Mizuki Tsuchiya

Source waters of the Pacific Equatorial Undercurrent

Abstract Hydrographic and direct current measurements were made north and east Papua New Guinea in June–August 1985 and January–February 1986 as part of the Western Equatorial Pacific Ocean Circulation Study (WEPOCS). Analyses of the data indicate that the major portion of the water in the Equatorial Undercurrent at its beginning north of Papua New Guinea is supplied from the south by a narrow western boundary undercurrent (New Guinea Coastal Undercurrent) transporting high-salinity, low-tritium, high-oxygen, low-nutrient water from the Solomon Sea northwestward along the north coast of Papua New Guinea through the Vitiaz Strait. The New Guinea Coastal Undercurrent has a maximum speed of 40–70 cm s −1 at a depth of about 200 m. It is a permanent feature despite the reversals of the wind and the surface current during the period of the northwest monsoon in austral summer. Its transport through the Vitiaz Strait is as high as 8 × 10 6 m 3 s −1 , which is of the same magnitude as the Equatorial Undercurrent transport at 143°E. The New Guinea Coastal Undercurrent revealed by the WEPOCS data is consistent with the low-latitude equatorward western boundary current implied in a calculation of the Sverdrup transport based on the observed wind-stress distribution for the tropical Pacific Ocean. High-salinity, low-tritium, low -oxygen, high -nutrient water which flows westward into the Bismarck Sea passing north of the Solomon Islands is entrained into the Equatorial Undercurrent north of New Ireland and returns to the east, resulting in a down-stream increae in the Undercurrent transport. Low-salinity, high-tritium, high-nutrient water of eastern North Pacific origin also contributes to the Equatorial Undercurrent in its source area west of the WEPOCS region. However, there is no evidence that northern waters are being continuously entrained into the Undercurrent in the WEPOCS region.

An eastern Atlantic section from Iceland southward across the equator

A long CTD/hydrographic section with closely-spaced stations was occupied In July- August 1988 from Iceland southward to 3°S along a nominal longitude of 20°W The section extends from the surface down to the bottom, and spans the entire mid-ocean circulation regime of the North Atlantic from the subpolar gyre through the subtropical gyre and the equatorial currents Vertical sections of potential temperature, sahnlty and potential density from CTD measurements and of oxygen, silica, phosphate and nitrate, based on d~screte water-sample measurements are presented and discussed in the context of the large-scale circulation of the North Atlantic Ocean The close spacing of hlgh-quahty stations reveals some leatures not described previously The more important findings include ( 1 ) possible reclrculation of the hghtest Subpolar Mode Water into the tropics, (2) a thermostad at temperatures of 8-9°C, lying below that of the Equatorial 13°C Water, (3) the nutrient distribution in the low-sallmty water above the Mediterranean Outflow Water that supports the previous conjecture of northern influence ot the Antarctic Intermedmte Water. 14) a great deal of lateral structure of the Mediterranean Outflow Water, with a number of lobes of high sahnlt), (5) an abrupt southern boundary ot the Labrador Sea Water at the Azores-Biscay Rise and a vertically well-mixed region to its south, (6) a sharp demarcation in the central Iceland Basra between the newest Iceland-Scotland Overflow Water and older bottom water, which has a significant component of southern water, (7) evidence that the Northeast Atlantic Deep Water is a mixture ot the Mediterranean Outflow Water and the Northwest Atlantic Bottom Water with very httle input from the Iceland-Scotland Overflow Water, (8) an usolated core of the high-salinity, low-silica Upper North Atlantic Deep Water at the equator, (9) a core of the high-oxygen, low- nutrient Lower North Atlannc Deep Water pressed against the southern flank of the Mld-Atlanuc Ridge just south of the equator, (10) a weak minimum of salinity, and well-defined maxima of nutrients associated with the oxygen minimum that separates the Middle and Lower North Atlantic Deep Waters south ot the equator, (11) a large body ol nearly homogeneous water beneath the Middle North Atlantic Deep Water between 2(1°N and the Azores-Biscay Rise, and (i 2) a deep westward boundary undercurrent on the southern slope of the Rockall Plateau

Water-mass distributions in the western South Atlantic; A section from South Georgia Island (54s) northward across the equator

