IILow-FrequencyVariability of theSeaCarl Wunsch11.1 IntroductionThe purposeful study of the time-dependent motion ofthe sea having periods longer than about 1 day is com-paratively recent. In the classic Handbuch of the early1940s, Sverdrup, Johnson, and Fleming (1942), onesearches in vain for more than the most peripheralreference to temporal changes on the large scale (oneof the few examples is their figure 110 showing theCalifornia Current at two different times). Until veryrecently, the ocean was treated as though it had anunchanging climate with no large-scale temporal var-iability. The reason for this is compelling and plain:until the electronics revolution of the past 30 years,the major oceanographic observational tool was theNansen bottle; using slow, uncomfortable ships, ittook essentially 100 years to develop a picture of thegross characteristics of the mean ocean. The more re-cent period, 1947 (Sverdrup, 1947) through about 1970(Stommel, 1965; Veronis, 1973b; and see chapter 5),was one of the intensive development of the theory oflarge-scale, steady models of the ocean circulation. Themethods were initially analytic, later numerical. Mostof these models were essentially low-Reynolds-num-ber, steady, sluggish, sticky, climatic oceans. In them,the role (if any) of small-scale, time-dependent proc-esses is simply parameterized by a positive eddy coef-ficient (Austauch) implying a down-the-mean-gradientflow of energy, momentum, heat, etc. The westward-intensification theories (Stommel, 1948; Munk, 1950)imply that any strong influence of such eddy coeffi-cients would be confined to the western boundariesand could be ignored in the interior ocean, except pos-sibly in the immediate vicinity of the eastward-movingfree-jet "Gulf Stream" (see Morgan, 1956). The result-ing models bear a remarkable resemblance to many ofthe gross features of the large-scale mean ocean circu-lation (see chapter 5).The culmination of these analytic and numericalmodels of the large-scale circulation coincided with anumber of developments that ultimately underminedthe momentary confidence that the models representedthe correct dynamics of the ocean circulation. Thesedevelopments were of two kinds-instrumental andintellectual.By 1970 instruments had been developed that madeit possible to obtain time-series measurements in theopen sea for periods far longer than a ship could pos-sibly remain in one location. These instruments in-cluded moored current meters, drifting neutrally buoy-ant floats, pressure gauges, and many others (Gould,1976; and see chapter 14). An additional "instrument"was the computer, which made it possible both tohandle the large data sets generated by time-series in-342Carl Wunschstruments and to explore new ideas by nonanalyticmeans. This computer impact has been felt, of course,in most branches of science.The intellectual developments that shifted the focusfrom the mean circulation to the time-dependent partwere also of various kinds. The analytic models seem-ingly had reached a plateau at which their increasinglyintricate features [e.g., essentially laminar boundarylayers of higher and higher order as in Moore and Niiler(1975)] seemed untestable and intuitively implausibleoutside the laboratory. Physical oceanography is alsoto some extent a mirror of meteorology; by 1970 mostoceanographers were at least vaguely familiar with thepicture of the atmosphere that had emerged over theprevious decades. In that fluid system, the view of therole of eddies had shifted from a passive means ofdissipating the mean flows (through purely down-gra-dient fluxes of momentum, energy, etc.) to a muchmore interesting and subtle dynamic linkage in whichthe mean flows (the climate) were in at least someparts of the system driven by the eddy fluxes (Jeffreys,1926; Starr, 1968; Lorenz, 1967). Because many of themeteorological results would apply to any turbulentfluid, there was reason to believe that the ocean couldalso exhibit such intimate dynamic linkages. But weshould note that even now much work is still directedat studying the mean circulation by essentially classi-cal (though improved) means, as if the variability werenot dynamically important (e.g., Schott and Stommel,1978; Wunsch, 1978a; Reid, 1978). The extent to whichsuch pictures of the mean circulation of the large-scaletracers will survive complete understanding of varia-bility dynamics is not now clear.In this chapter we shall review what is known aboutthe variability of the ocean. The expression "low-fre-quency variability," which is part of the title of thischapter, is a vague one used in a variety of ways byoceanographers, and encompassing a wide range ofthings. Here we mean by it anything with a time scalelonger than a day out to the age of the earth, althoughwe cannot really study by instrumental means phe-nomena with time scales longer than about 100 years.In spatial scale, it means phenomena ranging fromsome tens of kilometers to the largest possible globalocean oscillations. We shall, in common with recentpractice, also refer to the "eddy" field in the ocean.This word is often prefixed by "mesoscale" and is usedloosely to denote the subclass of variability encom-passing motions occurring on scale of hundreds of kil-ometers with time scales of months and longer. It is aconvenient shorthand and is meant to imply neitherany particular dynamics nor only flows with closedstreamlines. (The equivalent Soviet term is "synopticscale").There is little doubt that oceanographers were quiteaware, from the very beginning, of time variability inthe ocean. Maury (1855, p. 358) remarked that in draw-ing his charts he had disregarded "numerous eddiesand local currents which are found at sea." He alsonotes in particular (p. 188) the highly variable equato-rial currents of the Pacific Ocean. Even earlier, Rennel(1832) had quoted another observer (C. Blagden), asreferring to North Atlantic currents as "casual" (Swal-low, 1976).Most of the astute observers who worked at sea sinceMaury were very conscious of the difficulties of draw-ing conclusions about the mean circulation in the pres-ence of a
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