Seasonal Variation of the Semiannual 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. Strong coupling is observed during the first SAO cycle between equatorial and North Polar temperature. A model of this coupling via a mean meridional circulation suggests that planetary Rossby wave momentum deposition in the northern winter is the Underlying cause of the seasonal variation in the easterly phase of the SAO, This circulation can produce significant horizontal advection of angular momentum in the tropics even when the body force is confined to midlatitudes. At higher levels, the reverse component of the circulation or a reduced diabatic circulation combine with equatorial Kelvin and westerly gravity waves to produce the large westerly mean flow accelerations in the first SAO cycle.