We investigate tropical waves and their role in driving a quasi-biennial oscillation (QBO)-like signal in stratospheric winds in a global 7-km horizontal resolution AGCM. The Nature Run (NR) is a two-year global mesoscale simulation of the Goddard Earth Observing System (GEOS-5) AGCM. In the tropics, there is evidence that the NR supports a broad range of convectively generated waves. However, even with very high horizontal resolution and a healthy population of resolved waves, the zonal force provided by the resolved waves is still too low in the QBO region and parameterized gravity wave drag is the main driver of the NR QBO-like oscillation (NR-QBO). We find evidence that causes include both too coarse vertical resolution and too much dissipation. Nevertheless, the very high resolution NR provides an opportunity to analyze the resolved wave forcing of the NR-QBO. In agreement with previous studies, we find that large-scale Kelvin and small-scale waves contribute to the NR-QBO driving in eastward shear zones and small-scale waves dominate the NR-QBO driving in westward shear zones. Waves with zonal wavelength <1000 km account for up to a quarter of the resolved wave forcing in eastward shear zones and up to 70% of the resolved wave forcing in westward shear zones of the NR-QBO. We also find that the smallest-scale resolved waves (< 200 km) contribute up to 7% of the resolved wave forcing.

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*email: laura@nwra.com
*Preference: Oral