A long CTD/hydrographic section with closely spaced stations was made in February-April 1989 in the western Atlantic Ocean between 0”40’N and South Georgia (545) along a nominal longitude of 25W. Vertical sections of various properties from CTD and discrete water-sample measurements are presented and discussed in terms of the large-scale circulation of the South Atlantic Ocean. One of the most important results is the identification of various deep-reaching fronts in relation to the large-scale circulation and the distribution of mode waters. Five major fronts are clearly defined in the thermal and salinity fields. These are the Polar (49SS), Subantarctic (45S), Subtropical (41-425), Brazil Current (355) Fronts, and an additional front at 20-22s. The first three are associated with strong baroclinic shear. The Brazil Current Front is a boundary between the denser and lighter types of the Subantarctic Mode Water (SAMW), and the 20-223 front marks the boundary between the anticyclonic subtropical and cyclonic subequatorial gyres. The latter front coincides with the northern terminus of the high-oxygen tongue of the Antarctic Intermediate Water (AAIW) and also with the abrupt shift in density of the high-silica tongue originating in the Upper Circumpolar Water and extending northward. Two pycnostads with temperatures 20-24°C are observed between 10s and 2% with the denser one in the subtropical and the other lighter one in the subequatorial gyre. A weak thermostad centered at 4°C occurs in the AAIW between the Subtropical Front and the Subantarctic Front and shows characteristics similar to the densest variety of the SAMW. Another significant result is a detailed description of the complex structure of the deep and bottom waters. The North Atlantic Deep Water (NADW) north of 25s contains two vertical maxima of oxygen (at 2000 m and 3700 m near the equator) separated by intervening low-oxygen water with more influence from the Circumpolar Water. Each maximum is associated with a maximum of salinity and minima of nutrients. The deeper salinity maximum is only weakly defined and is limited to north of 18S, appearing more as vertically uniform salinity. South of 25s the NADW shows only a single maximum of salinity, a single maximum of oxygen, and a single minimum of each nutrient, all lying close together. The salinity maximum south of 25s and the deeper oxygen/salinity maximum north of 11s are derived from the same source waters. The less dense NADW containing the shallower extrema of characteristics turns to the east at lower latitudes and does not reach the region south of 25s. The

A Pacific hydrographic section at 88°W: Water‐property distribution

Full-depth conductivity-temperature-depth (CTD)/hydrographic measurements with high horizontal and vertical resolution were made in February-April 1993 along a line lying at a nominal longitude of 88°W and extending from southern Chile (54°S) to Guatemala (14°N). It crossed five major deep basins (Southeast Pacific, Chile, Peru, Panama, and Guatemala basins) east of the East Pacific Rise. Vertical sections of potential temperature, salinity, potential density, oxygen, silica, phosphate, nitrate, and nitrite are presented to illustrate the structure of the entire water column. Some features of interest found in the sections are described, and an attempt is made to interpret them in terms of the isopycnal property distributions associated with the large-scale ocean circulation. These features include: various near-surface waters observed in the tropical and subtropical regions and the fronts that mark the boundaries of these waters; the possible importance of salt fingering to the downward salt transfer from the high-salinity subtropical water; a shallow thermostad (pycnostad) developed at 16°–18.5°C in the subtropical water; low-salinity surface water in the subantarctic zone west of southern Chile; large domains of extremely low oxygen in the subpycnocline layer on both sides of the equator and a secondary nitrite maximum associated with a nitrate minimum in these low-oxygen domains; high-salinity, low-oxygen, high-nutrient subpycnocline water that is carried poleward along the eastern boundary by the Peru-Chile Undercurrent; the Subantarctic Mode and Antarctic Intermediate waters; middepth isopycnal property extrema observed at the crest of the Sala y Gomez Ridge; influences of the North Pacific and the North Atlantic upon deep waters along the section; and the characteristics and sources of the bottom waters in the five deep basins along the section.

Water-property distributions along an eastern Pacific hydrographic section at 135W

As part of the World Ocean Circulation Experiment, full-depth CTD/hydrographic measurements with high horizontal and vertical resolutions were made in June-August 1991 along a line extending from 34N to 33S at a nominal longitude of 135W with an additional short leg that connects it to the California coast roughly along 34N. The line spans the major part of the subtropical and intertropical circulation regime of the eastern North and South Pacific. The primary purpose of this paper is to present vertical sections of various properties from CTD and discrete water-sample measurements along this line and to give an overview of some important features as a basis for more comprehensive basin-scale studies. These features include: the frontal structures found in the surface-layer salinity field in the North Pacific; relatively high-salinity water that dominates the subpycnocline layer between the equator and 17N; troughs of the subpycnocline isopycnals for 26.8-27.5 σ θ found at 12N and 12.5S; a permanent thermostad at 9-10°C observed between 4.5N and 15N; the pycnostad of the Subantarctic Mode Water centered at 27.0-27.05 σ θ and developed south of 22S; two types of the Antarctic Intermediate Water representing the subtropical and equatorial circulation regimes; a thick tongue of high silica centered at 3000 m (45.8 σ 4 ) and extending southward across the entire section; deep (2000-3000 m) westward flows at 5-8N and 10-15S separated by an eastward flow at 1-2S; and dense, cold, oxygen-rich, nutrient-poor bottom waters, which are associated with fracture zones and believed to represent the pathways of eastward flows into the Northeast Pacific Basin of the bottom waters separated from the northward-flowing western boundary undercurrent. This work once again demonstrates the usefulness of long lines of high-quality, high-resolution hydrographic stations such as the one described herein in advancing the understanding of the large-scale ocean circulation.

