A climate shift in the mid‐1990s in summertime circulation over east Asia is described and the dynamics associated with the climate shift are discussed. The east Asian summer monsoon has a large interdecadal variability as well as interannual variability. It is suggested herein that the east Asian summer monsoon has undergone a decadal change in the mid‐1990s. After the mid‐1990s, there has been a significant decrease in the strength of zonal winds near the subtropical jet over the east Asia as well as a distinct increase in precipitation in the southeastern part of China. This decrease of the strength of zonal winds over east Asia could be understood as a barotropic response to a steady forcing associated with heating from increased precipitation. These decadal changes are significantly predominant only in the summertime. Concurrently, there has been a remarkable increase in the number of the typhoon passing through the southeastern part of China. It is suggested that the distinctive increase of the typhoon passing may be partly responsible for the increased precipitation in the same area after the mid‐1990s.

Prediction of Indian summer monsoon rainfall (ISMR) is at the heart of tropical climate prediction. Despite enormous progress having been made in predicting ISMR since 1886, the operational forecasts during recent decades (1989–2012) have little skill. Here we show, with both dynamical and physical–empirical models, that this recent failure is largely due to the models' inability to capture new predictability sources emerging during recent global warming, that is, the development of the central-Pacific El Nino-Southern Oscillation (CP–ENSO), the rapid deepening of the Asian Low and the strengthening of North and South Pacific Highs during boreal spring. A physical–empirical model that captures these new predictors can produce an independent forecast skill of 0.51 for 1989–2012 and a 92-year retrospective forecast skill of 0.64 for 1921–2012. The recent low skills of the dynamical models are attributed to deficiencies in capturing the developing CP–ENSO and anomalous Asian Low. The results reveal a considerable gap between ISMR prediction skill and predictability.

ABSTRACT:This study presents reviews of recent research on the structure and the multiscale variability in the East Asian monsoon. The boreal summer and winter seasons in the East Asian monsoon region exhibit significant intraseasonal, interannual and interdecadal variabilities. The interannual intensity of the East Asian summer monsoon (EASM) is mainly associated with the position of the centre of the Bonin High, which may be distinguished from the North Pacific anticyclone. The frequencies of heavy rainfall events and associated rainfall amounts increase, and extreme heavy rainfall is higher in August than in July, due to changes that occurred in the August rainfall-El Niño-Southern Oscillation (ENSO) relationship around the mid-1970s. This intraseasonal variability in EASM plays a more important role in the explanations of the interannual variability and climate change than does the annual mean. The interannual variability in the East Asian winter monsoon (EAWM) depends on the behaviour of the Siberian High (SH), Aleutian Low and the subtropical westerly jet stream. An EAWM index that takes into account the meridional shear of a 300 hPa zonal wind is a good indicator to represent the intensity of the EAWM. The Arctic Oscillation has a close relationship with the EAWM intensity on the decadal time scale. Distinct sub-seasonal variability is characterized with northward propagation and is observed in the interdecadal change in the monsoonal intraseasonal oscillation (ISO)-ENSO relationship. The preceding winter ENSO influenced the early summer northward propagating ISO (NPISO) activity before the late 1970s, whereas a strong NPISO-ENSO relationship appeared during the later summer after the late 1970s. The NPISO-ENSO relationship is robust owing to a tropical atmospheric bridge process involving the Walker Circulation and Rossby Wave propagation.

Future greenhouse warming is expected to influence the characteristics of global monsoon systems. However, large regional uncertainties still remain. Here we use 16 Coupled Model Intercomparison Project Phase 6 (CMIP6) models to determine how the length of the summer rainy season and precipitation extremes over the Asian summer monsoon domain will change in response to greenhouse warming. Over East Asia the models simulate on average on the earlier onset and later retreat; whereas over India, the retreat will occur later. The model simulations also show an intensification of extreme rainfall events, as well as an increase of seasonal drought conditions. These results demonstrate the high volatility of the Asian summer monsoon systems and further highlight the need for improved water management strategies in this densely populated part of the world.Plain Language Summary Future climate change is expected to influence the characteristics of the global monsoon system. However, large regional uncertainties still remain. Using 16 Coupled Model Intercomparison Project Phase 6 models, we determine the impact of greenhouse warming on the length of the summer rainy season and precipitation extremes over the Asian subregional monsoon domains (East Asia, western North Pacific, India, and Indo-China Peninsula). Over East Asia the models simulate on average an earlier inception and a later termination of the summer rainy season, whereas over India, the termination will occur later. The model simulations also show an intensification of extreme rainfall events, as well as an increase of seasonal drought conditions. Our results demonstrate the high volatility of the Asian summer monsoon system and further highlight the need for improved water management strategies in this densely populated part of the world.