Submesoscale Vorticity and Divergence in the Alboran Sea: Scale and Depth Dependence

Abstract

The statistics of submesoscale divergence and vorticity (kinematic properties, KPs) in the Alboran Sea (Mediterranean Sea) are investigated, using data from drifters released during two experiments in June 2018 and April 2019 in the framework of the Coherent Lagrangian Pathways from the Surface Ocean to Interior (CALYPSO) project. Surface drifters sampling the first meter of water (CARTHE and CODE) and 15 m drifters (SVP) are considered. The area of interest is dominated by processes of strong frontogenesis and eddy formation as well as mixing, related to the high lateral gradients between Mediterranean and Atlantic waters. Drifter coverage and distribution allow to investigate the dependence of KPs on horizontal scales in a range between 1 and 16 km, that effectively bridges submesoscale and mesoscale processes, and at two depths, of 1 and 15 m. For both experiments, the surface flow is highly ageostrophic at 1 km scale, with positive vorticity skewness indicating the presence of submesoscale features. Surface divergence quickly decreases at increasing scales with a slope compatible with a turbulent process with broadband wavenumber spectrum, suggesting the influence of surface boundary layer processes such as wind effects, waves and Langmuir cells at the smaller scales. Vorticity, on the other hand, has a significantly slower decay, suggesting interaction between submesoscale and mesoscale dynamics. Results at 15 m are characterized by reduced ageostrophic dynamics with respect to the surface, especially for divergence. Submesoscale processes are present but appear attenuated in terms of KP magnitude and skewness. The results are generally consistent for the two experiments, despite the observed differences in the mixed layer stratification, suggesting that submesoscale instabilities occur mostly at surface fronts associated with filaments of Atlantic and Mediterranean waters that are present in both cases. The results are compared with previous literature results in other parts of the world ocean and a synthesis is provided. Good agreement with previous surface results is found, suggesting some general properties for divergence and vorticity scale dependence. The importance of further investigating very high resolution frontal processes at scales of tens of meters, as well as processes of interaction with high wind effects is highlighted.

Oceanic Mesoscale Cyclones Cluster Surface Lagrangian Material

“…At first sight, the clustering asymmetry is unanticipated. Indeed, while in submesoscale-dominated regimes, clustering of material in cyclonic regions is documented and rationalized (Balwada et al, 2021;Berta et al, 2020;D'Asaro et al, 2018;Esposito et al, 2021), such an asymmetry is unexpected at the mesoscale − see the introductory review of literature in Section 1. Our model resolution formally allows an accurate representation of mesoscales but hardly permits submesoscales.…”

Comparing the Currents Measured by CARTHE, CODE and SVP Drifters as a Function of Wind and Wave Conditions in the Southwestern Mediterranean Sea

“…This might be expected, since the CARTHE drifter is closer to the sea surface and has a smaller drag area ratio compared to the CODE. We therefore conclude that the two types of surface drifters, CARTHE and CODE, measure the currents in approximately the same way and that their data can be combined to investigate the surface kinematics and dynamics [ 24 , 25 , 36 ].…”

“…In early spring 2019, more than 180 drifters were deployed during the second cruise (CALYPSO 2019; [ 23 ]). Most drifters were deployed in tight clusters in order to measure the near-surface currents and related differential kinematic properties (e.g., vorticity and divergence) at scales as small as 1 km [ 8 , 24 , 25 ]. Several DWSD TM drifters were deployed to measure surface wave characteristics in the study area.…”

The Alboran Sea circulation and its biological response: A review

“…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).…”