Regional circulation patterns inducing coastal upwelling in the Baltic Sea

Abstract

Atmospheric feedback involved in the occurrence of coastal upwelling in a small semi-enclosed sea basin, i.e., the Baltic Sea, was analysed, and the regional circulation conditions triggering upwelling in different coastal sections were identified. Upwelling in the summer season (June–August, years 1982–2017) was recognized on the basis of sea surface temperature patterns. Circulation conditions were defined using (1) the established daily indices of zonal and meridional airflow and (2) the synoptic situation at sea level distinguished by applying rotated principal component analysis to sea level pressure data. The 12 daily synoptic patterns differed substantially in the intensity and location of their pressure centres. The mean seasonal frequency of upwelling was generally higher along the western Baltic shores than along the meridionally oriented eastern shores and varied from less than 10 to over 30% along the more predestined coastal sections, i.e., the northwestern coast of the Gulf of Bothnia, the northern Gulf of Finland and the southern Swedish coast. Due to the variable orientations of coastlines, upwelling could occur under almost any prevailing wind direction, and thus, each of the classified synoptic patterns could induce upwelling in some coastal sections. As deduced from the pressure fields for each circulation pattern, mostly alongshore winds triggered upwelling, which is in line with the Ekman rule. With time, upwelling could also be induced by the stress of normal to the coastline seaward winds.