Joseph L. Reid

On the total geostrophic circulation of the Indian Ocean: flow patterns, tracers, and transports

Abstract The South Atlantic Ocean receives waters from the North Atlantic, the Weddell Sea, and from the Circumpolar Current through the Drake Passage. The circumpolar and North Atlantic waters are of widely different characteristics (temperature, salinity, oxygen and nutrients) but of overlapping density ranges, and as they enter the South Atlantic are caught up in the circulation imposed by the winds and thermohaline processes. Interleaving of these different characteristics takes place in various ranges of depth and density, and more than a dozen vertical extrema are created in the tracer fields. These are spread by lateral flow in recognizable layers. These patterns as seen on vertical sections and on isopycnals can be taken to indicate the sense of flow in various areas and depths, and I have used them, with the density field, to add the barotropic component to the baroclinic flow defined by the density field. This gives the total geostrophic flow at all depths in a manner that appears to be consonant with the tracer patterns, and that satisfies continuity of mass. The resulting flow field has the traditional western boundary current from the Weddell Sea to the equator, beneath a poleward flow north of 50°S. In the upper waters the flow pattern shows a cyclonic gyre in the Weddell Sea, a northward flow from the Antarctic along the western boundary from 55°S to 40°S, an anticyclonic gyre between 15°S and 40°S, and an eastward flow near 10°S. Below the upper layer the axis of the anticyclonic gyre shifts southward, to 35°S at 1500 m, and at greater depths the gyre is confined to the Brazil Basin, west of the Mid-Atlantic Ridge and north of the Rio Grande Rise. Below the crest of the Mid-Atlantic Ridge in the Brazil Basin waters from the south flow northward along the Ridge, and part of them crosses the Ridge at or near the equator and returns southward in the eastern basin, joined by waters from the North Atlantic. The northward abyssal flow in the Brazil Basin is not confined to the western boundary but extends some distance across the basin, diverted only slightly at 3500 m by the small deep remnant of the overlying anticyclonic gyre. From 3000 to 4000 decibars the flow within the Weddell Sea and the Argentine Basin form a single cyclonic gyre and the flow within the Cape Basin is cyclonic below 3000 m.

Abyssal characteristics of the World Ocean waters

Abstract The abyssal characteristics of the World Ocean, including not only temperature but salinity, density, oxygen, and silica, are displayed on both maps and vertical sections to examine the origins of the waters of some of the major basins. Although the coldest waters that appear at the bottom in each of the oceans have long been known to have come from the Southern Ocean, the characteristics indicate that the major component of the abyssal waters of the World Ocean does not derive directly from the abyssal Antarctic but from the shallower Circumpolar Water (CPW). The CPW is a mixture of Antarctic waters with the warm, saline, oxygen-rich, and nutrient-poor deep waters from the North Atlantic. As the CPW extends northward it is modified by mixing with the overlying waters, which in the North Atlantic and North Indian oceans are more saline and in the North Pacific less saline. Except for the Antarctic area, the northern North Atlantic Ocean is the major source of oxygen to the deep-ocean waters. The abyssal waters of the Northeast Pacific are farthest from regions of ventilation and are the most nearly uniform and may be the oldest of the abyssal waters.

On the Characteristics and Circulation of the Southwestern Atlantic Ocean

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 b...

On the contribution of the Mediterranean Sea outflow to the Norwegian-Greenland Sea

Abstract In an earlier paper dealing with the mid-depth (1000 m) circulation of the North Atlantic Ocean, the water from the Mediterranean outflow, seen as a high-salinity subsurface layer, was shown to flow northward along the coast of Europe as well as westward. The distribution of the core of high-salinity water has been examined along an isopycnal surface that passes through the core near the source. The isopycnal varies in depth in the North Atlantic in accordance with the general circulation and outcrops at the sea surface in the Labrador and Norwegian-Greenland seas. Near 60°N it is shallow enough to extend through the Faroe-Shetland Channel. The high salinity of the Mediterranean outflow extends along this isopycnal and contributes substantially to the salinity of the water passing northward into the Norwegian-Greenland Sea. It has been supposed previously that it is the upper waters of the Northeastern Atlantic Ocean that pass through this channel and contribute the high salinity of the Norwegian Current. From examination of the temperature, salinity, and oxygen of the various layers, it appears likely to be the Mediterranean core that contributes the characteristics of the Norwegian Current, but with heat exchange through the sea surface, precipitation, and the low-salinity contribution from the Baltic-North Sea all uncertain, these characteristics alone do not provide a clear answer. Consideration of the silica field, however, provides a more convincing argument that the deeper water in the depth range of the Mediterranean outflow water provides a major component of the water passing northward through the Faroe-Shetland Channel.

