Gravity Wave Forcing and the Simulation of Antarctic Polar Ozone in the Whole Atmosphere Community Climate Model

The heterogeneous chemistry module in the Whole Atmosphere Community Climate Model (WACCM) has been revised to bring the partitioning of condensed-phase HNO3 into agreement with observations. Because of this change, stratospheric ozone loss during Antarctic spring becomes very sensitive to temperature; and the standard version of WACCM, which underestimates polar temperatures in the lower stratosphere, simulates unrealistically low ozone column amounts in Antarctic spring. We address this “cold pole” problem by introducing additional wave forcing in the lower stratosphere via parameterized gravity waves that force stronger downwelling and adiabatic warming at high latitudes. Insofar as observational guidance is ambiguous regarding the waves that might be important in the Southern Hemisphere stratosphere, we have experimented with two approaches: (1) Introducing a spectrum of inertia-gravity waves (horizontal wavelength of ~300 km) specified independently of the existing mesoscale gravity wave spectrum; and (2) increasing the forcing of orographic gravity waves in the Southern Hemisphere. Both approaches produce additional wave forcing in the lower stratosphere and reduce substantially the Antarctic cold-pole problem without degrading most aspects of the simulation in the Northern Hemisphere stratosphere, or in the mesosphere and lower thermosphere of both hemispheres. However, one aspect of the standard simulation that does change significantly is the climatology of stratospheric sudden warmings in northern winter; in this regard, the results obtained by enhancing orographic forcing in the Southern Hemisphere are preferable.