Exploiting High-Density Zonal-Sampling of HIRDLS Profiles Near 60 degrees S to Investigate Missing Drag in Chemistry-Climate Models

This study exploits the high-density zonal sampling at the turnaround latitude of the High Resolution Dynamics Limb Sounder (HIRDLS) in the Southern Hemisphere to investigate the missing drag in chemistry-climate models near 60 degrees S. Gravity wave (GW) properties including amplitude, zonal wavenumber, vertical wavelength, and momentum flux are estimated with a wave analysis method based on the S-transform. Monthly means of GW properties compare well with estimates from previous studies. We further investigate the contribution to GW momentum flux above orographic and nonorographic regions and find that while fluxes are much larger locally over orographic regions, the contribution to the zonal mean is roughly 3 times smaller than the contribution over nonorographic regions. We also investigate the relationship with the zonal wind and find that GW momentum flux is highly correlated with the near surface winds over orographic regions. In addition to momentum flux, we also provide estimates of the zonal drag and use these estimates to evaluate the current GW parameterizations and resolved wave forcing in models participating in phase 1 of the Chemistry-Climate Model Initiative (CCMI-1). The HIRDLS zonal drag estimates suggest that the CCMI-1 models have insufficient zonal drag, especially in June, July, and August, and that the majority of the missing drag is over nonorographic regions. Our discussion includes implications for the Brewer-Dobson Circulation and ozone hole. Plain Language Summary Atmospheric gravity waves are small-scale perturbations that are generated by, for example, flow over orography or by nonorographic sources like convection. Theses waves affect the general circulation of the atmosphere and accounting for their effects in models is important. Most models are too coarse to fully resolve all scales of atmospheric gravity waves, and they must account for the missing effects of these waves through parameterizations. Gravity wave parameterizations have been developed and they are adjusted to obtain realistic temperatures and winds in models. Ideally parameterizations would be constrained well by observations, but many observational challenges make it difficult. Biases in the circulation and temperature in climate models, especially in the Southern Hemisphere winter, are usually attributed to deficiencies in current gravity parameterizations. In fact, the phenomenon has been called the "missing drag" in chemistry-climate models (CCMs). In this study we use satellite measurements of temperature from the High Resolution Dynamics Limb Sounder instrument to estimate the effects of gravity waves near 60 degrees South to determine the sources of the missing drag. The analysis suggests that a large part of the missing drag in CCMs is from nonorographic sources.