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Joseph L. Reid and Arnold W. Mantyla

Abstract

The large-scale oxygen distribution within the upper 1500 m of the North Pacific Ocean reveals an extra zone of low oxygen near 30–40°N in the east that is not easily compatible with a simple large-scale subtropical anticyclonic flow at mid-depth. Further examination of the relative flow patterns suggests that the large subtropical gyre generally supposed to obtain at the sea surface has a very strong return flow southward, just cast of the Kuroshio, and that this flow turns eastward near 20–25°N and extends eastward at least as far as 16°E. At greater depths, near 1000 m, it continues eastward all across the Pacific. The area of high steric height within the anticyclonic gyre at this depth is thus shaped like the letter C, with two branches extending eastward from the western boundary. Each branch has an eastward flow on its north side and a westward flow on its south side. The highest oxygen values at mid-depth are found near the western boundary, deriving from the South Pacific, and the two eastward flows carry the higher oxygen waters eastward as two tongues of higher oxygen values, leaving an area of lower oxygen near 30–40°N in the cast.

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Joseph L. Reid and Peter F. Lonsdale

Abstract

The Samoan Passage at about 10S, 169W appears to be the major channel through which the deep and abyssal waters flow northward from the South Pacific. The northward flow, Postulated from the distribution of characteristics, is confirmed by direct measurements of the currents. The density field and the water characteristics are consonant with an intensified deep western boundary current, whose quasi-geostrophic balance requires the densest water to lie shallowest on the western side of the Samoan Basin, and from which it appears to cascade suddenly into the deeper waters of the North Tokelau Basin. The density field and the water characteristics are also consonant with a southward flowing western boundary current lying immediately above the abyssal flow. It is proposed that this shallower flow, at depths somewhere between about 2000 and 3500 m, represents a return flow of water from the deep North Pacific, with high nutrient and low oxygen content.

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Rana A. Fine, Joseph L. Reid, and H. Göte Östlund

Abstract

The input of bomb tritium into the high-latitude Northern Hemisphere waters has demonstrated the spread of a tracer in three dimensions in the North Pacific Ocean. Subsurface tritium maxima in middle and low latitudes clearly show the importance of lateral mixing (along isopycnals) in the upper waters. The tritium pattern as mapped on isopycnal surfaces puts definite time bounds on the exchange between the subtropical anticyclonic gyre of the North Pacific and both the subarctic cyclonic gyre and the system of zonal flows in the equatorial region. The penetration of bomb tritium to depths below 1000 m in the western North Pacific Ocean shows that these waters have been ventilated at least partially in the past 17 years of the post-bomb era. From the tritium pattern the upper waters of the North Pacific can be divided into three regions: a mixed layer that exchanges rapidly with the atmosphere, a laterally ventilated intermediate region (between the mixed layer and at most the winter-outcrop isopycnal) that exchanges on decadal time scales with the atmosphere, and a deeper layer penetrated by vertical diffusion alone, with a longer atmospheric exchange time scale. The greatest percentage of the tritium inventory of the North Pacific is in the intermediate region. This indicates that such lateral ventilations, which take place from all high-latitude regions, are a major source of penetration for atmospheric constituents into the oceans on decadal time scales.

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Joseph L. Reid, Worth D. Nowlin Jr., and William C. Patzert

Abstract

The waters found within the southwestern Atlantic Ocean extend into it as separate lavers with markedly different characteristics. Along the western boundary the deeper waters, derived from the North Atlantic, are warm, highly saline, oxygen-rich and nutrient-poor. This North Atlantic Deep Water (NADW) lies within the density range of the Circumpolar Water (CPW) from the south, which is cooler, lower in salinity, very low in oxygen and very high in nutrients. Where the NADW and CPW meet in the southwestern Atlantic, the NADW separates the CPW into two layers above and below the NADW—each less saline, richer in nutrients and lower in oxygen than the NADW.

Above the upper branch of the CPW lies the Subantarctic Intermediate Water, which is lowest in salinity of all the layers. Beneath the lower branch of the CPW lies an abyssal layer derived from the mid-depths of the Weddell Sea. It is colder, less saline, lower in nutrients and higher in oxygen than the Circumpolar Water.

These layers appear to be separated vertically by density gradients which tend to be sharper at the interface than in the layers themselves. These maxima in stability, which result from the interleaving of water masses from different sources, extend over hundreds of kilometers: apparently vertical exchange processes are not strong enough to dissipate them.

Within the Argentine Basin the circulation of all except the abyssal layer appears to be anticyclonic and so tightly compressed against the western boundary that equatorward flow is observed just offshore of the poleward flow at the boundary. Waters from the north (within the Brazil current near the surface and from the North Atlantic at greater depths) flow southward along the western boundary and turn eastward near 40°S, part returning around the anticyclonic gyre and part joining the Antarctic Circumpolar Current. Likewise the Circumpolar Waters, which have entered from the Pacific, flow northward along the western boundary to about 40°S and then turn eastward, both above and below the NADW. The abyssal waters are derived from the Weddell Sea. Within the Argentine Basin they flow northward along the western boundary and turn eastward south of the Rio Grande Rise, and then southward on the western flank of the Mid-Atlantic Ridge; the abyssal flow is cyclonic beneath the anticyclonic upper circulation.

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