The EOFs of the Geopotential Eddies at 500 mb in Winter and Their Probability Density Distributions

Abstract

In this paper we examine the empirical orthogonal functions (EOFs) of the geopotential eddies at 500 mb in the northern extratropical atmosphere during the cold season, and discuss the time variability of the EOF coefficients and their probability density distributions. Our data sample includes 1152 5-day mean fields derived from 32 years of NMC and ECMWF analyses. The first EOF is representative of the wintertime stationary eddies: the second and the fourth show good similarity to the PNA and WA teleconnection patterns, respectively. The pattern of the fifth EOF resembles a zonal wavenumber-3 with a latitudinally confined amplitude. The probability density distributions, estimated by means of nonparametric statistical techniques, have been computed both for the time-dependent amplitudes of individual EOFs and for indices of the spatial variance explained by more than one pattern. The EOF-5 appeared to be particularly relevant in this analysis; its amplitude shows a bimodal distribution, and bimodality is also found in the distribution of the combined amplitude of the first few EOFs when the fifth is included. The difference between the composite map computed averaging the fields classified in the high-amplitude mode (according to EOFs 1?5) and the corresponding map for the low-amplitude mode shows a large-scale pattern with positive maxima in regions known to be preferred locations for blocking centers.