R. A. Barkley The Kuroshio current The Kuroshio is one of the world's major ocean currents, equal in volume to some 6000 large rivers. Its sudden variations can have disastrous effects on Japanese fisheries, but as oceano- graphers develop routine monitoring methods fish catches could increase threefold EVERY second the Kuroshio current side (looking downstream) of the should also learn more about the way in carries some 50 million tonnes of sea current's axis. The heat which is carried which both weather and climate respond water past Japan's southeast coast-a north by this flow influences the weather to changes in conditions at sea, and the flow equal in volume to about 6000 throughout the northern hemisphere. The ways in which marine animals and plants rivers the size of the Danube or Volga. Kuroshio therefore plays an indirect but are affected by their environment. But even this massive current would take important part in the everyday life of the some 250 years to equal the total volume fishermen and farmers of eastern Asia THE INFLUENCE exerted on ocean of the north Pacific. Thus, although the and also, to a lesser extent, in the lives of currents by the Earth's rotation was not Kuroshio is one of the major currents in most of the rest of man kind as well. generally appreciated until 1835, when the world's oceans, and plays a vital role The first European chart to show the G. de Coriolis, while studying equations in the circulation of the north Pacific, it Kuroshio was Varenius' "Geographia of motion in a rotating frame of occupies only a small fraction (less than Generalis" of 1650. Later, expeditions reference, discovered what is now called 0.1 per cent) of that ocean: a thin narrow headed by Captains James Cook Coriolis force. Coriolis showed how the band less than lOOkm in width and about (1776-80) and Krusenstern (1804) effects of the Earth's rotation could be lkm at maximum depth running for added to western knowledge about the incorporated into the Newtonian 3000km along the western edge of the Kuroshio. Although Japanese scientists equations of motion by adding two Pacific between the Philippines and the began to study the biology of the additional terms. One, the centrifugal east coast of Japan. Kuroshio in 1880, it was not until 1893, force of the Earth's rotation, is usually Asia's seamen have known the when Wade started a series of drift absorbed into a redefined term for Kuroshio since ancient times. They bottle experiments, that systematic 'gravitation', which includes gravity and named it Kuro-shio (which means 'black examination of its currents first began. centrifugal forces together and acts along stream' in the Japanese language) Today, in an attempt to learn more a vertical defined by the direction of a because of the deep ultramarine colour of about the Kuroshio. scientists from plumb bob. By this definition there is no the warm, high salinity water which is China, Indonesia, Japan, Korea, horizontal component of either force. The found flowing north on the right hand Philippines, Singapore, Thailand, Hong other term, the coriolis force, makes Kong, United States, the Soviet Union and Vietnam are engaged in a major ~~ international project called the Co- Dr Richard Andrew Barkley is Research PATH OF THE KUROSHIO is shownin Oceanographer at the Bureau of Commercial operative Study of the Kuroshio (CSK). heavy shading on this streamline chsrt Fisheries Biological Laboratory in Honolulu. By keeping a figurative finger on this prepared from geostrophic current measura- He has worked on the hydrodynamics of the pulse of the north Pacific, they hope to ments. The region of narrow intense flow Kuroshio-Oyashio front and his present learn more about western boundary east of Japan is the Kuroshio extension. research is concerned with distribution of currents in general, the Kuroshio in which differs from the Kuroshio in that it temperature. salinity and oxygen in the Pacific has no land boundary to generate frictional Ocean, and with the currents and mixing particular, and the whole north Pacific flow. Convergence and divergence is shown which control these distributions. Ocean as well. In the process, they by streamlines terminating and beginning 54 SCIENCE JOURNAL 3" 0' !O IO m 3 v 0" 0" 120" 130" 1 55 MARCH 1970 ' allowance for the effects of Conservation common to use the 1000 decibar surface driven' circulation was made by W. H. of angular momentum on a particle (at very nearly lo00 metres depth) as a Munk in his pioneer study of oceanwide which moves relative to the Earth's reference surface for measurements transports. He pointed out the surface. Coriolis force acts at right angles used in dynamic calculations. fundamental importance of wind sheer, to such motion and also at right angles to The theory of wind driven ocean or torque, which transmits