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This study finds a significantly decreasing (increasing) trend (1979–2018) of sea level pressure (SLP) over coastal East Asia (far North Pacific) during May and June, which resembles the negative (positive) Pacific Decadal Oscillation (PDO) (North Pacific Oscillation, NPO)‐like pattern. Thus, we examine May composite differences between early and late Indian summer monsoon (ISM) onset years. Sea surface temperature (SST) differences show that early (late) ISM onset is accompanied by a negative (positive) PDO‐like state. Whereas, in SLP, an intense low pressure over northwestern India, the Arabian Sea to Southeast Asia (far North Pacific, East Asia) is associated with the early (late) onset. During the early onset, warm surface air temperature (SAT) anomalies over southeast Russia propagate towards Central Asia to the Middle East and the northwest Indian subcontinent, strengthening the land‐sea thermal contrast, which subsequently strengthens the monsoon low. We thus show how the positive NPO, negative PDO, and high spring Bering Sea ice link with the SAT anomalies. In the early onset, more wave packets generated in response to these North Pacific anomalies propagate towards the Atlantic and European region, reaching and converging over the northwest Indian subcontinent. The warm surface anomalies possibly draw eastward‐propagating waves towards northwest India, thus intensifying the monsoon low there. Meanwhile, in the CAM5 PDO simulation, we observe a similar pattern of atmospheric responses, where warm SAT anomalies associated with stationary Rossby wave trains propagate from the North Pacific towards the North Atlantic to Central Asia and the northwest Indian subcontinent, strengthening the monsoon low. Terefore, our study highlights the crucial role of the North Pacific anomalies in modulating the ISM onset processes via atmospheric pathways. [ABSTRACT FROM AUTHOR]

The precipitation variability is subject to impacts of both ocean and land surface condition changes. The present study investigates the role of land surface condition in the interannual variation of the early rainy season (May–June) precipitation over the Indochina Peninsula. A precursory signal is identified in surface air temperature (SAT) during March–April over central Asia after the late 1970s. The air temperature anomalies extend to middle troposphere and persist to early summer (May–June) and modulate the land‐sea thermal contrast and lower‐level and upper‐level winds over the North Indian Ocean and thus affect the early rainy season precipitation over the Indochina Peninsula. The maintenance of SAT anomalies over central Asia is related to that of surface heat flux anomalies that in turn is attributed to the persisting atmospheric wind anomalies. The precursory signal is weak before the late 1970s due to the lack of persisting atmospheric wind anomalies. The interdecadal change in the persistence of central Asian SAT anomalies is associated with a difference in the distribution of the North Atlantic sea surface temperature (SST) anomalies. After the late 1970s, the North Atlantic SST anomalies with a southwest‐northeast distribution excite a wave train that extends from the North Atlantic to central Asia during May–June. Before the late 1970s, the wave train associated the North Atlantic SST anomalies with a south–north distribution is confined to the North Atlantic region. [ABSTRACT FROM AUTHOR]