1. Introduction
Storm surges, generated by extreme wind stress acting on shallow, continental shelf seas, can lead to severe coastal floods, particularly when they coincide with a high astronomic tide [
1]. They can result in devastating economic and social impacts, including loss of life, damage to property and disruption of essential services [
2,
3,
4,
5]. As climate changes, coastal areas are becoming increasingly more vulnerable to storm surges [
4]. To mitigate loss of life and damage to property by storm surges, great efforts have been made in monitoring and forecasting storm surges, such as enhancing tide-gauge networks, integrating satellite observations and improving numerical prediction of storm surge and inundation [
6].
In recent years, some studies have indicated that satellite altimetry is capable of observing and studying storm surge features. Scharroo et al. showed that Geosat follow-on sea level anomalies reached 90 cm at the coast of the Gulf of Mexico during Hurricane Katrina [
7]. Han et al. clearly showed the utility of satellite altimetry in observing and understanding storm surges, complementing tide-gauge observations for the analysis of storm surge characteristics and for the validation and improvement of storm surge models [
4]. Lillibridge et al. reported that the storm surge caused by Hurricane Sandy was captured by the HaiYang-2A (HY-2A) satellite [
8]. Recently, Chen et al. showed a detailed analysis of HY-2A satellite observations combined with tide-gauge data during the passage of Sandy. For this event, Montauk’s tide-gauge recorded a maximum surge of 173 cm, and HY-2A showed positive sea-level anomalies of about 183 cm during the storm event. Further analysis suggests that the continental shelf wave generated during the passage of Sandy and observed by altimetry and tide-gauges has a propagating speed of 6.5 m/s [
6]. The SARAL/Altika altimeter captured a storm surge event in the North Sea during Cyclone Xaver in December 2013 [
9]. Han et al. studied storm surge features in the Gulf of Mexico during Hurricane Isaac, as observed by Jason-1 and Jason-2 altimeters and tide-gauge data, showing the utility of the constellation of altimeter missions and prospects of the upcoming Surface Water and Ocean Topography (SWOT) mission [
10]. These studies have shown that satellite altimetry is very useful for observing and understanding features of storm surges. They have also indicated that it is highly opportunistic for a single satellite altimetry mission to capture storm surges due to infrequent sampling. Satellite altimetry does not provide along-coast sea surface height distribution. On the other hand, there are a few recent studies on using satellite altimetry data to improve storm surge simulation through data assimilation [
11,
12,
13]. These studies [
11,
12,
13] showed the positive impacts and challenges of assimilating altimetry data on storm surge hindcasts and forecasts in the Gulf of Venice.
Typhoons often cause storm surges off the eastern and southern coasts of China in summer and fall, for example along the coast of Fujian. Taiwan Strait, connecting the South China Sea and the East China Sea (
Figure 1), is a shallow water area between Fujian and Taiwan, with an average depth of about 60 m [
14]. The tide is very strong and dominated by semi-diurnal constituents [
14,
15,
16]. Storm surge models have been developed for Taiwan Strait and adjacent waters [
16,
17]. According to previous studies, the interaction between tide and storm surge is notable, especially in shallow waters where tidal range is large [
18,
19,
20]. In spite of the fact that the region is often hit by typhoons and storm surges, there has been little literature reporting on storm surges observed by satellite altimetry.
In this study, we use TOPEX/Poseidon (T/P) satellite altimetry observations to show cross-shelf variation of Typhoon Seth storm surge off Southeast China on 10 October 1994. The T/P satellite ground track had a pass nearby Xiamen at 05:46 UTC on 10 October 1994 (
Figure 1), when Typhoon Seth was located close to the southeastern coast of China. We then apply a state-of-the-art finite-volume community ocean model (FVCOM) [
21] to simulate and understand the Typhoon Seth storm surge off Southeast China. The novel aspect of this study is using satellite altimetry observations to calibrate the storm surge model by adjusting the model wind forcing fields. We further integrate the T/P satellite altimetry with the simulated results from the calibrated model to investigate features of the storm surge. Note that storm surges are not only the sea level rises directly forced by wind stress and/or low atmospheric pressure, but also coastally-trapped free propagating signals generated remotely by storms. On 1 October 1994, Seth started in an area near the Marshall Islands and then strengthened into a strong typhoon on 7 October moving northwestward east of Taiwan. As it moved through the Ryukyu Islands, winds gusted to 110 knots (200 km/h). At 00:00 on 10 October, it turned northeastward.
This paper is organized as follows. In
Section 2, we describe altimetry data, tide-gauge data, the FVCOM model and its setup, as well as the model calibration procedure.
Section 3 evaluates and calibrates the model results against observations.
Section 4 discusses tide-surge interactions and the mechanisms of storm surge propagation. We provide conclusions in
Section 5.
5. Conclusions
A 3D, barotropic, finite-volume coastal ocean model was developed to simulate and study the storm surge along the southeastern China during Typhoon Seth in October 1994. A novel aspect is that the model storm surge was calibrated against a TOPEX/Poseidon observed cross-shelf storm surge profile, by adjusting the model wind forcing fields in reference to the typhoon best-track data. Through the calibration process, we determined the baseline wind forcing fields, i.e., 1.3-times the NCEP wind fields, which agree approximately with the best-track data for the maximum sustained speed while have a spatial structure similar to the NCEP wind fields.
The model results from the baseline wind fields reduce the along-track RMS difference between the model and altimetric data from 0.16 m to 0.09 m. It reduces the RMS temporal difference from 0.21 m to 0.18 m between model and tide-gauge data at Xiamen. In particular, the baseline model produces a peak storm surge of 1.01 m at 6:00 10 October 1994 at Xiamen, agreeing with tide-gauge data. The model results also show that the nonlinear interactions between the storm surge and astronomical tides contribute to the peak storm surge by 34% and that the storm surge propagates southwestward as a coastally-trapped Kelvin wave.