Wind-Forced Submesoscale Symmetric Instability around Deep Convection in the Northwestern Mediterranean Sea

Abstract

During the winter from 2009 to 2013, the mixed layer reached the seafloor at about 2500 m in the northwestern Mediterranean Sea. Intense fronts around the deep convection area were repeatedly sampled by autonomous gliders. Subduction down to 200–300 m, sometimes deeper, below the mixed layer was regularly observed testifying of important frontal vertical movements. Potential Vorticity dynamics was diagnosed using glider observations and a high resolution realistic model at 1-km resolution. During down-front wind events in winter, remarkable layers of negative PV were observed in the upper 100 m on the dense side of fronts surrounding the deep convection area and successfully reproduced by the numerical model. Under such conditions, symmetric instability can grow and overturn water along isopycnals within typically 1–5 km cross-frontal slanted cells. Two important hotpspots for the destruction of PV along the topographically-steered Northern Current undergoing frequent down-front winds have been identified in the western part of Gulf of Lion and Ligurian Sea. Fronts were there symmetrically unstable for up to 30 days per winter in the model, whereas localized instability events were found in the open sea, mostly influenced by mesoscale variability. The associated vertical circulations also had an important signature on oxygen and fluorescence, highlighting their under important role for the ventilation of intermediate layers, phytoplankton growth and carbon export.

Slantwise Convection in the Irminger Sea

“…Given the harsh conditions in the Irminger Sea, a winter glider survey is the most promising avenue for better resolving this process in the future. Previous studies have observed the subduction of high oxygen and high chlorophyll waters associated with down‐front wind–forced instabilities (Bosse et al., 2021; Joyce et al., 2009; Thomas et al., 2013). This implies that slantwise convection may be an important contributor to the ventilation of the deep Atlantic.…”

“…Field campaigns elsewhere have resolved the horizontal structure of PV during down‐front wind events using towed instruments and gliders (Adams et al., 2017; Bosse et al., 2021; Carpenter et al., 2020; Joyce et al., 2009; Peng et al., 2020; Thomas et al., 2013, 2016; Thompson et al., 2016; Yu et al., 2019). Given the harsh conditions in the Irminger Sea, a winter glider survey is the most promising avenue for better resolving this process in the future.…”

“…Slantwise convection is the response to a mixed convective–shear instability called symmetric instability and results in mixing approximately along isopycnals (Haine & Marshall, 1998; Thomas, 2005). Evidence of slantwise convection has been observed at strong currents and persistent fronts such as the Gulf Stream, Kuroshio, and Antarctic Circumpolar Current (Joyce et al., 2009; Peng et al., 2020; Thomas et al., 2013, 2016; Viglione et al., 2018) as well as at transient fronts (Bosse et al., 2021; Buckingham et al., 2019; Thompson et al., 2016; Yu et al., 2019). Slantwise convection is thought to play a significant role in subtropical mode water formation (Joyce et al., 2013), in the reduction of the wind‐work available to the large‐scale ocean circulation (Thomas & Taylor, 2010), and in critically modifying turbulence in the ocean's surface boundary layer (D’Asaro et al., 2011).…”

“…and Antarctic Circumpolar Current (Joyce et al, 2009;Peng et al, 2020;Thomas et al, 2013Thomas et al, , 2016Viglione et al, 2018) as well as at transient fronts (Bosse et al, 2021;Buckingham et al, 2019;Thompson et al, 2016;Yu et al, 2019). Slantwise convection is thought to play a significant role in subtropical mode water formation (Joyce et al, 2013), in the reduction of the wind-work available to the large-scale ocean circulation (Thomas & Taylor, 2010), and in critically modifying turbulence in the ocean's surface boundary layer (D'Asaro et al, 2011).…”

Atmosphere-Ocean Coupled Variability in the Arabian/Persian Gulf

“…These meteorological phenomena occur at a rate of 10 events per year and substantially impact the natural environment and human health. They also cause abrupt changes in the Gulf 's circulation, mixing intensities, heat-budget, and SST patterns (Al Senafi and Anis, 2015;Li et al, 2020b) similar to the Mistral winds that blow towards the Gulf of Lion in the Mediterranean (Bosse et al, 2021), and the Bora wind events that blow towards the Adriatic Sea (Ferrarin et al, 2019). In addition, SST changes resulting from these meteorological events also likely play a major role in the formation and location of the Gulf Deep Water (GDW).…”