Chemistry-climate interactions in the Goddard Institute for Space Studies general circulation model 1. Tropospheric chemistry model description and evaluation

A tropospheric chemistry model has been developed within the Goddard Instit ute for Space Studies general circulation model (GCM) to study interactions between chemistry and climate change. The model uses simplified chemistry based on CO-NOx-HOx-O-x-CH4 and also includes a parameterization for isopre ne emissions, the most important non-methane hydrocarbon. The model reprodu ces observed annual cycles and mean distributions of key trace gases fairly well. It simulates preindustrial to present-day changes similar to those s een in other simulations. For example, the global tropospheric ozone burden increases 45%, within the 25% - 57% range of other studies. Annual average zonal mean surface ozone increases more than 125% at northern midlatitudes . Comparison between runs that allow calculated ozone to interact with the GCM and those that do not shows only minor ozone differences. The common us age of non-interactive ozone seems adequate to simulate ozone distributions . However, use of coupled chemistry does alter the tropospheric oxidation c apacity, enlarging the preindustrial to present-day OH decrease by about 10 % (-5.3% global annual average uncoupled, -5.9% coupled). Thus simulation o f changes in oxidation capacity may be systematically biased (though a 10% difference is within the uncertainty). Global annual average radiative forc ing from preindustrial to present-day ozone change is 0.32 W m(-2). The for cing p m tn increase by similar to 10% with coupled chemistry. Forcing grea ter than 0.8 W m(-2) is seen over the United States, the Mediterranean area , central Asia, and the Arctic, with values greater than 1.5 W m(-2) over p arts of these areas during summer. Though there are local differences, the radiative forcing is overall in good agreement with other modeling studies in both magnitude and spatial distribution, demonstrating that the simplifi ed chemistry is adequate for climate studies.