Transient upwelling in the central Baltic Sea

Abstract

The paper presents a theoretical study to explain the regular occurrence of a cold water upwelling cell at the southern east coast of the Gotland island in the central Baltic Sea. While for a circular island up- and downwelling patterns would rotate around the island, the responses around the elongated Gotland island with narrow tips at its southern and northern ends are different. The study uses the example of the response of a coastal ocean to a wind band to develop an understanding of important aspects of generation of Kelvin waves and how the waves change the response patterns. The basic idea is that Kelvin waves starting from the northern tip diminish the upwelling along a portion of the east coast, while the export of upwelling around the southern tip is hindered by a slow phase speed due to shallower waters and coastal curvatures. This results in the generation of the upwelling cell at the southern east coast. For a proof of the suggested mechanism and to provide a more realistic quantitative analysis, we use a numerical circulation model of the Baltic Sea (MOM4) with a horizontal grid scale of one nautical mile. The findings provide some insight, which can be applied to other systems with variable alongshore bathymetry.