The Sensitivity of Future Hydroclimate Projections to Topography and Large‐Scale Dust Forcing in the Luquillo Mountains of Eastern Puerto Rico

Abstract Precipitation over Puerto Rico is expected to decline in coming decades, threatening an important source of surface water supply. However, earth system models neither resolve fine‐scale orographic impacts nor the radiative effects of Saharan dust aerosols on precipitation over the island's mountains. Here, we use the Weather Research and Forecasting model coupled with chemistry (WRF‐Chem) to downscale global climate model projections to a convection‐resolving 4‐km resolution for both dust and no‐dust scenarios. We compare the regional hydroclimate during three periods: historical (2014–2018), mid‐ (2045–2049) and end‐21st century (2085–2089), using output from an RCP8.5 simulation of the Community Earth System Model, version 1. For each period, the coarsest parent domain is simulated both with and without dust emission, transport and radiative effects, and the ensuing thermodynamic perturbations passed to the 4‐km domain via nesting. The downscaled output indicates that precipitation over eastern Puerto Rico is expected to decline through the end of the century, leading to an increase in consecutive dry days and a depletion of soil moisture that is evident across all seasons. The inclusion of dust exacerbates the drying signal during late summer when Saharan air outbreaks are active but ameliorates drying in other seasons relative to its historical simulation. Rainfall decreases rapidly by the mid‐21st century, and the declines plateau through 2100 when dust is included, whereas there is a modest rebound to wetter conditions when dust is ignored. Although the highest elevations exhibit resilience to future drying, there is a clearer drying signal at low elevations.