Distribution of nitrate, phosphate and silicate in the world oceans

Abstract This study describes the global horizontal distributions of the plant nutrients phosphate, nitrate and silicic acid, with depth, on a one-degree latitude-longitude grid. The source of the data is a subset of the National Oceanographic Data Center. Nutrients in surface waters are enriched in upwelling and high latitude regions and are generally depleted at mid-latitudes. The depletion at mid-latitudes is associated with the subtropical anticyclonic gyre systems. With increasing depth, the nutrient content increases in the water column. At depths of more than 1000m, the nutrient distributions are associated with different water masses which have their own inherent characteristics.

On the influence of the Norwegian-Greenland and Weddell seas upon the bottom waters of the Indian and Pacific oceans*

The bottom waters of the North Pacific and North Indian oceans have temperature and salinity distributions that suggest origins from the extreme waters of the Norwegian-Greenland and Weddell seas. We attempt to trace these waters from their sources to the abyssal Pacific and Indian oceans by examining distributions of temperature and salinity along a stratum defined by density parameters. We assume that the major flow and mixing will take place along such surfaces, though the results make plain that vertical mixing is also important. The density stratum we have chosen to examine extends from the sea surface in the Norwegian-Greenland Sea and from near the surface in the Weddell Sea to depth of about 3500 m in the central oceans and below 4000 m in the North Pacific. The cold and saline water of the Norwegian-Greenland Sea is traced along the density stratum through the Denmark Strait, where vertical mixing raises both temperature and salinity to their maximum values in the central North Atlantic. From there the temperature and salinity decrease monotonically southward toward the Weddell Sea, partly by lateral mixing with the cold, low-salinity waters on this stratum where it lies near the sea surface in the Weddell Sea, and partly by vertical mixing with the underlying Antarctic Bottom Water. From the southern South Atlantic the high values of temperature and salinity (the stratum now lies close to the vertical maximum in salinity) extend eastward with the Antarctic Circumpolar Current into the Indian and Pacific oceans, with monotonically decreasing temperature and salinity as further vertical mixing erodes the maximum in salinity, until the salinity maximum is found at the bottom in the North Pacific Ocean. The stratum we have defined terminates at abyssal depths in the northern Indian and Pacific oceans; since water must rise somewhere to balance the sinking in regions of bottom-water formation, there must be upward flow across the stratum elsewhere. The tremendous areal extent of the salinity maximum, however, suggests that the upward flow through the stratum must be minimal except in the North Indian and North Pacific oceans, where stability is shown to be very low at the depth of the stratum.

The Pacific shallow oxygen maximum, deep chlorophyll maximum, and primary productivity, reconsidered

Abstract In the open mid-latitude Pacific in summer, a subsurface oxygen maximum with saturation ratios reaching 120% or more is found below the maximum of primary productivity and above the deep chlorophyll maximum. This feature appears to arise from the photosynthetic production of oxygen, which is prevented from escaping to the atmosphere by a density cap created by surface warming during the summer. In high and low latitudes, or in areas where most photosynthesis takes place nearer the surface or where the density cap is weaker, this accumulation may not be so effective and the degree of saturation may be less, even though the primary productivity may be greater. The excess oxygen (above saturation) found in the subsurface maximum can be used to estimate minimal rates of primary productivity at those depths. Productivity estimated from oxygen accumulation is much greater than the primary productivity measured by the 14 C method. This suggests that the 14 C method substantially underestimates true productivity not only in these regions but generally.

