A knowledge of ocean currents is of great importance in all branches of marine work and the study of motion within the sea is the main task of physical oceanographers. Although the general pattern of surface currents is reasonably well known, the eddies and day-to-day changes found in them still require investigation. Currents which are almost as fast and as variable as those on the surface have been found well below the surface and are still far from being completely understood. A variety of forces can produce motion within the sea and there is great interplay between these forces; consequently ocean water movements are very complex. They range from small-scale currents found near shorelines, the result of wave action, to large-scale oceanic currents which are related to the wind circulation and to density differences within the sea.
When the wind blows persistently in a particular direction, the wind stress on the sea surface causes the water in the upper layer to move bodily and a current is thereby formed. These wind-induced currents may be permanent in regions of strong prevailing winds but they do not flow in the same direction as the wind. In the Northern Hemisphere they are deflected to the right of the wind direction and in the Southern Hemisphere to the left by an angle varying between 30 and 60 degrees. This deflection is an effect of the earth's rotation.
The density of sea water depends both upon its temperature and upon its salinity and, since these vary from place to place, the density also varies and gives rise to currents. These density currents are not independent of winds, however, because the wind-induced currents themselves alter the density distributions. Similarly, if warm water is driven from low latitudes into higher latitudes it will be cooled, and thus a limit is set on the differences of density which can be attained. The positions of the land masses and the shape and depths of the ocean basins also affect the currents.
Various methods have been devised to enable currents to be measured directly. Much useful information has been gained by observing the effect of surface currents upon the navigation of ships. Drift bottles and drift cards have also been used, the cards being placed either in a ballasted bottle or in a plastic waterproof envelope and released at sea. The finder fills in details of the recovery position and forwards the card to the investigator. Much of the knowledge gained about the coastal currents of New Zealand was obtained by the use of drift cards.
Subsurface currents are generally measured with current meters. These instruments, which measure current velocity and direction, are used more particularly in coastal waters since they are operated either from a ship or a buoy. Their principal disadvantage is the fact that they measure the current velocity relative to the ship or buoy and even an anchored ship moves at the end of its anchor line, particularly in deep water. A promising method for measuring deep currents has recently been developed in England. A small float fitted with a device which produces an ultrasonic signal, is ballasted to float at the required depth. A ship fitted with a suitable sound receiver can then track the movements of the float and so obtain a direct measurement of the deep currents. This method demands precise position-finding by the ship because the movements of the float are measured relative to the ship's movements.
In view of the difficulties encountered in measuring currents directly, oceanographers often employ an indirect method in which hydrodynamical equations of motion are used. The dynamic computation of currents depends upon the accurate measurement of temperature, salinity, and depth, and techniques are used which enables these measurements to be made. A metal bottle fitted with a valve at each end is attached to a wire and lowered with the valves open to the approximate desired depth. A weight which slides down the wire is then released. This strikes a release mechanism on the bottle which turns over and the valves close to obtain a sample of water. Normally a number of bottles are attached to the wire at various depths. The samples obtained can then be analysed for any desired properties. Salinity can be determined by chemical analysis but physical methods are often used, and a standard of accuracy of ±0.02%0 can be readily obtained. The average value of salinity within the oceans is 35%0 (%0 means parts per thousand). Reversing thermometers are mounted on the sampling bottle and when the bottle turns upside down the mercury columns break and record the temperature at the depth of sampling. Corrections are applied to the thermometer readings to allow for the expansion of mercury after it is brought on deck. Two types of thermometer are used on each bottle. One type is protected from the water pressure by a strong exterior glass sheathing. The other type is not protected from the pressure and the bulb of mercury is therefore compressed by an amount which depends upon the depth. Thus the unprotected thermometer gives a higher reading than the protected thermometer and the difference amounts to about 0.01°C for each metre of depth. The standard of accuracy for each type is about ±0.01C.
A shallow, well-mixed layer, approximately 50 to 200 metres thick, forms the surface layer of the oceans, the mixing resulting from the effect of wind and waves. The vertical differences of temperature and salinity within this layer are very small but the water properties can change fairly rapidly because the layer is subjected to solar radiation, evaporation, and precipitation. At the bottom of this layer there is a region where the vertical temperatures decrease rapidly over a small depth, with consequent sharp changes in the density of the water. This region is called the thermocline and mixing cannot proceed rapidly there because of the steep density gradient which exists. As a result, water properties such as temperature and salinity do not change quickly below the thermocline. These conservative properties are used to classify oceanic water into various “water masses”, and the movement of these water masses can be traced over long distances. This fact is used as a subsidiary method of determining currents. Different water masses exist at different depths and a vertical section may consist of an upper, intermediate, deep, and bottom water mass.
