The Evolution of Physical Oceanography in the Last Hundred Years

The last hundred years have seen a revolution in our understanding of ocean processes. I have selected a subset of seven topics. Technological developments generally led (rather than followed) new ideas. The advent of satellite remote sensing led to global sampling with adequate spatial sampling density (two quite different achievements, both of enormous consequence). Computers permitted the timely reduction of very large datasets of disparate observations into digestible displays, including also meaningful estimates of the uncertainties of the measured fields. Underlying all these development is a century of transition from grossly inadequate expeditionary sampling to an appreciation of a rational sampling strategy. At the opening ceremony of the Lisbon 1998 EXPO, David Halpern assigned me to speak on "Oceanography Before Satellites" and allocated five minutes to do so. Yes, there was oceanography before satellites (Figure 1). I now have a generous eight pages to review the last hundred years. I will describe events the way I remember them; if in fact they were different, they ought to have been this way. The dawn of the century found physical oceanographers in an expedition mode taking Nansen-casts and interpreting the observations in terms of a steadystate circulation in exact geostrophic balance. The following quote may sound vaguely familiar: "Oceanography... has undergone a rather rapid development during the last decades tending from a more descriptive science towards working according to exact mathematical-physical principles" (written by Albert Defant 1 during the closing days of the Meteor Expedition 1925-27.) Defant was referring to the general principles of Physikalische Hydrodynamik 2 developed in twenty five years under the leadership of V. Bjerknes, built on the work of his father C.A. Bjerknes (and continued by his son J. Bjerknes). At one time or another the "Bergen Figure. 1. The Red Sea parted, allowing Moses and the Isrealites to escape the pursuing soldiers of the Pharaoh (by permission of Pictures Now! Powered by Wood River Media, Inc. 1998 Wood River Media. June, 1998 ht tp://www.lycos.com/pictu rethis/religion/judais m/history~bible_stories~crossing_the red sea/310521.html). School" included J.W. Sandst6m, T. Bergeron, H. Solberg, W. Ekman, H.U. Sverdrup and C.G. Rossby. Sverdrup (my teacher) turned down a permanent position in Bergen (an unheard-of act of independence) for adventure in the Arctic and a career centered on measurement and analysis. Physikalische Hydrodynamik differs from "pure" hydrodynamics in three important ways: stratification, rotation and turbulence. Stratification was handled by multi-layered (rather than continuously stratified) media, and there are only two brief references to turbulence; connection to reality was handled by arbitrarily 1Albert Defant 1961, Physical Oceanography, Pergamon Press, New York, translated from Dynamische Oceanographie, J.Springer, Berlin 1929. W.Bjerknes, J.Bjerknes, H.Solberg and T.Bergeron 1943, Physikalische Hydrodynamik, Edwards Brothers (translated from original publication in 1933 by Springer). Oceanography • VoL 15 • No. 1/2002 135 amplifying the coefficients of molecular viscosity and diffusivity. This started on a day in 1900 when Nansen shared his intuition about wind-dr iven currents with Bjerknes; the student Ekman was called into the discussion and produced the theory of the Ekman spiral that very evening. But it took some hundred years until the coherent structure of a spiral could be unambiguously identified in the "noisy" lavers of the upper oceanL I am not aware of any major advances until mid° century. To illustrate what has happened since, I have selected seven topics. Someone else would have selected seven different topics. And for every achievement here noted, three have been left out. This is no way to