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 to observe. However, three-dimensional models, forced by realistic atmospheric conditions and river runoff, have reached such a state of accuracy that the highly fluctuating current field and the 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 Sea are 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 as they cross the North Atlantic and thus has a direct impact on the climate in Europe. To relate the local wind field over the Baltic Sea to the large-scale atmospheric circulation, we defined a Baltic 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 exchange through the Danish Sounds. Based on three-dimensional model calculations, it is shown that different phases of the NAO during winter result in major changes of horizontal transports in the deep basins of the Baltic Sea and in upwelling along the coasts as well as in the interior of the basins. During NAO<sup>+</sup> phases, strong Ekman currents are produced with increased up-and downwelling along the coasts and associated coastal jets, whereas during NAO<sup>-</sup> phases, Ekmandrift and upwelling are strongly reduced, and the flow field can almost entirely be described by the barotropic stream function. The general nature of the mean circulation in the deep basins of the Baltic Sea, obtained from a 10-yr model run, can be described by the depth integrated vorticity balance derived from the transport equation for variable depth.
Highlights
On the long-term mean, the water balance of the Baltic Sea is controlled by in- and outflows through the entrance area, river runoff and the net precipitation
The purpose of this paper is to study the variations of horizontal transport and upwelling in the Baltic Sea in response to changes in remote and local atmospheric forcing conditions
Three-dimensional models have reached a sufficient state of accuracy that a coherent picture of the circulation of the entire Baltic Sea and its variabilities can be described
Summary
On the long-term mean, the water balance of the Baltic Sea is controlled by in- and outflows through the entrance area, river runoff and the net precipitation (precipitation minus evaporation). Hanninen et al (2000) investigated the Atlantic control on oceanographic and ecological changes in the Baltic Sea. Statistically significant relations between the ice extent of the Baltic Sea, changes in air temperature and the large-scale atmospheric circulation have been described by Omstedt and Chen (2001). A 10-yr simulation with a coupled sea ice– ocean model forms the basis of a statistical analysis of the correlation between remote and local atmospheric forcing and the response of the Baltic Sea. Two different winter periods from December to March, where the NAO has a local maximum and minimum, respectively, are analysed with respect to changes in the circulation and upwelling, and compared with the general circulation derived from a long-term average. A simplified North Sea is connected to the Skagerrak in order to provide characteristic North Sea water masses in case of inflow conditions, and to take up sea level elevations due to different forcing conditions
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