Large horizontal surface velocity shears in the ocean obtained from images of refracting swell and in situ moored current data

Aerial photographs showing strong refraction of swell near the northern California coastline were obtained on September 8, 1982. The possibility that this refraction is due to a large horizontal current shear is discussed; a maximum shear value of the order of 10−3 s−1, with the velocity directed along a linear slick‐like front, is calculated from the wave data in the photo. In situ current data obtained at 90 m depth at a nearby midshelf site on two surface moorings separated by 390 m also show occasional periods of large horizontal velocity shear. On January 4, 1982, a maximum horizontal shear of order 10−3 s−1 was observed for several hours. These separate observations taken together indicate that large horizontal surface velocity shears of order 10−3 s−1 occur over the northern California continental shelf, and that large surface current shear can be usefully detected and measured by high flying high resolution sensors.