Future Changes to Rainfall Extremes Over Puerto Rico in a Convection-Permitting Model

Islands in the Caribbean are vulnerable to anthropogenic warming due to sea level rise and their reliance on rainfall for agriculture. These islands are particularly prone to rainfall extremes, such as the 1,029 mm of daily maximum rain in Puerto Rico due to Hurricane Maria in 2017. Rainfall extremes mostly occur in the early rainy season (ERS) from April–June and late rainy season (LRS) from August–November. While global climate models project reduced rainfall in the Caribbean by the end of the century, they are too coarse to properly resolve the complex coastline and terrain of Puerto Rico and associated convection that is often induced by sea-breeze convergence and orographic uplift. Here, we resolve this issue by running the Model for Prediction Across Scales-Atmosphere (MPAS-A) using a 60–3 km global variable mesh centered over the Caribbean to downscale extreme rainfall days from coarser transient simulations during 2001–2021 and 2041–2061. This model configuration allows for the evaluation of dynamical and thermodynamic future changes at convection-permitting scales over Puerto Rico using MESACLIP as forcing data, although these simulations underestimate extreme rainfall amounts. Results show that by mid-century, rainfall extremes increase in the ERS but decrease in the LRS, mainly associated with changes in isolated convection. Stronger upward motion and sea breeze convergence support future increased rainfall in the ERS, while stronger subsidence likely reduces LRS rainfall extremes. These results suggest that more attention needs to be given to the increasing risk of ERS rainfall extremes over Puerto Rico.