This study investigates the evolution of a transverse coastal upwelling front in the Baltic Sea from both surface and subsurface perspectives. Unlike the classical signature of coastal upwelling, where cooler water remains confined along the coastline, satellite sea surface temperature observations reveal the occurrence of cross-shore jets of upwelled water that extend tens of kilometres offshore at distinct locations. These jets are observed across the Gulf of Finland, extending from the Estonian to the Finnish coast.

Two key aspects are examined. First, in-situ Lagrangian drifter observations are used to demonstrate the impact of this sub-mesoscale front on particle pathways and transport properties. Second, by combining the satellite surface expression of the fronts with regional bathymetry, we analytically predict their spatial evolution.

The results show that during the active phase, transverse jets transport cooler, nutrient-rich subsurface waters to the surface while simultaneously reducing the average surface current speeds in surrounding regions. During the relaxation phase, a different dynamical regime emerges, characterised by enhanced mixing and the formation of filaments and eddies. Bathymetric analysis indicates that these transverse jets preferentially occur over steeper bottom slopes (>0.0075).

Furthermore, we identify the origin of the upwelled waters as the intermediate water mass layer, rather than the bottom layer. This distinction is critical for understanding nutrient pathways, as phosphorus-rich waters are most likely sourced from intermediate depths. Excess phosphorus is a known driver of cyanobacteria blooms in the Baltic Sea, which is known as one the world’s largest hypoxic areas. These findings demonstrate that analysis of the surface expression of sub-mesoscale features can provide key insights into their subsurface structure and biogeochemical impact.