Abstract
Two deep‐sea moorings were deployed 780 km off the coast of southern Taiwan for 4–5 months during the 2010 typhoon season. Directional wave spectra, wind speed and direction, and momentum fluxes were recorded on two Extreme Air‐Sea Interaction buoys during the close passage of Severe Tropical Storm Dianmu and three tropical cyclones (TCs): Typhoon Fanapi, Super Typhoon Megi, and Typhoon Chaba. Conditions sampled include significant wave heights up to 11 m and wind speeds up to 26 m s−1. Details varied for large‐scale spectral structure in frequency and direction but were mostly bimodal. The modes were generally composed of a swell system emanating from the most intense storm region and local wind‐seas. The peak systems were consistently young, meaning actively forced by winds, when the storms were close. During the peaks of the most intense passages—Chaba at the northern mooring and Megi at the southern—the bimodal seas coalesced. During Chaba, the swell and wind‐sea coupling directed the high frequency waves and the wind stress away from the wind direction. A spectral wave model was able reproduce many of the macrofeatures of the directional spectra.
Plain Language Summary
Wind blowing over the ocean generates waves. Tropical cyclones, with their cyclonic wind pattern and extremely high winds, generate some of the most dangerous and complex wave fields on the planet. Using buoy measurements, which survived the close passages three intense tropical cyclones off the coast of Taiwan in 2010, we look at the details of these complex wave fields. We found that the wave field is generally made up of a system of longer waves which emanate from the most intense region of the storm and a system of shorter waves which are produced by the local winds. These two wave systems often propagate in different directions. As a storm passes, sometimes these two wave systems overlap and combine and sometimes they remain separate. It is unclear exactly what determines whether or not they combine, but it is certainly a nonlinear phenomenon. During one passage, the short and long seas combined shifting the direction of the short waves, which in turn interacted with the atmosphere by shifting the direction of the wind stress. This result was surprising and may help to improve our ability to model tropical cyclone intensity.
Key Points
Directional wave spectra were observed under the close passage of intense tropical cyclones
Seas were bimodal but coalesced during the two most intense passages; however, seas were young, refuting the dominance of shape stability
During Chaba, swell and wind‐sea coupled and directed high frequency waves and wind stress away from the wind direction
