Observations of the Agulhas Current by along-track interferometric synthetic aperture radar

Abstract

Synthetic aperture radar (SAR) offers the possibility to observe the sea surface circulation with very high spatial resolution. These observations are particularly relevant in coastal areas and shelf seas. SAR has been routinely providing valuable information on sea surface winds and waves for many decades. During the last decade, a new application of SAR measurements based on the analysis of the Doppler shift has emerged, opening the possibility to measure directly the surface currents. There are still however many unresolved questions and challenges. One of the challenging questions is the wave-current interaction and its effect on the wind and wave retrieval. The Agulhas Current is the strongest western boundary currents in the southern hemisphere. The region of the Agulhas Current is characterized by a complex upper ocean dynamics involving a wide range of mesoscale and submesoscale processes. It thus provides an ideal natural laboratory for oceanographers and remote sensing sensors and techniques. Unique acquisitions of the interferometric SAR TanDEM-X over the Agulhas Current with very high spatial resolution (100 - 200 m) are analyzed. Maps of SAR-derived surface velocity are compared to model data. The SAR velocity images accurately capture the boundary and the intensity of the Agulhas Current. Moreover, these maps show unprecedented fine structure of the Agulhas Current and its interaction with the wave field. The pattern depicted by the backscatter images is on the other hand very variable from scene to scene depending on the wind and sea state. Only in particular cases, the current structure can be discerned from the backscatter. The influence of the Agulhas Current on the wind and wave field retrieval is investigated. The inversion of the backscatter to wind speed without taking the current into account lead artificially high estimates of SAR-derived wind speeds. Note that the wind and wave field retrieval will also impact the current retrieval via the wave-induced Doppler shift. These SAR observations are analyzed together with collocated existing products of ocean surface wind, ocean surface current, sea surface temperature and significant wave height. Our analysis indicates that wave-current interactions in regions of strong current shear produce very different signatures of sea surface roughness. The roughness signatures depend on the wave propagation direction relative to the current. A case of particularly enhanced roughness, probably due wave breaking events, is discussed.