Inshore circulation in the Southern California Bight, 1974–1977

Abstract From September 1974 through April 1977 nine cruises were made between Dana Point and Del Mar in the Southern California Bight to study the circulation in the inshore region within about 40 km of the coast. Observations were usually along three lines, 37 km apart, nearly normal to the coast. On each line STD (salinity-temperature-depth) stations were occupied at intervals of 2 km over the sharp bottom slope in the inshore area and at intervals of 7 km in the deep water farther offshore. The geostrophic flow at the sea surface relative to 500 dbar was found to be toward the south-southeast along the coast in March 1975, March and June 1976, and April 1977 and toward the north-northwest in September 1974, September and December 1975, and October 1976. It was weak and showed hardly any significant pattern near the end of January 1977. This variation appears to represent the seasonal variation of the California Current, which extends close to the coast in March and April and stays offshore during the rest of the year. At depths of 200 to 300 m, near the core of the California Undercurrent, the geostrophic flow relative to 500 dbar was predominantly north-northwestward throughout the study period. Although there was a significant variation in its speed, it was not possible to determine its time scale. In October 1976 and January 1977, temperature in the upper 300 m was higher (by 2 to 3°C at 0 to 100 m) than that normally expected during these months. At the same time, temperature-salinity curves shifted toward high salinity in the temperature range 9 to 12 or 13°C (depth 100 to 300 m) in October 1976, and the shift (maximum exceeded 0.3%) extended from 9°C (300 m) to the surface temperature in January 1977. This anomalous distribution is believed to be a coastal manifestation of a large-scale warming of the surface water in the eastern North Pacific that persisted from September 1976 through February 1977.

Flow path of the Antarctic Intermediate Water in the western equatorial South Pacific Ocean

Abstract The flow path of the Antarctic Intermediate Water north and east of Papua New Guinea is investigated with use of hydrographic data collected in 1985–1986. The most original form of the Antarctic Intermediate Water in the study region is transported from the Solomon Sea to the equator by a narrow western boundary undercurrent (New Guinea Coastal Undercurrent) flowing northwestward through the Vitiaz Strait and along the north coast of Papua New Guinea. On the basis of this finding and previous studies by REID (1965, Johns Hopkins Oceanographic Studies, 2) and Reid and Mantyla (1978, Journal of Physical Oceanography, 8, 946-951), which indicated that the Antarctic Intermediate Water crosses the equator in the far western Pacific and the major part continues northward along the western boundary of the North Pacific, it is concluded that the New Guinea Coastal Undercurrent plays an important role in transporting the Antarctic Intermediate Water from the South Pacific to the North Pacific.

Thermostads and circulation in the upper layer of the Atlantic Ocean

Abstract There are three major permanent thermostads with roughly the same potential densities in the upper layer of the Atlantic Ocean. One is the thermostad of the 13°C Water in the equatorial Atlantic. The original type of the 13°C Water is formed in the thermocline in the eastern sector of the South Atlantic subtropical gyre by vertical mixing of dense, low-salinity water from the winter outcrop farther south and overlying less dense, high-salinity water. There might also be a lateral contribution of relatively high-salinity water from the Indian Ocean. The original 13°C Water thus formed is transported northwestward along the northern edge of the subtropical gyre and fed into the North Brazilian Current, which flows equatorward along the coast of Brazil. In the region of the equator, the Equatorial Undercurrent and the subsurface North and South Equatorial countercurrents branch off from the North Brazilian Current and carry the 13°C Water eastward to the thermostad region. Vertical mixing does not explain the development of the thermostad, but is found to be essential in determining the ultimate characteristics of the 13°C Water. The other two thermostads are those of the 18°C Water in the Sargasso Sea and the Subantarctic Mode Water in the western South Atlantic. Unlike the 13°C Water, both of these mode waters are formed as thermostads in the surface layer by winter convection, but vertical mixing in the subtropical gyres may play a role in determining their characteristics. All the three thermostads appear to be required to balance the system of flows in opposing directions.

The subthermocline phosphate distribution and circulation in the far eastern equatorial Pacific Ocean

Abstract Measurements of nutrient salts were made in two periods, November to December 1981 and February to March 1982, along nearly the same tracks in the far eastern Pacific between the Galapagos Islands and the coast of South America. The subthermocline circulation was deduced principally from the distribution of phosphate. The major flow patterns were found to be basically the same for the two periods. Along the 85°W meridian, the major observed zonal currents were the Equatorial Undercurrent within 2° of the equator, the westward return flows on both sides of the Undercurrent, the subsurface South Equatorial Countercurrent centered at about 6°S, and additional eastward flows at about 4° and 8°S. East of 85°W, the subsurface South Equatorial Countercurrent veered southeastward and crossed the 10.5°S parallel in the vicinity of 81°W. Significant decreases in the nutrient concentrations occurred in the domain of the Equatorial Undercurrent from November to December 1981 to February to March 1982. These decreases can be explained by an increase in the eastward speed of the Undercurrent during February to March 1982.