On the Role of the Seasonal Cycle

Abstract

The seasonal cycle is defined as the projection of an atmospheric time series onto a suitably defined subset of orthogonal basis functions, the choice of which depends on the length of the series involved. This procedure guarantees that any atmospheric covariance can be expressed as the sum of a seasonal cycle part and a transient part, where transient refers to departures from the seasonal cycle, not the lime mean. Results for the seasonal cycle contribution to the zonally averaged fluxes of momentum (200 mb) and heat (850 mb) and for the zonally averaged height variance (500 mb) in NMC data are given for 1) the winter season (seasonal cycle defined in terms of a few Legendre functions), 2) 7-year mean (seasonal cycle defined as annual and semiannual Fourier harmonies), and 3) the winter season, but utilizing the Fourier basis act appropriate for the 7-year time series. The latter calculation indicates that the interannual variability of the momentum flux is dominated by the interactions between the seasonal cycle and the meteorologically low-frequency flow. Possible applications to other problems are discussed.