10
Bruce A. Warren
a layout of the circulatory gyres, geostrophic estimates of transports of most of the
major ocean currents, and a Merzian rendition of the vertical–meridional circulation.
Chapter XV was a tour de force that has never been equaled (and probably could not
be, given the vastly increased amount of observational and theoretical material that a
mind needs to digest now). It was the treatise on the general ocean circulation for its
day, and while much has been learned since then, it is still good reading.
In 1968 Jerome Namias remarked to me that air-mass analysis had been an
intermediate stage in meteorology, that with the ability to run dynamical models
of the atmospheric circulation, nobody talked air masses anymore. To some extent,
watermassology is becoming superannuated too, as oceanographers have become
capable of measuring routinely and abundantly much more than the traditional water
properties, and as general circulation models reach toward usefulness. But it seems
to me that, given the stable characteristic features of the water-property fields, at least
the nomenclature will endure as useful, convenient shorthand.
5. PHYSICS
Aristotle went out with the seventeenth century, but discussions afterwards about
circulation mechanics, despite Newtonian physics, tended to be qualitative and sterile,
exemplified by the late-nineteenth-century squabble between Croll and Dr. Carpenter
about the roles of wind and density forcing (M. Deacon, 1971, pp. 320–328). By
the time of the Second World War circulation physics consisted essentially of just
two ideas: geostrophy, which says nothing about what drives circulation, and the
Ekman spiral, which showed that the direct effect of the wind stress was confined to
a thin surface layer, and therefore left it still a mystery how the wind could drive the
manifestly deep-penetrating circulatory gyres.
After the war Sverdrup (1947) demonstrated that the vertically integrated flow
in the open ocean was driven by the curl of the wind stress, and Stommel (1948) discovered that the extreme asymmetry of the circulatory gyres was due to the meridional
variation in the local vertical component of the earth’s rotation vector. Carrier (Munk
and Carrier, 1950) made the mathematics more tractable by casting the asymmetry as
a western-boundary layer superposed on a lower-order interior flow field. [So far as I
know, Charney (1955) was actually the first to use (though very unobtrusively) the now
commonplace term, “boundary current.”] Stommel (1957) then resolved the vertically
integrated motion into near-surface Ekman transport and deep-reaching geostrophic
flow; and he showed that what the wind-stress curl does is to force convergence of
the Ekman transport, and that the resulting vertical velocity in turn drives a divergent
geostrophic flow at depth. So at last the basic mechanism by which wind drives circulation was grasped, and it was the starting point for the subsequent flowering of
steady-state and time-dependent circulation theory.
Meteorologists had assumed that the contribution of the ocean circulation to the
global meridional energy transport was negligible compared to that of the atmosphere;
Bruce A. Warren
a layout of the circulatory gyres, geostrophic estimates of transports of most of the
major ocean currents, and a Merzian rendition of the vertical–meridional circulation.
Chapter XV was a tour de force that has never been equaled (and probably could not
be, given the vastly increased amount of observational and theoretical material that a
mind needs to digest now). It was the treatise on the general ocean circulation for its
day, and while much has been learned since then, it is still good reading.
In 1968 Jerome Namias remarked to me that air-mass analysis had been an
intermediate stage in meteorology, that with the ability to run dynamical models
of the atmospheric circulation, nobody talked air masses anymore. To some extent,
watermassology is becoming superannuated too, as oceanographers have become
capable of measuring routinely and abundantly much more than the traditional water
properties, and as general circulation models reach toward usefulness. But it seems
to me that, given the stable characteristic features of the water-property fields, at least
the nomenclature will endure as useful, convenient shorthand.
5. PHYSICS
Aristotle went out with the seventeenth century, but discussions afterwards about
circulation mechanics, despite Newtonian physics, tended to be qualitative and sterile,
exemplified by the late-nineteenth-century squabble between Croll and Dr. Carpenter
about the roles of wind and density forcing (M. Deacon, 1971, pp. 320–328). By
the time of the Second World War circulation physics consisted essentially of just
two ideas: geostrophy, which says nothing about what drives circulation, and the
Ekman spiral, which showed that the direct effect of the wind stress was confined to
a thin surface layer, and therefore left it still a mystery how the wind could drive the
manifestly deep-penetrating circulatory gyres.
After the war Sverdrup (1947) demonstrated that the vertically integrated flow
in the open ocean was driven by the curl of the wind stress, and Stommel (1948) discovered that the extreme asymmetry of the circulatory gyres was due to the meridional
variation in the local vertical component of the earth’s rotation vector. Carrier (Munk
and Carrier, 1950) made the mathematics more tractable by casting the asymmetry as
a western-boundary layer superposed on a lower-order interior flow field. [So far as I
know, Charney (1955) was actually the first to use (though very unobtrusively) the now
commonplace term, “boundary current.”] Stommel (1957) then resolved the vertically
integrated motion into near-surface Ekman transport and deep-reaching geostrophic
flow; and he showed that what the wind-stress curl does is to force convergence of
the Ekman transport, and that the resulting vertical velocity in turn drives a divergent
geostrophic flow at depth. So at last the basic mechanism by which wind drives circulation was grasped, and it was the starting point for the subsequent flowering of
steady-state and time-dependent circulation theory.
Meteorologists had assumed that the contribution of the ocean circulation to the
global meridional energy transport was negligible compared to that of the atmosphere;