Interannual variability of the winter stratosphere and troposphere in the Northern Hemisphere

Interannual variations of the winter stratosphere and troposphere in the Northern Hemisphere are investigated by focusing attention on the long-term variability for the period from 1958 to 1994. Two modes of variability are first extracted by applying an empirical orthogonal function (EOF) analysis to the winter mean 50 hPa height data. The connection of these modes of variability with the troposphere and oceans is then investigated by calculating correlation coefficients between the time coefficients of the EOF and (i) the 500 hPa geopotential height, and (ii) sea surface temperatures (SSTs). The first stratospheric EOF mode alternates between the polar regions and mid-latitudes, and can be traced into the troposphere. The tropospheric circulation pattern associated with this mode is identified as changes in the meridional propagation of stationary planetary waves. Comparison with general circulation model (GCM) results suggests that this mode is essentially produced by atmospheric dynamical processes. Time coefficients of the EOF 1 mode exhibit an increasing trend. This mode appears to be related to the recent tropospheric circulation changes that have occurred over the Euro-Atlantic region and the eastern Asian sectors. The second EOF, on the other hand; exhibits a quite high correlation with SSTs in the equatorial eastern Pacific: and its variability is identified as linked to the El Nino/Southern Oscillation (ENSO) cycle.