Abstract
Abstract. Observations of tsunamis away from shore are critically important for improving early warning systems and understanding of tsunami generation and propagation. Tsunamis are difficult to detect and measure in the open ocean because the wave amplitude there is much smaller than it is close to shore. Currently, tsunami observations in deep water rely on measurements of variations in the sea surface height or bottom pressure. Here we demonstrate that there exists a different observable, specifically, ocean surface roughness, which can be used to reveal tsunamis away from shore. The first detailed measurements of the tsunami effect on sea surface height and radar backscattering strength in the open ocean were obtained from satellite altimeters during passage of the 2004 Sumatra-Andaman tsunami. Through statistical analyses of satellite altimeter observations, we show that the Sumatra-Andaman tsunami effected distinct, detectable changes in sea surface roughness. The magnitude and spatial structure of the observed variations in radar backscattering strength are consistent with hydrodynamic models predicting variations in the near-surface wind across the tsunami wave front. Tsunami-induced changes in sea surface roughness can be potentially used for early tsunami detection by orbiting microwave radars and radiometers, which have broad surface coverage across the satellite ground track.
Highlights
The need for a reliable system for early tsunami detection and warning was made painfully clear by the hundreds of thousands of lives lost to the tsunami generated by the 9.3 magnitude Sumatra-Andaman earthquake that swept the Indian Ocean on 26 December 2004 (Stein and Okal, 2005; Lay et al, 2005; Titov et al, 2005)
Satellite altimeters afford a unique opportunity to study the effects of a tsunami wave on the ocean surface through concurrent measurements of the surface height and the radar backscattering strength
The same conclusion has been reached from statistical analyses of several different attributes of the radar backscattering data: (I) magnitude of the radar backscattering strength anomaly and its representative spatial scale, as given by the number of zero crossings in a data segment of a fixed spatial extent; (II) magnitude of the radar backscattering strength anomaly filtered to the range of spatial scales representative of a tsunami wave; and (III) correlation of the radar backscattering strength and the sea surface height anomalies filtered to the range of spatial scales representative of a tsunami wave
Summary
The need for a reliable system for early tsunami detection and warning was made painfully clear by the hundreds of thousands of lives lost to the tsunami generated by the 9.3 magnitude Sumatra-Andaman earthquake that swept the Indian Ocean on 26 December 2004 (Stein and Okal, 2005; Lay et al, 2005; Titov et al, 2005). Quality is high and there are few data points excluded by intrinsic quality controls Results obtained using this segment of the Jason-1 records are presented in Sect. The Jason-1 records obtained on ascending path 129 for cycle 109 outside the vicinity of the tsunami leading front, i.e. to the south of about 6◦ S and to the north of 2◦ S (Fig. 1), contain valuable information regarding surface m27anifestat2i6ons of the Sumatra-Andaman tsunami. Tsunami manifestations in the radar backscattering strength should extend to the north of 2◦ S as long as the tsunami amplitude remains strong Analysis of this segment of the record is complicated by increased frequency of data points being excluded by intrinsic quality controls (Fig. 4). Godin et al.: Variations in sea surface roughness induceFdigbuyreS3umatra-Andaman tsunami
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