Secular Variation of Northern Hemisphere 50 kPa Geopotential Height

Abstract

There is accumulating evidence in the literature that different short-period climate regimes (subclimates) may have characterized the Northern Hemisphere during the past 40 years. We, therefore, investigate the 40-yr record of 50 kPa height (1946?85) and analyze the time series of zonal anomalies stratified by season. We find that there appears to be two contiguous regimes-with a rather abrupt transition during the early 1960s?which had significantly different means, trends and degrees of variability. The results are compared with those from recent investigations of Northern Hemisphere surface and/or tropospheric temperature variation. The possibility of a ?climatic jump? during the early 1960s is discussed. Our results raise the question of an appropriate period to use for determining ?normals,? whether for standard level surfaces or, more generally, for calculating the statistics of the general circulation, both in the horizontal and vertical.