Ocean-atmosphere coupled mesoscale model simulations of precipitation 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. In addition, South American meteorology and climate are also made further complicated by ENSO, a powerful coupled ocean-atmosphere phenomenon. Modelling studies in this region have typically resorted to either atmospheric mesoscale or atmosphere-ocean coupled global climate models. The latter offers full physics and high spatial resolution, but it is computationally inefficient typically lack an interactive ocean, whereas the former offers high computational efficiency and ocean-atmosphere coupling, but it lacks adequate spatial and temporal resolution to adequate resolve the complex orography and explicitly simulate precipitation. Explicit simulation of precipitation is vital in the Central Andes where rainfall rates are light (0.5-5 mm hr-1), there is strong seasonality, and most precipitation is associated with weak mesoscale-organized convection. Recent increases in both computational power and model development have led to the advent of coupled ocean-atmosphere mesoscale models for both weather and climate study applications. These modelling systems, while computationally expensive, include two-way ocean-atmosphere coupling, high resolution, and explicit simulation of precipitation. In this study, we use the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST), a fully-coupled mesoscale atmosphere-ocean modeling system. Previous work has shown COAWST to reasonably simulate the entire 2003-2004 wet season (Dec-Feb) as validated against both satellite and model analysis data when ECMWF interim analysis data were used for boundary conditions on a 27-/9-km grid configuration (Outer grid extent: 60.4°S to 17.7°N and 118.6°W to 17.4°W).

We now evaluate COAWST model simulations using MIROC5 CMIP5 model for both its input and boundary conditions for an entire year (October 2003 – October 2004) and will evaluate its ability to simulation both seasonal precipitation patterns and weak mesoscale-organized convection in the Central Andes. Model validation will compare COAWST model output against ECMWF-interim analysis and the TRMM 3B42 precipitation product. To elucidate the impact of two-way ocean coupling, another simulation featuring one way feedback (ocean to atmosphere) was also completed. Both simulations successfully reproduced the seasonal cycle of precipitation in the Central Andes and in the Western Amazon and reproduced most of the key features that characterize the South American climate (i.e., Bolivian High, Argentinian Low, low-level jet, etc.). Precipitation associated with the monsoon trough however tended to be too weak due to an overabundance of upwelling along the equatorial zone, especially in the two-way coupled simulation where SSTs were up to 4K colder than in ECMWF-interim analysis. Unlike in Northeastern Brazil, COAWST simulations produced reasonable estimates of overall accumulated precipitation (as compared to TRMM) and also for the diurnal and seasonal cycles in the Central Andes. Accurate simulations in the Central Andes indicate COAWST did likely reproduce the key Rossby Wave response between strong convection in the Western Amazon and the strength of the Bolivian High which is a key moisture transport mechanism for the Central Andes. When evaluated at particular points throughout the Central Andes, COAWST simulated precipitation days (days with > 5 mm/day) generally was within 30 days of that shown in TRMM 3B42. Finally, probability and cumulative distribution functions of precipitation over Tropical South America demonstrates COAWST simulated precipitation during the wet season was generally too light, but over higher terrain regions including the Central Andes rainfall histograms more closely resembled TRMM and was likely associated with more accurate simulations of orographically-forced precipitation.