4:45 PM - 5:00 PM
[ACG43-12] The effect of shallow mixed layer on the development of the boreal summer intraseasonal oscillation over the western North Pacific in 2024
Keywords:air-sea interaction, boreal summer intraseasonal oscillation, tropical western Pacific
The role of air-sea interaction on the development process of atmospheric convection associated with the boreal summer intraseasonal oscillation (BSISO) is investigated. The field observation using Research Vessel (R/V) Mirai was conducted in the tropical western North Pacific in early summer 2024. In addition to the onboard observation, three wave gliders were deployed to form the observation network surrounding the R/V Mirai. Convective activities around the station observation point (15°N, 132°E) were suppressed during the early part of the intensive observation, then activated in late July, which coincided with the BSISO's transition to the convectively active phase. Along with the enhanced convection, a typhoon GAEMI developed in the vicinity of the station point. A few days before the activation of the BSISO convection, the oceanic mixed layer rapidly became shallower simultaneous with low salinity signals in the upper 40 meters, which can be partially attributed to increased precipitation. The shallower mixed layer can lead to the rapid SST warming since the supply of heat from solar radiation was confined to the shallow mixed layer. The mixed layer heat budget analysis shows that the primary contribution to the warming was the rapid increase in surface heat flux. In addition, meridional advection also contributed to the warming, coinciding with a shift of the ocean currents toward the south. These results suggest that the shallow mixed layer associated with the low-salinity signal in the upper ocean and an abrupt change in the direction of surface currents may have been effective in activating convection, leading to the northward migration of the BSISO.