Sea Surface Mixed Layer during the 10–11 June 1994 California Coastally Trapped Event

Abstract

A midlevel, coastally trapped atmospheric event occurred along the California coast 10?11 June 1994. This feature reversed the surface wind field along the coast in a northerly phase progression. Along the central California coast, the winds at the coastal stations reverse before the corresponding coastal buoy offshore, then followed hours later by passage of the leading edge of an overcast stratus cloud. The sea surface temperature was much colder in the narrow strip along the coast. The cloud characteristics may be accounted for by a sea surface mixed layer (SSML) model beginning with the wind reversal and growing with the square root of time. Heat is lost from the SSML to the sea surface. A cloud forms when the air temperature at the top of the SSML is equal to the dewpoint. It is suggested that a bore develops on the top of the SSML, increasing the thickness of the SSML and the progression speed of the cloud to 8 m s?1. There is evidence that an undular bore with a leading cloud develops in the thinner inshore SSML. Advancing beyond Monterey Bay, horizontal density contrast is believed to have caused the bore to change character to a gravity current with a narrower cloud that passed a point inshore before the winds reversed at the buoys. The last trace of a disturbed boundary layer ended at Point Arena where strong northerly winds prevented any further northerly progression and contributed to a cyclonic eddy that was formed in the lee of the point. Caution is suggested in the interpretation of stratus cloud phase progression without coastal wind measurements.