Physical Linkage Between Indian and East Asian Summer Monsoons

Abstract

Observational evidence is presented to show the existence of a boreal summer teleconnection between the Indian summer monsoon (ISM) and East Asian summer monsoon (EASM). Based on station rainfall data, the dominant patterns of variability in monthly and seasonal rainfall over India-East Asia region are investigated mainly through composite analysis. The association between the midlatitude circulation and Indian monsoon rainfall on interannual time scale has also been examined by using 54-year NCEP/NCAR reanalysis data. The major results are as follows: (1) Associated with the year-to-year fluctuation of Indian summer rainfall, a well-organized upper level teleconnection pattern is obvious over the Eurasia continent with two anomalous anticyclonic (cyclonic) circulations in the strong (weak) monsoon year. One center is located over West Tibet, and the other with an equivalent barotropic structure resides in northeast Asia. This teleconnection establishes a linkage between two monsoon systems. (2) Within the summer season from May to September, above mentioned midlatitude teleconnection pattern undergoes a different structure with the strongest intensity in June and August. Hence, depending on the condition of Indian monsoon, the 'window of linkage' between Indian subcontinent rainfall and East Asian monsoon seems to open only in June or August. (3) On the other hand, a global anomalous wavetrain with favored longitudinal phase has been found in the upper and middle troposphere in each summer month. During June and August, this geographically fixed wavetrain characteristic of a circumglobal feature coincides with the ISM-EASM teleconnection. Based on all these observational results, three possible scenarios are proposed to explain the establishment of the teleconnection. And it seems that the upper level climatological westerly jet, Indian monsoon heating and midlatitude stationary wave activity are important factors that control the establishment of the teleconnection. On intraseasonal time scale, simultaneous and lagged correlation statistics have been calculated between height in northern hemisphere and convection over north India. It is found that prior to the breakout of the convection in north India two anomalous ridges have existed over north of Pakistan and northeast Asia, respectively. A plausible mechanism of this interaction between the westerly flow in the midlatitude and Indian monsoon is also briefly discussed.