3 On the Mid Depth Circulation of the World Ocean Joseph L Reid 3 1 Introduction There is a large part of the ocean circulation for which we have very little information and very vague concepts This is the great domain of the mid depth ocean We have considerable information about the flow at and quite near the sea surface and some inferences about the abyssal flow derived mostly from the traditional patterns of characteristics at the bottom Recently some attention has been focused on the deep western boundary currents where the flow is strong enough to be detected both in the density field and in some cases by direct measurement But for the greater part of the volume of the ocean beneath the upper kilometer and away from the western boundary currents and above the abyssal waters we have little information on or understanding of the circulation Most treatments of the deep water as well as the abyssal water have dealt in terms of the western boundary flow and a general meridional flow is all that has emerged from most of the studies WiUst 1935 for example assumed a principally thermohaline meridional flow to obtain from the abyssal layer up through his Subantarctic Intermediate Water at depths above 1 kin with no recognizable pattern of gyral flow analagous to the surface circulation It seems worthwhile to consider what information there is for this great volume of water This study will begin with a general discussion of the earlier ideas on this problem It will review briefly the recent work of the last 10 years or so which has begun to make substantial contributions and will display and discuss some world wide mid depth patterns of characteristics and of geostrophic vertical shear There is no simple distinction between the upper waters the deep and abyssal waters and what I shall call the mid depth waters A working definition will be that the mid depth waters are those that are found between about 1 and 3 km in middle and low latitudes and their source waters which are shallower in high latitudes Warren s study this volume chapter 1 of the deep circulation includes some of these waters of course and I have tried to avoid duplication Some duplication remains however in part for immediate clarity and in part for different emphasis 3 2 The Circulation of the Upper Waters and Their Contribution to the Mid Depths Our first information about general ocean circulation came from the experience of mariners crossing the great oceans They found the best routes for eastward travel to be in the zone of the west winds and for westward travel in the trades and noted early the western boundary currents As the information accumu70 Joseph L Reid lated these findings led by the middle of the nineteenth century to the general concept of subtropical anticyclonic gyres subarctic gyres and various zonal flows near the equator The variability of this general pattern was learned early and is most clearly presented in the sailing directions coast pilots and atlases prepared by the various hydrographic offices For example the typical atlas of surface currents of the northwestern Pacific Ocean U S Navy Hydrographic Office 1944 provides information 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 octant While this can give no information on the frequency of the variations each measurement represented a mean of 12 to 24 hours or longer the presence of variation is clearly shown everywhere and the general findings of Fuglister 1954 Dantzler 1976 and Wyrtki Magaard and Hager 1976 are to some degree anticipated But in spite of the variability and the smoothing effects in taking its mean certain major features of the gross field stand out On this particular atlas the strongest of these are the Kuroshio and the North Equatorial Current The West Wind Drift around 40 N is also clear though weaker But in the area between the Kuroshio West Wind Drift and the North Equatorial Current the return flow from the Kuroshio toward the southwest described by Sverdrup Johnson and Fleming 1942 is only marginally discernible In a later compilation of the average drift Stidd 1974 it is somewhat clearer This surface circulation had been generally accepted as wind driven but the depth to which it extended or to which any wind driven current extended was not known It is not clear what was generally believed or why but the impression left from reading the various papers on this subject is that it was very shallow over most of the ocean Information about the subsurface circulation arose from a different source Measurements of water characteristics began in the eighteenth century Prestwich 1875 reviewed them and the various interpretations that had been made The measurements were mostly of temperature with some of salinity Very few had reached abyssal depths though there were enough to identify the Antarctic and Greenland Seas as sources of abyssal water He concluded that all of the water from top to bottom is in a state of movement and that high latitude cold waters flow equatorward at abyssal depths from both north and south in the Atlantic but only from the south in the Pacific and Indian Oceans and that these sources account for the low subsurface temperatures of the central oceans He did not however consider only such a simple convection model but worked out some more detailed parts of the system as well His most interesting interpretations are of the details of the shallower subsurface flows He noted that zones of maximum surface temperature and salinity in the Atlantic and Pacific are not exactly at the equator but in two zones roughly parallel to it north and south that the waters between 10 N and 10 S in the upper 200 m are colder than those to the north and south and that this must result from a rising of the deeper colder waters in that zone where they are moved poleward as they are warmed He noted the excessive salinity of the Mediterranean and the very high temperature at great depth He explained the high temperature compared to that in the Atlantic by the presence of the sill at the Straits which excluded the colder waters of high latitudes and winter overturn within the Mediterranean that gave the bottom waters the same temperature as the surface minimum value He noted that the salt balance had been explained by surface inflow and subsurface outflow and noted that water with characteristics similar to those within the
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