Frontal Dynamics in the Alboran Sea: 2. Processes for Vertical Velocities Development

Abstract

[Abstract] Significant lateral density gradients occur throughout the year in the Alboran Sea, giving rise to two main fronts: the Western Alboran Gyre Front (WAGF) and Eastern Alboran Gyre Front (EAGF), where large vertical velocities often develop. To improve the understanding of the processes that underlie the development of the vertical velocities in the fronts, the periods of development were analyzed in the perspective of the frontogenesis, instabilities, non-linear Ekman, and filamentogenesis mechanisms, using multi-platform in-situ observations and a high-resolution realistic simulation in spring 2018. The spatio-temporal characteristics of the WAGF indicate a wider, deeper, and longer-lasting front than the EAGF. Additionally, the WAGF shows stronger and deeper upwelling and downwelling regions. The WAGF vertical velocities (up to |55| m/day) are amplified by an across-front ageostrophic secondary circulation generated by: (a) frontal intensification explained by frontogenesis, which shows a sharpening of buoyancy gradients associated with the Atlantic Jet, (b) nonlinear Ekman effects, that are enhanced by the persistent western wind blowing along the frontal direction, and (c) submesoscale instabilities (symmetric and ageostrophic baroclinic instabilities). The EAGF vertical velocities (up to |38| m/day) are amplified by two asymmetrical ageostrophic cells developed across the front with a narrow upwelling region in the middle. The cell's circulation is explained by frontal intensification produced by filamentogenesis through a cold filament advection to the Mediterranean Sea interior, that is characterized by pointy isopycnals at the center of the filament. This mechanism is observed in both the model and glider observations.

[Plain Language Summary] The processes responsible for vertical velocities development at the main fronts of the Alboran Sea were analyzed combining a high-resolution realistic simulation and glider observations in spring 2018. The two fronts were found to be governed by different spatio-temporal dynamics. In the Western Alboran Gyre Front, the vertical velocities are amplified by an across-front ageostrophic circulation generated by the intensification of the front, which is the result of an increase of horizontal density gradients, and along down-front winds. In turn, submesoscale instabilities processes strengthen the vertical velocities in the frontal region, but weaken the front. On the contrary, in the Eastern Alboran Gyre Front, the vertical velocities development is associated with an across-front complex pattern described by two asymmetric ageostrophic motions produced by the frontal intensification, which are the consequence of a cold filament advected in the interior of the Mediterranean Sea. As a result, pointy isopycnals are observed in the front.