Influence of long-term regional and large-scale atmospheric circulation on the Baltic sea level

Abstract

The connection between variations in the North Atlantic Oscillation (NAO) index and theBaltic sea level has been investigated for the period 1825—1997. The association between theNAO and the strength of the zonal geostrophic wind stress over the Northwest Atlantic suggestsan NAO impact on Baltic sea level variations, because the monthly mean sea level mainly isdetermined by externally driven variations caused by wind conditions over the North Sea.Several period bands were found to have high correlation between oscillations in the winter(JFM) NAO index and the Baltic Sea winter mean sea level. The correlation was, however, higher in the 20th century than in the 19th. During the last two decades, the correlation betweenthe NAO index and the sea level has been exceptionally high. The winter mean of a regionalatmospheric circulation index had a correlation with the Kattegat winter mean sea level of 0.93.With the Baltic sea level the correlation was 0.91, compared with the NAO index correlationfor the same period of 0.74. The regional index also showed a high correlation with the meansummer and mean autumn sea levels, when the corresponding seasonal NAO indices showeda weak connection. The temporal variation of the connection with the NAO index implies aregional atmospheric circulation occasionally differing from the large-scale circulation associatedwith the NAO. Seasonal means of the sea level in Stockholm do, however, reflect theregional wind climate to a large extent, and the Baltic sea level is a useful proxy for identificationsof climatic dependencies in the region.