CLIMATE ANOMALIES NORTH OF 55-DEGREES-N ASSOCIATED WITH TROPICAL CLIMATE EXTREMES

The relationship between extremes in tropical climate phenomena, e.g. ENSO and Indian summer monsoon, and the lower tropospheric temperature north of 55-degrees-N is examined. For this purpose we carry out a dy namic statistical analysis applied to the data set of monthly mean 500 -1000 hPa thickness from January 1949 to December 1991 (German Weather Service (DWD) analyses). To test the null hypothesis of identical hor izontal mean distributions of the polar temperature field in irrespect ive of the sign of extreme tropical episodes a multivariate significan ce test is performed and the level of recurrence is estimated. In orde r to improve the signal-to-noise ratio, the data are projected onto th e eigenmodes of an advection-diffusion model. Comparing composites of warm versus cold ENSO events, the model indicates a significant differ ence in the winter lower tropospheric temperature pattern in northern latitudes. This pattern contains a warmer lower troposphere over the n orthern North American continent in winter, corresponding to other stu dies. The multivariate analysis further shows that the Indian summer m onsoons with deficient rainfall are preceded by a strong cold anomaly in the lower troposphere over the northern Asian continent early in th e year, corresponding to the known hypothesis of Asian-snow-monsoon co upling.