Spatial Variability of Seasonal Sea Level Pressure and 500 mb Height Anomalies

Abstract

The spatial variability of seasonal mean sea level pressure (SLP) and 500 mb height anomalies are determined using eigenvector analysis. Previously, spatial variability had only been analyzed in such a manner during winter months, and the time coefficients of the eigenvectors of SLP and 500 mb heights were not compared. The eigenvectors of SLP and 500 mb heights obtained here are compared during each season, and the vectors are compared between seasons. Surface and 500 mb level eigenvector coefficients are found to be correlated and they are correlated in some seasons to air temperatures at surface stations. The data used were National Meteorological Center analyses of daily SLP and 500 mb heights between 1946 and 1977. The eigenvectors are determined for the covariance matrices of seasonal mean pressure and height departures from the 31 or 32 year long term normal over a 132 grid-point network. The coefficients of the first eigenvector of 500 mb heights are most highly correlated to the coefficients of both the first and second winter SLP eigenvectors and they are highly correlated to mean winter air temperatures at stations in the southeastern United States. The first two winter SLP eigenvectors qualitatively resemble patterns of pressure variability associated with the North Atlantic and North Pacific oscillations. Spring and autumn eigenvectors suggest that 500 mb anomalies will lie over surface anomalies and have the same sign and configuration. This tendency for upper level anomalies to have the same location and configuration as those at the surface is not suggested by the summer eigenvectors and the time coefficients of the eigenvectors at both levels are not highly correlated either. The coefficients of the first summer 500 mb eigenvector shift to more positive values around 1963, a change which is correlated to observed air temperature variability in the eastern Canadian Arctic. In winter, spring, and autumn both the SLP and 500 mb height eigenvectors repeatedly indicate that an opposition in pressure exists between the northern North Pacific and the south-central North Pacific regions.