Bruce A. Warren

A hydrographic section across the subtropical South Indian Ocean

Features of the water-property and circulation fields at the southern limit of the continentally bounded Indian Ocean are described on the basis of a transoceanic hydrographic section occupied along roughly Lat. 32°S by the R.R.S. Charles Darwin in November-December 1987. Primary observations consisted of 106 full-depth CTD/O2 stations with discrete measurements of the concentrations of dissolved silica, phosphate and nitrate. The section lies in the southern part of the South Indian subtropical gyre; water-property features in the upper kilometer indicate that the northward interior flow is predominantly in the eastern half of the ocean there, consistent with the forcing pattern of wind-stress curl. The southward return flow is the Agulhas Current, whose transport at Lats 31–32°S is estimated as 85 × 106 m3 s−1. Circumpolar Deep Water flows northward to fill the greater deep Indian Ocean by means of western-boundary currents in the Crozet Basin, Central Indian Basin and Perth Basin. North Atlantic Deep Water entering directly from the mid-latitude South Atlantic is almost entirely confined to the south-western Indian Ocean (Mozambique Basin, Natal Valley) by the topography of the Madagascar Ridge and Mozambique Channel. Geostrophic transport figures are presented based on a zero-velocity surface constructed along the section from the tracer-property evidence of where deep water was moving northward and where southward. Ekman transport, deduced from shipboard acoustic-Doppler profiler measurements, as well as synoptic and historical wind stress data, is found to be small (about 1 × 106 m3 s−1 northward). Net transport (geostrophic and Ekman) across the section is estimated to be 7 × 106 m3 s−1 southward, which implies a similarly sized Indonesia throughflow. Ambiquity in the geostrophic referencing scheme, and the magnitude of baroclinic eddy noise on the section, suggest this figure in uncertain by at least ±10 × 106mm3 s−1. The calculations obtain a figure for net transport of water below 2000 dbars of 27 × 106 m3 s−1 northward, which specifies an average upwelling speed at the 2-km level north of 30°S of 6.9 × 10−5 cm s−1. This estimate, perhaps uncertain by 20–30%, nonetheless contributes to growing evidence for an anomalously vigorous meridional circulation in the Indian Ocean. The associated calculations of heat and fresh water flux divergences demonstrate that the Indian Ocean thermohaline circulation essentially expresses a conversion of bottom and deep water to mid-depth thermocline, and near-surface water.

Maintenance of the low-oxygen layer in the central Arabian Sea

Abstract An intermediate depth layer, approximately 1 km thick, in the northwestern Indian Ocean contains essentially no detectable dissolved oxygen. Previous suggestions for primary causes of this feature have been: (a) very slow movement within the layer, allowing a long time for organic decomposition to consume the oxygen; (b) very large local consumption rates, resulting from enormous productivity in the surface layer; or (c) low oxygen concentrations in the waters entering the layer from the south, due to their long transit from their sea-surface sources. Observations reported here of a transient anthropogenic trace gas, trichlorofluoromethane (F-11 or freon 11), however, demonstrate that the residence time for water in the low-oxygen layer is not espciallt long, about 10 years. Concurrent summertime measurements of surface productivity, while high, preclude an exceptional mean consumption rate at depth. An oxygen budget for the layer supports the idea that the near-zero concentration is maintained by moderate consumption applied to waters with initially low oxygen concentration that pass through the layer at moderate speed.

Transindian hydrographic section at Lat. 18°S: Property distributions and circulation in the South Indian Ocean

Abstract Water-property distributions and circulation in the South Indian Ocean are described and discussed with reference to color profiles of temperature, salinity, potential density (σθ,), and the concentrations of dissolved oxygen, silica, and phosphate as observed on a section of 59 closely spaced, deep hydrographic stations along Lat. 18°S, Madagascar to Australia, during July to August 1976. In the upper water, the origin and disposition of the salinity-maximum, oxygen-maximum, and salinity-minimum layers combine to demonstrate a general northward component of flow in the interior of the South Indian Ocean, penetrating to the depth of the intermediate oxygen minimum at about 1000 m. The northward volume transport is estimated geostrophically as about 20 × 106 m3 s−1, roughly consistent with an estimate of Sverdrup transport, based on the July mean wind-stress curl for the zone 16 to 20°S, of 16 × 106 m3 s−1. Southward flow along the east coast of Madagascar appears large enough to balance all the northward interior flow. The deep Indian Ocean is filled entirely from the Antarctic, but the Central Indian Ridge and Ninetyeast Ridge divide the flow into three separate circulation systems, each with its own western-boundary-current field, in the West Australian Basin, Central Indian Basin, and, at Lat. 18°S, Mascarene Basin. In the boundary-current systems of the Mascarene and West Australian basins, at least, the primary northward flow is accompanied by weak southward flow well off the bottom and well east of the boundary. Because the Central Indian Basin is closed off to the south at moderately deep levels, only the water at depths of 2000 to 3500 m in the boundary current there flows directly northward from the Antarctic; the lower deep water of that basin derives from the boundary current in the West Australian Basin by overflow across deep low-latitude saddles on the Ninetyeast Ridge. The net northward volume transport below 2000 m in each western-boundary-current system is estimated geostropically, on the basis of zero-velocity surfaces inferred from the tracer-property distributions, as: Mascarene Basin, 5 × 106 m3 s−1; West Australian Basin, 6 × 106 m3 s−1; and Central Indian Basin, 8 × 106 m3 s−1 (the latter figure including the very deep flow derived from the Ninetyeast Ridge overflow).

