum flux forcings at the scale of the eddy.