The South Pacific Ocean
Water Masses of the South Pacific Ocean. Above the deep water the water masses of the Pacific Ocean are of more complicated character than those of the other oceans. Subantarctic Water is of small significance in the Atlantic and Indian Oceans, but the South American
Temperature-salinity relations within the Subantarctic Water of the South Pacific. The depths of the shallowest values are indicated. Locations of stations and boundaries of water masses are shown on the inset map. Abbreviations: D, Dana; C, Carnegie, Di, Discovery, W.S., William Scoresby, SA-A.S.P., Subantarctic South Pacific; W.S.P. Western South Pacific; E.S.P., Eastern South Pacific.
Turning first to the South Pacific Ocean, one finds to the south of lat. 40°S the Subantarctic Water mass which, in the upper layers, is characterized by a salinity between 34.20 ‰ and 34.40 ‰ and a temperature between 4° and 8°. This is evident from the T-S curves in fig. 194, according to which the water at Dana station 3642 off New Zealand was of the same character as the water at the Discovery station 967 half way between New Zealand and South America. Part of this
Temperature-salinity relations in the western and eastern South Pacific. The depths of the shallowest values are shown. Squares represent winter surface values in the region of the Subtropical Convergence. Locations of stations used and boundaries of water masses shown on inset map in fig. 194. W.S.P., Western South Pacific; E.S.P., Eastern South Pacific.
In the western part of the South Pacific Ocean a water mass is encountered that is similar to the Central Water of the Indian Ocean. This is evident from fig. 195, in which the corresponding temperature and salinity values at a number of Dana stations are entered, together with those from Discovery station 898 off Tasmania and Planet station 331 in lat. 2°S and long. 152°E. A comparison with fig. 188, p. 685, shows that the Indian Ocean Central Water and the water in the western Pacific are practically identical.
The large squares in the figure show the corresponding values of surface temperatures and salinities in August in lat. 35° to 45°S, long. 150°E to 160°W (according to Schott's charts, 1935). Again a region is found
In the eastern South Pacific the few available data indicate the existence of another water mass which has, between temperatures of 10° and 18°, a salinity nearly 0.50 ‰ lower than that of the western body of water. The definition of this water mass is mainly based on the observations at a number of Carnegie stations, four of which are shown in fig. 195. At Dana station 3580, located in 19°S, 163°W, the water is intermediate in character between the eastern and western masses, and the boundary region between these water masses can therefore be placed approximately along the meridian of 165°W. The squares in fig. 195 (right) show corresponding values of surface temperature and salinity in August in lat. 35° to 40°S and long. 150° to 120° W.
In the South Pacific Ocean we therefore encounter three distinctly different upper water masses: the Subantarctic Water mass, which is more or less uniform to the south of 40°S and which changes its characteristics as it moves north along the coast of South America, and the western and the eastern South Pacific Central Water masses, which are separated from each other by a region of transition in about 165°W. The eastern Central Water mass does not extent north of 10°S but the western may extend nearly to the Equator, as indicated by the observations at Planet station 331. The existence of two characteristically different central water masses indicates that in the South Pacific Ocean the circulation is split up in two large cells, the location of which appears to be related to the prevailing winds. In the southern winter the atmospheric pressure shows two distinct areas of high pressure over the Pacific Ocean, one eastern with its center in approximately lat. 28°S and long. 100°W, and one western which extends partly over the Pacific Ocean with its center over eastern Australia. The surface currents (chart VII), which only in part reflect the circulation in the deeper layers, and in part show the wind drift of surface water, indicate in most seasons the existence of a region of weak and irregular currents in about 160°W which is the approximate region where the transition between the two central water masses takes place.
Below the upper water masses Antarctic Intermediate Water is present, in the east probably to within 10° to 15° from the Equator, and in the west to the Equator. In the South Pacific no salinity maximum is found below the intermediate water, but the salinity increases toward the bottom or remains constant below a depth of 2500 or 3000 m.
