Mechanistic Study On The Influence Of Global Warming On The Asian Monsoon And Its Relationship With ENSO

Asian monsoon is the most energetic monsoon system around the globe.Understanding the change of the Asian monsoon and its variability in a warmer climate is critical for the mitigation and adaptation of billions of people living in the region.Using the outputs from the multiple CMIP5/6 models and idealized model simulation,this paper investigates the mechanisms for the response of Asian summer monsoon(ASM)circulation to global warming.The changes of ASM under global warming scenarios are further compared to that in past warm interglacial epochs.Besides,the impact of global warming on the relationship between the Asian monsoon and El Ni?o-Southern Oscillation(ENSO)under global warming is investigated,by exploring the change of the anomalous western North Pacific anticyclone(WNPAC).The main conclusions are summarized as follow:(1)Although both the current state-of-art CMIP5 and CMIP6 models project a robust increase of monsoon rainfall in both the East Asian summer monsoon(EASM)and South Asian summer monsoon(SASM)regions due to the increase of atmospheric moisture,their circulation changes differ markedly.A robust strengthening(weakening)of EASM(SASM)circulation is projected.By separating fast land warming and slow oceanic processes in response to increased CO₂ radiative forcing,the results demonstrate that EASM circulation strengthening is attributed to the fast land warming and associated Tibetan Plateau thermal forcing.In contrast,SASM circulation weakening is primarily attributed to an El Ni?o-like oceanic warming pattern in the tropical Pacific and associated weakened Walker Circulation and suppressed precipitation over the Maritime Continent.(2)Precipitation was claimed to increase over ASM region during past warm interglacial epochs and also under future global warming scenarios.However,moisture budget analysis shows that the increased ASM rainfall during interglacial epochs primarily results from the dynamic process associated strengthened monsoon circulation,but it is caused by thermodynamic process associated with increased mean moisture under global warming scenario.Further analyses show that the effect of direct CO₂ radiative forcing on the ASM is an analogue to that in interglacial epochs driven by enhanced land-ocean thermal contrast,both of which are characterized by strengthened vertical and horizontal monsoon circulation.However,the above effect is overwhelmed by the substantially increased sea surface temperature(SST)under global warming,which is absent during interglacial epochs.The substantial SST warming acts to weaken the monsoon circulation by decreasing the land-ocean thermal contrast and enhancing the atmospheric static stability.The results demonstrate that the interglacial epoch is not an analogue of the ASM response to global warming,and the lack of SST warming is responsible for their difference.(3)Projected by the multiple CMIP5 models,the intensity of the anomalous WNPAC during El Ni?o mature winter remains almost unchanged under global warming.Taken the anomalous WNPAC as a two-step response to the diabatic heating anomaly over the equatorial central-eastern Pacific,the theoretical analyses and idealized numerical simulation based on GFDL?dry model are performed to explore the underlying mechanisms.The results demonstrate that the competition between the enhanced diabatic heating anomaly over the equatorial central-eastern Pacific and the enhanced mean state static stability results in a weakly strengthened low-level northeasterly wind anomaly over the western North Pacific(WNP).Under an enhanced negative meridional gradient of the mean state low-level specific humidity,the negative moist enthalpy advection anomaly is enhanced over the WNP,which favors the enhanced negative precipitation anomaly.The negative precipitation anomaly over the WNP is thus enhanced due to atmospheric internal processes rather than the changed local SST anomaly.Finally,the compensation between the enhanced local diabatic heating anomaly over the WNP and the enhanced mean state static stability results in an almost unchanged anomalous WNPAC.