Deep circulation in the eastern South Atlantic Ocean

Abstract Tracer properties on sections of closely spaced hydrographic stations across the Angola Basin of the South Atlantic Ocean along Lats 11° and 24°S suggest a three-layer description of the deep circulation there. Below 4 km the basin is closed off in the south, so water enters only from the north; the interior flow is southward, and the western-boundary current (above the eastern flank of the Mid-Atlantic Ridge) is southward at 11°S but northward at 24°S, as required by the Stommel-Arons dynamics. At depths roughly between 2400 and 4000 m the basin seems to be supplied only from the south, the western-boundary current is everywhere northward, and the interior flow is southward. Near the 2-km level the Mid-Atlantic Ridge is too deep to be an effective western boundary; the flow seems to he broadly southeastward across the full basin at l1°S, but at 24°S a topographically guided current flows northward above the ridge to supply southward interior flow. A hydrographic section along the Greenwich meridian illustrates an eastward jet at depths between 1300 and 3200 m that extends near Lat. 22°S from the western-boundary curent to a gap in the Walvis Ridge, through which the jet introduces water to the Cape Basin. Geostrophic estimates of the volume transports of these circulation clements, calculated with reference to zero-velocity surfaces construed from the tracer fields, are consistent in direction with the inferred flow patterns, but the values may be somewhat erroneously high, as they imply dubiously large upward velocities. A tongue of oxygen-poor, nutrient-rich water is found at depths between 3000 and 4500 m at 11°S but not at 24°S. It is strongest at the African continental rise, and extends some 1000 km westward. Its origin is attributed to decay within the ~ediment of detritus originating mainly from the Congo River plume, and its form to the deep horizontal flow field.

Current System South and East of the Grand Banks of Newfoundland

Abstract During April–June 1972 three ships conducted a survey of the region between the Grand Banks and the Mid-Atlantic Ridge, including a grid of hydrographic stations, and two long lines of near-bottom current-meter moorings across the Gulf Stream and North Atlantic Current, respectively. The purpose was to map the property distributions and current field where the Gulf Stream branches, in greater detail and with less ambiguity than hitherto; that material is described here. Worthingtons hypothesis that the primary current system there is not a branching Gulf Stream but portions of two separate (and nongeostrophic) gyres is criticized at length in terms of the observed property distributions; it is shown that, given a moderate degree of lateral mixing, they are consistent with the branching, geostrophic flow field, and that there is no need to abandon established physics in order to rationalize them. The deep motions recorded by the current meters on the North Atlantic Current line were roughly sugge...