Currents of the South Pacific Ocean. The only major current of the South Pacific Ocean which has been examined to some extent is the Peru Current. Following the nomenclature proposed by Gunther (1936), the name Peru Current will be applied to the entire current between the South American Continent and the region of transition
As is evident from the character of the waters (fig. 194), the origin of the Peru Current has to be sought in the subantarctic region, part of the Subantarctic Water which flows towards the west across the Pacific Ocean being deflected towards the north when approaching the American Continent. The total volume of water in the current does not appear to be very great. On the basis of a few Discovery stations it is found that the transport lies somewhere between 10 and 15 million m3/sec, and this figure includes transport of the upper water layers and of Antarctic Intermediate Water. The western limit of the current appears to be diffuse and cannot be well established on the basis of the available data, but it is probable that the current extends to about 900 km from the coast in 35°S. The northern limits, according to Schott (1931), can be placed a little south of the Equator where the flow turns toward the west.
The current being wide and the transport small, the velocities are quite small. This must be true particularly in the case of the Peru Oceanic Current which, however, is little known. The Peru Coastal Current is better known, partly because numerous observations of surface currents and surface temperatures are available off the coasts of Peru and Ecuador between the Equator and 10°S, and partly because of the extensive examination of the coastal waters which was conducted by the R.R.S. William Scoresby in 1931 and which has been discussed in detail by Gunther (1936). Within the Peru Coastal Current upwelling represents a very conspicuous feature. The upwelling is caused by the southerly and south-southeast winds which prevail along the coasts of Chile and Peru and carry the warm and light surface waters away from the coast, resulting in cold water being drawn from moderate depths toward the surface. On the basis of numerous sections close to the coast, Gunther concludes that the upwelling water comes from depths between 40 and 360 m, the average depth being 133 m. The upwelling therefore represents only an overturning of the upper layers and no water from greater depths is ever drawn to the surface.
The process of upwelling will naturally influence the current parallel to the coast because the distribution of density is altered, as explained on p. 500. The William Scoresby observations clearly show that the upwelling is an intermittent process, greatly influenced by local winds, and that reversal of the wind direction frequently leads to subsidence, that is, to re-establishment of the stratification characteristic of the undisturbed conditions.
According to Schott and to Gunther, the most active upwelling occurs in certain regions separated by regions in which the upwelling is less
In early winter, April to June, the shoreward-directed branch of the northern swirl is well developed and carries water of high temperature in toward the coast in latitudes 9° to 12°S. It may even appear as an inshore warm current which brings great destruction to the animal life of the coastal waters. In the discussion of the California Current it will be shown that during the period of upwelling this current is similarly characterized by a series of swirls on the coastal side of the current, whereas from November to February, when there is practically no upwelling, a warm countercurrent flows to the north along the coast.
Close to the coasts of Peru and Chile a subsurface countercurrent flows south, as is evident from the section prepared by Gunther, showing the distribution of salinity between the surface and a depth of 400 m at a distance of approximately 180 km from the coast and between lat. 3° and 36°S. The subsurface current appears to originate at the equatorial end of the section, where it is present at a depth of less than 100 m, but sinks gradually when progressing to the south, being found at nearly 300 m in lat. 36°S. The water of this countercurrent is of the type which has been called Pacific Equatorial Water (p. 706), as is evident from the T-S curves in fig. 194, in which the data from two stations within the countercurrent, stations WS 638 and WS 612, have been plotted. To the north of lat. 25°S, where excessive evaporation increases the salinity, the salinity of the subsurface countercurrent is lower than that of the surface water, whereas to the south of about 25°S the salinity of the countercurrent is higher than that of the surface waters. Consequently, the upwelling brings relatively low-salinity water to the surface off the coast of Peru, but relatively high-salinity water to the surface off
At the northern boundary of the Peru Coastal Current certain characteristic seasonal changes take place. During the northern summer the Peru Coastal Current extends just beyond the Equator where it converges with the Equatorial Countercurrent, the waters of which in summer mainly turn towards the north. In winter this countercurrent is displaced further to the south and part of the warm but low-salinity water of the Countercurrent turns south along the coast of Ecuador, crossing the Equator before converging with the Peru Coastal Current. The warm south-flowing current along the coast is known as El Niño and is a regular phenomenon in February and March, but the southern limit mostly lies only a few degrees to the south of the Equator. Occasionally, major disturbances occur which appear to be related to changes in the atmospheric circulation (Schott, 1931). In disturbed years, such as in 1891 and in 1925, El Niño extends far south along the coast of Peru, reaching occasionally past Callao in 12°S. According to Schott, the duration of El Niño periods in 1925 was as follows:
|Off Lobitos||4°20′S||Jan. 20 to April 6||76|
|Off Puerto Chicana||7°40′S||Jan. 30 to April 2||63|
|Off Callao||12°20′S||March 12 to March 27||15|
|Off Pisco||13°40′S||March 16 to March 24||8|
These figures show that the warm surface waters of the equatorial area slowly penetrated to the south, but withdrew much more rapidly, because the time interval between the appearance of the warm water off Lobitos and off Pisco was 44 days, whereas the time interval between the disappearance of the warm water at the two localities was only 13 days. The surface temperature of the water in March, 1925, was nearly 7° above the average, as is evident from the following compilation:
|Locality||Average temperature in March (°C)||Temperature in March, 1925 (°C)|
Details as to the surface salinity are not available, but normally the surface salinity between 5°S and 15°S is above 35.00 %0, whereas the waters of El Niño have a salinity between 33.00 and 34.00 %0.
