In-phase transition from the winter monsoon to the summer monsoon over East Asia: Role of the Indian Ocean

Analysis of observations shows that the weak East Asian winter monsoon (EAWM) to weak East Asian summer monsoon (EASM) transitions mainly occur in El Nino decaying years whereas strong EAWM (denoting stronger northerly winds along East Asian coast) to strong EASM (denoting less precipitation along the Meiyu-Baiu rainband) transitions all occur in non-El Nino-Southern Oscillation (ENSO) years during the period 1979-2009. This new finding implies that ENSO is not indispensable to the in-phase EAWM to the EASM transitions. The present study reveals an important role of the Indian Ocean in the strong EAWM to strong EASM transitions and proposes a possible mechanism for these transitions. A strong EAWM induces more precipitation over the Maritime Continent, and the associated anomalous heating excites a Gill-Matsuno type pattern in the tropics. The resultant wind and cloud changes enhance latent heat flux and reduce downward shortwave radiation over the northwestern Indian Ocean in winter, which leads to SST cooling. The cold SST anomalies persist to summer and excite an anomalous cyclone over the subtropical western North Pacific, leading to a strong EASM. The above processes also operate in the weak EAWM to weak EASM transitions during which El Nino impacts dominate but with additional contributions from the EAWM. The results of observational analysis are confirmed by numerical experiments with a coupled model.