A humidity index for the summer monsoon transition zone in East Asia

The East Asian summer monsoon transition zone (EASMTZ) is a narrow belt-shaped region in northern China, where the East Asian summer monsoon (EASM) transits to the mid-latitude westerlies. Existing EASM intensity indices have difficulties in reflecting the summer monsoon activity in EASMTZ. Therefore, as a complement to EASM index, the humidity index (HI) is constructed based on specific humidity threshold that can describe the seasonal migration of EASM, considering both its northward extent and temporal duration in the EASMTZ. The proposed HI, which captures the leading mode of summer rainfall in China, can provide reliable measurement of the atmosphere humidity associated with summer monsoon in the EASMTZ. HI is closely related to the significant anomalies in western Pacific subtropical high and East Asian subtropical westerly jet, which control the summer monsoon moisture transported into the EASMTZ. These circulation anomalies are linked to ENSO. Correlation analyses show that a high (low) HI tends to occur during the cold (warm) phase of ENSO, accompanied by a warm-to-cold (cold-to-warm) phase transition. There are two possible mechanisms that are responsible for the linkage between HI and ENSO. One is the upper-level zonal wind anomaly wave trains which transfers ENSO-related zonal wind anomaly northwards into EASMTZ through thermal wind balance and Eddy-driven mean meridional circulation. The other involves the Indian summer monsoon and mid-latitude circumglobal teleconnection. ENSO may exert influences on the humidity in EASMTZ via Indian summer monsoon and circumglobal teleconnection when the interaction between Indian summer monsoon and ENSO is active.