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

Abstract. Submesoscale (SMS) processes in a stratified coastal environment of the Gulf of Finland (Baltic Sea) were investigated using glider missions and a realistic simulation with a grid spacing of 0.125 nautical miles. The study period covered the transition from developing to established seasonal stratification. SMS variability, defined as small-scale anomalies of temperature and salinity cancelling each other's contribution to density (spice), was concentrated around the base of the upper mixed layer (UML) in spring and within the seasonal thermocline in late summer. We suggest that this shift was caused by the changes in surface heat flux and wind forcing. Both observations and simulations revealed events of high SMS variability, which can be related to background mesoscale dynamics and changes in wind forcing. We examined the event in which elevated spice in the subsurface layer did not coincide with the high Rossby number O(1) in the surface layer but instead appeared offshore of a coastal baroclinic current. Sloping isopycnals and intensified vertical velocities pointed to active SMS subduction. We propose that frontal instabilities, likely related to flow–topography interactions, drive the lateral and downward transport of surface waters, contributing to tracer redistribution below the thermocline.