Journal of the Marine Biological Association of India

Volume 14 Issue 1

Sea surface features.

E. C. Lafond and K. G. Lafond

Photographs of the sea surface made from satellites and high-flying airplanes throughout the world can be used to note special differences in the appearance of the sea surface. Roughness patterns can reveal processes taking place at sub-surface depths. Areas of upwelling, internal waves, current shears, surface wave trains, and coastal runoff have all been ascertained from photographs. Cloud patterns also provide information on wind and water properties, especially around India.

Oceanographic tower studies of surface features relating to various types of sub-surface motion including internal waves, turbulence, convergence circulation and differential flow have been conducted in shallow water off Mission Beach, California. Other examples of surface features related to internal waves have been recorded in the Bay of Bengal and the Andaman Sea.

One of the most persistent anomalies in the California area is sea surface slicks. Composed of a thin layer of organic film they concentrate into long lines or streaks, the orientation of which is related to convergence circulation caused by internal wave motion. Surface-active material is made up mainly of fatty esters, free fatty acids, fatty alcohols and hydrocarbons. The main source of this natural slick material is metabolic products and decomposition of organic matter in the sea. Because of its oily nature, it absorbs fat-soluble substances such as pesticides. Thus slicks, and the water motion associated with them, afford a means of concentrating and transporting pollutant materials.

Although the greatest amount of surface-active material comes from the biosphere, some components arise in the atmosphere; other fractions, such as man-made waste and drainage, are derived from land. The shoreward transport of slick material over the southern California continental shelf is estimated to be 6-9 ml per linear meter of coastline per day.

Slicks are more visible when the internal wave period is large and its height is great. When the wave height is 2 m or less, slicks are visible only 30 per cent of the time; when 5 m or greater, a slick is always present. Infrared sensors have shown that slicks are normally colder by an average of 0.2°C (ranging 0.1-0.5°C) than adjacent water. This is attributed to greater evaporation of a thin film on organic particles. Such temperature changes afford a means of identifying slick boundaries.

Slicks can propagate by internal wave motion, water currents, and wind. Under light winds, they nearly always occur 1/4 of a band width behind the crest of the internal wave. Measurements of the changing position of oil spill slicks in the open sea indicate they move at rates equal to 3-4 per cent of the wind speed. The most important evidence data are scattered, the least squares fit of 120 observations shows a decrease in width with increased wind. Extrapolation of the curves shows the slick width approaching zero as wind speed approximates 13 knots. Greater wind speeds cause the band slicks to break up and Langmuir-type circulation then becomes dominant.

Under light wind conditions, the direction of propagation of internal waves can be deduced from movement of the rough and smooth bands, since the rough band is over the crest and the smooth band is wavelength behind. Thus visible sea surface features can be used to establish sub-surface oceanographic processes.

Date : 30-06-1972