The delayed onset of South American monsoon under global warming in convection-permitting regional climate simulations.

Under a warmer scenario, several monsoon regimes are projected to have a delayed onset
of the rainy season. We employ state-of-the-art convection permitting regional climate
model (CPRCM) simulations performed at the UK Met Office to explore potential drivers of
this projected delay over South America. The simulations correspond to a present-day
climate (CPRM-PD) and an RCP8.5 scenario (CPRCM-2100). CPRCM-PD is downscaled
from an atmospheric general circulation model (AGCM) simulation forced with sea surface
temperatures (SSTs) for the period 1998-2007. CPRCM-2100 is driven by an AGCM
simulation forced with SSTs and greenhouse gas concentrations corresponding to an
RCP8.5 scenario. In CPRCM-2100, the onset of the rainy season is delayed, with several
regions exhibiting a delay of up to one month. The rainfall during September and October
shows approximately 50% decline over Central East Brazil, accompanied by coherent
changes in atmospheric thermodynamics. A larger relative increase in near-surface moist
static energy (MSE) is required of atmospheric destabilization in the RCP8.5 scenario, which
however, crosses the necessary threshold for significant rainfall to begin only in late
October/early November. The increase in MSE is primarily due to low-level moisture
enhancement during the onset phase which is also found to be delayed in the RCP8.5
scenario. Precipitation-moisture relationship over the region during the onset phase
indicates a 20% increase (relative to present-day) in near-surface specific humidity
requirement for a daily rainfall rate of 5 mm/day in the RCP8.5 scenario. However, there is a
substantial reduction in evapotranspiration during September and October, in addition to
the absence of any significant changes in moisture flux convergence. This hampers the
moisture build-up and delays the transition to the rainy season in these months. The decline
in evapotranspiration is despite larger soil moisture content in the soil column which
suggests reduced plant transpiration. An increase in stomatal closure in the future
environmental conditions leads to this decline in the RCP8.5 simulation. These changes are
also accompanied by changes in both surface and top of the atmosphere energy fluxes. The
results call for the urgency to develop land use policies to mitigate climate change effects,
given the increasing intensity of droughts in Brazil during recent times. The findings also
highlight the role of local processes in modulating climate projections and the necessity to
improve their representation in climate models.