Persistent Reduction in Precipitation Over the Sierra de Luquillo Following Severe Defoliation from Hurricane Maria.

Hurricane Maria made landfall along the southeast coast of Puerto Rico on 20 September 2017 as a high category four hurricane with sustained winds of 69 m s^-1. Among the most evident traces of the storm’s devastation was the widespread defoliation that it left in its wake. The loss of vegetation was particularly evident in the El Yunque National Forest, where tree mortality and stem breakage far exceeded that observed during previous major hurricanes. This destruction of the forest canopy may have lasting consequences for local hydrometeorological conditions, as the vegetation loss effectually diminishes a natural water reservoir and source of recycled water vapor that helps to maintain a consistent moisture supply to the cloud forest that tops the Luquillo Mountains. Here, we assessed the impact of defoliation on local atmospheric conditions via a numerical modeling experiment conducted using the using the Weather Research and Forecasting Model (WRF). Our analysis compared a control simulation, in which vegetation was undisturbed, to an experimental run, in which we adjusted the leaf area index (LAI) to reflect observed vegetation loss during Hurricane Maria, then allowed for the LAI values to recover over time. Our results show that defoliation is associated with a strong decrease in precipitation along the area of landfall and extending throughout El Yunque National Forest in the months immediately following Maria’s departure. Moreover, the defoliated model run exhibits an area of reduced precipitation over the southeastern Luquillo Mountains that persists into the following midsummer dry period (i.e., July-August 2018). This area of persistent rainfall deficiency aligns with negative soil moisture anomalies, increased partitioning toward sensible heat flux at the expense of latent heat flux, and a decline in relative humidity that strengthen into the midsummer dry period. We show that this drying of the boundary layer is accompanied by an increase in the lifting condensation level. Thus, these results are supportive of a moisture feedback, in which the drying signal in the Luquillo Mountains is perpetuated by accumulating soil moisture anomalies that contribute to an increase in cloud base and a decrease in precipitation.