TRENDS AND INTER-DECADAL VARIATIONS OF THE SURFACE AND LOWER-TROPOSPHERIC TEMPERATURE IN THE NORTHERN-HEMISPHERE FROM 1964 TO 93

In this study, we demonstrate remarkable warming trends in northern wi nter and spring not only at the surface but also in the lower troposph ere during a recent 30 year period (1964-93). In winter, major warming has happened over central Siberia and the northwestern Canada/Alaska region. The vertical structure of the warming tendency is somewhat dif ferent between these two regions. In spring, tropospheric warming is m ost noticeable over the northern part of North America. A three-dimens ional rotated EOF analysis of the temperature anomaly field in these s easons confirmed the warming trends as the most dominant mode at the s urface and in the lower troposphere. The rotated EOF analysis also sho ws that an interdecadal-scale fluctuation with abrupt changes in the p eriods of 1976/77 and 1988/89 is another dominant mode in the temperat ure field in winter. The spatial structure of this mode shows a wavenu mber-three spatial pattern with positive factorloadings over North Ame rica, Northern Europe and the eastern part of Eurasia. In spring, a fl uctuation of 10-13 year period is noticed in the second largest mode, which suggests a possible relation with the solar cyle of the same per iod. Possible forcing factors of these tropospheric temperature trends and fluctuations are briefly discussed.