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

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.

Frontal Dynamics in the Alboran Sea: 1. Coherent 3D Pathways at the Almeria‐Oran Front Using Underwater Glider Observations

“…Their results confirm the underestimation of observational q Ertel, but also validate the same spatial patterns that obtained by the 2D assumption using glider observations (Figure A1; Garcia‐Jove et al. (2022)). $$${q}_{\text{Ertel}}=-\frac{1}{{\rho }_{0}}\left[\left(f+\frac{\partial \upsilon }{\partial x}-\frac{\partial u}{\partial y}\right)\frac{{\partial }_{\sigma \theta }}{\partial z}-\frac{\partial \upsilon }{\partial z}\frac{{\partial }_{\sigma \theta }}{\partial x}+\frac{\partial u}{\partial z}\frac{{\partial }_{\sigma \theta }}{\partial y}\right]$$$ $$${q}_{\text{Ertel}}=-\frac{1}{{\rho }_{0}}\left[\left(f-\frac{\partial u}{\partial y}\right)\frac{{\partial }_{\sigma \theta }}{\partial z}+\frac{\partial u}{\partial z}\frac{{\partial }_{\sigma \theta }}{\partial y}\right]$$$ …”

“…A detailed analysis of this simulation is provided in the companion paper Garcia‐Jove et al. (2022).…”

“…In their companion paper, Garcia‐Jove et al. (2022), use numerical simulations to assess the impact on the evaluation of q Ertel of the glider trajectory angle with respect to the front. Their results confirm the underestimation of observational q Ertel, but also validate the same spatial patterns that obtained by the 2D assumption using glider observations (Figure A1; Garcia‐Jove et al.…”

“…Finally, in Section 5, we present the conclusions. A companion paper by Garcia-Jove et al (2022) examines the mechanisms that generate and strengthen the ASC generation in the AO front through a modeling approach.…”

The Alboran Sea circulation and its biological response: A review

“…The footprint of submesoscale dynamics in the Almerıá-Oran front was revealed by the subduction of Chl up to 60 m below the deep Chl maximum. Although observational evidence is still required, submesoscale vertical velocities are expected to be stronger around the edges of the WAG, given the sharper density fronts of this region (Oguz et al, 2014;Garcıá-Jove et al, 2022). The role of submesoscale dynamics in overall phytoplankton production and export in the ocean, and in the AS in particular, is currently a subject of active research (Troupin et al, 2019;Esposito et al, 2021;Zarokanellos et al, 2022).…”