On the origins of deep and bottom waters of the Indian Ocean

The characteristics of the deep and bottom waters of the Indian Ocean, when illustrated on potential-density anomaly surfaces, indicate that the waters enter from both the Atlantic and Pacific Oceans. The paths of spreading are constrained by the complex topography, and characteristics are seen to be altered by exchange with the overlying and underlying water and with the sediments, especially in the northern Indian Ocean. The Weddell Sea contributes to the densest waters found in the western basins and the Ross Sea and Adelie coast to the densest waters found in the eastern basins. Both dense water varieties are altered by and incorporated in the less dense water above; initially, water carried by the circumpolar current, then water from the north Atlantic, and finally by deep water whose characteristics are derived in the northern Indian Ocean. Contact with the sediments increases the silica content of the bottom water in the Southern Ocean. In the northern Indian Ocean the sediments alter the silica of the water at the bottom and, together with enhanced salinity from diffusion of saline overflows from the marginal seas above, imprint unique markers to the deep water that flows back to the south. At middepths the series of ridges between Madagascar and Australia confine the flow to a series of gyres that carry characteristics from the circumpolar current equatorward and the northern Indian Ocean characteristics southward. Within the circumpolar current, low-oxygen deep water from the Pacific is carried across the Atlantic and into the Indian Ocean south of Africa. Part flows around the cyclonic Weddell Sea Gyre, and part extends across the Southern Ocean. Water from another Pacific source can be seen near 2000 m extending westward from the Tasman Sea, south of Aus- tralia and across the Indian Ocean, and perhaps to the Aghulas Current region southeast of Africa. In this study we show some maps of deep and bottom water characteristics of the Indian Ocean, which may be used in a qualitative sense to infer pathways of spreading of water below a depth of about 2000 m. The Indian Ocean has numerous ridges and basins (Figure 1) that severely restrict possible pathways of deep and bottom wa- ter flow. In the following we will begin by looking at char- acteristics at the bottom of the ocean to reveal the path- ways of spreading of the densest bottom waters from basin to basin. Then a series of six potential-density anomaly surfaces at progressively shallower depths from about 4000 m up to about 2000 m will be shown to see what character- istics influence the deep waters and how pathways of flow are constrained by the bathymetry of the Indian Ocean. The choice of the specific density levels (Table 1) was based primarily on various property extrema shown by the 18oS Atlantis II Cruise 93 colored sections that accompany Warrens (1981) detailed discussion of the major features of the central Indian Ocean between Madagascar and Aus- tralia. The sections, approximately midway between Antarctica and the northern Indian Ocean boundaries, are well positioned to show features originating from both the north and the south, as well as some brought into the Indi- an Ocean by the Antarctic Circumpolar Current. The full- ocean area isopycnal maps provide essential evidence on the origins and fates of features seen in the sections and,

Circulation of Tritium in the Pacific Ocean

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 isopy...

The shallow salinity minima of the Pacific Ocean

Abstract From the high latitudes of the Pacific Ocean, where surface salinity is low, two shallow subsurface salinity minima extend equatorward along the eastern boundary currents and turn westward with the Trade Wind Drift. The density at the source is such that the waters sink beneath the lower-density, more saline waters within the subtropical anticyclonic gyres and the intertropical zones. The shallow minima are relatively minor features within the intertropical zones and lie beneath the high-salinity layers extending from the high-salinity cells of the anticyclones and above the major salinity minima of the Intermediate Water. Vertical mixing eventually eliminates the shallow minima, but only after some thousands of kilometers of flow. Because the high-salinity cell of the North Pacific is at a higher latitude than that in the South Pacific, the northern minimum can extend farther west (to the Philippine Islands) in the intertropical zone; the southern minimum extends (along 15°S) only to about 160°W. Both summer and winter maps of characteristics along an isanostere that lies near the minima have been prepared, and they indicate that in the western Pacific both the North Equatorial Counter-current and the South Equatorial Countercurrent shift northward in winter.