Formation and decay of a longshore baroclinic jet associated with transient coastal upwelling and downwelling: A numerical study with applications to the Baltic Sea

Abstract

A numerical study is undertaken on the formation and decay of a longshore baroclinic jet (LBJ) caused by a transient wind stress favorable for upwelling and downwelling. The two‐pycnocline type of stratification, typical for the Baltic Sea summertime, results in a nonmonotonic growth of the LBJ velocity with wind stress and time and a two‐branch structure of the jet, with domination of a branch related to the seasonal thermocline (permanent halocline) in the case of fully developed downwelling (upwelling). The LBJ associated with the fully developed upwelling (downwelling) is found to display baroclinic instability, and its decay is accompanied by selective formation of mostly cyclonic (anticyclonic) eddies. If upwelling or downwelling is not fully developed, the LBJ formed in a flat bottom basin does not display baroclinic instability unless the β effect is included. The β effect causes a detachment of the LBJ jet from the shoreline in the eastern half‐basin and thereby stimulates baroclinic instability. As a result, mesoscale eddies are generated only in the eastern half‐basin, and only if the basin diameter D is large enough to satisfy the inequality D > , where RI is the baroclinic Rossby radius, f is the Coriolis parameter, and β = df/dy. Effect of topography on baroclinic instability makes it possible to develop eddies, even if the LBJ related to partially developed upwelling or downwelling is considered at β = 0.