Main Component in Altimetry-Observed Response to a Tropical Cyclone

Abstract The ocean response to a tropical cyclone (TC) generally consists of near-inertial and geostrophic components, and the geostrophic response is impliedly believed to be ignorable. This study divides and compares the geostrophic and near-inertial components in the sea surface height (SSH) response obtained by a two-layer theory and numerical experiments, revealing that the geostrophic component is dominant and the near-inertial component ignorable. This conclusion results from 1) the high ratios (approximately 70%–90%) of the geostrophic component in the upwelling where the geostrophic response occurs and/or 2) the small absolute near-inertial amplitudes, usually below 3 cm, which satellite altimeters cannot capture. The findings are first presented in a two-layer model and then confirmed to be robust using numerical experiments by considering the effects of the nonlinear process, vertical dispersion of near-inertial internal waves, and vertical structure of the geostrophic response. By testing parameter dependence, the findings are insensitive to ocean stratification and TC characteristics including moving speed, intensity, and latitude, thus being generally appropriate for any TC case. The generality is physically related to the nature of Rossby adjustment and the dispersion of TC-induced near-inertial internal waves. Finally, the evidences are provided by the SSH response to Typhoon Mawar (2023) captured by the Surface Water and Ocean Topography satellite, which demonstrates the main features of the geostrophic response and no near-inertial wavelike signals. This study lays a solid foundation for dynamically understanding altimetry-observed response, highlighting the vital role of the geostrophic response. Significance Statement The ocean response to a tropical cyclone generally consists of wave and quasi-steady components, and the quasi-steady component is impliedly believed to be ignorable. Based on the theory, numerical experiments, and altimetry observations, this study finds the high ratios of the quasi-steady component in sea surface height response and the relatively small wave amplitudes. Thus, the quasi-steady response is the main component, and the wave component is ignorable in the sea surface height response to a tropical cyclone observed by satellite altimeters. This study lays a solid foundation for dynamically understanding altimetry-observed response to a tropical cyclone and proves the vital importance of the quasi-steady response.