On the deep western-boundary current in the Southwest Pacific Basin

Abstract The principal system of deep western-boundary currents in the subtropical South Pacific is that in the Southwest Pacific Basin, which transports Circumpolar Deep Water northward and Pacific Deep Water (at mid-depths) southward. The WOCE PCM9 current-meter array was placed across this system at Lat. 32° 30′S in order to measure the mean transports of those components and their variations. The array, consisting of 60 current meters on 20 moorings, extended 1000 km eastward from the Tonga–Kermadec Ridge, and remained in place for 22 months, between February 1991 and December 1992. The instruments were situated approximately at 2500 m, 4000 m, and close to the bottom. CTD sections (including dissolved oxygen and nutrients) were occupied along the array during its deployment and recovery, and, in between, by the WOCE transpacific section P6. Density sections were used to construct objectively-mapped fields of geostrophic velocity, which were adjusted using current-meter data as integration constants to provide snapshots of the full velocity field. The resulting adjusted transports sometimes differed substantially from relative geostrophic transports, but agreed quite well with transports calculated from current records alone. A time series of volume transport was derived from objectively-mapped three-day-averaged currents. The boundary-current system at PCM9 was essentially 700 km wide, with flow most intense on the flank of the Tonga–Kermadec Ridge, where the maximum mean velocity vector had a magnitude of 9.6 cm s −1 . The time-averaged transport, integrated horizontally across the array and from 2000 m to the bottom, was 16.0×10 6 ±11.9×10 6 m 3 s −1 northward. Of this roughly 15.8×10 6 ±9.2×10 6 m 3 s −1 was northward flow of Circumpolar Deep Water, and 0.2×10 6 ±5.1×10 6 m 3 s −1 was northward flow of Pacific Deep Water. Even for the 22-month mean, however, the velocity field was strikingly banded vertically, and there was little impression of a zero-velocity surface following the demarcation between Circumpolar Deep Water and Pacific Deep Water across the section; only in the horizontally integrated sense was there a correspondence between water masses and the variation of transport with depth. The very large variability in transport is associated with prominent oscillations of periods near 50 days, 20 days, and 10 days, as well as with strong events distributed irregularly across the array that lead to a concentration of spectral energy in a band between 40 and 200 days. The origins of these disturbances are not known. While unexpectedly large changes in the density field near the Tonga–Kermadec Ridge were observed from one cruise to another, the huge fluctuations in transport seemed to be connected more with velocity signals varying only slowly with depth. No measurable changes in water-mass properties were detected by the cruises during the 22 months of deployment, but the salinity was about 0.01 lower at the salinity maximum in the Circumpolar Deep Water than it had been 25 years earlier. The direct, long-term transport measurement suggests that the total upwelling at 2000 m north of 30° S is 13×10 6 m 3 s −1 , corresponding to an areally-averaged vertical velocity of 1.0×10 −5 cm s −1 . This is substantially smaller than earlier values, and it helps to reduce estimates of the global deep upwelling closer to those of the global deep downwelling. The small value of Pacific Deep Water transport in the boundary-current system, relative to that of Circumpolar Deep Water, implies, within the framework of the Stommel–Arons dynamics, that little of the deep water entering the Pacific from the Antarctic returns southward at mid-depths. If so, then some present-day circulation schemes and budgetary constructions need to be re-assessed.

Water masses and patterns of flow in the Somali Basin during the southwest monsoon of 1964

Abstract Closely spaced hydrographic sections made during August–September 1964 in latitudes 3°S–12°N, and between the East African coast and longitude 56°E, define in detail a complex structure of water masses in the Somali Basin under the southwest monsson. Reference to observations made elsewhere in the Indian Ocean permits clear identification of the source waters responsible for this structure. In the near-surface water the distributions of temperature and salinity show the course and lateral extent of the Somali Current, the offshore movement of cold water upwelled near the Somali coast, and two warm saline inflows from the Gulf of Aden and the Arabian Sea. At depths greater than 2000 m, small differences in temperature-salinity characteristics reveal a narrow northward flow adjacent to the continental slope, roughly paralleling the Somali Current. No definite inferences can be drawn concerning flow patterns at intermediate depths, both because of the apparent small scale of horizontal variation there, which is not resolved by the station spacing, and because of inherent ambiguity in core methods when applied to flows which may reverse seasonally.

Deep Currents in the Central Subarctic Pacific Ocean

Abstract Sections of closely spaced CTD stations along Longs. 165°W, 175°W and 175°E, in combination with 14-month current records from the central longitude, define two deep, nearly zonal currants, with speed increasing upward, in the subarctic Pacific. One flows eastward above the Aleutian Rise and Aleutian Trench, and appears to be a concentration of geostrophic flow forced by the bottom topography. The other flows westward along the Aleutian Island Arc, and is the northern-boundary current predicted by deep-circulation theory. Both currents reach to the sea surface, the boundary current being simply the deep part of the Alaskan Stream. The current records were too few to permit better than rough estimates of volume transports but to the extent that they could be combined with thermal-wind calculations they suggest, at 175°W, (1) a transport of 28 × 106 m3 s−1 for the Alaskan Stream, of whch 5 × 106 m3 s−1 was found below 1500 m, and (2) a transport of around 20 × 1O6 m3 s−1 for the eastward jet, of wh...

On the Obscurantist Physics of “Form Drag” in Theorizing about the Circumpolar Current

Abstract The authors point out that, since the “form-drag” force balance commonly advanced for the Antarctic Circumpolar Current is really just a statement that northward Ekman transport in the circumpolar Drake Passage zone is compensated by deep southward geostrophic flow, the balance is actually irrelevant to the magnitude of the current itself. It is thus misleading to ascribe a role to form drag in its physics. Sverdrup dynamics seems to offer a more promising analysis of the real Circumpolar Current–as proposed long ago.

The geothermal heating of the abyssal subarctic Pacific Ocean

Abstract Recent deep CTD-O 2 measurements in the abyssal North Pacific along 175°W, 152°W, and 47°N indicate large-scale changes in the O-S characteristics in the deepest kilometer of the water column. Geothermal heat flux from the abyssal sediments can be invoked as the agent for causing large-scale modification of abyssal temperatures (but not salinities) in the subarctic Pacific Ocean. East-west and north-south thermal age differences of about 100 years are inferred using a spatially uniform geothermal heat flux of 5 x 10 -2 WrmW m -2 .