Generation of subsurface cyclonic eddies in the southeast Baltic Sea: Observations and numerical experiments

Closely spaced CTD transects performed in the summertime reveal simultaneous downward/upward bendings of temperature/salinity contours in the seasonal thermocline/permanent halocline of the Stolpe Channel and the Gulf of Gdansk, which may be interpreted as geostrophically balanced cyclonic eddies in the intermediate layer. To examine processes capable of forming the observed cyclonic eddies, a numerical simulation based on the Princeton Ocean Model (POM) has been initiated. The subsurface cyclones in the Stolpe Channel were satisfactory simulated in model runs under easterly/northerly wind conditions. Their formation was shown to result from the adjustment of the high potential vorticity (PV) outflow (from the Bornholm Basin via the Stolpe Sill) to low potential vorticity environment by vortex stretching (so‐called the PV outflow hypothesis by Spall and Price [1998]). In accordance with the real wind conditions, a cyclonic eddy observed in the intermediate layer of the Gulf of Gdansk was satisfactorily reproduced in a model run with the westerly wind shutdown, which implies westward transport throughout the Stolpe Channel and thereby excludes the PV outflow hypothesis. The subsurface cyclone simulated in the Gulf of Gdansk was traced to form in the course of relaxation of the coastal downwelling baroclinic jet.