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trally buoyant floats by Swallow (who credits Stommel for suggesting this idea) soon demonstrated that variability in space and time was the rule, not the exception (though more intense near the boundary currents). There was an urgent need for a systematic exploration of the ocean variability. The development of deep-ocean mooring technology provided such an opportunity, and the Mid-Ocean Dynamics Experiment (MODE) starting in 1973 under the leadership of Stommel and Robinson defined the parameters of variability. (Soviet oceanographer Brekhovskikh got there first, but failed to get definitive results because of a high failure rate of current meters.) We now think of this mesoscale variability as the ocean weather and the underlying circulation as the ocean climate (itself subject to slow variations that are discussed later). Climate came first, weather later—rather the opposite of what happened in meteorology.

The era coincides with a flowering of geophysical fluid dynamics (GFD). Nearly everyone in GFD had their initiation at the summer sessions in Walsh Cottage at Woods Hole first organized by W. Malkus. Mesoscale variability was incorporated into general circulation models (GCMs). We recall the excitement of seeing B. Holland's first spontaneously unsteady wind-driven circulation model.

I believe that the numerical modeling reached a plateau in later decades as a result of a dependence on semiempirical nonphysical parameterization. Ironically, modeling came to the rescue, but in the new form of process-oriented modeling (as opposed to simulations of actual conditions), leading to appreciation of the ventilation of deep layers, of constant potential vorticity pools, and so forth. A resurgence of theoretical thinking has evolved into an indispensable complement to big numerical models.

Internal Waves

On a smaller scale, internal waves (long recognized as a curiosity) became part of the oceanographic mainstream. At periods of less than a day, internal waves are the principal contributors to the velocity variance. This development owes a great deal to the application of power-spectral analysis, which in turn was made possible only by the computer revolution. Fifty years ago no oceanographer knew how to handle the wiggly records associated with random-phase wide-band processes. (Yet acousticians and opticians had done so for many years.) We could manage the discrete tidal line spectrum, and get away with the analysis of narrow-band processes such as distant swell, but we failed miserably in the analysis of storm waves or internal waves. Most ocean processes are wide-band!

In 1931, Ekman took some current measurements with a string of Ekman meters suspended on a vertical mooring. When I met Ekman in Oslo in 1949, he expressed disbelief that currents separated vertically by as little as 100 m could bear so little resemblance, and he delayed publishing an analysis until shortly before his death. But there is nothing mysterious in the result; processes with vertical bandwidth Dk are incoherent at separations exceeding ?z=?K-1 !

Among the seamark achievements are the recognition of an astounding spectral universality (within a factor 2) under a wide variety of conditions (still not understood) and of the role played by internal waves in ocean mixing processes. The transformation to internal solitons (solibores) in near-shore regions (first recognized on satellite images) is becoming an important component in coastal studies.

Edge Waves

There exists a class of wave motion that is coastally trapped. Wave crests and troughs extend perpendicular to shore and diminish exponentially with distance from shore. Propagation is in a direction parallel to shore. There are two scales: the rotationally trapped Kelvin edge waves, and the gravitationally trapped Stokes edge waves. Both were discovered in the nineteenth century and considered curiosities. Referring to the latter, Lamb writes: ''it does not appear that the type of motion here referred to is very important." In fact, these curiosities are the very centerpiece of a rapidly developing coastal dynamics—one that is amazingly different and almost isolated from the deep ocean dynamics. It has turned out that the linear edge waves provide a linear core to the highly nonlinear coastal and littoral dynamics.

Gravitational edge waves are excited by incoming surface waves depending in a complex (but predictable) way on the character of the wave system. The edge waves, in turn, determine the littoral dynamics, the bar formation and cusps in the beach profile, and the spacing of rip currents. For a given medium size of sand grain and representative values of wave height, period, and direction, it is now possible to predict an equilibrium beach profile. Crucial elements in this development were the radioactive and fluorescent tagging of sand grains and the spectral representation of the incoming wave system. In a larger sense the underlying parameter space depends on the type of coast as determined by plate tectonics, and a mass balance determined by river discharge, cliff erosion, and the presence of submarine canyons.

Surface Waves

This is another old subject that was revived by modern spectral analysis. In 1957, Miles and Phillips in two seamark papers6,7 discussed the generation of waves by wind, and a year later Phillips8 introduced the famous k-4

6  

 phillips, O.M. 1957. On the generation of waves by turbulent wind. J. Fluid Mech. 2:417-445.

7  

 Miles, J.W. 1957. On the generation of surface wave. by shear flows. J. Fluid Mech. 3:185-204.

8  

 phillips, O.M. 1958. The equilibrium range in the spectrum of wind-generated waves. J. Fluid Mech. 4:426-434.



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