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

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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. It then slows down as it expensive and time consuming to make transport. A balance is reached between reaches the convergent lows, where the and more difficult to interpret, the coriolis force and wind stress at the sea air rises and much of the moisture returns dynamic method is little affected by surface. which is a precise analogy to the to the sea as rain. In winter, the high transient motion due to surface waves, balance existing between coriolis force pressure band moves south some 20° of tides and winds, which are often much and the horizontal pressure gradient in latitude and splits into eastern and stronger than the movements of water in geostrophic flow. western cells, with a low pressure band, the permanent current system. In 1947. H. U. Sverdrup, the the Aleutian low, all across the Pacific Geostrophic current calculations have Norwegian oceanographer and meteorol- farther north. one major drawback-they only give ogist, used Ekman's concepts to calculate On streamline charts (such as the information on the flow relative to some the wind driven transport in equatorial map on page 55). constant amounts of reference surface which is presumed to currents of the Pacific Ocean. The water flow between any two streamlines be nearly motionless. The problem arises following year H. Stommel showed that (within the accuracy of the methods because the dynamic method depends changes in the coriolis effect with latitude used), so velocity of flow is inversely upon an integration, and the constant of (the horizontal component of Coriolis proportional to the distance between integration-the absolute pressure at force varies as the sine of the latitude, SO adjacent streamlines. In addition, such some level surface common to all the that it reaches a maximum value at the charts show convergences and oceanographic stations used in the pole. and vanishes at the equator), were divergences, flow of water downward out calculationsis not normally known, and responsible for the narrow swift currens of the layer in question, or upward from cannot be measured directly with the along the western boundaries. below, by showing streamlines terminat- required accuracy. In practice it is A major advance in the theory of wind ing (convergence) or beginning

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EKMAN TRANSPORT in the north Pacific to one-tenth as thick at the perimeters of a year. It is not surprising, then, that the is shown in the map above, where the the gyre, and cold dense, deep water is ocean's density structure is remarkably prevailing winds in July are shown in found much closer to the surface there. constant, and that currents such as the colour, and the surface current streamlines Density gradients due to convergence Kuroshio hardly respond to changes in in black. Off the coast of California, and the northeast coasts of China and Japan. and divergence of the Ekman transport the local winds. Currents near the the wind blows along the coast, inducing cause horizontal pressure gradients Equator, on the other hand, represent offshore transport of water. The opposite which generate the geostrophic flow much smaller accumulations of energy situation, where Ekman transport carries shown on the map on page 59. Such and respond much more readily to surface water towards the coast can be seen on the eastern side of the Gulf of Alaska gradients are largest in the upper 100 to changes in the wind. 200 metres and diminish to negligible values at 1 to 2km depth, due to AFTER DECADES of effort by Japanese Compensating displacements of denser, oceanographers. supplemented by stu- (divergence) at a coast or in midocean. In deep water. dies made during international expedi- the following discussion it will be The effectiveness of Coriolis force in tions and by individuals from many convenient to use the term 'Sverdrup limiting the flow of water out of the thick, countries, it is now possible to piece unit to indicate a flow of one million warm water lens at the convergence can together a reasonably satisfactory cubic metres per second. be understood by comparing currents and description of the Kuroshio proper. Water According to the map on this page horizontal pressure gradients in the enters the Kuroshio over a broad front Ekman transport removes water from the central gyre with those at the Equator, 1OOOkm in width (130N to about 23ON at equatorial currents and the northem gyre where coriolis force vanishes. At the longitude 125OE) then accelerates and and transports it into the central gyre. Equator a pressure gradient only one per narrows. Some water leaves the right Some 45 streamlines enter the gyre at its cent as strong as that across the hand side of the Kuroshio as soon as it perimeter (from the coasts, between Kuroshio produces comparable current begins to turn towards the east, but islands and across looand 500N). What velocities. Why? Because flow at the narrow, intense flow persists for 1500 to happens to this massive flow? Most of Equator- is not geostrophic and is 2000km after the current leaves Japan's it-bout 40 Sverdrup units--+baporates therefore free to speed up until friction east coast, after which there is a marked into the cod, dry air flowing out of the and inertia forces are large