A high-emission future scenario in a sub-kilometric simulation of the Northwestern Mediterranean destabilizes the circulation and induces a seasonal Balearic Gyre

Abstract. Surface waters in the western Mediterranean are warming at a rate that is nearly three times faster than that of the global ocean. The induced increase in stratification already results in surface circulation changes. In this study, we analyse a simulation of the Western Mediterranean conducted at a resolution finer than 500 m north of 42° N and forced by a regional climate simulation under the SSP5-8.5 scenario. We focus on the evolution of surface dynamics in the northwestern Mediterranean Sea throughout the 21st century. The dynamics of the system are assessed using a scale decomposition of the kinetic energy (KE) into seven terms. The standing-mean, transient-mean, standing-eddy and transient-eddy KEs are described by four terms. Three cross-terms quantify the covariance between standing fluctuations and transient motions and diagnose meso- to sub-mesoscale KE coupling. These also enable the assessment of the seasonal variability of the dynamics. The method is applied to a 4.5 km reanalysis for comparison with the present and future outputs of the HR scenario. When confronted with the reanalysis, present-day outputs show a more extended Northern Current (NC) that maintains its strength until it reaches the Balearic Sea. Because the northernmost extension of the Algerian Eddies falls within the parent-model relaxation zone, it is not represented. Instead, the cyclonic Northern Gyre is more clearly closed between Menorca and Corsica, across scales and in both mean and transient components. In the future scenario, the NC widens especially offshore due to the increased stratification; large- and particularly small-scale turbulence intensify. However, the most prominent change occurs in the Balearic Sea, where a substantial large-scale gyre is observed to develop, related to the NC destabilization. This Balearic Gyre (BG) induces an increase in smaller-scale KE, both stationary and transient. The BG is seasonally maintained (destabilized) during summer (winter) thanks to a persistent (eroded) core of Atlantic Water (AW). Interannual events show that the gyre is more powerful when the AW in the core of the BG reaches a depth greater than 150 m in some winters. Finally, during autumn, wind-driven eddying of the NC in the Ligurian Sea is expected to intensify due to its widening.