Surface Wave Breaking Caused by Internal Solitary Waves: Effects on Radar Backscattering Measured by SAR and Radar Altimeter

Jorge Magalhães,José Da Silva,Werner Alpers,Adriana Santos-Ferreira

doi:10.5670/oceanog.2021.203

Abstract

Breaking surface waves play a key role in the exchange of momentum, heat, and gases between the atmosphere and the ocean. Waves break at the ocean’s surface at high or medium wind speeds or in the absence of wind due to shoaling of the seafloor. However, surface waves also break due to interactions with internal solitary waves (ISWs). In this paper, we revisit surface wave breaking caused by ISWs and how ISWs are manifested in synthetic aperture radar (SAR) images acquired by the TerraSAR-X and Sentinel-1 satellites and in high-resolution radar altimeter data acquired by the SAR altimeter (SRAL) onboard the Sentinel-3A satellite. X-band TerraSAR-X images acquired at low wind speeds suggest that meter-scale surface breaking waves resulting from large-scale ISWs are associated with large modulations in backscatter at HH and VV polarizations that cannot be explained by present theories. Furthermore, Sentinel-1 C-band SAR satellite images acquired at moderate to high wind speeds also exhibit large radar signatures from surface wave breaking at VV and VH cross-polarizations. Finally, new observations from the Sentinel-3 SRAL altimeter show clear evidence of significant wave height (SWH) variations along the propagation paths of ISWs. The SWH signatures are unique in showing that the surface wave energy does not return to its unperturbed level after an ISW passes, most likely because intense meter-scale wave breaking results in surface wave energy dissipation. In summary, these results show that surface wave breaking contributes significantly to radar remote sensing of ISWs.

Highlights

Surface waves are a familiar feature of the sea surface, but the ocean environment exhibits an extensive array of other wave phenomena, including the more counterintuitive internal waves (IWs) that propagate within the ocean
We presented evidence that internal solitary waves (ISWs)-induced surface wave breaking leaves measurable signatures in synthetic aperture radar (SAR) and SAR altimeter data that cannot be fully explained by present theories
For SARs, we have documented in three distinct images exceptionally large increases in NRCS values associated with large-scale ISWs: one in a dual co-polarization (VV and HH) X-band TerraSAR-X SAR image, and two others in coand cross-polarization (VV and VH) C-band Sentinel-1 SAR images

Summary

Introduction

Surface waves are a familiar feature of the sea surface, but the ocean environment exhibits an extensive array of other wave phenomena, including the more counterintuitive internal waves (IWs) that propagate within the ocean. Unlike the smaller surface waves, IWs may span up to hundreds of kilometers in the horizontal and 200 meters in amplitude, and they may induce the largest vertical velocities in the ocean (of the order of 1 m s–1). These waves are perpetually generated in the ocean, either by the wind (Levine, 2002; Guthrie et al, 2013) or by tides that oscillate over rough bottom topography (Jackson et al, 2012). ISW research increasingly relies on satellite remote sensing, which allows quasi-continuous monitoring on a global scale (Jackson et al, 2012; da Silva et al, 2015)

Objectives

Conclusion