Dynamics of the North Balearic Front during an autumn tramontane and mistral storm: air–sea coupling processes and stratification budget diagnostic
2018
Abstract. The North Balearic Front forms the southern branch of the cyclonic
gyre in the northwestern Mediterranean Sea. Its dynamics exhibit significant seasonal variability. During autumn, the front spreads northward
during the calm wind periods and rapidly moves back southward when it is
exposed to strong northerly wind events such as the tramontane and mistral.
These strong winds considerably enhance the air–sea exchanges. To investigate
the role of air–sea exchanges in the dynamics of the North Balearic front, we
used observations and a high-resolution air–sea coupled modelling system. We
focused on a strong-wind event observed in late October 2012, which was
well-documented during the Hydrological Cycle Mediterranean Experiment. The
coupled model was able to correctly reproduce the 4 ∘ C sea surface
temperature drop recorded in the frontal zone together with the observed
southwestward displacement of the front. The comparison between the weak wind
period preceding the event and the strong-wind event itself highlighted the
impact of the wind regime on the air–sea coupling. During the low-wind period
the coupling is thermal and dynamical whereas during the strong-wind period
the coupling is mainly thermal. The effect of air–sea exchanges on the
stratification variations in the frontal zone was investigated with a
stratification budget diagnosis. The stratification variations are controlled
by diabatic air–sea buoyancy flux, adiabatic Ekman buoyancy flux, and
advective processes. During the strong-wind period, the Ekman buoyancy flux
was found to be 3 times greater than the air–sea buoyancy flux and thus
played a major role in the de-stratification of the frontal zone. The role of
Ekman pumping and inertial wave in the advective processes is also discussed.
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