Forcing-dependent submesoscale variability and subduction in a coastal sea area (Gulf of Finland, Baltic Sea)

Abstract. Submesoscale (SMS) processes within a stratified coastal environment are characterized based on glider missions and a realistic simulation with the SMS permitting grid spacing of 0.125 nautical miles. The study period covered the conditions of the developing and established seasonal stratification in the Gulf of Finland (Baltic Sea). The tracer variance maximum was detected around the upper mixed layer (UML) depth and shallower than the depth of the maximum density gradient in spring and in the seasonal thermocline in late summer. We suggest that atmospheric forcing is the key driver of these disparities. In spring and early summer, predominantly positive net surface heat flux promoted water column stratification, and wind-induced mixing often did not reach the lower portion of the UML – likely SMS flows due to the horizontal buoyancy gradients developed there. In conditions, when the net surface heat flux was close to zero or negative, wind forcing had a more prominent role in shaping dynamics within the thermocline. High SMS tracer variance was consistently observed and simulated in the subsurface layers at the offshore side of a coastal baroclinic current characterized by slanted isopycnals. We propose that topography-related instabilities of frontal currents can create favourable conditions for SMS subduction transporting tracers from the sea surface near coastal boundary towards offshore and downwards, below the seasonal thermocline.