Rapid Intensification of Typhoon Hato (2017) over Shallow Water

Iam-Fei Pun1, Johnny Chung Leung Chan2, I-I Lin3, Kelvin Chan4, James Price5, Dong Shan Ko6, Chun-Chi Lien7, Yu-Lun Wu3 and Hsiao-Ching Huang8, (1)National Central University, Graduate Institute of Hydrological and Oceanic Sciences, Taoyuan, Taiwan, (2)City University of Hong Kong, Hong Kong, Hong Kong, (3)National Taiwan University, Taipei, Taiwan, (4)Sun Yat-sen University, Zhuhai, China, (5)Woods Hole Oceanographic Inst, Woods Hole, MA, United States, (6)Naval Research Laboratory, Stennis Space Center, MS, United States, (7)National Taiwan University, Department of Atmospheric Sciences, Taipei, Taiwan, (8)NTU National Taiwan University, Taipei, Taiwan
Abstract:
On 23 August, 2017, Typhoon Hato rapidly intensified by 10 kt within 3 h just prior to landfall in the city of Macau along the South China coast. Hato’s surface winds in excess of 50 m s−1 devastated the city, causing unprecedented damage and social impact. This study reveals that anomalously warm ocean conditions in the nearshore shallow water (depth < 30 m) likely played a key role in Hato’s fast intensification. In particular, cooling of the sea surface temperature (SST) generated by Hato at the critical landfall point was estimated to be only 0.1–0.5 °C. The results from both a simple ocean mixing scheme and full dynamical ocean model indicate that SST cooling was minimized in the shallow coastal waters due to a lack of cool water at depth. Given the nearly invariant SST in the coastal waters, we estimate a large amount of heat flux, i.e., 1.9k W m−2, during the landfall period. Experiments indicate that in the absence of shallow bathymetry, and thus, if nominal cool water had been available for vertical mixing, the SST cooling would have been enhanced from 0.1 °C to 1.4 °C, and sea to air heat flux reduced by about a quarter. Numerical simulations with an atmospheric model suggest that the intensity of Hato was very sensitive to air-sea heat flux in the coastal region, indicating the critical importance of coastal ocean hydrography.