The extreme development of El Niño leads to disastrous catastrophes of both oceanographic and meteorological character. The decrease of the temperature of El Niño toward the south indicates that the waters are mixed with the ordinary cold coastal waters, and during this mixing process the organisms in the coastal current, from plankton to fish, are destroyed on a wholesale scale. Dead fish later cover the beaches, where they decompose and befoul both the air and the coastal waters. So much hydrogen sulphide may be released that the paint of ships is blackened, a phenomenon known as the “Callao painter.” More serious is the loss of food to the guano birds, many of which die of disease or starvation or leave their nests, so that the young perish, bringing enormous losses to the guano industry. The meteorological phenomena which accompany El Niño are no less severe. Concurrent with a shift in the currents a shift of the tropical rain belt to the south takes place. In March, 1925, the precipitation at Trujillo in 8°S amounted to 395 mm, as compared to an average precipitation in March of the eight preceding years of only 4.4 mm. In 1941 the rainfall was again excessive, but no details are available. These terrific downpours naturally cause damaging floods and erosion. In the 140 years from 1791 to 1931 twelve years were characterized by excessive rainfall at Piura in lat. 5°S and twenty-one years by moderate rainfall which was, however, greatly in excess of the average. During the remaining nearly one hundred years the rainfall was close to nil. A greater development of El Niño is therefore not an uncommon phenomenon, but the catastrophic developments appear to occur on an average of once in twelve years. The records reveal no periodicity because the interval between two disastrous years varies from one year to thirty-four years.
El Niño is not the only current that brings warm water to the coast of Peru with subsequent destruction of the organisms near the coast. High temperatures off the coast appear to be an annual occurrence in the months of April to June, in about lat. 9° to 12°S, that is, at and to the north of Callao. These high temperatures are due, as pointed out by Gunther, to the greater development of the warm branch of the northern swirl; the water that approaches the coast is in this case offshore oceanic surface water of high temperature and relatively high salinity (p. 703). The disastrous effect on the marine organisms is very much milder than that of El Niño, but otherwise similar in character. It may lead to the killing of plankton and fish and to the migration of guano birds, but is ordinarily observed mainly by reason of a change in the color of the coastal water and development of hydrogen sulphide. Locally, these changes are known by the name of aguaje, also used synonymously with
On leaving the coast the waters of the Peru Current join the waters of the South Equatorial Current, which flows all the way across the Pacific towards the west but is known only as far as surface conditions are concerned. The subsurface data are inadequate for computation of velocities and transports, and it is therefore not possible to give any numerical values. Some features of this current will, however, be dealt with when discussing the currents of the equatorial regions of the Pacific, and it will be shown that the cold water along the Equator does not represent a continuation of the Peru Coastal Current but is due to a divergence along the Equator within the South Equatorial Current.
The other currents of the South Pacific Ocean are even less known, but from the character of the water masses it appears that two current systems exist, the nature of which may be revealed by future exploration. One big gyral appears to be present in the eastern South Pacific; in the western South Pacific annual variations are so great that in many regions the direction of flow becomes reversed, as is the case off the east coast of Australia. No chart of the transport by the current can be prepared.