Simulating Future Precipitation Characteristics of the Central Andes with a Coupled, Ocean-Atmosphere Regional Climate Model

Future IPCC global climate models (GCMs) scenarios are used to describe changes to seasonal precipitation patterns and rainfall intensity over the 21^stCentury. Due to their coarse resolution and simplified moist physics, GCMs poorly represent mesoscale-organized convection, its associated radiative processes, and its diurnal cycle. For this study, we use the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) model, a fully-coupled mesoscale atmosphere-ocean modeling system, to physically downscale IPCC future climate scenarios at resolutions that permit mesoscale-organized convection. Previous work using COAWST has demonstrated its ability to reproduce the current climate’s seasonal cycle (Oct 2003 – Oct 2004) in the Central Andes using historical CMIP5 model data as 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).  

Our investigation into the future precipitation regimes (spatial distribution, precipitation rates, and diurnal cycle) of the Central Andes utilizes model output from four CMIP5 GCMs (MIROC5, GFDL-ESM2M, IPSL, and CCSM) as input for COAWST. Although small in sample size, these four GCMs demonstrate variable degrees of climatic sensitivity and skill at predicting monsoonal patterns which creates a representative sample of future GCM climate predictions.

Because running COAWST on climatological time scales is a computationally expensive endeavor, our study is based upon a series on one year “snapshots” spread roughly 30 years apart starting in 2003. Owing to their different boundary conditions, the spread of precipitation days (days with 5 mm or more) amongst model simulations range between 35 and 98 days for a given year. Model simulation sets (MIROC5, GFDL, etc.) do not yield a consistent tendency in precipitation days with time, yet the ensemble mean decrease (relative to 2003) for each analysis period and reaches 18 days (18.1%) by 2087. Variability in precipitation days and the decreasing ensemble mean appear to be associated with differences in simulated afternoon and evening convection in the western Amazon which is dynamically linked to the intensity of the Bolivian High, a key circulation feature associated with moisture transport into the Central Andes. Regardless of these differences, COAWST simulations show little to no change in the timing of diurnal precipitation, yet the magnitude of diurnal precipitation, the total annual precipitation, and the number of precipitation days tended to decrease in future years in the Central Andes. Specifically, in 2087, annual precipitation is simulated to have fallen 100 mm and there were 30 fewer precipitation days when compared to 2003. Cumulative distribution functions of regional precipitation show increased variability in annual precipitation where both low and high-end precipitation are seen to increase, yet locations with moderate annual precipitation decrease.