Diurnal SST warming and the boreal summer intraseasonal oscillation in the Philippine Sea: contrasts between early and late summer

The relationship between diurnal fluctuations of sea surface temperature (SST) in the Philippine Sea and the boreal summer intraseasonal oscillation (BSISO) and its difference between early and late summer are investigated using four-year (2017-2020) data of a moored buoy deployed at 137°E, 13°N. A strong diurnal cycle of SST is observed frequently during convectively suppressed phases of the BSISO, simultaneous with a weakening of surface winds and enhanced insolation. In addition, abrupt SST warming along with the strong diurnal cycle occurred in the middle of May in 2018 and 2019, in conjunction with the first BSISO of the year. Interestingly, the convectively suppressed phases of second BSISO showed small SST warming albeit the strong diurnal cycle, which can be attributed to a deepening of the warm isothermal layer. In the remaining two years, 2017 and 2020, SST increased gradually with small diurnal SST amplitude, which is presumably attributed to continuous surface winds with moderate strengths mixing the upper ocean. Moreover, the first BSISO was observed in late summer. These results suggest that the abrupt SST warming associated with the strong diurnal cycle is linked to the seasonal onset of the BSISO.