Abstract
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.
Highlights
The surface circulation in the northwestern Mediterranean Sea (NWMS) is formed by a cyclonic oceanic gyre (Fig. 1)
This case study focused on the evolution of North Balearic Front (NBF) dynamics and stratification during IOP16 (26–29 October 2012), www.ocean-sci.net/15/179/2019/
After IOP16, the observations showed a southward displacement of the NBF, of several tens of kilometres, with a rapid decrease in sea surface temperature (SST), larger than 4 ◦C in the frontal zone
Summary
The surface circulation in the northwestern Mediterranean Sea (NWMS) is formed by a cyclonic oceanic gyre (Fig. 1). The tramontane and mistral are two strong, dry and cold northwesterly/northerly winds which are channelled by the topography, between the Pyrenees and the Massif Central for the tramontane and between the Massif Central and the Alps for the mistral (Fig. 1) These winds produce strong air–sea exchanges (Flamant, 2003; Hauser et al, 2003) which lead to important diabatic buoyancy losses for the sea and which impact the NBF position in autumn (Seyfried et al, 2017). The atmospheric response to the front creates wind stress divergence and curl impacting the sea by Ekman pumping (Chelton and Xie, 2010) These air–sea feedbacks in frontal zones may have a marked influence on the diabatic and Ekman buoyancy fluxes (Thomas and Lee, 2005).
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