Spatial and seasonal characteristics of recent decadal trends in the northern hemispheric troposphere and stratosphere

Linear trends in the northern hemispheric 500 hPa geopotential height, lower tropospheric temperature, and zonal-mean zonal winds are investigated for the period 1965-1993. There is a general opinion that decadal trends in the atmospheric circulation are produced by changes in the equatorial sea surface temperatures (SSTs). Tn order to determine other factors, a trend analysis is conducted in the present study to remove the variabilities related to the equatorial SSTs as ''known factors'' by means of a linear regression. The results of the analysis suggest that during the winter, decadal trends originate from two different sources of variability. Trends over Pacific Ocean are related to changes in the equatorial SST. The decadal trends over Eurasian-Atlantic sector, however, are related to the stratospheric polar-night jet and the meridional propagation of planetary waves, which can be considered internal modes of variability in the winter circulation. During the summer, trends in the subtropical lower-tropospheric temperature are explained as a direct response to changes in the equatorial SST. Trends over eastern Siberia, however, can not be explained in a similar manner. Instead, a relation to trends in the extent of spring snow cover is suggested. Accordingly, the possibility of producing long-term variations through interactions among the atmosphere and land surface processes is also discussed.