enough to drop in velocity. This region of narrow atmospheric high. But Ekman transport balance the pressure gradient. At higher intense flow east of Japan is called the more than makes up for this loss; the latitudes Coriolis force allows much larger Kuroshio extension, and it differs from excess 'piles up' in the Subtropical horizontal pressure gradients to build up the Kuroshio in that there is no land Convergence and sinks to depths of 100 before the flow reaches speeds where boundary on the left hand side to to 200 metres. From there, much of this friction and inertia forces become generate a frictional boundary layer. water spreads radially outward but some important limiting factor. Comparison of velocity profiles (plots of it is mixed downward, warming the The potential energy stored in the of velocity across the surface current) of underlying water. Over long periods of mid-latitude gyre is many hundreds of the Kuroshio and the Kuroshio extension time, a 'lens' of lower density water times greater than the kinetic energy of shows that, although both have about one kilometre thick has its currents, and represents an essentially identical velocity profiles on accumulated in the centre of the gyre. accumulation of energy five to ten times their right hand sides, the velocity The low density layer is only one quarter larger than the energy added by winds in gradient on the left hand side of the

MARCH 1970 57 the velocity goes up from a few tens of Kuroshio is at least six times greater (a balance of angular momentum can Only centimetres per second to one metre per change of 2m/s in 8km) than that in the be attained on the western boundary, second as the current narrows east of Kuroshio extension (2m/s in 50km). which accounts for the westward Other things being equal, this would intensification in the current systems of Taiwan. From Taiwan to Japan Some result in six times greater frictional stress the world oceans. dilute water from the Asian shelf, on the on the left side of the Kuroshio. Such In the Kuroshio extension the flow left of the current, is entrained by the velocity profiles support the theoretical adjusts to conditions in the ocean's flow, which speeds up even more, to view that the Kuroshio and Kuroshio interior, where large velocity gradients, velocities of 1.5 to 3m /s. Transport just extension are the major nongeostrophic and the angular momentum associated off Japan's south coast amounts to 35 portions of the flow in the central and with such gradients, cannot persist. Sverdrup units, but there may be northern gyres. where important Friction is no longer concentrated at the significant flow below 1000 metres since adjustments in the distribution of energy boundary, nor are inertial forces the current extends to considerable take place. restricted to the right hand side of the depths at that point, and so the total In the Kuroshio energy is dissipated stream. Instead, large eddies and transport may be as high as 45 to 50 through friction (on a small scale, since meanders dissipate kinetic energy Sverdrup units. the frictional boundary layer is only about throughout the path of the flow, and Just as the current reaches Japan's ten kilometres wide). On average, the redistribute angular momentum at the southeast tip, it flows over the shallow dissipation rate must be in balance with same time. The transport decreases In-Bonin Ridge which extends due south the mean rate at which wind adds downstream as the flow fans out and from Honshu, Japan's main island. The energy to the central gyre. becomes the broad, slow West Wind Kuroshio undergoes complex and little However, friction not only dissipates Drift between 155O and 160°E. understood fluctuations near this ridge. energy, it also generates counter- Fifty Sverdrup units of flow approach Once past the Izu-Bonin Ridge, the clockwise angular momentum at a rate the east coast of Mindanao in the Kuroshio may turn north along Japan's which more than compensates for the Philippines and half of this volume turns east coast for a short distance, or it may decrease in the Earth's (counter- north into the Kuroshio. Most of this flow continue to flow almost due east. In clockwise) angular momentum as the consists of warm (20°-28"C) water in the either case, it joins the Oyashio current, current flows north. To balance this upper 200 metres; there IS a layer with which flows southward from the excess, clockwise angular momentum is relatively high salinity (near 35 parts per Kamchatka Peninsula. Together these generated on the right hand side of the thousand) between 100 and 200 metres two currents leave the coast and form the current, where the velocity decreases with slightly more dilute water (34.5 Kuroshio extension. Transports here towards the right in what is sometimes parts per thousand) above that. amount to about 45 Sverdrup units (up to termed an inertial boundary layer. As H. The depth of the high velocity flow 60 on occasion) though only 25 to 35 Stommel pointed out in 1948, this increases from 200 to 400 metres, and Sverdrup units are within the high

