INTERNATIONAL WORKSHOP ON Tropical Cyclone - Ocean Interaction in the Northwest Pacific, 2026
ABSTRACT SUBMISSION FORM
From Inherited Waves to Local Forcing: Frequency-Dependent Wave Adjustment during Typhoon Saudel
Authors: Wyn Pauly1*, Hyodae Seo1, Tunggul Bhirawa2, Satoshi Mitarai2
1 University of Hawaii at Manoa, Honolulu, United States of America 2 Okinawa Institute of Science and Technology, Onna, Japan
*Corresponding author: rwpauly@hawaii.edu
Session:
- ✓ Oceanic and Air-Sea Interaction Processes in Tropical Cyclones I
□ Oceanic and Air-Sea Interaction Processes in Tropical Cyclones II
□ Improving Tropical Cyclone Predictability and Coupled TC-Ocean Prediction Models
□ Oceanic and Air-Sea Interaction Processes in Tropical Cyclones III
□ Poster Session
□ Future direction of high-resolution ocean modeling and Rapidly Intensifying Typhoon Modeling
□ Other: ______________________
Abstract:
Tropical cyclone wave fields evolve under rapidly changing wind speed and direction while simultaneously containing waves generated under earlier forcing conditions. Because wave adjustment and propagation timescales vary across the spectrum, the observed sea state under tropical cyclones may reflect both instantaneous local forcing and the history of the evolving wind field. However, how wave adjustment varies across frequency and what characterizes the transitional sea state between locally adjusted and inherited wave components remain poorly constrained by in situ observations. Understanding this frequency-dependent adjustment is important for interpreting wave observations and representing air–sea interaction under tropical cyclone conditions.
Here, we investigate the frequency-dependent adjustment of the wave field during Typhoon Saudel using directional wave observations from four drifting Sofar Spotter buoys deployed off Okinawa. The buoys sampled the storm from 24 August to 3 September 2026, including observations in the front-left and back-left quadrants within 200 km of the storm center. The dataset contains 366 wave spectra at hourly frequency, spanning ERA5 wind speeds of approximately 5–27.5 ms-1, with significant wave heights of up to 9 m. We examine how the spectral shape, directional spreading, and wind–wave directional alignment vary across frequency and wind speed.
The observations reveal a non-monotonic evolution of variability among spectra as wind forcing increases. Spectra have relatively similar energy distributions across frequency under weaker winds (<10ms-1), become increasingly heterogeneous at intermediate wind speeds, and converge again under the strongest forcing. The greatest variability among spectra follows the evolving spectral peak rather than remaining fixed at a particular frequency, indicating that the enhanced variability is associated with changes in the dominant wave system. At intermediate wind speeds (10-15ms-1), differences in the distribution of energy across frequency are consistent with a transitional sea state in which locally generated waves coexist with wave energy inherited from earlier forcing. At higher wind speeds (>15ms-1), the high-frequency spectra converge toward a common form, with this convergence extending progressively toward lower frequencies as wind speed increases.
Directional observations show a corresponding frequency-dependent transition. High-frequency waves are closely aligned with the local wind, while waves near and below the spectral peak can remain substantially misaligned. As wind speed increases, wind alignment extends toward the spectral peak, while lower-frequency waves retain greater directional separation from the local wind. Directional spreading also varies across frequency and wind speed, suggesting that convergence in spectral shape does not necessarily correspond to convergence in directional structure.
Overall, these observations suggest that tropical cyclone wave adjustment proceeds progressively across the spectrum: short waves respond rapidly to local forcing, while longer waves retain signatures of prior forcing and propagation over longer timescales. The coexistence of locally adjusted and inherited wave energy is most apparent at intermediate wind speeds, where variability among spectra is greatest. These results provide observational evidence that tropical cyclone sea states encode forcing history differently across frequency and across spectral properties, supporting a view of wave memory as a frequency-dependent continuum rather than a binary separation between locally generated wind sea and swell.
Keywords: Tropical cyclones; Directional wave spectra; Wind-wave interaction; Wave adjustment
Presentation Preference:
- ✓ Oral Presentation
- Poster Presentation