[A-CG05_30AM2] Continental-Oceanic Mutual Interaction: Global-scale Material Circulation through River Runoff

[ACG05-08] Heat budget analysis on cooling events associated with typhoon passages in Seto Inland Sea, Japan

*Yusuke UCHIYAMA¹, Tatsuya NISHII¹ (1.Graduate School of Engineering, Kobe University)

Keywords:typhoon, estuary, heat budget analysis, EOF analysis, ROMS

Typhoons alter coastal oceans significantly through several mechanisms including the enhanced vertical mixing through increased surface wind and waves, resulting in extensive cooling of the upper oceans. In the present study, we investigate impacts of typhoon passages with a detailed ocean modeling in particular on the temperature structure in Seto Inland Sea, the largest semi-enclosed estuary in Japan. We develop a synoptic, double nested downscaling ROMS model (Shchepetkin and McWilliams, 2005; 2008) forced by the assimilative JCOPES oceanic reanalysis (Miyazawa et al., 2009) and JMA GPV-MSM atmospheric reanalysis for the surface momentum, heat and radiation fluxes by exploiting a bulk formula developed for COAMPS. The horizontal grid refinement occurs from 1/12 degree (JCOPE2) to 2 km (ROMS-L1) and to 600 m (ROMS-L2), where the L2 model running for about two years (2012-2013) is our test bed for the comprehensive heat budget analysis. The persistent clockwise estuarine circulation and the eastward-flowing Kuroshio are key features that cause the overall circulations of the estuary.In the fall 2012, SST is found to decrease about two degrees for a two-week period during three consecutive typhoons passing nearby. The first EOF mode of the modeled SST corresponds to the seasonal cooling along with mixed-layer deepening, whereas the effects of the typhoons appear in higher modes. Kuroshio interacts with the topography to form standing cyclonic cold-core eddies as extracted in the second mode, resulting in intermittent eastward cold-water transport beyond the headlands. The third and forth modes jointly represent cold water formation associated with storm-driven coastal upwelling that propagates with the estuarine circulation. Similar EOF modes are detected in the SST during the fall 2013 when two typhoons attacked the study area.In the falls in 2012 and 2013 after the mid September when a series of typhoons pass by the estuary, the heat budget analysis exhibits that the net heat flux at surface becomes negative to induce prominent surface cooling and cold-water formation in the upper ocean. Whereas divergence of the horizontal advective heat flux is crucial in the daily-averaged heat budget, the surface net heat flux is essential to long-term temperature variation. Latent heat flux is found to play a primary role in the negative net surface flux as well as decrease of downward shortwave (solar) radiation. Unstable lowest atmospheric planetary boundary layer leads to pronounced changes in the latent heat flux in response to surface wind and abrupt decrease of the near-surface humidity after the typhoon passages.