Regional Analysis of Northern Hemisphere 50 kPa Geopotential Heights from 1946 to 1985

Abstract

In Knox et al., the interannual variation of the Northern Hemisphere 50 kPa geopotential height field averaged between 30° and 80°N was investigated for the 40-year period from 1946 to 1985. We presented strong statistical evidence supporting the notion that a rather abrupt transition in the climate system took place during the early 1960s. There was no attempt to compare the spatial distribution of the 50 kPa height difference between Regime 1 (1946–62) and Regime 2 (1963–85). As a sequel to the first paper, we investigate the spatial characteristics of the transition height field. We find that the difference in the 50 kPa height field between Regime 1 and Regime 2 is characterized by low frequency circulation modes of the Pacific/North American (PNA) teleconnection pattern, the North Atlantic Oscillation (NAO), and an Arctic oscillation. There was an increase (in the residual sense) of the frequency and amplitude of the positive phase of the PNA in Regime 2 relative to Regime 1. Fourier analysis is applied to interpret the regime changes in terms of planetary and long waves during the winter season. The change in the Arctic circulation is primarily associated with an amplification of the wave 2 component in its normal phase location, while in the midlatitudes the primary contributor is wave 1, again in its normal location. We also examine the 40-year time series of 50 kPa height at the three centers of the winter PNA and confirm a strong negative correlation between the first two centers and a significant positive correlation between the first and third. To assess the current trend, the 50 kPa anomaly field averaged over the 1981–87 period is examined. The winter season shows an eastward shift of the North Pacific Ocean cooling pattern and amplified warming over most of North America, the maximum centered over western Canada. The NAO phase changed to negative. Our results are discussed in relation to the interregime sea surface temperature change over the North Pacific Ocean and to the increase in frequency and amplitude of ENSO events during Regime 2. Overall, there is a reinforcement of the earlier evidence for the two subclimate regimes.