Equatorial Circulation Changes up to the Mesosopause during Stratospheric Sudden Warming Events

Abstract:

Significant general circulation changes are brought about in the equatorial region as well as in the polar region during stratospheric sudden warming (SSW) events. Many studies have been devoted for the region up to the stratopause level from both observational and theoretical aspects. However, observational evidence is still fragmentary in the region above the stratopause, because global data capable of comprehensive analyses are still insufficient for the region. In this study, we make global gridpoint data derived from Aura Microwave Limb Sounder (MLS) observations for geopotential and temperature fields up to the mesopause level. Using these data, we make dynamical analyses for equatorial zonal wind and temperature changes since June 2004 to present. During SSW events, poleward flows of the residual mean meridional circulation are enhanced to lead to equatorial temperature perturbations consisting of a cooling in the upper stratosphere and a warming in the lower mesosphere. These temperature perturbations are found to strengthen the equatorial semiannual oscillation (SAO) with two separate out-of-phase amplitude maxima centered near the stratopause (SSAO) and the middle mesosphere (MSAO) through the thermal wind balance at the equator. In particular, the large amplification of the SAO is clearly observed during mid-winter SSW events when the SSAO and the MSAO have eastely and westerly maxima respectively. Moreover, we compare the results with simulated ones by a general circulation model.