Ocean surface velocity

Abstract

Introducing direct observations of scatter velocity from space, SAR-derived Doppler anomalies, when corrected for wind effects, are found to regularly provide estimates of the surface velocity. With a persistent surface current flow direction versus the radar look direction, the greater Agulhas Current and the Pacific equatorial current become ideal for the examination of the Doppler shift measurements. Comparisons with estimates derived from surface drifters, geostrophic current derived from altimetry, and modeled surface current yield very promising results. The current regimes are depicted with greater detail than generally possible from altimetry, due to the geostrophic assumption, and from surface drifters due to inhomogeneous distribution. In particular, the SARderived range Doppler velocity seems to be able to retrieve the variation of the equatorial and counter-equatorial surface currents across the intertropical convergence zone. Similarly the full strength and persistence of the Agulhas Current with variable dominance of shear, convergence, and divergence zones are detected in contrast to altimetry that tends to suppress the strength of the highly topographically steered Agulhas Current (Grandlingh, 1983). In summary, the results are considered promising for strengthening the use of SAR in quantitative studies of the ocean currents. Combined with surface drifters and altimeter-derived surface geostrophic currents, monitoring of the dynamics of intense current regimes may certainly be advanced. However, promising and exciting as these results are, the SAR-based Doppler velocity retrievals are in need of careful validation. A dedicated field campaign is therefore highly recommended. Execution of a campaign would also be very timely in view of the Sentinel-1 SAR mission, expected to be launched in 2014, to ensure continuation of C-band SAR data from ESA’s ERS-2 and ENVISAT satellites. Important applications driving Sentinel-1 include marine vessel detection, oil spill mapping, and sea ice monitoring. With adequate validation and retrieval accuracy of the new method and products reported here, Sentinel-1 could be expected to provide additional consistent information on winds, waves, and currents.