Seasonal variation of the semi annual oscillation

Abstract

Observations of zonally averaged temperature from the Nimbus 7 Stratospheric and Mesospheric Sounder (SAMS) in 1979-82 demonstrate that a significant seasonal variation or asymmetry exists in the equatorial semiannual oscillation (SAO) in the sense that the "first' semiannual cycle beginning in the Northern Hemisphere winter (December-May) is much stronger than the "second' cycle beginning six months later (June-November). Calculation of balanced winds from the satellite data indicates a corresponding seasonality in SAO wind regimes: equatorial easterlies are stronger in December-February than in June-August and are followed by stronger westerly mean flow accelerations and, as a result, stronger westerlies in March-May than in September-November. This observation is in agreement with previous studies of the semiannual oscillation. -from Authors