Interdecadal Variability in the Interface Between the Indian Summer Monsoon and the East Asian Summer Monsoon

This study investigates the interdecadal variability (IDV) of the interface between the Indian summer monsoon and the East Asian summer monsoon (IIE). Results suggest that the IDV of the IIE is characterized by a uniform zonal movement associated with seesaw variations between the Indian summer monsoon (ISM) and the East Asian summer monsoon (EASM). The IDV of the IIE is closely linked to two air-sea coupled modes: one resembles the Asian-Pacific Oscillation (APO) and the other resembles the North Atlantic tripole (NAT) pattern. Both the APO-like and NAT-like patterns facilitate an eastward shift of the IIE during their positive phase. The APO-like mode strengthens the western North Pacific subtropical high (WNPSH), which further enhances the climatological southwesterly wind over East Asia and leads to an intensification of the EASM. The simultaneous anticyclonic anomaly over the India and Bay of Bengal (BOB) region weakens the ISM. The NAT-like mode forces a wave train of alternating cyclonic and anticyclonic anomalies across Eurasia, causing southwesterly anomalies over East Asia that favors a stronger EASM. The atmospheric response is reversed during the negative phases of the APO-like and NAT-like patterns. Recent decades have seen a weakening influence of the APO-like mode, but a growing impact of the NAT-like mode on the IIE-IDV. The interdecadal transition of the IIE-APO and IIE-NAT relationships has a smaller influence on the IIE than that of the phase change of the APO-like and NAT-like patterns. Modeling results confirm the influence of the APO-like and NAT-like patterns on the IIE-IDV.