[A-CG30] Extratropical oceans and atmosphere

[ACG30-P09] Highest annual and seasonal precipitation of Japan observed by TRMM PR over Yakushima Island: Role of surface wind convergence and Kuroshio SST

*Rakesh Teja Konduru^1,2, Masato I. Nodzu², Jun Matsumoto^2,3, Moeka Yamaji⁴ (1.Data Assimilation Team, RIKEN Center for Computational Science, Kobe, Japan, 2.Department of Geography, Tokyo Metropolitan University, Minami-Osawa, Tokyo, Japan, 3.Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan, 4.Japan Aerospace Exploration Agency, Tsukuba, Japan)

Keywords:TRMM Precipitation Radar, Island climate, air-sea interactions, precipitation, climatology, Baiu Season

This study investigated spatiotemporal variations in precipitation over Yakushima Island in the Kyushu region of Japan using ultra-high-resolution (1-km; UHPET) satellite and in-situ observations. We revealed that the wettest spot is in the southeast region of Yakushima (a circled area in the figure). In-situ observations at Yakusugi land (10) and Yodogawa Mountain trail entrance (8) show annual precipitation of 10,048 and 9,139 mm/year. UHPET satellite estimates show > 6500 mm/year of rain to the southeast of the Yakusugi land. Yakushima Island undergoes substantial seasonal rainfall variations with a bimodal peak in rainfall. The first seasonal rainfall peak is seen in the Baiu season over most of the Island, and the highest seasonal rainfall (> 1500 mm/month ) is recorded over the southeast region. These seasonal changes in rainfall can be linked with the seasonal changes in the surface wind convergence in the lower atmosphere over the Tokara Strait south of Yakushima. We used Japan Meteorological Agency’s MSM reanalysis dataset and J-OFURO3 to analyze the atmosphere and ocean dynamics. Surface wind convergence is strong in the Baiu and summer seasons than in winter and early Baiu seasons over the south of Yakushima at Tokara strait. This surface wind convergence, along with Baiu frontal system, can support the development of large precipitation systems, bringing rainfall to Yakushima. During late summer, typhoons enhance precipitation in the eastern areas of Yakushima and contribute to the second seasonal peak. In winter and early Baiu season, the SST is higher in the Kuroshio region at Tokara Strait than in the adjacent ocean and acts as a source of moisture. A north-south meridional SST gradient across the Kuroshio SST front creates divergence over low SSTs and convergence over warmer SSTs because of surface pressure adjustments. Surface wind convergence over the Kuroshio warm waters results in the convergence of surface moisture, which can support the generation of small to medium precipitation systems along the Kuroshio warm waters at the Tokara Strait. These precipitation systems can influence the southern regions of Yakushima Island. This study explains for the first time the causes of seasonal precipitation over Yakushima Island's southeast region that accumulate significant annual rainfall. We need detailed investigations on the precipitation mechanism for each season, which will be done in the future.