STRUCTURE AND SEASONALITY OF INTERANNUAL AND INTERDECADAL VARIABILITY OF THE GEOPOTENTIAL HEIGHT AND TEMPERATURE-FIELDS IN THE NORTHERN-HEMISPHERE TROPOSPHERE

Spatial Patterns and seasonality of interannual and interdecadal variability in the 500-hPa geopotential height, sea level Pressure, and 1000-500-hPa thickness field are examined based on NMC analyses over the Northern Hemisphere extratropics from 1946 onward. The leading empirical orthogonal function (EOF) of wintertime seasonal mean 500-hPa height is closely related to the Pacific/North American (PNA) teleconnection pattern. The time series of its expansion exhibits a trend from predominantly negative values (below-normal heights over western Canada) in the early part of the record to predominantly positive values during the 1980s. The leading EOF of sea level pressure, which is similar in summer and winter, contains elements of the North Atlantic Oscillation (NAO) and a zonally symmetric ''seesaw'' between polar and temperature latitudes. Its expansion coefficient exhibits relatively little memory from season to season or from year to year. The leading EOFs of summertime and annual-mean 500-hPa height and thickness are of the same polarity throughout almost the entire hemisphere. The time series of their expansion coefficients are strongly correlated with the time series of hemispheric-mean thickness. We refer to such modes as manifestations of a ''background field'' whose linear dependence on hemispheric-mean temperature is more important than the details of its spatial structure. Such a background field is evidently present year-round, but it shows up most clearly during summer when the PNA pattern and other regional teleconnection patterns are weakest. The fact that it is more pronounced in the thickness field than in the geopotential height field suggests that it is primarily associated with thermal, rather than dynamical, variability. The time series of its expansion coefficient is dominated by variations on the interdecadal time scale: it accounts for half the hemispherically integrated variance of the thickness field associated with perturbations with periods longer than five years. The large and possibly spurious drop in hemispherically averaged thickness and 500-hPa height in the NMC analyses between 1955 and 1963 contributes substantially to the variance associated with the background field, but the leading modes of the summertime and annual-mean thickness fields remain strongly correlated with one another and with hemispheric-mean thickness even when the years prior to 1963 are excluded from the record. Surface air temperature data exhibit qualitatively similar behavior, as does the extratropical Northem Hemisphere 830-515-hPa thickness field derived from a 100-year GCM simulation in which sea surface temperature is prescribed in accordance with the climatological mean annual cycle. The long-term trend in the wintertime PNA-like pattern, with rising heights and temperatures over western Canada, has contributed substantially to the rather large rise in hemispheric-mean wintertime surface air temperature since the late 1970s but it has had little if any effect on the hemispheric mean temperature aloft, or on summertime surface air temperatures, which did not rise enough to completely offset the declines in the 1950s and early 1960s.