The Effects of Interactive Stratospheric Chemistry on Climate Simulations in a Coupled Atmosphere-Ocean General Circulation Model

Stratospheric ozone depletion in the last decades has profound effects on stratospheric and tropospheric climate, especially in the summer Southern Hemisphere. To date, many coupled atmosphere-ocean general circulation models (AOGCMs) prescribe the evolutions of the stratospheric ozone layer using monthly and zonally averaged ozone field. However, the prescribed ozone field does not capture some important aspects of the ozone layer: it lacks zonal asymmetry, underestimates Antarctic ozone hole, and does not realistically represent the mesospheric ozone diurnal cycle. The impacts of these biases on simulations, particularly on ocean and sea ice, are not well understood. The purpose of this study is to determine the effects of using interactive stratospheric chemistry instead of prescribed ozone on simulations of climate and climate change in an AOGCM. We compare two sets of ensemble simulations for the 1960-2010 period using different versions of the Goddard Earth Observing System 5 (GEOS5) – AOGCM: one with interactive stratospheric chemistry, and the other with prescribed monthly and zonally averaged ozone and 6 other stratospheric radiative species calculated from the interactive chemistry runs.

Consistent with previous studies using prescribed sea surface temperatures and sea ice concentrations, the interactive chemistry runs simulate a deeper Antarctic ozone hole and consistently larger changes in surface pressure and winds at the Southern Hemisphere high latitudes than the prescribed ozone runs. It is also found that interactive chemistry changes stratospheric circulations, which leads to a weaker Brewer-Dobson circulation and higher temperature in the upper troposphere and lower stratosphere compared to the prescribed ozone case. The use of a coupled atmosphere-ocean model in this study enables us to determine the impacts of interactive chemistry on simulations of ocean circulation and sea ice. Interactive chemistry produces statistically significant lower sea surface temperature in the north Pacific and north Atlantic. In the Southern Ocean, the interactive chemistry runs have larger circulation trends with stronger changes in northerly and westerly surface flow near the Antarctica continent and stronger upwelling near 60^oS. Using interactive chemistry also simulates a larger decrease of Antarctic sea ice concentrations. We will discuss what causes these differences. Our results highlight the importance of using interactive chemistry in order to correctly capture the influences of stratospheric ozone forcing on climate simulations.