2.1.1 Different views of the ocean
Some readers of the wider literature on the general
circulation of the ocean come to recognize that the
writing suffers from a kind of multiple personality
disorder. As with the actual psychiatric situation,
the different personalities do not always recognize
the existence of the other individuals. One can
identify several of these personalities:
1 The descriptive oceanographers’ classical ocean.
This ocean circulation is large scale, steady and
laminar. Its origins lie with the technology of
the mid-nineteenth to late-twentieth century,
which was almost entirely ship-based. To obtain
a description of the ocean, one had to lump
together observations spanning many decades
and treat them as though they were simultaneous. Out of 100 years of observations arose the
many attempts to depict ‘the’ oceanic general
circulation as in the diagrams of Defant (1941),
Wüst (1935), Sverdrup et al. (1942) and many,
many others. A recent example is Schmitz and
McCartney (1993). The extreme version of this
approach is the Broecker (1991) ‘global conveyor belt’, in which the caricature of the circulation renders it as a geological structure. It is
an important corollary of this view that there is
a unique general circulation – usually given in
terms of mass fluxes – with all other property
fluxes such as temperature or nutrients computed as the product of the mean flows and
the local, supposed smoothly varying, property
distributions.
2 The analytical theorists’ ocean. The beginnings
are often traced to Sverdrup (1947), Stommel
(1948) and the large literature that followed. A
recent depiction of this ocean is in Pedlosky
(1996). This impressive theoretical construct is
very similar in its framework to the classical
descriptive one. The ocean is thought of as
essentially quasi-steady, in which features such
as the Gulf Stream have nearly fixed paths. Any
time variability is, as in 1, thought of as ‘noise’.
There are many similarities between the pictures
derived from 1 and the theory as it emerged in
2, although the resemblance is rarely made
quantitative; usually appeal is made only to
pictorial similarity. (For a crude example of a
quantitative test, that of the Sverdrup relationship, see Wunsch and Roemmich, 1985; Hautala
et al., 1994.)
There is an important field called ‘geophysical
fluid dynamics’, whose goal might be described
as the reduction of geophysical systems to terms
sufficiently simple so as to produce a deep understanding of the various dynamical proesses. To
some degree, the analytical theorists’ ocean (2) is
that of geophysical fluid dynamics. Sometimes,
however, the original simplifications are forgotten and the reduced models are then confused
with the real system.
3 The observers’ highly variable ocean. Here the
focus is on elements of the ocean circulation
that are time variable and that often also exhibit
very strong spatial structures over very short
distances (the lowest baroclinic Rossby radius
of order 30 km or less). The emphasis, again for
reasons of technological capability, has until
very recently been on regionally varying elements. This view of the ocean emerged during
2.1
Global Problems and Global Observations
Carl Wunsch
47
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
Some readers of the wider literature on the general
circulation of the ocean come to recognize that the
writing suffers from a kind of multiple personality
disorder. As with the actual psychiatric situation,
the different personalities do not always recognize
the existence of the other individuals. One can
identify several of these personalities:
1 The descriptive oceanographers’ classical ocean.
This ocean circulation is large scale, steady and
laminar. Its origins lie with the technology of
the mid-nineteenth to late-twentieth century,
which was almost entirely ship-based. To obtain
a description of the ocean, one had to lump
together observations spanning many decades
and treat them as though they were simultaneous. Out of 100 years of observations arose the
many attempts to depict ‘the’ oceanic general
circulation as in the diagrams of Defant (1941),
Wüst (1935), Sverdrup et al. (1942) and many,
many others. A recent example is Schmitz and
McCartney (1993). The extreme version of this
approach is the Broecker (1991) ‘global conveyor belt’, in which the caricature of the circulation renders it as a geological structure. It is
an important corollary of this view that there is
a unique general circulation – usually given in
terms of mass fluxes – with all other property
fluxes such as temperature or nutrients computed as the product of the mean flows and
the local, supposed smoothly varying, property
distributions.
2 The analytical theorists’ ocean. The beginnings
are often traced to Sverdrup (1947), Stommel
(1948) and the large literature that followed. A
recent depiction of this ocean is in Pedlosky
(1996). This impressive theoretical construct is
very similar in its framework to the classical
descriptive one. The ocean is thought of as
essentially quasi-steady, in which features such
as the Gulf Stream have nearly fixed paths. Any
time variability is, as in 1, thought of as ‘noise’.
There are many similarities between the pictures
derived from 1 and the theory as it emerged in
2, although the resemblance is rarely made
quantitative; usually appeal is made only to
pictorial similarity. (For a crude example of a
quantitative test, that of the Sverdrup relationship, see Wunsch and Roemmich, 1985; Hautala
et al., 1994.)
There is an important field called ‘geophysical
fluid dynamics’, whose goal might be described
as the reduction of geophysical systems to terms
sufficiently simple so as to produce a deep understanding of the various dynamical proesses. To
some degree, the analytical theorists’ ocean (2) is
that of geophysical fluid dynamics. Sometimes,
however, the original simplifications are forgotten and the reduced models are then confused
with the real system.
3 The observers’ highly variable ocean. Here the
focus is on elements of the ocean circulation
that are time variable and that often also exhibit
very strong spatial structures over very short
distances (the lowest baroclinic Rossby radius
of order 30 km or less). The emphasis, again for
reasons of technological capability, has until
very recently been on regionally varying elements. This view of the ocean emerged during
2.1
Global Problems and Global Observations
Carl Wunsch
47
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER