Diminished Impact of the East Asian Winter Monsoon on the Maritime Continent Rainfall After the Late‐1990s Tied to Weakened Siberian High–Aleutian Low Covariation.

Strong East Asian winter monsoon (EAWM) can enhance the Maritime Continent rainfall (MCR) via frequent southward penetration of cold air along the coast of East Asia. However, such impact diminished after the late‐1990s, primarily due to the weakened covariation between the Siberian high and Aleutian low (AL). During 1979–1998, the co‐variability between the Siberian high and AL facilitates a strong east‐west pressure gradient over East Asia. The resultant northerlies invade southward and facilitate the enhanced MCR. In contrast, such covariation is weak during 2000–2018, and it impedes the southward invasion of northerly winds and diminishes EAWM's impact. Further inspections indicate that the weakened covariation is primarily caused by the reduced variability of the AL. The underlying mechanism is confirmed by the historical simulation of version 6 Model for Interdisciplinary Research on Climate (MIROC6) that participates in phase 6 of the Coupled Model Intercomparison Project and numerical experiments with European Center‐Hamburg atmospheric model version 6.3 (ECHAM6). Plain Language Summary: East Asian winter monsoon (EAWM) is an important extratropical system to drive the variations of the Maritime Continent rainfall (MCR) via frequent southward penetration of cold air along the coast of East Asia. However, the EAWM's impact becomes diminished after the late‐1990s, and it is primarily rooted in the weakened covariation between the Siberian high (SH) and Aleutian low (AL). During 1979–1998, the co‐variability between the SH and AL causes a strong east‐west pressure gradient over East Asia. The resultant northerly winds invade southward along the coast of East Asia and facilitate the enhanced MCR. In contrast, during 2000–2018, the covariation is weak and diminishes EAWM's impact. Further analysis indicates that it is the weakened AL variability that determines the weakening of the covariation between the SH and AL. The phenomenon and the underlying mechanism are confirmed by the climate models. This study highlights the interdecadal variations of the EAWM in linking the Asian winter climate. Key Points: The impact of the East Asian winter monsoon on the Maritime Continent rainfall diminished after the late‐1990sIt arises from the weakened Siberian high–Aleutian low covariation that weakens northerly winds reaching the tropicsThe phenomenon and mechanisms are reproduced and validated by numerical experiments with two climate models [ABSTRACT FROM AUTHOR]