Amplitude Modulation of the Quasi-Biennial Oscillation by Convectively Coupled Equatorial Waves

Abstract

The quasi-biennial oscillation (QBO) is a dominant mode of stratospheric circulation characterized by downward propagation of easterly and westerly phases of zonal wind with a periodicity of 26-32 months. Vertically propagating waves such a mixed-Rossby gravity (MRG), Kelvin waves and mesoscale gravity waves excited mainly by convective processes in the troposphere are considered as the major factors that cause the QBO. However, recent studies could not establish any statistical relationship between the activity of the convectively coupled MRG and Kelvin waves and the QBO and we assume it is mainly due to improper identification and isolation of these waves. A combination of wavenumber-frequency filtering and empirical orthogonal functional analysis (EOF) of outgoing longwave radiation (OLR) shows that convectively coupled MRG and Kelvin waves exhibit a strong geographical preference and seasonal amplitude variability. Previous studies have not considered this information when evaluating the relationship between these waves and the amplitude of the QBO zonal winds. Using a phase space representation of the QBO, we show that the seasonality of the easterly and westerly phases of the oscillation at 30 hPa displays a significant overlap with the seasonality of the MRG and Kelvin waves. In contrast to previous studies, frequency distribution for the zonal wind at 30 hPa reveals a clear preference for higher QBO easterly amplitudes during strong MRG seasons and higher QBO westerly amplitudes during strong Kelvin seasons.