A Climatological Analysis of Oscillations of Kelvin Wave Period at 50 mb

Abstract

Zonal and meridional wind and temperature spectra for periods of oscillation ranging from 8 to 40 days are determined at 6-month intervals from daily observations of 50-mb wind and temperature for the 12 years 1959?71 at seven stations ranging from Ascension Island in the south to Caribou, Maine, in the north. In the mean for the 12 years, tropical zonal wind spectra possess significant peaks at periods near 13 days, whereas the tropical meridional wind spectra exhibit no peaks in the period interval 8?40 days, confirming the existence of Kelvin waves at 50 mb during this time. Coherence estimates suggest that these waves may be traced as far north as the Tropic of Cancer. In the tropics, zonal wind and temperature oscillations of 10?20 day periods are twice as large a few months after the quasi-biennial east wind maximum as after the west wind maximum, whereas the meridional wind oscillations are relatively large near the time of quasi-biennial west wind maximum. The phase difference between zonal wind and temperature fluctuation of Kelvin wave period, nominally one-quarter cycle, is a minimum shortly after the time of quasi-biennial cast wind maximum, and was anomalously small during the transition from a quasi-biennial to a quasi-triennial zonal wind regime in 1963. At 50 mb in the tropics, the inferred upward flux of westerly momentum at Kelvin wave period is nearly three times as large at the time of quasi-biennial cast wind maximum as at the time of west wind maximum, supporting the concept that Kelvin waves are intimately related to the downward propagation of the quasi-biennial west wind regime. This upward momentum flux was also anomalously small during 1963. Along with the study of these long-term anomalies, worthy of further investigation is the observation that the temporal variations in magnitude of zonal wind oscillations of 10-20 day period are basically out of phase in tropical and temperate latitudes.