Ocean circulation

Abstract

Although the main features of the surface circulation have been known for hundreds of years, and the water masses below the surface were mapped out by the great systematic surveys made in the period between the two world wars, the amplitude of the deep circulation is still uncertain, and the detail of the subsurface water movements is still in the main unknown. This lack of knowledge is well illustrated by the recent discovery of the Pacific Equatorial Undercurrent, or Cromwell Current. Flowing eastwards with a speed of 2 to 3 knots at 100 m depth below the equator, its transport, according to Knauss (i960) amounts to 40 million cubic metres of water per second, and it extends for at least 3500 miles. The presence of this remarkable feature was not predicted; nevertheless, many notable advances have been made during the last 10 years in the theoretical treatment of the circulation of the ocean, and some predictions are still awaiting observational test. The vertically integrated mass transport of the Gulf Stream was computed by Munk (1950) in terms of the wind stress on the sea surface, and much subsequent study has been devoted to the wind-driven circulation. The internal thermohaline circulation, driven by density differences in the water, remained relatively neglected until Stommel (1958) devised a model which successfully accounted for several known features of the ocean circulation.