Effects of remote and local atmospheric forcing on circulation and upwelling in the Baltic Sea

Abstract

Due to the ephemeral nature of the atmospheric conditions over the Baltic Sea, the flow fieldis highly variable, and thus, changes in the resulting circulation and upwelling are difficult toobserve. However, three-dimensional models, forced by realistic atmospheric conditions andriver runoff, have reached such a state of accuracy that the highly fluctuating current field andthe associated evolution of the temperature and salinity field can be described. In this work, effects of remote and local atmospheric forcing on circulation and upwelling in the Baltic Seaare investigated. Changes in the characteristics of the large-scale atmospheric wind field overthe central and eastern North Atlantic can be described by the North Atlantic Oscillation(NAO). The NAO is related to the strength and geographical position of weather systems asthey cross the North Atlantic and thus has a direct impact on the climate in Europe. To relatethe local wind field over the Baltic Sea to the large-scale atmospheric circulation, we defined aBaltic Sea Index (BSI), which is the difference of normalised sea level pressures between Osloin Norway and Szczecin in Poland. The NAO is significantly related to the BSI. Furthermore, the BSI is highly correlated with the storage variation of the Baltic Sea and the volume exchangethrough the Danish Sounds. Based on three-dimensional model calculations, it is shown thatdifferent phases of the NAO during winter result in major changes of horizontal transports inthe deep basins of the Baltic Sea and in upwelling along the coasts as well as in the interior ofthe basins. During NAO+ phases, strong Ekman currents are produced with increased up-anddownwelling along the coasts and associated coastal jets, whereas during NAO- phases, Ekmandrift and upwelling are strongly reduced, and the flow field can almost entirely be described bythe barotropic stream function. The general nature of the mean circulation in the deep basinsof the Baltic Sea, obtained from a 10-yr model run, can be described by the depth integratedvorticity balance derived from the transport equation for variable depth.