Occurrence frequencies of sea-surface slicks at long and short time-scales in relation to wind speed

Abstract

For 2 years photographic records of the sea surface were taken in a coastal area. These were used, in conjuction with records of the wind speed and direction, to determine the frequency of formation of slicks as an index of surface microlayer presence. Parallel collection of sea-surface samples provided evidence for a relation between the pictures and accumulation processes of organic matter in surface microlayers. Results show that slicks are present for more than 25% of the recording time; this frequency may reach more than 60%, depending on the hour and season. Slick formation is dependent on wind speed and slicks never exist at wind speeds greater than 7 m s −1; below this upper limit the highest probability of slick formation corresponds with wind speed values between 2 and 4 m s −1, but in many cases, no slicks are formed under low wind speed conditions. (Slicks were found to require considerable time to form, following a decrease in wind speed, and to disperse, following an increase in wind speed. This dependence explains most of the seasonal and diurnal variations in slick occurrence.) Slicks are more frequent in the morning, and less frequent at midday and afternoon, because of wind induced by heat flux between the sea and continent in coastal areas. Despite the fact that it was not possible to take photographic records during the night, a high probability of slick formation during the night can be deduced from the recorded decrease in wind speed.