Enhanced Tropospheric Biennial Oscillation of the East Asian Summer Monsoon since the Late 1970s

Abstract

Abstract The tropospheric biennial oscillation (TBO) of the East Asian summer monsoon (EASM) has major impacts on East Asian climate. Here it is shown that, since the late 1970s, the TBO signal of EASM has strengthened significantly. The EASM TBO in wind anomalies undergoes a transition from a cyclone over the western North Pacific (WNPC) in the preceding summer to an anticyclone over the western North Pacific (WNPAC) in the following summer, with the anomalies strengthening remarkably after the late 1970s. Correspondingly, the biennial components of precipitation anomalies in eastern China show different distributions. Both observational and numerical simulation analyses demonstrate that these changes are caused by the westward shift of El Niño warming and enhanced Indo-Pacific and Atlantic–Pacific coupling. The positive sea surface temperature (SST) anomalies are associated with the TBO of the EASM shift toward the central Pacific after the late 1970s, which favors the strengthening of the WNPC and causes a weakened EASM. In following summer, both the north Indian Ocean and tropical North Atlantic SST warming have been closely coupled with El Niño since the late 1970s, which favors the strengthening of WNPAC and causes an intensified EASM. Together, these changes provide more a favorable background state for the transition of circulation anomalies over the western North Pacific, giving rise to enhanced biennial variability in EASM in the late 1970s.