Development of the Boreal Summer Intraseasonal Oscillation Over the Western North Pacific Associated With Self‐Induced Fresh and Shallow Mixed Layers in 2024

Abstract Air‐sea interaction plays an important role in the development of the boreal summer intraseasonal oscillation (BSISO). To elucidate this air‐sea interaction, a field observation using the research vessel (R/V) MIRAI was conducted in the tropical western North Pacific for 1 month in early summer 2024. In addition to the onboard observations, three wave gliders were deployed to form the observation network surrounding the R/V MIRAI. Convective activities around the station (15°N, 132°E) were suppressed during the early part of the observation period, then activated in late July, which coincided with the BSISO's phase shift to the convectively active phase and Typhoon GAEMI developed in the vicinity of the station. During several days before the activation of the BSISO convection, termed the BSISO transition period, low‐salinity signals were observed in the upper 40 m with heavy precipitation. At the same time, the oceanic mixed layer (ML) became dramatically shallow, accompanied by rapid sea surface temperature warming. Furthermore, a transition from shallow to deep convection was observed. The heat budget of the oceanic ML shows that the primary source of the warming was the rapidly increased surface heat flux term, suggesting that the supply of heat from insolation was confined to the shallow ML. In addition, meridional advection also contributed to the warming, coinciding with an intensification of southward currents. These results suggest that the air‐sea interaction between fluctuations in the oceanic ML and the development of atmospheric convection leads to the northward migration of the BSISO through moisture buildup in the atmosphere.