Understanding the Asian summer monsoon response to greenhouse warming: the relative roles of direct radiative forcing and sea surface temperature change

It is now widely accepted that the global hydrological cycle will become more intensified in a warmer climate, as a consequence of the increase in tropospheric water vapor following the Clausius–Clapeyron relationship, leading to the so-called “wet-get-wetter, dry-get-dryer” pattern of change (Held and Soden 2006). Because of energetic constraints (Takahashi 2009; O’Gorman et al. 2012), the rate of precipitation increase is less than the rate of water vapor, and tropical atmospheric circulation weakens as climate warms (Held and Soden 2006; Vecchi and Soden 2007). However on the regional scale, hydroclimate projections from state-of-the-art climate models show large uncertainty and model spread, particularly in the tropics and over the monsoon regions (Turner and Annamalai 2012; Christensen et al. 2014). The “warmer-get-wetter” mechanism has been proposed to explain the spatial distribution of rainfall change in the tropics, relating to sea surface temperature (SST) pattern (Xie et al. 2010; Chadwick et al. 2013; Ma and Xie 2013). Kent et al. (2015) find that the uncertainty of regional precipitation change in the tropics is predominantly related to spatial shifts in convection and convergence, associated with SST pattern and land-sea thermal Abstract Future hydroclimate projections from state-ofthe-art climate models show large uncertainty and model spread, particularly in the tropics and over the monsoon regions. The precipitation and circulation responses to rising greenhouse gases involve a fast component associated with direct radiative forcing and a slow component associated with sea surface temperature (SST) warming; the relative importance of the two may contribute to model discrepancies. In this study, regional hydroclimate responses to greenhouse warming are assessed using output from coupled general circulation models in the Coupled Model Intercomparison Project-Phase 5 (CMIP5) and idealized atmospheric general circulation model experiments from the Atmosphere Model Intercomparison Project. The thermodynamic and dynamic mechanisms causing the rainfall changes are examined using moisture budget analysis. Results show that direct radiative forcing and SST change exert significantly different responses both over land and ocean. For most part of the Asian monsoon region, the summertime rainfall changes are dominated by the direct CO2 radiative effect through enhanced monsoon circulation. The response to SST warming shows a larger model spread compared to direct radiative forcing, possibly due to the cancellation between the thermodynamical and dynamical components. While the thermodynamical response