Evidence for the influence of the Quasi-Biennial Oscillation on the Semiannual Oscillation in the tropical middle atmosphere

The differences in the SAO associated with the QBO are roughly con-fined to the latitudes where the QBO has appreciable amplitude, suggesting that the mechanism is controlled by verti-cal coupling. The westerly phases of the SAO and AO show downward propagation with time. This analysis suggests that forcing by dissipation of waves with westerly momentum is responsible for the westerly acceleration of both the SAO and AO.

The timing and structure of the easterly phases of the SAO and AO near the stratopause are consistent with the response to meridional advection of momentum across the equator during solstices; it is not apparent that lo-cal wave processes play important roles in the easterly phases in the region of the stratopause.