Persistence of the 500 mb Height Field during Northern Hemisphere Winter

Abstract

The persistence of the planetary-scale circulation during the Northern Hemisphere winters from 1965/66 to 1981/82 is documented. National Meteorological Center analyses of 500 mb geopotential height for the Northern Hemisphere poleward of 20°N are used. Instead of constructing auto-correlation maps based on lime series at grid points, time series are constructed that show the pattern correlations between daily hemispheric maps. These time series provide information on the day-to-day changes in persistence of the winter circulation and allow comparisons of the intraseasonal and interannual variability of persistence. The hemispheric circulation is usually quite persistent from day-to-day; correlations between successive maps usually fall within the range 0.7 to 0.9. As the time between the maps is increased the correlations between these maps decreases at a rate faster than that expected of an autoregressive (red-noise) process. The hemispheric circulation rarely resembles closely the circulation a few days before unless synoptic scale waves are removed. Subjectively defined criteria are used to identify quasi-stationary regimes i.e. periods during which the planetary-scale circulation is more persistant than usual. A total of 58 regimes encompassing 25% of the 2040 days am identified. Multiple regimes are evident during several winter. The quasi-stationary regimes exhibit considerable diversity in their spatial configurations. Comparison of the regimes using principal component analysis suggests that the most frequently reoccurring regime consists of a superposition of a wavenumber 3 pattern at roughly 50°N upon zonally symmetric components at middle and polar latitudes of opposing signs. However, this principal component explains only 17% 6f the variance contained in the 58 regimes.