3On the Mid-DepthCirculationof the World OceanJoseph L. Reid3.1 IntroductionThere is a large part of the ocean circulation for whichwe have very little information and very vague con-cepts. This is the great domain of the mid-depth ocean.We have considerable information about the flow atand quite near the sea surface, and some inferencesabout the abyssal flow derived mostly from the tradi-tional patterns of characteristics at the bottom. Re-cently, some attention has been focused on the deepwestern boundary currents, where the flow is strongenough to be detected both in the density field and insome cases by direct measurement. But for the greaterpart of the volume of the ocean-beneath the upperkilometer and away from the western boundary cur-rents and above the abyssal waters-we have little in-formation on, or understanding of, the circulation.Most treatments of the deep water as well as the abys-sal water have dealt in terms of the western boundaryflow, and a general meridional flow is all that hasemerged from most of the studies. WiUst (1935), forexample, assumed a principally thermohaline meri-dional flow to obtain from the abyssal layer up throughhis Subantarctic Intermediate Water, at depths above1 kin, with no recognizable pattern of gyral flow anal-agous to the surface circulation.It seems worthwhile to consider what informationthere is for this great volume of water. This study willbegin with a general discussion of the earlier ideas onthis problem. It will review briefly the recent work (ofthe last 10 years or so), which has begun to makesubstantial contributions, and will display and discusssome world-wide mid-depth patterns of characteristicsand of geostrophic vertical shear.There is no simple distinction between the upperwaters, the deep and abyssal waters, and what I shallcall the mid-depth waters. A working definition willbe that the mid-depth waters are those that are foundbetween about 1 and 3 km in middle and low latitudesand their source waters, which are shallower in highlatitudes. Warren's study (this volume, chapter 1) ofthe deep circulation includes some of these waters, ofcourse, and I have tried to avoid duplication. Someduplication remains, however, in part for immediateclarity and in part for different emphasis.3.2 The Circulation of the Upper Waters and TheirContribution to the Mid-DepthsOur first information about general ocean circulationcame from the experience of mariners crossing thegreat oceans. They found the best routes for eastwardtravel to be in the zone of the west winds and forwestward travel in the trades, and noted early the west-ern boundary currents. As the information accumu-70Joseph L. Reidlated, these findings led, by the middle of the nine-teenth century, to the general concept of subtropicalanticyclonic gyres, subarctic gyres, and various zonalflows near the equator.The variability of this general pattern was learnedearly and is most clearly presented in the sailing direc-tions, coast pilots, and atlases prepared by the varioushydrographic offices. For example, the typical atlas ofsurface currents of the northwestern Pacific Ocean(U.S. Navy Hydrographic Office, 1944) provides infor-mation by averages in 1°x 1°squares, but for 5°x 5°areas provides summations by octants in direction,with average speed and fractions of time for each oc-tant. While this can give no information on the fre-quency of the variations (each measurement repre-sented a mean of 12 to 24 hours or longer), the presenceof variation is clearly shown everywhere, and the gen-eral findings of Fuglister (1954), Dantzler (1976) andWyrtki, Magaard, and Hager (1976) are to some degreeanticipated.But in spite of the variability and the smoothingeffects in taking its mean, certain major features of thegross field stand out. On this particular atlas thestrongest of these are the Kuroshio and the North Equa-torial Current. The West Wind Drift around 40°N isalso clear, though weaker. But in the area between theKuroshio-West Wind Drift and the North EquatorialCurrent, the return flow from the Kuroshio toward thesouthwest described by Sverdrup, Johnson, and Flem-ing (1942) is only marginally discernible. In a latercompilation of the average drift Stidd, 1974), it issomewhat clearer.This surface circulation had been generally acceptedas wind driven, but the depth to which it extended, orto which any wind-driven current extended, was notknown. It is not clear what was generally believed, orwhy, but the impression left from reading the variouspapers on this subject is that it was very shallow overmost of the ocean.Information about the subsurface circulation arosefrom a different source. Measurements of water char-acteristics began in the eighteenth century. Prestwich(1875) reviewed them and the various interpretationsthat had been made. The measurements were mostlyof temperature with some of salinity. Very few hadreached abyssal depths, though there were enough toidentify the Antarctic and Greenland Seas as sourcesof abyssal water. He concluded that all of the water,from top to bottom, is in a state of movement, andthat high-latitude cold waters flow equatorward atabyssal depths from both north and south in the At-lantic, but only from the south in the Pacific and IndianOceans, and that these sources account for the lowsubsurface temperatures of the central oceans.He did not, however, consider only such a simpleconvection model, but worked out some more detailedparts of the system as well. His most interesting inter-pretations are of the details of the shallower subsurfaceflows. He noted that zones of maximum surface tem-perature and salinity in the Atlantic and Pacific are notexactly at the equator but in two zones roughly parallelto it, north and south; that the waters between 10°Nand 10°S in the upper 200 m are colder than those tothe north and south; and that this must result from arising of the deeper, colder waters in that zone, wherethey are moved poleward as they are warmed.He noted the excessive
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