Scales and dynamics of Submesoscale Coherent Vortices formed by deep convection in the northwestern Mediterranean Sea

Abstract

Since 2010, an intense effort in the collection of in situ observations has been carried out in the northwestern Mediterranean Sea thanks to gliders, profiling floats, regular cruises, and mooring lines. This integrated observing system enabled a year‐to‐year monitoring of the deep waters formation that occurred in the Gulf of Lions area during four consecutive winters (2010–2013). Vortical structures remnant of wintertime deep vertical mixing events were regularly sampled by the different observing platforms. These are Submesoscale Coherent Vortices (SCVs) characterized by a small radius (∼5–8 km), strong depth‐intensified orbital velocities (∼10–20 cm s−1) with often a weak surface signature, high Rossby (∼0.5) and Burger numbers O(0.5–1). Anticyclones transport convected waters resulting from intermediate (∼300 m) to deep (∼2000 m) vertical mixing. Cyclones are characterized by a 500–1000 m thick layer of weakly stratified deep waters (or bottom waters that cascaded from the shelf of the Gulf of Lions in 2012) extending down to the bottom of the ocean at ∼2500 m. The formation of cyclonic eddies seems to be favored by bottom‐reaching convection occurring during the study period or cascading events reaching the abyssal plain. We confirm the prominent role of anticyclonic SCVs and shed light on the important role of cyclonic SCVs in the spreading of a significant amount (∼30%) of the newly formed deep waters away from the winter mixing areas. Since they can survive until the following winter, they can potentially have a great impact on the mixed layer deepening through a local preconditioning effect.

Observations of open-ocean deep convection in the northwestern Mediterranean Sea: Seasonal and interannual variability of mixing and deep water masses for the 2007-2013 Period

“…The scale of variability associated with these changes in the upper ocean stratification (Figure a) is typically of 5–10 days and are similar to the scale of variability observed in the horizontal velocities at all levels (Figure b). This suggests an important role of submesoscale and mesoscale circulation features such as eddies during the restratification phase [ Houpert ; Bosse et al ., ]. Similar eddies were also observed in other places such as the Labrador Sea [ Lilly and Rhines ; Lilly et al ., ; Hátún et al ., ; De Jong et al ., ] or the Irminger Sea [ Fan et al ., ], where they play an important role in the restratification of the deep convection area.…”

“…These SCVs induce the propagation of columnar buoyancy perturbations typically of 0.05–0.15 m 2 .s −2 as discussed in Bosse et al . []. The eddy‐induced buoyancy flux would then represent about 0.5–1.4% of the total integrated buoyancy flux over the winter (typically 1.15 m 2 .s −2 ).…”

Deep sediment resuspension and thick nepheloid layer generation by open‐ocean convection

“…The formation of cyclonic SCVs is likely favored by a winter mixed layer reaching the bottom. They are characterized by small radius (5–10 km) due to the weak stratification of the ocean deep levels and peak velocities located at great depths [ Bosse et al ., ; Damien et al ., ].…”

“…During the summer MOOSE‐GE 2012 cruise, two cyclonic SCVs were observed along the continental slope in the vicinity of the convection region [ Durrieu de Madron et al ., ; Bosse et al ., ]. The vertical profiles of density and large particle abundance in an eddy identified as a cyclonic SCV by Bosse et al .…”

A New Glider-Compatible Optical Sensor for Dissolved Organic Matter Measurements: Test Case from the NW Mediterranean Sea

“…This structure, which has also a signature in the humiclike plot (Figure 7g), is associated with a local "doming" of the isopycnals that suggests a subsurface eddy. Recent observations of submesoscale coherent vortices (SCV) that resembles the structure observed here were reported for the same geographical area (Bosse et al, 2015(Bosse et al, , 2016. However, their origins are likely different since SCV reported in these studies are observed deeper (200-500 m) and are associated with high-salinity anomalies.…”