Dynamical ocean responses to Typhoon Malakas (2016) in the vicinity of Taiwan

Saffir-Simpson Category-4 Typhoon Malakas crossed the Kuroshio, the Western Boundary Current of the North Pacific Ocean, and moved along the path of the Kuroshio for eight days between September 12 and 20, 2016. Malakas induced a 3-4 degrees C sea surface cooling and an increase in sea surface Chlorophyll-a concentration to 0.8 mg/m(3). The total cooling area was approximately 18,000 km(2). Satellite observations show that Malakas triggered two upwelling regions associated with two mesoscale eddies, both to the northeast of Taiwan. The Regional Oceanic Modeling System (ROMS) was employed to offer insights into the dynamical mechanisms on the ocean response to a typhoon. The numerical model results reveal that a coastal upwelling event was induced by southeasterly winds blowing prior to the arrival of Malakas' outer band, and caused a surface cooling near Taiwan's northeast coast. A branch of the Taiwan Warm Current interacted with the offshore Kuroshio current to form an eddy prior to the typhoon's arrival. Malakas then facilitated the development of the near-coastal eddy. An open ocean eddy was triggered by inertial-Ekman pumping under the forcing effect of Malakas. The interaction between the southward current caused by Malakas and the northeastward flowing Kuroshio maintained these two induced mesoscale eddies. Numerical results show a near-inertial oscillation (IO) of about a 26-h period with an e-folding time scale of similar to 5 days was magnified and modulated by Malakas. The typhoon-induced oceanic phenomena were very well revealed in this combined observational and numerical modeling study. Plain Language Summary Typhoon Malakas passed through and affected the ocean to the northeast of Taiwan from September 12 to 20, 2016. Two surface cooling regions were observed by geostationary satellites after Typhoon Malakas passed by the area. The revelation of the factors responsible for inducing the oceanic surface cooling is crucial in understanding atmosphere-ocean dynamic processes and managing the local ecosystem resources. A numerical ocean model was employed in an attempt to replicate the oceanic phenomena that were observed. The model results suggest that a coastal upwelling event and an oceanic eddy were induced near the northeast coast of Taiwan prior to the arrival of the typhoon and subsequently enhanced by Malakas. An open ocean eddy was also triggered to the northeast of Taiwan by Malakas. The dynamic balance analyses of numerical model results reveal that inertialEkman pumping caused by Malakas was the critical factor in the induction of these phenomena. Eddyinduced upwelling was triggered and subsequently cooled down ocean surface temperatures. In addition, the variant typhoon wind magnified a near-inertial oscillation of about a 26-h period, in the current field, with a similar to 5-day e-folding time scale, to the northeast of Taiwan, which was imbedded into the growth and decay of the eddies.