140
Joseph Pedlosky
so there is much to explain. It is provocative to note that the very word does not even
appear in the index of that great tome, The Oceans, by Sverdrup, Johnson, and Fleming
(1942) which was for a long time the definitive work on oceanography although the
word appears a few times
1 in the text. More frequently, the thermocline is referred
to there as a “discontinuity layer” or “transition layer.” This makes the thermocline
appear as a curious secondary feature separating zones of greater oceanographic
interest. Similarly in Defant’s two-volume treatise on physical oceanography (1961),
the thermocline is noted largely as a transition zone between what are supposed to
be oceanographic analogues of the atmosphere’s troposphere and stratosphere. The
existence of such a transition layer is taken, misleadingly, as a natural process similar
to the seasonal thermocline in lakes. Indeed, the first use of the word “thermocline”
appears in the limnology literature of the late nineteenth century, a fact for which I
am indebted to my colleague Bruce Warren.
It does seem like one of those examples, not uncommon in science, in which a
phenomenon is noted but that it might be a problem requires first that it be posed as
a problem, i.e., why should there be a permanent thermocline in the ocean? It is a bit
like becoming used to looking at the Rocky Mountains and taking their presence for
granted. It may not be natural for most people in their neighborhood to ask why they
are there but once the question is asked it is easy to realize the importance of finding
the answer.
Of course it is a danger of amateur historiography to overlook past insights
into a scientific problem that contributed to a background understanding but which
often enter into oblivion because these insights were not united to a powerful enough
method to obtain a complete solution to the problem. So, for example, the insightful
work of Montgomery (1938), in which it is pointed out that the water mass properties
at depth in the subtropical gyre can be traced back along an isopycnal to the properties
of surface water, was an idea clearly influenced by the meteorological interest of the
time in isentropic analysis of atmospheric motions suggested by Rossby (1937). This
idea is seen in even more explicit form in the interpretive work of Iselin (1936,
1939) whose much reproduced schematic shows explicitly how water from the mixed
layer sliding down along isopycnal surfaces sets the vertical distribution of density
(and so implicitly also its horizontal distribution) in the subtropical gyres. These very
profound insights were, however, innocent of a powerfully enough unifying dynamical
foundation that could go beyond a quasi-diagnostic explanation to provide the basis
for a theoretical understanding of thermocline structure. Interestingly enough, the key
ingredient has turned out to be potential vorticity conservation and this was much on
the minds of Rossby and Montgomery who had collaborated on its application to
atmospheric motions. A similar application to oceanic motion had to wait another
40 years.
2
1 My colleague, Bruce Warren, has counted ten appearances of the word in the book.
2 I am excluding the remarkable work of Welander, discussed below.
Joseph Pedlosky
so there is much to explain. It is provocative to note that the very word does not even
appear in the index of that great tome, The Oceans, by Sverdrup, Johnson, and Fleming
(1942) which was for a long time the definitive work on oceanography although the
word appears a few times
1 in the text. More frequently, the thermocline is referred
to there as a “discontinuity layer” or “transition layer.” This makes the thermocline
appear as a curious secondary feature separating zones of greater oceanographic
interest. Similarly in Defant’s two-volume treatise on physical oceanography (1961),
the thermocline is noted largely as a transition zone between what are supposed to
be oceanographic analogues of the atmosphere’s troposphere and stratosphere. The
existence of such a transition layer is taken, misleadingly, as a natural process similar
to the seasonal thermocline in lakes. Indeed, the first use of the word “thermocline”
appears in the limnology literature of the late nineteenth century, a fact for which I
am indebted to my colleague Bruce Warren.
It does seem like one of those examples, not uncommon in science, in which a
phenomenon is noted but that it might be a problem requires first that it be posed as
a problem, i.e., why should there be a permanent thermocline in the ocean? It is a bit
like becoming used to looking at the Rocky Mountains and taking their presence for
granted. It may not be natural for most people in their neighborhood to ask why they
are there but once the question is asked it is easy to realize the importance of finding
the answer.
Of course it is a danger of amateur historiography to overlook past insights
into a scientific problem that contributed to a background understanding but which
often enter into oblivion because these insights were not united to a powerful enough
method to obtain a complete solution to the problem. So, for example, the insightful
work of Montgomery (1938), in which it is pointed out that the water mass properties
at depth in the subtropical gyre can be traced back along an isopycnal to the properties
of surface water, was an idea clearly influenced by the meteorological interest of the
time in isentropic analysis of atmospheric motions suggested by Rossby (1937). This
idea is seen in even more explicit form in the interpretive work of Iselin (1936,
1939) whose much reproduced schematic shows explicitly how water from the mixed
layer sliding down along isopycnal surfaces sets the vertical distribution of density
(and so implicitly also its horizontal distribution) in the subtropical gyres. These very
profound insights were, however, innocent of a powerfully enough unifying dynamical
foundation that could go beyond a quasi-diagnostic explanation to provide the basis
for a theoretical understanding of thermocline structure. Interestingly enough, the key
ingredient has turned out to be potential vorticity conservation and this was much on
the minds of Rossby and Montgomery who had collaborated on its application to
atmospheric motions. A similar application to oceanic motion had to wait another
40 years.
2
1 My colleague, Bruce Warren, has counted ten appearances of the word in the book.
2 I am excluding the remarkable work of Welander, discussed below.