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. Plain Language Summary: We investigated the interdecadal variability (IDV) of the interface (IIE) between the Indian summer monsoon (ISM) and the East Asian summer monsoon (EASM). We found a significant IIE‐IDV characterized by a uniform zonal movement. The IIE‐IDV is closely linked to two air‐sea coupled modes: one resembles the Asia‐Pacific oscillation (APO) and the other resembles the North Atlantic tripole (NAT) mode. Both modes facilitate the eastward shift of the IIE during their positive phase. The APO‐like mode drives the IIE to shift eastward by strengthening the western North Pacific subtropical high (WNPSH), whereas the NAT‐like mode affects the IIE‐IDV by inducing a wave train of alternating cyclonic and anticyclonic anomaly across Eurasia. The effect of APO‐like mode has weakened in recent decades, but the NAT‐like mode have an increasing impact on the IIE‐IDV. The stronger the influence, the weaker the air‐sea coupled modes. The model results confirmed the main features of IIE‐IDV and the guiding role of APO‐like and NAT‐like modes. Key Points: The interface between the Indian summer monsoon and the East Asian summer monsoon shows a uniform zonal movement at interdecadal timescaleThe Asian–Pacific Oscillation and the North Atlantic tripole modes have a major impact on the interface between the two monsoon systemsThere is an interdecadal transition in the relationship between the monsoon interface and exogenous modulators [ABSTRACT FROM AUTHOR]