Publication Date:
2022-05-25
Description:
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
Description:
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.
Description:
The author began this research in the M.I.T.-W.H.O.I Joint Program while
supported by the U. S. Offce of Naval Research through a Secretary of the Navy
Graduate Fellowship in Oceanography. Support for collection and analysis of the
data taken during R.R.S. Charles Darwin cruises 86-19 and 87-25 was provided by
the U. S. National Science Foundation under grants OCE8800135 and OCE8513825
to D. B. Olson at the University of Miami and by the U. S. Offce of Naval Research
under contract N00014-87-K-0001, NR083-004 and grant N00014-89-J-1076 to B.
A. Warren at W.H.O.I. Collection of data taken during R.Y. Moana Wave cruise 89-
3 was supp6rted by the U. S. National Science Foundation under grant OCE881691O
to H. L. Bryden and J. M. Toole at W.H.O.I. Collection of data taken during
the U.S.-P.R.C. Toga cruises was supported by N.O.A.A. under grant NA85AA-DACU7.
Keywords:
Ocean circulation
;
Moana Wave (Ship) Cruise MW89-3
;
Charles Darwin (Ship) Cruise CD86-19
;
Charles Darwin (Ship) Cruise CD87-25
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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