Near-equatorial deep circulation in the Indian and Pacific oceans
Johnson, Gregory C.
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KeywordOcean circulation; Moana Wave (Ship) Cruise MW89-3; Charles Darwin (Ship) Cruise CD86-19; Charles Darwin (Ship) Cruise CD87-25
Theory and observations of deep circulation in the near-equatorial Atlantic, Indian and Pacific Oceans are reviewed. Flow of deep and bottom water in the near-equatorial Indian and Pacific oceans, the two oceans with only a southern source of bottom water, is described through analysis of recent CTD data. Zero-velocity surfaces are chosen through use of water-mass properties and transports are estimated. Effects of basin geometry, bottom bathymetry and vertical diffusivity as well as a model meridional inertial current on a sloping bottom near the equator are all discussed in conjunction with the flow patterns inferred from observations. In the western equatorial Indian Ocean, repeat CTD surveys in the Somali Basin at the height of subsequent northeast and southwest monsoons show only small differences in the strength of the circulation of the bottom water (potential temperature θ ≤1.2°C). A deep western boundary current (DWBC) carrying about 4x106 m3 s-1 of this water is observed moving north along the continental rise of Africa at 3°S. The cross-equatorial sections suggest that the current turns eastward at the equator. The northern sections show a large mass of the coldest water in the interior east of the Chain Ridge, augmenting the evidence that the DWBC observed south of the equator turns east at the equator rather than remaining on the boundary, and feeds the interior circulation in the northern part of the basin from the equator. The circulation of deep water (1.2°C< θ ≤ 1.7°C) in the Somali and Arabian Basins is also analyzed. A DWBC flowing southward along the Carlsberg ridge in the Arabian Basin is described. In the central equatorial Pacific Ocean a recent zonal CTD section at 10°N, allows estimation that 5.0x106 m3 s-1 of Lower Circumpolar Water (LCPW, θ ≤ 1.2°C) moves northward as a DWBC along the Caroline Seamounts in the East Mariana Basin. In the Central Pacific Basin, 8.1x106 m3 s-1 of LCPW is estimated to move northward along the Marshal Seamounts as a DWBC at this latitude. An estimated 4.7x106 m3 s-1 of the LCPW moves back southward across 10°N in the Northeast Pacific Basin along the western flank of the East Pacific Rise and an equatorial jet is observed to flow westward from 138°W to 148°W shifting south of the Line Islands at 2.5°S, 159°W. The net northward flow of LCPW across 10°N in the Pacific Ocean is estimated at 8.4x106 m3 s-I. The net southward flow of the silica-rich North Pacific Deep Water (NPDW, 1.2 < θ ≤ 2.0°C) in the central Pacific Ocean estimated at 2.7x106 m3 s-1 is also discussed. In the Indian Ocean, the eastward equatorial flow in the the bottom water of the Somali Basin differs from the prediction of a flat-bottom uniform-upwelling Stommel-Arons calculation with realistic basin geometry and source location. The behavior of a uniform potential vorticity meridional jet on a sloping bottom is examined in an attempt to explain the observed behavior at the equator. The inertial jet does not cross the equator in a physically plausible fashion owing to the constraint of conservation of potential vorticity. Mass and heat budgets for the bottom water of the Somali Basin are of interest with respect to the equatorial feature. Upwelling through the θ = 1.2°C surface is estimated at 12±4x10-5 cm s-1 and a rough heat budget for the deep Somali Basin results in an estimate of vertical diffusivity of 9±5 cm2 s-1 at 3800 m. Numerical model results indicate that large vertical diffusivities result in eastward jets in the bottom water at the equator. In the Pacific Ocean the DWBC observed flowing northward south of the equator crosses the equator with transport nearly intact, albeit split into two at 10°N by the tortuous bathymetry. However the southward flow along the East Pacific Rise in the Northeast Pacific Basin and the westward equatorial jet this flow feeds are puzzling. The basin depth decreases equatorward and eastward, which may allow some southeastward flow in the Stommel-Arons framework. However, the equatorial jet is still unexplained. The estimated vertical velocity and diffusivity at 3600 db of 2±2x10-5 cm s-1 and 4±3 cm2 s-1 for the area between 12°8 and 10°N are much smaller than estimates in the Somali Basin. Thus the two oceans, similar in their single southern source of bottom water, have DWBC's which behave remarkably differently near the equator. In the Somali Basin of the Indian Ocean the DWBC appears to turn eastward at the equator, with large vertical upwelling velocity and large vertical diffusivity estimates for the bottom water of the basin. In the Pacific Ocean the DWBC appears to cross the equator, but there is a puzzling westward flowing equatorial jet in the bottom water of the Northeast Pacific Basin.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1990
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