Storm-driven submesoscale motions over sloping topography: Insights from the Baltic Sea

While surface submesoscale processes have been extensively studied, their counterparts in the bottom boundary layer remain largely unexplored. However, a few recent studies have indicated that these subsurface structures, emerging mainly through flow-topography interactions, are instrumental not only for the turbulent boundary mixing but also for the forward cascade of mesoscale energy towards dissipation and the lateral exchange of boundary waters with the oceanic interior. These studies have investigated the genesis of submesoscales primarily in the open ocean, focusing in particular on the vicinity of strong permanent current systems, coastal jets, or dense water outflows. Here, we use the Baltic Sea as a natural laboratory to demonstrate that interior submesoscales can arise even in semi-enclosed, strongly stratified basins away from the major current systems, in regions where tides and coastal jets are virtually absent, and ephemeral wind-driven currents typically dominate over short timescales. Using realistic high-resolution numerical simulations, we demonstrate that submesoscale vortices and filamentary structures are ubiquitous in the interior, below the mixed layer, especially during storm events. High cyclonic/anticyclonic vorticities are generated at the lateral boundaries close to the bottom, as the interior flow interacts with the sloping topography, with the strong vorticity anomalies being subsequently transported from the boundary into the stratified interior in the form of eddies, fronts, and filaments. Negative potential vorticity patches, indicative of submesoscale overturning instabilities, also develop from these flow-topography interactions. Our results show that surface and subsurface submesoscales coexist but remain largely isolated in this strongly stratified environment. By analyzing a series of sequential storm events, we show that winds indirectly energize the interior submesoscale motions by accelerating the boundary currents, with the strongest structures forming during severe storm episodes. Reversal of surface winds reverses the currents, significantly affecting the submesoscale generation sites and the mixing hotspots that exhibit, consequently, transient behavior. The intense winds also induce coastal upwelling and downwelling with the up-and downwelling sites evolving into pronounced mixing hotspots, presenting enhanced dissipation rates, resulting predominantly from the susceptibility of the flow to submesoscale overturning instabilities. These findings highlight the broader significance of storm-forced submesoscale dynamics in wind-driven marine and limnic systems, extending their relevance beyond the Baltic Sea context.