[A-CG32] Extratropical oceans and atmosphere

[ACG32-P08] Impact of the seasonal march of sea surface temperature in the East China Sea on the heavy rain event that occurred in Kyushu Island Japan in August 2021

*ATSUYOSHI MANDA¹, Terunobu Sakagami¹, Masami Nonaka², Hisashi Nakamura³, Satoshi Iizuka⁴ (1.Mie University, 2.JAMSTEC, 3.RCAST, The University of Tokyo, 4.NIED)

The rapid rise in sea surface temperature (SST) around the East China Sea at the end of July, following the rainy season, has been identified as a potential factor intensifying heavy rainfall in northern Kyushu Island, Japan. Conversely, during the peak summer month of August, meteorological disturbances triggering heavy rainfall are typically suppressed, making such events less likely. This study investigates the impact of SST on the mid-August 2021 heavy rainfall event, which was accompanied by a prolonged stagnant front, creating atmospheric synoptic conditions similar to those during the rainy season. Higher sea surface temperatures during this event may have contributed to increased precipitation. The study aims to elucidate the influence of post-rainy season SST seasonal variation in the East China Sea on heavy rainfall events through numerical simulations using a cloud-permitting model. Two numerical experiments were conducted, namely, (CNTL) experiment and the sensitivity experiment (referred to as 0702), where SST is replaced with climatology in early July. The 72-hour rainfall averaged over Kyushu Island in CNTL was approximately 20% higher than in 0702. On August 13 and 14, a meso-low passed near Kyushu, with the rainfall difference between the two experiments expanding. Additionally, CNTL exhibited a tendency for the meso-low to develop more than 0702. Comparing the horizontal distribution of vertically integrated water vapor flux during the period of enhanced precipitation due to the approaching meso-low, the inflow into northern Kyushu was greater in CNTL. On the other hand, comparing static instability on the upwind side of the precipitation area, CNTL was relatively more unstable on August 14, while stabilizing on August 13. This suggests that the strengthening of turbulent heat flux due to the increase in sea surface temperature did not directly enhance static instability around the precipitation area on August 13. Instead, it implies that the enhancement of horizontal water vapor flux through the development of the meso-low increased precipitation.