Gravity wave forcing and the Southern Hemisphere cold pole bias in WACCM

Rolando Garcia* and Anne Smith, Doug Kinnison
NCAR

The Whole Atmosphere Community Climate Model (WACCM) simulates Southern Hemisphere winter and spring temperatures that are too cold compared with observations. We attempt to address this “cold pole” problem by introducing additional forcing in the stratosphere via parameterized gravity waves. 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 can change significantly is the climatology of stratospheric sudden warmings in Northern Hemisphere winter; in this regard, the results obtained by enhancing orographic forcing in the Southern Hemisphere appear to be preferable.



*email: rgarcia@ucar.edu
*Preference: Oral