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MIT 12 000 - The Water Masses of the World Ocean

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2The Water Massesof the World Ocean:Some Resultsof a Fine-Scale CensusL. V. Worthington2.1 IntroductionIn his original brief monograph, Helland-Hansen (1916)introduced the concept of a water mass as being definedby a temperature-salinity (T-S) curve. He found thatover a large area of the eastern North Atlantic a "nor-mal" T-S curve could be drawn. He showed that vari-ations from this curve could be attributed to the intru-sion of alien water masses that had originatedelsewhere. The use of the T-S diagram has been almostuniversal in physical oceanography since Helland-Han-sen introduced it. It is not only a powerful descriptivetool, but observers at sea routinely plot T-S diagramsand use them as a check on the tightness of theirsampling bottles and the correct function of their ther-mometers.The term "water mass" has been very loosely usedby numerous authors. According to Sverdrup, Johnson,and Fleming (1942), a water mass is defined by a seg-ment of a T-S curve, and a "water type" by a singlevalue of temperature and salinity that usually falls ona T-S curve. Thus a T-S curve is made up of an infinitenumber of "water types." These definitions will beadhered to in this chapter as far as is possible. Ocean-ographers have used other methods to describe theocean, both before and after Helland-Hansen's intro-duction of the T-S diagram, and these methods will bebriefly discussed below, but I will deal primarily withthe world water masses as defined by T-S diagrams.The most important advance in water-mass analysissince Helland-Hansen came with the introduction byMontgomery (1958), Cochrane (1958), and Pollak (1958)of the volumetric T-S diagram, in which the volumesof all the world water masses were estimated. Thevolume of the world ocean, including adjacent seas, is1369 x 106 km3. Montgomery, Cochrane, and Pollakwere able to divide the individual and world oceansinto bivariate classes of temperature and salinity, eachof which contained an assigned volume. For example,the most abundant class found by Montgomery (1958)in the world ocean was T = 1.0-1.5°C, S = 34.7-34.8%o; he calculated that this relatively small classcontained 121 x 106 km3, or 9% of the water in theocean.Wright and Worthington (1970) produced a volumet-ric census of the North Atlantic that was a direct de-scendant of Montgomery's (1958) work. This later cen-sus was motivated by the introduction, pioneered bySchleicher and Bradshaw (1956), of the very accuratesalinometers based on the measurement of electricalconductivity. The precision of data obtained with thesesalinometers enabled Wright and Worthington (1970)to divide the North Atlantic into much smaller classesthan Montgomery and his colleagues had used: Wrightand Worthington's smallest class (below 2C) was0.1°C x 0.01Oo. Fifty of these classes make up one of42L. V. WorthingtonMontgomery's classes (0.5°C x 0.1%o). This fine-scalecensus had clear advantages over the coarser-scale cen-sus that inspired it, and in consequence I undertook acensus of the world-ocean water masses using the fine-scale classes that Wright and Worthington (1970) in-troduced. The greater part of this paper will be devotedto the presentation of the results of this census, withsome discussion of the formation of these watermasses.2.2 Methods of Describing the OceansThe simplest and the most universally used method ofdescribing the oceans has been the preparation of ver-tical profiles of temperature, salinity, dissolved oxygen,or some other variables, constructed from oceano-graphic sections made across an ocean, or part of anocean, from a ship or a number of ships. Ocean-widetemperature profiles have been drawn by oceanogra-phers since Thomson's (1877) treatment of the Chal-lenger sections, but the standard of excellence for thiskind of presentation was set by Wiist and Defant (1936)in their atlas of the temperature, salinity, and densityprofiles from the Atlantic Meteor expedition of 1925-1927 and by Wattenberg (1939), who prepared the ox-ygen profiles. These vertical profiles were drawn incolor, with detailed bottom topography provided. Theatlas by Wist and Defant (1936) provided the model forFuglister's (1960) atlas of vertical profiles of tempera-ture and salinity from the transatlantic sections madeby various ships and observers during the InternationalGeophysical Year. Later, Worthington and Wright(1970) drew similar profiles, for sections made by theErika Dan in the northern North Atlantic in 1962.They also included dissolved-oxygen profiles modeledon those of Wattenberg, which Fuglister had been un-able to do because of the poor quality of oxygen anal-yses made from Woods Hole ships during the Interna-tional Geophysical Year.Probably the finest example in this form is that ofthe vertical profiles by Stommel, Stroup, Reid, andWarren (1973) for the transpacific sections at 28°S and43°S from Eltanin in 1967. These profiles are shown insix color plates; the variables are temperature, salinity,oxygen, phosphate, nitrate, and silicate. The stationand sample-bottle spacing for these sections were care-fully planned so as not to miss any important baro-clinic gradient or variation in nutrient concentration.Composite vertical profiles are often drawn fromdata provided by a number of ships from different yearsor even different decades. Such sections are, of course,less useful for dynamical studies, but sometimes pro-vide an excellent description of the water. A fine ex-ample is that of Wiist's much cited north-south tem-perature, salinity, and oxygen profiles in the Atlantic(Wiist, 1935, plate XXIII). Reid (1965) used the samemethod to construct zonal and meridional profiles oftemperature, salinity, oxygen, and phosphate across thePacific.Helland-Hansen and Nansen (1926) drew verticalprofiles of temperature and salinity superimposed oneach other in their work on the eastern North Atlantic.This method has been followed by others, notably Tait(1957). I find such profiles difficult to read, but thatmay be idiosyncratic. Helland-Hansen and Nansen(1926) also introduced vertical profiles


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