Why do inverse eddy surface temperature anomalies emerge ? The case of
the Mediterranean Sea
Abstract
It is widely accepted that the signature of anticyclonic (cyclonic)
eddies on the sea surface temperature corresponds to a warm (cold) core
anomaly. Nevertheless, this statement has been put to question by recent
regional studies showing the existence of inverse eddy SST anomalies:
cold-core anticyclones and respectively warm-core cyclones. This study
shows that the emergence of these inverse signatures is a seasonal
phenomenon that affects the life cycle of mesoscale eddies in the
Mediterranean Sea. We use remote-sensing observations and in-situ data
to analyse the eddy-induced SST anomaly over a 2 years period
(2016-2018). We build an eddy core surface temperature index to quantify
the amount of Cold Core Anticyclones and Warm Core Cyclones all over the
year and especially during the spring re-stratification period. We find
that these inverse eddy signatures could reach a peak of 70 % in May
and June, both for cyclones and anticyclones. Besides, in order to
understand the underlying dynamical processes, we construct a simple
vertical column model to study the impact of the seasonal air-sea fluxes
on the surface stratification inside and outside eddies. 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 surface temperature inversion in
the eddy core. This simplified model, suggests that vertical mixing
modulation by mesoscale eddies might be the key mechanism that leads to
the eddy-SSTA seasonal inversion in the ocean.