Zonal Propagation Characteristics of Large–Scale Fluctuations in the Mid–Latitude Troposphere

Abstract

The general character of zonally propagating fluctuations in the mid-latitude troposphere is illustrated using the quadrature spectrum of the zonal sine and cosing Fourier coefficients. It is shown that for low frequencies (periods around 20 days) and low wavenumbers, fluctuations in the geopotential field tend to be westward, while those in the temperature field are eastward. Evidence is provided, by means of empirical orthogonal function analysis, that this situation is explained by the existence of two distinct modes of fluctuation, which together account for most of the large-scale, low-frequency variance. One mode has substantial surface amplitude, is nearly barotropic in structure, and propagates westward with respect to the earth. The other mode is more baroclinic and propagates eastward.