Forced and internal modes of variability of the East Asian summer monsoon
Abstract. The modern instrumental record (1979–2006) is analyzed in an attempt to reveal the dynamical structure and origins of the major modes of interannual variability of East Asian summer monsoon (EASM) and to elucidate their fundamental differences with the major modes of seasonal variability. These differences are instrumental in understanding of the forced (say orbital) and internal (say interannual) modes of variability in EASM. We show that the leading mode of interannual variation, which accounts for about 39% of the total variance, is primarily associated with decaying phases of major El Nino, whereas the second mode, which accounts for 11.3% of the total variance, is associated with the developing phase of El Nino/La Nina. The EASM responds to ENSO in a nonlinear fashion with regard to the developing and decay phases of El Nino. The two modes are determined by El Nino/La Nina forcing and monsoon-warm ocean interaction, or essentially driven by internal feedback processes within the coupled climate system. For this internal mode, the intertropical convergence zone (ITCZ) and subtropical EASM precipitations exhibit an out-of-phase variations; further, the Meiyu in Yangtze River Valley is also out-of-phase with the precipitation in the central North China.
In contrast, the annual cycle forced by the solar radiation shows an in-phase variation between the ITCZ and the subtropical EASM precipitation. Further, the seasonal march of precipitation displays a continental-scale northward advance of a southwest-northeastward tilted rainband from mid-May toward the end of July. This coherent seasonal advance between Indian and East Asian monsoons suggests that the position of the northern edge of the summer monsoon over the central North China may be an adequate measure of the monsoon intensity for the forced mode. Given the fact that the annual modes share the similar external forcing with orbital variability, the difference between the annual cycle and interannual variation may help to understand the differences in the EASM variability on the orbital time scale and in the modern records.