Recent interdecadal changes in the Tropospheric Biennial Oscillation of the East Asian summer monsoon

Abstract

This paper focuses on the interdecadal changes in the biennial East Asian summer monsoon (EASM) transition related to the Tropospheric Biennial Oscillation (TBO), which has major impacts on East Asian climate. During 1958–2020, the TBO signal in the EASM exhibits noticeable interdecadal changes around the late 1970s and late 1990s. Among the three periods [1958–1979 (P1), 1980–2000 (P2), 2001–2020 (P3)], the TBO signal is the strongest in P2 and the weakest in P1. Such changes are closely associated with changes in the amplitude and position of wind anomalies over the western North Pacific (WNP). Further analysis indicates that the regime shifts in ENSO properties, including intensity, period, and location of ENSO-related sea surface temperature (SST) anomalies, play a crucial role. Compared with P1, the warm SST anomalies in the central Pacific and North Indian Ocean SST associated with the slowly decaying ENSO together induce the stronger WNP wind anomalies in both P2 and P3. However, such SST warming is relatively weaker in P3 compared with those in P2, which is consistent with the weak intensity and short period of ENSO in P3. In comparison, the anomalous circulation over the WNP is thus relatively weaker in P3 than in P2. Therefore, these anomalous SSTs associated with the shift in the ENSO regime are suggested to induce circulation anomalies over the WNP that subsequently contributed to the changes in the EASM TBO around the late 1970s and late 1990s.