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rOTAL TRANSPORT by gaoatmphii cur- grounds are located on both sides of the waters, as offshore currents move in the north Pacific. Tranaporc in th. frontal zone in the Kuroshio extension, so inshore at speeds of 1 -2m /s, or mpre. -upper lo00 met- is shown incdour(bach this complex feature of the western north Meanders in the Kuroshio south of line mpcrsente 6 Sd~punits), and in the uppar 100 mares in black (each line Pacific is of particular interest to Japan's Japan have been studied intensively. The mpmunts 1 Sd~punit). The map shows fishing fleet. A pioneer in fisheries axis of the current may shift onshore or the intense narrow flow at the westem oceanography, Professor Michitaka Uda, offshore lOOkm or more in a matter of boundary, the change to broad slow moving has studied such fronts and the fisheries weeks. For example. in 1959 the ibw downstream. near longitude 160OE. associated with them since 1930. He and the own broader dower moving water Kuroshio off Japan's south coast (at tranqmt which occurs toward the aooulh and his colleagues in Japan's unique 133%) began to move offshore in March system of fisheries universities have or April. shifting from its initial position contributed much to our knowledge of the 20km offshore to a distance of 140km in velocity core of the Kuroshio extension, sea as an environment. They have taken about one month. This meander also where speeds of two metres per second particular interest in variability in the drifted rather slowly downstream, or more are &en observed. occurrence of various kinds of fish, and reaching the central portion of the south the time the current has reached changes in the environment which coast (136OE) by the end of May and the 160°E, towawls the end of the Kuroshio cause much of this variability. eastern portion (139OE) by ea* August, extension, it consists of a mosaic of The path of the Kuroshio as shown in Bt which time the Kuroshio's axis was water types: warm saline water from the the map on page 55 is very nearly its back within 25 Km of the con# original source off the Philippines, coastal average position, but the current can farther upstream (133OE). where the and shelf water of lower salinity, and cdd undergo marked and fairly rapid changes disturbance was first observed. Such dilute water from the Oyashio, with in speed and in the location of its axis. meanders may appear and disappear summer temperatures of 3O to 10%. and Apart from changes due to tides, short within a few months, but they may also salinities sometimes as low as 33.8 term changes due to major shifts in the remain more or less stationary for more parts per thousand. axis of the Kuroshio can occur as it flows than a decade. When the meander Some mixing occurs in the core of the past southern Japan. Meanders develop develops, cold water is brought up Kuroshio extension, but on the whde in the current which occasionally bring toward the surface between the Kuroshio there are two distinct types of water in high velocity flow unusually close to the and the coast and temperatures drop to the current: warm saline water on the coastline, and part of the Kuroshio's flow as much as 10°C below normal. This right, and cdd dilute water on the left. may be diverted into nearby bays. where change has profound effects on coastal The convoluted front separating these it can flush out much of the coastal water and offshore fisheries, since the area two types of water is often very sharp within a matter of days. These sudden involved is fairly large-about the size of and active, with strong velocity gradients and as yet unpredictaMe events cause the Bay of Biscay. Familiar species of and shape contrasts in the properties of widespread damage to boats and fishing fish move away and are replaced by the water on the two sides. Rich fishing gear anchored in the normally quiet others, and so fishermen must either

MARCH 1970 59 move to new grounds or market what cloud cover and rainfall. Similarly. cold experience with changes in the ocean in they can catch on the old grounds. air from Siberia flows out over the Pacific various fishing areas and the The only nontidal changes in the in winter, to encounter warm water consequences of such changes in the Kuroshio which appear to be at all regular carried north by the Kuroshio; these past. It is still too early to evaluate the are annual changes in velocity and temperature contrasts trigger formation system's effectiveness, except to note transport, which are easily obscured by of numerous cyclonic lows in the that the information provided to the the irregular variations discussed above. atmosphere over the Kuroshio. The lows fishermen is very much in demand. Japanese scientists generally agree that carry stormy weather east to northeast The potential value of fishery the speed is greatest from May until across the northern Pacific Ocean forecasting systems can be judged from August, with a second maximum in towards the cqasts of Alaska, Canada and the fact that boats in some fisheries must January and February. But an analysis by the United States. spend 80 per cent or more of their time Y. V. Pavlova has shown that the annual Improved knowledge of the ocean as scouting for fish. If this time could be cycle is rather more complex, at least for an environment can help fishermen reduced by half, each ship in such a geostrophic currents. Speed and locate and catch protein to feed an fishery could increase its catch as much transport not only vary with season, they increasingly hungry world. Fishermen as threefold. vary in different ways from place to place could make direct use of forecasts of the We have seen that there are many along the Kuroshio. Off