Can Global Warming Strengthen the East Asian Summer Monsoon?

Abstract

The Indian summer monsoon (ISM) tends to be intensified in a global-warming scenario, with a weakened linkage with El Niño–Southern Oscillation (ENSO), but how the East Asian summer monsoon (EASM) responds is still an open question. This study investigates the responses of the EASM from observations, theoretical, and modeling perspectives. Observational and theoretical evidence demonstrates that, in contrast to the dramatic global-warming trend within the past 50 years, the regional-mean EASM rainfall is basically dominated by considerable interannual-to-decadal fluctuations, concurrent with enhanced precipitation over the middle and lower reaches of the Yangtze River and over southern Japan and suppressed rainfall amount over the South China and Philippine Seas. From 1958 through 2008, the EASM circulation exhibits a southward shift in its major components (the subtropical westerly jet stream, the western Pacific Ocean subtropical high, the subtropical mei-yu–baiu–changma front, and the tropical monsoon trough). Such a southward shift is very likely or in part due to the meridional asymmetric warming with the most prominent surface warming in the midhigh latitudes (45°–60°N), which induces a weakened meridional thermal contrast over eastern Asia. Another notable feature is the enhanced ENSO–EASM relationship within the past 50 years, which is opposite to the ISM. Fourteen state-of-the-art coupled models from the Intergovernmental Panel on Climate Change show that the EASM strength does not respond with any pronounced trend to the global-warming “A1B” forcing scenario (with an atmospheric CO₂ concentration of 720 ppm) but shows interannual-to-decadal variations in the twenty-first century (2000–99). These results indicate that the primary response of the EASM to a warming climate may be a position change instead of an intensity change, and such position change may lead to spatial coexistence of floods and droughts over eastern Asia as has been observed in the past 50 years.