A History of Thermocline Theory
141
In the years that followed there were few attempts to deal with the structure of
the thermocline except for some linear models (Stommel and Veronis, 1957; Pedlosky,
1969; Gill, 1985) that attempted to describe the capture of the wind-driven motion
to an upper region of the ocean while accepting an already specified background
stratification as a starting point for the linearization of the theory. Such treatments,
interesting in their own right, are clearly incapable of addressing the fundamental
question of the density structure that defines the thermocline. They do, however, have
the virtue of indicating the beta-effect as being the important dynamical mechanism
for limiting the depth of penetration below the upper sea surface of the wind-driven
motion.
The principal issue is easily stated. The ocean is heated nonuniformly but
persistently at the sea surface. How is it, then, that the signature of that forcing
penetrates to only about 20% of the total ocean depth even though the heating has
gone on for millions of years? Clearly, a dynamical process is required to trap the
thermal signal to the upper ocean, but what specifically is the operating mechanism?
MODERN THEORIES, 1959 TO 1980
It is a weakness of human perception to define “modern,” in the sense of “advanced,”
as that which is simply contemporaneous with one’s own time. Nevertheless, I think it
is fair to mark the modern era of thermocline theory with the advent of two remarkable
papers that appeared simultaneously (Robinson and Stommel, 1959, and Welander,
1959) that did denote a real step forward in development. As remarkable as the papers,
published back-to-back in an issue of Tellus, are themselves for their pioneering
approach to the problem of the thermocline, it is the foreword to the first paper (and
serving as a foreword to both) that immediately captures the reader’s attention. The
foreword alerts the reader to the inconsistency of the basic physical assumptions of
the two papers, declines to suggest a test that would distinguish which is correct
(since they both seemed to predict a reasonable thermocline structure), and leaves
it to the reader to choose between them. The Robinson and Stommel paper focused
on an explanation of the thermocline as a lake-like balance between the downward
diffusion of heat from the surface and the advection of cold water. It is interesting
to note that in the model the zonal variation of temperature was ignored along with
the accompanying zonal advection of temperature. As we know, a surface condition
in which the temperature is independent of longitude does not, in fact, produce a
solution independent of longitude but that assumption allowed the authors to find
a similarity transformation, which, after a type of linearization, led to solutions for
the density anomaly which exponentially decreased with depth. One of the most
interesting aspects of the paper is its emphasis on obtaining an estimate of the vertical
velocity at the base of the thermocline in the hope of establishing a rational basis for
the driving mechanism of the abyssal circulation as proposed by Stommel (1958).
Another point of interest, in hindsight, is the discussion of the size of the turbulent
141
In the years that followed there were few attempts to deal with the structure of
the thermocline except for some linear models (Stommel and Veronis, 1957; Pedlosky,
1969; Gill, 1985) that attempted to describe the capture of the wind-driven motion
to an upper region of the ocean while accepting an already specified background
stratification as a starting point for the linearization of the theory. Such treatments,
interesting in their own right, are clearly incapable of addressing the fundamental
question of the density structure that defines the thermocline. They do, however, have
the virtue of indicating the beta-effect as being the important dynamical mechanism
for limiting the depth of penetration below the upper sea surface of the wind-driven
motion.
The principal issue is easily stated. The ocean is heated nonuniformly but
persistently at the sea surface. How is it, then, that the signature of that forcing
penetrates to only about 20% of the total ocean depth even though the heating has
gone on for millions of years? Clearly, a dynamical process is required to trap the
thermal signal to the upper ocean, but what specifically is the operating mechanism?
MODERN THEORIES, 1959 TO 1980
It is a weakness of human perception to define “modern,” in the sense of “advanced,”
as that which is simply contemporaneous with one’s own time. Nevertheless, I think it
is fair to mark the modern era of thermocline theory with the advent of two remarkable
papers that appeared simultaneously (Robinson and Stommel, 1959, and Welander,
1959) that did denote a real step forward in development. As remarkable as the papers,
published back-to-back in an issue of Tellus, are themselves for their pioneering
approach to the problem of the thermocline, it is the foreword to the first paper (and
serving as a foreword to both) that immediately captures the reader’s attention. The
foreword alerts the reader to the inconsistency of the basic physical assumptions of
the two papers, declines to suggest a test that would distinguish which is correct
(since they both seemed to predict a reasonable thermocline structure), and leaves
it to the reader to choose between them. The Robinson and Stommel paper focused
on an explanation of the thermocline as a lake-like balance between the downward
diffusion of heat from the surface and the advection of cold water. It is interesting
to note that in the model the zonal variation of temperature was ignored along with
the accompanying zonal advection of temperature. As we know, a surface condition
in which the temperature is independent of longitude does not, in fact, produce a
solution independent of longitude but that assumption allowed the authors to find
a similarity transformation, which, after a type of linearization, led to solutions for
the density anomaly which exponentially decreased with depth. One of the most
interesting aspects of the paper is its emphasis on obtaining an estimate of the vertical
velocity at the base of the thermocline in the hope of establishing a rational basis for
the driving mechanism of the abyssal circulation as proposed by Stommel (1958).
Another point of interest, in hindsight, is the discussion of the size of the turbulent