Two distinct upper water masses are present in the immediate vicinity of New Zealand, the Sub-Antarctic Water Mass lying to the south and the Subtropical Western South Pacific Water Mass lying to the north. These two water masses meet in a region called the Subtropical Convergence Region which is often (but not always) characterised by comparatively sharp changes in temperature and salinity between the warmer, more saline subtropical water and the cooler, less saline Sub-Antarctic Water.
The Sub-Antarctic Water Mass extends southwards to somewhere between about 54° s and 62 s where another major boundary, the Antarctic Convergence, separates the Sub-Antarctic Water from the colder Antarctic Water. The Sub-Antarctic Water Mass thus lies in the westerly wind belt of the so-called Roaring Forties and Fifties, and the main movement of this water is towards the north-east. This movement is called the West Wind Drift.
The subtropical waters move mainly westwards under the influence of the south-east trade winds and this movement is called the Trade Wind Drift. The Australian continent bars the westward movement of part of this Trade Wind Drift and the water is deflected to move southwards off the east coast, thus forming the East Australian Current. When the subtropical water transported by this current meets the north-east-moving Sub-Antarctic Water, it turns and moves eastwards across the Tasman Sea as the Tasman Current.
The surface water movements just described give rise to the coastal current pattern illustrated in the map below. It must be pointed out, however, that the circulation pattern is generalised and that at any one time a particular current may be either strongly or weakly developed. Much more investigational work will be necessary before a complete description of the currents can be given.
Three currents arise from the movement of subtropical water in the Trade Wind Drift. These are the East Auckland Current, the West Auckland Current, and the East Cape Current, all of which are southgoing. The East Cape Current lies offshore except where it meets the coast some distance south of East Cape.
The Tasman Current gives rise to two coastal currents, the Westland Current which flows northward along the west coast of New Zealand until it meets the West Auckland Current, and the Southland Current which flows east through Foveaux Strait and then north along the Otago coast. The water in these currents may be described as being modified subtropical water since it has been transported well south over a long distance and has different characteristics from the subtropical water found in, say, the East Cape Current. A branch of the Westland Current enters Cook Strait from the west and is called the D'Urville Current, so named because the famous navigator, Dumont d'Urville, on an occasion in 1827, was the first to note its existence when his ship was unexpectedly carried by the current well into Cook Strait from the west. The Canterbury Current is a cool, north-flowing current which contains water from the Southland Current, together with Sub-Antarctic Water of the West Wind Drift which has upwelled from below the surface. This current can extend as far north as Gisborne.
Immediately below the upper water masses there is another water mass, the Antarctic Intermediate Water. This is derived from a mixture of Antarctic and Sub-Antarctic Waters which sink at the Antarctic Convergence and spread north almost as far as the Equator. The vertical distribution of salinity shows a minimum value at depths somewhere between 800 and 1,200 metres, and this salinity minimum marks the core of the Antarctic Intermediate Water. As this water moves towards the north it mixes with the over and underlying waters and the salinity minimum becomes less and less pronounced.
Two water masses present below the intermediate layer are Deep Water and Antarctic Bottom Water. The Antarctic Bottom Water is formed near the Antarctic continent and, being very cold and dense, it sinks and spreads northwards. This water has been traced well north of the Equator. The Deep Water originates mainly in the high latitudes of the North Atlantic Ocean where surface waters are cooled. The consequent increase in density causes the cooled water to sink and it spreads southwards. South of the Equator the Deep Water continues in southward movement above the northgoing Antarctic Bottom Water and below the northgoing Antarctic Intermediate Water, and eventually helps to replace the water that is moving away from the Antarctic Ocean. It may be seen, therefore, that a huge process of turnover operates within the ocean. The rates involved in this turnover process are not yet known, but long-term studies involving the radioactive carbon-dating of sea water are being carried out both in New Zealand and overseas.
by Norman MacKillop Ridgeway, New Zealand Oceanographic Institute, Wellington.