Seasonal Variations of the 40–50-Day Oscillation in Atmospheric Angular Momentum

Abstract

Seasonally varying spectral and cross-spectral calculations are carried out on multiyear time series of vertically and zonally averaged daily zonal wind fields to describe the seasonal cycle of the 40?50-day oscillation of atmospheric angular momentum. Intraseasonal variability (including 40?50-day fluctuations) of global momentum is largest in late boreal winter and smallest in boreal autumn; however, the 40?50-day spectral peak is most pronounced in boreal summer when lower-frequency intraseasonal variance is depressed. The 40?50-day spectral peak in global momentum is much less pronounced and apparently is restricted to a narrower frequency band, than corresponding peaks in zonal wind spectra from individual tropical rawinsonde stations. Contributions to global momentum fluctuations from three near-equal-area latitude bands (tropics, Northern Hemisphere, and Southern Hemisphere) are compared, confirming that intraseasonal momentum fluctuations are tropical in origin. The variance of extratropical momentum at this time scale is about an order of magnitude less than the tropical momentum variability. Coherent tropical?extratropical interactions are found principally in boreal winter, with the highest coherence between the tropics and Northern Hemisphere. The corresponding phase difference between tropical and Northern Hemisphere momentum is suggestive of poleward propagation of momentum out of the tropics.