angular the Earth's axis of rotation, and thus has currents concerns itself primarily with the momentum to the sea surface and thus no effect on the energy of motion but forces which generate, control and finally generates the major circulation systems. only modifies its direction. dissipate energy in the sea. Most of this Munks study was based on a linear The vertical component of Coriolis energy appears in two forms: kinetic steady state mathematical model in force is so small when compared to the energy of motion-the currents per which friction played a large part. K. force of gravitation that it can always be s-nd potential energy of position Hidaka has explored such theoretical neglected in ocean current theory. In the -the 'pressure head' due to slopes in the models extensively since 1949. sea, however, forces which act along the sea's surface and the internal density In more recent years the importance of horizontal are generally weak. Indeed, structure. There are also others, primarily large scale friction in currents such as the frictional and inertial forces are far thermal energy, which affect the Kurpshio and Gulf Stream has been smaller than those we are accustomed to distribution of density and thereby questioned by a number of theoreticians, on land-by at least five orders of influence currents. who regard it as characteristic of the magnitudeand they have little or no Those forces which carried Kuroshio climatic-mean flow, but not of the effect on any but the fastest moving water through nets set by Japanese instantaneous current. That is, meanders ocean currents. The forces exerted by fishermen this morning were generated and eddies formed from time to time by wind stress are often even weaker. Thus weeks, months, or even years earlier by the current could be considered as a form the horizontal component of the weak winds blowing over the entire north of turbulent friction of the magnitude Coriolis force becomes an important Pacific Ocean. Even a small gust of wind required in Munks and Hidaka's theories, factor when considering ocean currents. rippling the quiet waters off Baja, if their effects were averaged over large In most Ocean currents the horizontal California, for example, contributes to the areas over long periods of time. On a pressure gradient is found to be in oceanwide accumulation of energy which 'shorter time scale, and over smaller balance with Coriolis force. Such currents drives the inexorable flow of millions of distances, such meanders and eddies are called geostrophic or 'Earth-balanced' tonnes of water past Asia's coasts. A must be treated individually, which currents. Thus it is possible to obtain a major problem in ocean theory is to requires the use of nonlinear time useful approximation (to within 15 per determine the way in which energy is dependent mathematical models. Such cent) to actual currents by making transferred from the winds to the shallow models present formidable mathematical measurements of horizontal pressure wind driven upper layers of the ocean; difficulties which in some cases can only gradients. To do this, a series of from there, into the geostrophic currents; be overcome by the use of numerical temperature and salinity measurements and, finally, into the narrow sheer zone computer methods. is made to depths of lkm or more at on the left hand side of the Kuroshio, several locations (called 'stations') across where energy is dissipated. PREVAILING SURFACE WINDS over the a current. These observations define the When V. W. Ekman provided. in 1905. north Pacific in summer Uescribe a field of density (which depends on a theoretical explanation for Nansen's clockwise circulation around the temperature, salinity and pressure, or observation that Arctic icebergs tend to mid-latitude high pressure cell, from depth), from which horizontal pressure drift to the right of the wind, he laid a which the air spirals outward, away from gradients can be computed. A chart major cornerstone for all subsequent the high, towards the low pressure showing this field of pressure gradients is studies of wind driven flow. Ekman regions over Asia and Alaska, and equivalent to a chart showing streamlines showed that a steady wind produces towards the climatic Equator, the dol- of geostrophic flow. This procedure is transport of water at right angles to the drums. near 5ON. Initially, this is cool, referred to as the dynamic method, and wind's direction (toward the right in the dry air which descends towards the sea was first used by J. W. Sandstrom and B. northern hemisphere, and to the left in surface and speeds up, removing both Helland-Hansen in 1903. Unlike direct the southern hemisphere). This move- heat and moisture from the surface water current measurements, which are more ment of water is usually called Ekman layers as it goes.