Sea-surface slick occurrence in the open sea (Mediterranean, Red Sea, Indian Ocean) in relation to wind speed

The frequency, occurrence and extent of sea-surface slicks were studied in open seas during a ship crossing from the eastern Mediterranean to La Réunion Island (Indian Ocean), via the Red Sea, by taking sea-surface photographs every 1 min 45 s and relating them to wind speed and direction records. During 10% of the recording time, slicks covered from 20% to 80% of the observed sea surface, and they covered the entire photographic field during 1% of the recording time. Slicks were observed only at wind speeds below 5 m/s, but in some cases, they were absent at lower wind speeds. Conversely, slicks were never observed at wind speeds higher than 6 m/s. Compared to that in coastal waters, the frequency of occurrence of slicks was lower (11% against 30%) at the surface of the open sea, mainly as a consequence of persistent bad weather (southwest monsoon period). Nevertheless, the records provide evidence that slicks are not rare in the open sea and that slick-forming meteorological conditions found in coastal areas are similar to those of open seas. These include the upper wind speed limit and the percentage of the sea surface covered by slicks in relation to wind speed. The daily rhythm of slick formation in coastal waters, due to diurnal changes in wind speed and direction, is absent in the open ocean.