The Roles of Equatorial Trapped Waves and Internal Inertia-Gravity Waves in Driving the Quasi-Biennial Oscillation. Part I: Zonal Mean Wave Forcing

The roles of equatorial trapped waves (EQWs) and internal inertia gravity waves in driving the quasi-biennial oscillation (QBO) are investigated using a high-resolution atmospheric general circulation model with T213L256 resolution (60-km horizontal and 300-m vertical resolution) integrated for three years. The model, which does not use a gravity wave drag parameterization, simulates a QBO. Although the simulated QBO has a shorter period than that of the real atmosphere, its amplitudes and structure in the lower stratosphere are fairly realistic. The zonal wavenumber/frequency spectra of simulated outgoing longwave radiation represent realistic signals of convectively coupled EQWs. Clear signals of EQWs are also seen in the stratospheric wind components. In the eastward wind shear of the QBO, eastward EQWs including Kelvin waves contribute up to similar to 25%-50% to the driving of the QBO. The peaks of eastward wave forcing associated with EQWs and internal inertia gravity waves occur at nearly the same time at the same altitude. On the other hand, westward EQWs contribute up to similar to 10% to driving the QBO during the weak westward wind phase but make almost zero contribution during the relatively strong westward wind phase. Extratropical Rossby waves propagating into the equatorial region contribute similar to 10%-25%, whereas internal inertia gravity waves with zonal wavelength less than or similar to 1000 km are the main contributors to the westward wind shear phase of the simulated QBO.