Atmosphere-Ocean Coupled Mesoscale Model Simulations of Future Precipitation Climate in the Central Andes

The meridional extent and complex orography of the South American continent contributes to a wide diversity of climate regimes ranging from hyper-arid deserts to tropical rainforests to sub-polar highland regions. Previous future climate change studies in South America have largely relied upon global climate models (GCMs) which are both spatially and temporarily coarse, and apply simplified model physics. While GCMs are capable of resolving general South American climate features (i.e., ITCZ, South Atlantic Convergence Zone (SACZ), the Bolivian High, etc.), their low spatial and temporal resolution do not allow for adequate resolution of the strong gradients between climate regimes and the complex orography. Such limitations are particularly problematic in the Tropical Andes where both the pastoral agriculture and water availability are particularly sensitive to the distribution and intensity of seasonal precipitation and glacial runoff. Recent computational advances however now make it possible to run regional climate simulations using prognostic mesoscale atmosphere-ocean coupled models, such as the coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system.

Using COAWST, this study investigates how climate change impacts the spatial distribution, precipitation rates, and diurnal cycle of precipitation patterns in the Central Andes. To justify the application of COAWST to future climate simulations, it was first run during the 2003-2004 wet season (Dec-Feb) and validated against both satellites and model analysis data. This analysis not only showed COAWST to reasonably simulate synoptic climate features (i.e., ITCZ, Bolivian High, etc.), but also the probability density functions of both precipitation and outgoing longwave radiation provided by the Tropical Rainfall Measure Mission (TRMM) 3B42 precipitation product and Clouds and the Earth Radiant Energy System (CERES) SYN1deg product, respectively.

To simulate future climate, COAWST simulations are conducted for 1 year utilizing IPCC climate simulation data for the atmosphere and ocean boundary conditions. The present work parameterizes all atmospheric physical processes (microphysics, radiation, cumulus, etc.) on the outer 27-km domain covering all of South America, but the inner 9-km domain covering the Central Tropical Andes and the Amazon explicitly resolves convection. The ocean model uses a single 8-km domain covering all of South America and applies open boundary conditions. The higher resolution and better realization of atmospheric and ocean processes, orography, and moisture transport in COAWST allows for more realistic simulations of precipitation and convection then can be provided with GCM. Additionally, these runs provide data not possible in current GCMs such as precipitation rates and its diurnal cycle and also more accurate realizations of surface variables.