Winter NAO significantly influences Baltic SST values throughout spring and summer

The aim of the study was to test the possibility of improving prediction skills of Baltic SST values throughout the year using past NAO index value. This could be possible due to the strong effect of NAO on Central and North European winter (DJFM) temperatures and the large thermal capacity of a sea, even as shallow as the Baltic.

 

First, the correlations of monthly NAO index and SST fields have been calculated. Significant (p<0.05) correlations were noted only in the central and southern Baltic in January, February and March, unlike in the air temperature - not shown – when significant correlations last from December to March. This may suggest one month delay in NAO affecting SST. To test this delay directly correlations with one month delay were performed showing significant values for five months of the year (from December to April SST temperatures).

 

Next, the main test of the study was performed by calculating the lagged correlations of average winter (DJFM) NAO values with SST fields. The results show that winter NAO significantly affects SST first in the central and southern basins (April to June) and later (July-September) also in the Bothnian Bay in the north. In some regions (mainly close to the Gulf of Riga) the influence of winter NAO is significant even in October, seven months after the end of the DJFM period when NAO influences the air temperatures. This shows that the effect must be caused by changes of heat content of the sea water. Also the fact that the effect of winter NAO spreads northwards during the summer can be explained with advection due to sea currents.

 

The results show that winter NAO values can be used to improve the skill of seasonal predictions of Baltic SST values in spring and summer. They also imply that if the observed significant correlation of the winter NAO values with global temperature will continue in the next decades, the prevailing positive winter NAO values should result in the spring and summer Baltic SST warming at a faster rate than the adjacent land.