Effects of Surface Wave Breaking Caused by Internal Solitary Waves in SAR Altimeter: Sentinel-3 Copernicus Products and Advanced New Products

Abstract

We address surface wave breaking caused by oceanic Internal Solitary Waves (ISWs) and how ISWs are manifested in the SAR altimeter onboard Sentinel-3A and -3B satellites by means of their effects in Significant Wave Height (SWH). Two different regions of the ocean are selected, namely the tropical Atlantic Ocean off the Amazon shelf and the Banda Sea in the Indian Ocean, where there are scenes of Sentinel-3 OLCI acquired simultaneously with an along-track SAR mode altimeter, which include signatures of large amplitude ISWs. New data of unfocused SAR (UF-SAR 20 Hz) and fully focused SAR (FF-SAR 160 Hz) modes are analyzed, which are retracked in full range and over a reduced range of bins (truncation carried out dynamically ten gates away from the estimated epoch position). At first order, in scales of 1–3 km, a strong decrease in the normalized radar cross section (NRCS) over the rough part of the ISWs is observed followed by a small increase in the smooth part relative to the unperturbed ocean background. A second order ISW signature, in scales of 20 km, is noted: the SWH is attenuated after the passage of an ISW, considering length scales of about 10 km before and after the ISW crest. The SWH signatures are unique in showing that the surface wave energy does not return to its unperturbed level after the passage of an ISW, admittedly because intense meter-scale wave breaking results in surface wave energy dissipation. Furthermore, Sentinel-2 MSI images are analyzed and provide insights into this same phenomenon: white-capping resulting in a radiance increase at all (visible) wavelengths. Modulation of breaking waves owing to ISWs is demonstrated by estimates of the fraction of breaking waves in the presence of internal waves.

Using a Tandem Flight Configuration between Sentinel-6 and Jason-3 to Compare SAR and Conventional Altimeters in Sea Surface Signatures of Internal Solitary Waves

“…To investigate ISW sea surface signatures in SAR and conventional altimeters we focus mainly on the Banda Sea in the Indian Ocean. This region has been previously identified as a hotspot for large-scale ISWs [26,27], and used in [15] to document ISW sea surface signatures in the SAR altimeter aboard Sentinel-3. In addition to the Banda Sea, added value case studies are also presented in nearby regions, given that these seas are known hotspots for ISWs (e.g., the Celebes and South China Seas, e.g., [28,29]).…”

“…The details of satellite altimetry needed to interpret ISW sea surface signatures have already been described in the literature and the reader is referred to the studies presented in [12][13][14][15][16] and references therein. Nonetheless, it is important to recall and discuss some fundamental differences between SAR and conventional altimetry needed to interpret their ISW sea surface signatures during the Sentinel-6/Jason-3 tandem period.…”

“…When properly aligned, Santos-Ferreira et al [13][14][15] have shown that SAR altimeters aboard Sentinel-3 can unambiguously detect the sea surface signatures of large-scale ISWs (i.e., with scales of a few kilometres), including in radar backscatter, SSHAs, and SWHs. As illustrated in Figure 1 (see also Figure 4 in [13]), we recall that according to theory, an ISW should have characteristic signatures in these signals.…”

“…Internal Solitary Waves (ISWs) in the ocean naturally provide this contrasting scenario in sea surface roughness. It has already been well-documented that these waves, which propagate along the ocean's pycnocline, can increase and decrease sea surface radar backscatter in scales of a few kilometres with quasi-instantaneous transitions between them-both in conventional altimeters [12] as well as in SAR altimeters [13][14][15].…”

Characterization of Polarized SAR Scattering of Breaking Waves Caused by Internal Solitary Waves

“…Breaking waves may result from several formation mechanisms, such as those related to microbreaking of decimeter-scale waves and the formation of nonlinear structures, or those related to strong breaking of meter-scale waves and crests overturning. Resonance occurs for an ISW when its phase velocity matches the group velocity of the surface waves ( C sw group = C IW phase ) [26]. Due to the accumulation of energy, the crest will be violently overturned and broken even at modest wind.…”