Wind-driven salinity tongue migration in the Gulf of Finland according to NEMO and ERA5 reanalyses

Two hypothesis that can be responsible for the wind-driven migration of the salinity tongue in the bottom layer of the Gulf of Finland (GoF) are tested based on the NEMO and ERA5 reanalyses. The first hypothesis authored by Krauss and Brügge (1991) implies along-channel wind developing upwelling and downwelling jet-like currents in the wind direction at opposite lateral boundaries of the channel, and a compensatory countercurrent in the deep layer (the so-called coastal jet hypothesis). The second hypothesis implies the cross-channel wind developing the along-channel Ekman transport in the upper layer, and a countercurrent in the deep layer (so-called Ekman transport hypothesis). The salinity tongue migration velocity along the GoF thalweg as estimated from the NEMO output is found to be extremely correlated with the along-thalweg projection of the wind stress calculated from the ERA5 output. This is a convincing evidence in favor of the coastal jet hypothesis as opposed to the Ekman transport hypothesis to correctly describe the mechanism of the wind-driven migration of salinity tongue in GoF. Based on a theory of coastal upwelling/downwelling in a two-layer fluid, some analytical considerations and related empirical estimates are presented to explain the prevailing coastal jet mechanism in GoF.