1958 Cochrane 1958 and Pollak 1958 still stand but I believe that their classes are too large One can see that by merging the present classes into divisions as large as 0 5 o of salinity as Cochrane and Pollak have done or 1 0 o as Montgomery has done the high volume warm prongs figure 2 9 would be artificially blurred and merged with one another One or two colleagues have asked whether I could not use a logarithmic volume scale in such presentations as figures 2 2 2 5 so that the warm water masses if included could be made to stand out more clearly but one of the principal virtues of the volumetric T S diagram is that it displays the relative abundances of the water masses as they actually exist The concentration of water in the most abundant North Pacific class exceeds that in the warm water prongs shown in figure 2 9 by a ratio of about 25 973 to 10 or less This is analogous to comparing the elevation of Mount Everest to that of Water Street Woods Hole near the original building of the Woods Hole Oceanographic Institution In fact if we were able to sample and measure salinity more perfectly the apparent elevations shown in the deep water in figure 2 8 would probably be even higher The feelings I have about the census are compounded equally of fascination and frustration The frustration is the result of the decrease in the rate of acquisition of new high quality data This decrease is due in part to the trends in modem physical oceanography in which the dramatic improvements in direct current measurements have understandably taken priority over routine measurements of water properties on a large scale It is also clear that there is a long delay as much as 5 years between the time hydrographic data are obtained at sea and the time these data become available on tape from the National Oceanographic Data Center in part because some investigators take a long time to turn their data in to the Data Center I have been reluctant to obtain new data informally from friendly colleagues however because I do not think that the Data Center should be bypassed at present its function would be impaired if data were only exchanged between a cabal of skilled observers The fascination results from the precise but peculiar way in which the water masses of the oceans are arranged particularly the deep water masses that make up the greater part of the oceans Why for instance are the big exclusive North Pacific classes fresher than existing circumpolar and South Pacific waters Are they fossil water masses that were formed in some past millennium when the oceans were somewhat fresher or are they still undergoing a change toward the fresher as the result of slow vertical mixing across density surfaces with the still fresher water that lies above them at the present time I do not think that we can supply answers to such questions at present and an swers will not be availableeven in the future without painstaking observations There are indications this style of observations may be coming back into vogue The authorless Scripps data report of the INDOPAC expedition Scripps Institution of Oceanography Reference 78 21 is an excellent example It should be worthwhile to reactivate this census which was closed as of June 1977 when more high quality data of this kind are available from NODC and I shall probably propose to do so at some time in the future 2 4 The Formation of Water Masses There is only one hypothesis about water mass formation that is universally agreed upon that is that the cold dense water that fills the great ocean basins has been formed at high latitudes The manner in which the thermocline halocline is formed is under dispute and there are almost as many notions of the rate at which all the various water masses are formed as there are investigators Given the extraordinary regularity of the T S curves that are found in much of the oceans it is natural to assume that these curves are the result of vertical mixing between two end water masses Very simply stated this assumption implies that the bottom water as all agree has been formed at high latitudes that the surface water at middle and low latitudes has received its T S characteristics from the atmosphere by the uneven processes of evaporation and heating and that the remainder of the water column is a mixture of surface and bottom water Wiist 1935 clearly recognized that this was an oversimplification and his use of the corelayer method reflects his conviction that different water masses can be traced to a small number of moreor less point sources at the sea surface over a wide range of latitude The notion that all the thermocline water masses can be traced to the sea surface is generally attributed to Iselin 1939a He constructed a T S diagram from winter observations at the surface of the western North Atlantic and found that it corresponded closely to the T S diagram obtained from a typical hydrographic station in that ocean It is worth noting that Wiist 1935 p 3 anticipated Iselin in the South Atlantic by 4 years He wrote The vertical structure of the Subantarctic Intermediate Water with its horizontal spreading at depths is analogous to a vertical figure of the horizontal arrangement of temperature and salinity at the surface of the formation region Wiist did not dwell on this subject further and it is clear that he regarded core layers as more important as indices of ocean circulation In his 1939a paper Iselin stressed lateral mixing as responsiblefor the T S curve in the western North Atlantic Sverdrup in chapter XV of The Oceans Sver57 Water Masses of the World Ocean A Fine Scale Census drup Johnson and Fleming 1942 amplified Iselin s concept he suggested that subtropical convergences were the dominant source of the waters in the thermocline halocline In these convergences according to Sverdrup surface water sinks in late winter over a wide range of latitude He compared late winter seasurface T S points to the T S curves obtained from subsurface hydrographic data and found a close correspondence in the south Indian Ocean the eastern and western South Pacific and the western North Pacific just as Iselin 1939a had done for the North Atlantic In this type of water mass formation very little change takes place in each individual water typewhen 10 C water outcrops at the sea surface it sinks again at 10 C or nearly so not at 40 C In vertical mixing models the water types are constantly changing In the most violent of these Stommel 1958
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