Study of Ocean Circulation since 1935
173
approximate the actual flow, but it cannot give useful values of the transport, and using
such a reference for flow at greater depths is likely to be very misleading in many cases.
The use of geopotential anomaly, or dynamic topography, began in the 1920s,
with McEwen et al. (1930), Smith et al. (1937), and Koenuma (1937) in the Pacific,
Helland-Hansen and Nansen (1927), Parr (1935), and Montgomery (1938) in the
Atlantic, and Gordon et al. (1978) in the Antarctic. These all used some deep isobar
as a reference. They recognized that the speeds they calculated were only relative and
hoped for some solution.
Dietrich (1936) had proposed that the vertical oxygen minimum near 800 m
beneath the Gulf Stream might indicate a depth of low or minimum flow in accord with
W¨ ust’s (1935) deep flow pattern. But Rossby (1936) showed that the vertical shear
there, measured relative to 800 m, would have meant too much southward transport
beneath it and too little northward transport above it. Did anyone accept a southward
flow there? It seems to have been neglected by everyone except Defant (1941a,b).
After examining Dietrich’s and W¨ ust’s studies, Defant (1941b) proposed that
there might be a reference surface that varied in depth from area to area to accommodate the different layers and sources. He also noted that though such a surface can
be selected, it must be recognized as an assumption. He chose such a surface for the
Atlantic Ocean and mapped the flow at depths from the sea surface to 2000 m. He
referred the geostrophic shear to a surface of zero flow by proposing that a depth of no
or minimum shear could be a depth of no flow, and thus a reference for calculating the
speed. His reference surface was shallow near the equator (about 800 m) and sloped
downward both north and south to about 2000 m. This, I believe, was to minimize
the convergence in meridional flow.
But he made the reference surface slope up to the west near the western boundary. I believe that he did this because, after seeing W¨ ust’s (1935) work and working
with W¨ ust on the Meteor Atlas (W¨ ust and Defant, 1936), he knew that there is a
deep southward flow beneath or near the Gulf Stream. The vertical shear there is
monotonic, and a change in the sense of flow calls for a zero reference depth, in this
case near 1500 m, with northward flow above and southward flow beneath. He could
not extend his work below 2000 m because the data were scarcer there, and the fields
were weaker, and would require greater accuracy in the measurements.
This choice of zero shear as zero flow did not receive any support. Not only
was his method rejected, but also his results, and the whole concept of calculating
geostrophic flow from the density field became suspect.
This is unfortunate, because his resulting “absolute” topography (Defant,
1941b) seems remarkably like the major features of the Atlantic flow that we recognize today. At the surface it shows the Gulf Stream and the Brazil Current, the two
great anticyclonic gyres north and south of the equator, the North and South Equatorial currents and the Equatorial Countercurrent, and the subarctic gyre. It shows the
narrowing of the anticyclonic gyres below 800 decibars, which had not been recognized, and the deep southward flow near the western boundary, the western boundary
undercurrent, W¨ ust’s “deep current.”
173
approximate the actual flow, but it cannot give useful values of the transport, and using
such a reference for flow at greater depths is likely to be very misleading in many cases.
The use of geopotential anomaly, or dynamic topography, began in the 1920s,
with McEwen et al. (1930), Smith et al. (1937), and Koenuma (1937) in the Pacific,
Helland-Hansen and Nansen (1927), Parr (1935), and Montgomery (1938) in the
Atlantic, and Gordon et al. (1978) in the Antarctic. These all used some deep isobar
as a reference. They recognized that the speeds they calculated were only relative and
hoped for some solution.
Dietrich (1936) had proposed that the vertical oxygen minimum near 800 m
beneath the Gulf Stream might indicate a depth of low or minimum flow in accord with
W¨ ust’s (1935) deep flow pattern. But Rossby (1936) showed that the vertical shear
there, measured relative to 800 m, would have meant too much southward transport
beneath it and too little northward transport above it. Did anyone accept a southward
flow there? It seems to have been neglected by everyone except Defant (1941a,b).
After examining Dietrich’s and W¨ ust’s studies, Defant (1941b) proposed that
there might be a reference surface that varied in depth from area to area to accommodate the different layers and sources. He also noted that though such a surface can
be selected, it must be recognized as an assumption. He chose such a surface for the
Atlantic Ocean and mapped the flow at depths from the sea surface to 2000 m. He
referred the geostrophic shear to a surface of zero flow by proposing that a depth of no
or minimum shear could be a depth of no flow, and thus a reference for calculating the
speed. His reference surface was shallow near the equator (about 800 m) and sloped
downward both north and south to about 2000 m. This, I believe, was to minimize
the convergence in meridional flow.
But he made the reference surface slope up to the west near the western boundary. I believe that he did this because, after seeing W¨ ust’s (1935) work and working
with W¨ ust on the Meteor Atlas (W¨ ust and Defant, 1936), he knew that there is a
deep southward flow beneath or near the Gulf Stream. The vertical shear there is
monotonic, and a change in the sense of flow calls for a zero reference depth, in this
case near 1500 m, with northward flow above and southward flow beneath. He could
not extend his work below 2000 m because the data were scarcer there, and the fields
were weaker, and would require greater accuracy in the measurements.
This choice of zero shear as zero flow did not receive any support. Not only
was his method rejected, but also his results, and the whole concept of calculating
geostrophic flow from the density field became suspect.
This is unfortunate, because his resulting “absolute” topography (Defant,
1941b) seems remarkably like the major features of the Atlantic flow that we recognize today. At the surface it shows the Gulf Stream and the Brazil Current, the two
great anticyclonic gyres north and south of the equator, the North and South Equatorial currents and the Equatorial Countercurrent, and the subarctic gyre. It shows the
narrowing of the anticyclonic gyres below 800 decibars, which had not been recognized, and the deep southward flow near the western boundary, the western boundary
undercurrent, W¨ ust’s “deep current.”