Regionalizing The Impacts Of Wind- And Wave-Induced Currents On Surface Ocean Dynamics: A Long-Term Variability Analysis In The Mediterranean Sea

Effects of wind and waves on the surface dynamics of the Mediterranean Sea are assessed using a modified Ekman model including a Stokes-Coriolis force in the momentum equation. Using 25 years of observations, we documented intermittent but recurrent episodes during which Ekman and Stokes currents substantially modulate the total mesoscale dynamics by two nonexclusive mechanisms: (a) by providing a vigorous input of momentum (e.g., where regional winds are stronger) and/or (b) by opposing forces to the main direction of the geostrophic component. To properly characterize the occurrence and variability of these dynamical regimes, we perform an objective classification combining self-organizing maps and wavelet coherence analyses. It allows proposing a new regional classification of the Mediterranean Sea based on the respective contributions of wind, wave, and geostrophic components to the total mesoscale surface dynamics. We found that the effects of wind and waves are more prominent in the northwestern Mediterranean, while the southwestern and eastern basins are mainly dominated by the geostrophic component. The resulting temporal variability patterns show a strong seasonal signal and cycles of 5-6 years in the total kinetic energy arising from both geostrophic and ageostrophic components. Moreover, the whole basin, specially the regions characterized by strong wind- and wave-induced currents, shows a characteristic period of variability at 5 years. This can be related to climate modes of variability. Regional trends in the geostrophic and ageostrophic currents show an intensification of 0.058 +/- 1.43.10(-5) cm/s per year.Plain Language Summary The ocean dynamics plays a decisive role in the global balance of essential variables, such as heat, CO2, or primary production, as well as in the dispersion of pollutants. However, the physical processes that control the mesoscale dynamics and its variability in the surface of the Mediterranean Sea is not fully understood. Therefore, we have analyzed the regional contribution of the geostrophic and the wind- and waves-induced currents using a classification method based on a machine-learning algorithm. We find that the effect of wind and waves is stronger over regions of the northwestern Mediterranean, while the southwestern and eastern basin is mainly dominated by geostrophy. We observe that regions where wind and wave dominate the dynamics co-vary with the main Mediterranean climate modes of variability. The geostrophic currents show an intensification with a clear shift in 2002, which suggests that this positive trend could be a part of a large decadal oscillation.