Though various theories have been developed to explain the origin of ocean currents, yet no single theory has ever succeeded fully. The mathematical treatment of the ocean currents involves the hydrodynamic laws and equations, static’s, kinematics, dynamics and thermodynamics and various equations of motion etc.
The credit for making first attempts at a quantitative explanation of ocean currents goes to K. Zoppritz, who introduced many simplifications, but there was no agreement with observations. He did not consider the turbulence, related friction and the rotation of the earth in his theory.
H. Mohan later introduced the deflecting force of the earth’s rotation. In the year 1898, V. Bjerknes developed the circulation theory which was mainly concerned with currents caused by density differences in the ocean water.
Another theory of wind-induced currents was advanced by V.W. Ekman in 1905. This was known as the drift theory. In the year 1929, A. Defant, an eminent oceanographer, explained the conditions of stable stratification of moving water masses of the oceans.
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However, the aforesaid theories laid the foundation for the understanding of ocean currents, when the oceanographers clearly understood the main forces causing and maintaining the currents and their modification by the rotation of the earth, friction and the bottom topography of the oceans.
But the quantitative explanation of the large-scale general circulation of the surface currents was lacking. Even a number of small-scale characteristics of the motion were not explained.
Later on, C.G. Rossley and H.U. Sverdrup did commendable work by introducing the most significant phenomenon of lateral mixing of ocean water into the equation of motion.
By 1947 oceanographers from Sweden, Japan, U.S.A. and Germany introduced mathematical procedures that made possible a more complete consideration of the hydrodynamic equations.
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The evolution of various theories with regard to ocean currents increases our understanding of the very complicated interplay of forces that affect the ocean currents in numerous ways.
However, even the latest theory has failed to give a complete explanation of ocean currents, because with increasing observations at sea and the ultra-modern observational techniques new problems appear.
To be able to explain ocean currents, the field of mass, the field of gravity, the pressure field, and the distribution of internal forces in the ocean must be known.
However, in view of the difficulties enumerated above, in the following pages the causes and the character of ocean currents have been described in simple non-mathematical language.