Normal Modes of the Atmosphere as Estimated by Principal Oscillation Patterns andDerived from Quasi-Geostrophic Theory

摘要

The Principal Oscillation Pattern (POP) analysis is a technique to empiricallyidentify time dependent spatial patterns in a multivariate time series of geophysical or other data. In order to investigate medium scale and synoptic waves in the atmosphere it was applied to tropospheric geopotential height fields. The data was subjected to zonal Fourier decomposition and to time filtering so that variations with periods between 3 and 25 days were retained. Analyses were performed separately for each zonal wavenumber 5 to 9 on both hemispheres in Winter. The most significant POP's are very similar in time and spatial structure to the most unstable waves in the stability analysis. They describe the linear growth phase of baroclinic, unstable waves which propagate eastward with periods of 3 to 7 days. Since the POP's are purely derived from observations the results indicate the appropriateness of the assumptions usually made in linear stability analysis of zonally symmetric flows to explain high frequency atmospheric fluctuations. The POP Analysis reveals patterns which are not found in the linear stability analysis. These can possibly be attributed to the nonlinear decay phase of baroclinic waves. Eliassen-Palm cross sections help to clarify the interpretation of the POP's in terms of the life cycle of nonlinear baroclinic waves.