A Spectral Climatology

Abstract

Using 5 years of daily initialized height fields from the National Meteorological Center, expressed as coefficients of spherical harmonies, a climatology of the annual cycle has been formulated for the 1000, 700, 500 and 250 mb surfaces. The global analyses were first separated into separate Northern and Southern Hemisphere analyses, with a rhomboidal truncation at wavenumber 12. The daily values of each of the spectral coefficients were fit with the first four annual harmonics. Only those harmonics and Spectral coefficients were retained which explain a statistically significant amount of variance in time and space. The resulting mean height fields for both the Northern and the Southern hemispheres compare very favorably with established climatologies in spite of the limited length of the record on which they are based and the use of operational analyses. The statistical selection of spatial and temporal harmonies which contribute significantly to the annual mean and the annual cycle offers a unique insight into the structure of the climate in the two hemispheres.