Precision digitally processed and corrected synthetic aperture radar (SAR) image spectra along a large portion of SEASAT orbit 1339 (September 28, 1978) have revealed a number of unique potentials, but also some substantial limitations of spaceborne SAR for global ocean wave monitoring. SAR appears to be capable of monitoring the spatially evolving ocean wave number spectrum with high statistical reliability. The large‐scale (few hundred kilometer) spatial evolution of the spectrum is generally consistent with the location (in time and space) of wave‐generating sources, even when multiple wave systems are present. On a more local scale of just a few tens of kilometers, the nearshore wave number spectrum responds to local depth changes but is too noisy to respond reliably to local current changes. The SEASAT SAR was severely restricted in its ability to sense azimuthtraveling (along‐track) waves in high sea states. Azimuth waves corresponding to ocean wavelengths shorter than about 200 m were either undetected or severely attenuated in the higher sea states (Hs≥2 m); azimuth waves shorter than 100 m are not detectable in even the lowest sea states. Moreover, there is a tendency for attenuated wave systems to be shifted toward the range direction. This effect will significantly impede studies of spatial evolution of SEASAT SAR spectra without some additional correction strategy.

Abstract. In September 1991, the ERS 1 synthetic aperture radar (SAR) collected a set of four colocated images over the western Gulf Stream (GS). The SAR images were supplemented by satellite infrared imagery and measurements of the marine atmospheric boundary layer (MABL) from both a pair of surface buoys and a suite of instruments on the Ukrainian research vessel R/V Vernarisky, which between September 15 and 19 made six transects across the GS northern boundary within the SAR swaths. The influence of wind variations within the MABL was evident in each of the four SAR images. Evidence for thermal wind effects is seen in the changing character of the imagery as the mean wind across the sea surface temperature boundary changed direction from pass to pass. Both in situ data and analytical models suggest that much of the day-to-day SAR image variability was the result of changes in the character of the MABL, especially evident near the northern GS boundary where the sign of the atmospheric stability fluctuated over short spatial scales. Because many of the governing parameters of the MABL rapidly decorrelate, significant changes in the SAR imagery occurred during the 3 days between passes. Nevertheless, at least some manifestation of the GS was evident in the SAR imagery on each of the four passes.

SIR-B synthetic aperture radar ocean-wave spectra for a 200-km pass crossing the Agulhas current off the coast of Africa are analyzed. A significant enhancement of one spectral peak along the northern edge of the current is attributed both to amplification of the waves by refraction-dominated wave-current interaction and to transient specular backscatter contributions spatially correlated to the waves. 2 m s-• toward the southwest. The brightened area in columns 9 to 13 is thus presumably associated with a phenomenon along the north edge of the current. The three bright lines in column 17 are spurious.Finally, weather information, nearshore wave measurements, and current measurements in the Agulhas were gathered along with the SAR data. Surface weather charts (provided by the South African Weather Bureau) for 1200 UT on October 6 and 7, shown schematically in Figure 3, indicate passage of a cold front over the observation area a day before the pass. Across the front, the winds shifted from east-southeasterly at 10-15 kt (5.1-7.7 rn s-•) to faster southwesterly winds up to 25 kt (12.8 m s--•). Nearshore wave measurements were provided by the University of Cape Town, with the assistance of the South African National Research Institute of Oceanology. Most important was a pair of measurements from a waverider buoy (in 27 m of water) at East London at 1200 and 1800 UT on October 7. These indicated an 11.3-s swell period and a 6.5-s wind sea period; the significant wave height was 2.6 m. Information on the Agulhas itself showed the current to be in essentially a normal, undisturbed state. A current measurement along the north edge of the Agulhas, taken a week and a half before the SIR-B overflight indicated a current vector of 2.6 rn ß s-• directed approximatel• along the 200-m depth contour (a typical situation), though the high-speed core of the current was somewhat closer to shore than normal. A METEOSAT infrared image from October 6 (Figure 4) shows the current, a band of warm water originating in the tropics, to be approximately aligned with the coast. In particular, there is no evidence of a .major current meander. 15,389 15,390 IRVINE AND TILLEY: OCEAN WAVE SPECTRA AND WAVE-CURRENT INTERACTION

Seasat synthetic aperture radar ocean wave spectra for a 900‐km pass are analyzed and interpreted in the context of both their probable generation sources and their surface current and bathymetric modifiers. Systematic vector wavenumber variations of several times the standard error of determination (∼1.5 % in magnitude and 0.9° in direction) occur along the entire 900‐km pass. The large‐scale spatial variation of a 200‐m swell system can be accurately accounted for as a result of dispersion from a distant storm. The more local variations are qualitatively well correlated in position with known currents and bathymetry but show systematic biases that appear partly due to an environmentally dependent instrument transfer function in the regions of high current and highest sea state. There is also substantial evidence that a large angular deviation in the center of the pass is the result of a mesoscale eddy just to the east.

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