Effects of the North Atlantic Oscillation on water temperature in southern Baltic coastal lakes

Abstract

The effects of the North Atlantic Oscillation (NAO) on water temperature (WT) in Lakes Jamno, Gardno, and Łebsko were studied between 1961 and 2000. Both sets of data (NAO and WT) consisted of monthly and seasonal values. Correlation and regression analyses were used and the relationships were tested for statistical significance (Fisher-Snedecor test, coefficients of correlation and determination). Positive and statistically significant (even at α=0.01) relationships were demonstrated for the winter months (December, January, February and March) and in October. The strongest relationships were demonstrated for winter (December–February) in lakes Jamno and Gardno (correlation coefficients of 0.73 and 0.74). Extreme winters, particularly the very mild ones, tended to diminish the strength of the relationships. Very mild winters were characterised by relatively favourable solar (sunshine) conditions as well as high air and water temperatures, while the NAO Index values were relatively low. Relationships for spring (March–May) and summer (June–August) were not significant. In those seasons, the effects of atmospheric circulation (NAO) on WT were thus low, while the influences of solar factors were strong. Asynchronous relationships bear some predictive value, particularly when the predictive variable is the January NAO Index (NAOJan). These relationships were significant at α=0.01, out to April (NAOJan vs. WTApr). Attention is also drawn to local factors (air temperature, geostrophic wind, solar conditions, and sea water intrusions into lakes) which affected the strength of the relationships investigated.