Simulated Delay in the Onset of the Rainy Season in Central-East Brazil under Global Warming Influenced by Plant Physiological Response

Abstract The projected delay in the onset of the South American monsoon over central-east Brazil under global warming is explored in a pair of convection-permitting regional climate model (CPRCM) simulations performed at the Met Office, corresponding to the present day and an RCP8.5 scenario. We also examine the corresponding driving general circulation model (GCM) simulations to understand the impact of explicitly resolving convection within the CPRCM. The transition to the rainy season is associated with the buildup of lower-tropospheric moist static energy and moisture, the demands for which are enhanced in the RCP8.5 scenario. However, significant reductions in evapotranspiration and the absence of moisture flux convergence enhancement in September and October render unfavorable conditions for the onset of the rains in the future climate. The reduced evapotranspiration is irrespective of an increase in column soil moisture in the RCP8.5 scenario. A stomatal response to altered environmental conditions in the future climate contributes to this decline in evapotranspiration. These changes lead to a delayed and more abrupt onset in the RCP8.5 scenario. The direction of change in the onset is similar in both the CPRCM and the driving GCM, although the CPRCM is drier than the GCM during the onset phase. The findings from this study highlight the role of plant physiology in modulating climate projections and the necessity to improve their representation in climate models.