the southern tip Kuroshio's flow, because the entire reasons for undertakings such as the of Japan, for example, maximum current system is a series of fishing Co-operative Study of the Kuroshio. They transport within the Kuroshio occurs in grounds which move about with changes range from the most abstract, through September and again in March or April, in the flow. Various species of tuna, the coldly practical to the mundaneim- while maximum velocities are observed in sardine and anchovy, mackerel, squid provements in ocean current theory, July and January. Just east of Japan, and many other commercially important better weather predictions, more efficient according to Pavlova, maxima in species are each found in specific zones ways to catch fish, and improved charts transport occur during June and in and near the Kuroshio. For example, of the oceans. All these and more will December, with velocity maxima in fronts where coastal and offshore waters result from studies of the Kuroshio and August and February. meet are often good fishing grounds. other parts of the world ocean. But Species such as sardines are caught on regardless of motive, form or content, the WHAT NOW REMAINS to be learned the coastal side of these fronts, while goal of these studies can be summarized about currents such as the Kuroshio? mackerel and tuna occur in abundance in in much simpler terms: the search for Perhaps most valuable would be the warmer offshore waters. Changes in man's ultimate tool, knowledge. information on fine structure and on the marine environment appear to fluctuations over periods of a month or influence both the timing and the paths less. To determine how fluctuations of fish migrations. propagate from place to place, For more than 50 years, Kitahara, Uda measurements must be made simul- and their fellow fisheries oceanographers taneously at several points along the in Japan have studied the response of current, and at least a few direct fish to their environment. They have set measurements must be made of the up an extensive network for collecting currents at all depths. Many develop- and reporting temperatures at the sea ments in ocean current theory await the surface and at various depths, results of such observations, which will movements of schools of fish near also be needed for forecasts of conditions various fishing grounds, catch rates, and in and near the Kuroshio to serve the ocean current information for making needs of fishermen and meteorologists. fishing condition forecasts. But the effort required, for a complete Six regional fisheries research survey of the Kuroshio, in terms of ships, laboratories and 38 prefectual fisheries time and operating costs, is staggering experiment stations are responsible for FURTHER READING and detailed rapid surveys of the collecting, analysing and distributing Kuroshio must wait until more efficient WATER CHARACTERISTICS OF THE information on individual fisheries, such KUROSHIO by J. Masuzawa (in The Oceano- tools, such as instrumented buoys, as the salmon, sardine or albacore. Ships graphical Magazine, 17. 37. 1965) become commonly available. at sea send information on their catches THE SEASONAL VARIATION OF THE Once routine monitoring of the marine and the environment to the appropriate KUROSHIO CURRENT (In Russian) by Y. V. environment becomes commonplace, we laboratory by radio. There these Pavlova (in Okeanologia, 4. 625, 1964) DESCRIPTION OF THE KUROSHIO can expect marked improvements in long reports are compiled and analysed (PHYSICAL ASPECT) by D. Shoji (in range weather forecasts and in catch to produce charts of fishing conditions, Proceedings of Symposium on the Kuroshio, rates of commercial fisheries, which will conditions in the ocean, and forecasts of Oceanographical Society of Japan and rapidly repay the original investment of various kinds, which are sent to the UNESCO, Tokyo, 1965) effort and funds. Even rather minor fishing fleet by mail, radio and facsimile. ON THE VARlABlLllY OF THE VELOCITY OF THE KUROSHIO VOL 1 by D. Shoji and improvements in weather forecasts can Some charts and forecasts are prepared H. Nitani (in The Journal of the Oceano- bring significant savings to farmers, at 10 day intervals. and others are sent graphical Society of Japan, 22, 192 7966) public utilities, cargo ships, airlines and Out Once a month. Research and cargo THE INFLUENCE OF FRICTION ON INERTIAL others who use forecasts in scheduling ships also provide information on MODELS OF OCEANIC CIRCULATION by operations or planning routes. temperature, salinity, currents and other R. W. Stewart (in Studies on Oceanography, Farmers in northern Japan may have Hidaka Jubilee Committee, Tokyo, 1964) factors in the environment for use in the ON THE NATURE OF THE KUROSHIO, tTS good or poor harvests depending on the fishing condition broadcasts. ORIGIN AND MEANDERS by M. Uda (in extent to which the Kuroshio flows north Forecasts are based on long term Studies on Oceanography Hidaka Jubilee along Japan's east coast, before joining trends, the time when fishing begins or Committee, Tokyo, 7964) AN INTRODUCTION TO PHYSICAL OCEANO- the cold Oyashio water, since water ends on various grounds, data on age and temperatures offshore strongly influence GRAPHY by W. S. von Arx (Addison-Wesley Size composition of the catch, and on Publishing Co, Reading, Massachusetts, 19621

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