Atmospheric and Oceanic Environmental Factors for Typhoon Development in Northwestern Pacific Ocean and South China Sea: Case Study of Super Typhoon Rai (2021)

Tropical cyclones (TCs) are natural hazards that impact ecosystems and human society especially in the Northwestern Pacific (NWP), which is the most active region. TC development in the NWP is affected by both atmospheric and oceanic environments. Typhoons typically occur during the summer season but, due to climate variability, they now occur almost year-round in the NWP region. There has also been an observed increase in strength and frequency of super typhoons (STYs) during the winter season in the recent years. Typhoon Rai devastated the Philippines in December 2021. It underwent rapid intensification (RI) from Category 1 to Category 5 shortly before making landfall. It weakened as it traversed through but unexpectedly re-intensified to Category 5 in the South China Sea (SCS), becoming the third recorded STY in the region. This study aims to investigate the atmospheric and oceanic environmental factors in the NWP and SCS regions to explain the conditions for RI of STY Rai, and examine the ocean response to the TC event. The TC track data is from the Regional and Mesoscale Meteorology Branch, Cooperative Institute for Research in the Atmosphere, Colorado State University, USA, which provides 6-hourly TC location and intensity. Atmospheric variables such as geopotential, winds, and relative humidity are from European Centre for Medium-Range Weather Forecasts Reanalysis v5. Oceanic variables such as sea surface temperature (SST), sea surface height anomaly (SSHA), temperature and salinity profiles, and mixed layer depth (MLD) are from Remote Sensing Systems, Copernicus Marine Environment Monitoring Service, and Argo floats. Boundary of western North Pacific subtropical high denoted by 5880 m geopotential height and atmospheric steering flows describe the environment for the typhoon track. Vertical wind shear (VWS) and relative humidity (RH) conditions were favorable for RI in NWP (6-8 ms-1 VWS and 71-76% RH) but they were relatively less favorable for RI in SCS (8-11 ms-1 VWS and 64-66% RH). Pre-TC ocean conditions were favorable for typhoon development in both NWP and SCS with 27-30°C SST and >4 cm SSHA. TC-induced cooling, decrease in SSHA, and deepening of MLD were observed after RI events. The results indicate that oceanic environment may have more contribution to intensification than the atmospheric environment.