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50 Years of Ocean Discovery: National Science Foundation 1950—2000
experiment in 1969 (another Stommel brainchild) provided direct measurements of convective overturning. Prior to MEDOC there had been very little direct observational evidence for deep water formation.
THE CLIMATE REVOLUTION
In 1960, NSF awarded J. Bjerknes a grant of $30,000 per year for three years to study "Sea Surface Temperature and Atmospheric Circulation." This was the beginning of ENSO, a combined ocean (El Niño) and atmosphere (Southern Oscillation) phenomenon.
Milankovitch long ago computed long-term variations in the orbital parameters of the Earth-Sun-Moon system with periods from 20,000 to 100,000 years. In a remarkable development pioneered by Imbrie, the terms have now been detected in the ocean sediment record, and they provide important information concerning the atmosphere-ocean response to harmonic forcing.
Hasselmann pioneered an approach that in some sense is opposite to that of Milankovitch. He suggested a "random walk" of the climate state in response to random pulses associated with short-term "weather." The character of such random walks is that they lead to large long-time departures from the mean. It has been demonstrated that the random-walk excitation accounts for the dominant part of the observed climate variance.
The coupled ocean-atmosphere system is capable of complex feedback systems. A number of these have been identified: ENSO, the Pacific "decadal variation," and the North Atlantic Oscillation. It would appear that the three phenomena can account for a significant fraction of the ambient variance. El Niño has a recognizable linear component in a highly nonlinear equatorial dynamics: an equatorially trapped wave moving eastward at a rate of order 0.1 m/s (playing a role somewhat similar to the edge waves in highly nonlinear coastal and littoral dynamics). There has been significant progress in ENSO prediction.
Greenhouse warming has occupied center stage, largely because mankind can do something about this component of climate variability. Model predictions now have error bars of the same order as the predicted mean change. There is urgent need for observational testing. The inevitable result will be an improved modeling and an increased understanding of ocean processes.
In all of the climate problems, a first-order consideration is the oceanic and atmospheric equator-to-pole heat flux (3.7 × 1015 W across 24°N) required to maintain the global heat balance. In 1955, Sverdrup estimated that the ocean contributed 1.4 × 1015 W, and this was mostly in the wind-driven circulation. We now estimate that the ocean carries more than half the total load, with comparable contributions from the wind-driven and thermohaline circulations. Quite a change!
THE TECHNOLOGY REVOLUTION
We all agree that there has been a technology revolution in ocean sciences; Larry Clark's paper, later in the volume, presents highlights of this revolution. It probably would have made more sense if I had organized this review along those lines; more often than not, new ideas have come out of new technology, rather than the other way around.
High-speed computers led to an explosion in the 1950s in every branch of physical oceanography (I have already listed a few examples). Readily available analysis of noisy records led at last to a sensible and reproducible description of surface waves and internal waves. It opened the door to objective analyses of extensive and diverse data sets, matched field processing of ocean acoustic transmissions, and the application of inverse theory to ocean measurements for an objective approach to estimating the validity of a given set of assumptions. Sadly, oceanographers had long found support for their favorite theory without such an objective assessment. In reviewing some past experiments designed to answer certain questions, one finds that the proposed measurements could not possibly have decided the issue with any reasonable degree of probability even if all measurements had worked (which is not always the case).
We have already referred to the revolution associated with the development of a deep-sea mooring technology. A similar case can be made for drifters, particularly those with a programmed depth strategy z(t), which have spearheaded a Lagrangian renaissance led by T. Rossby, D. Webb, and R. Davis. The oceans are a remarkably good propagator of sound (but not of electromagnetic energy), and this has played a profound role in ocean exploration starting with the acoustically navigated Swallow floats. The application of inverse methods has made possible the interpretation of the entire recorded field of an acoustic transmission in terms of the properties of the intervening water.
We must not overlook low-tech developments. A U.S. patent for the O-ring was awarded to Niels Christensen in 1939 (so Rita cannot claim credit for this seamark). Until the mid-1960s we used to load our gear into numerous boxes and carry them aboard the vessels, only to find that a crucial item had been left ashore. I think Frank Snodgrass was the first to build portable laboratories with the equipment assembled and pretested. The portable laboratory (Figure 1) is then brought aboard, ready for action. Decks of all oceanographic vessels now provide bolt-downs 2 feet on center for securing the portable laboratories. In about the same period we learned how to drop unattached instruments to the relatively benign environment of the deep seafloor, later to be recalled acoustically. There was a psychological block to overcome; it is not easy to let go of a line from which you have a year' s budget of equipment hanging.
Satellites constitute the most important technology innovation in modern times. Oceanographers are a conserva