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Global Water Masses : Summary and Review

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Global Water Masses : Summary and Review )' ~ ----------------------------------~O~C=EA~N~O~LO~G~I~C~A~A~C~T~A~1~9~86~-~V~O~L.~9~-~N~o~4~~~-----·j Water masses Global distribution Temperature Global water masses: Salinity Masses d'eau Répartition globale summary and review Température Salinité W. J. EMERY a, J. MEINCKE b a Department of Oceanography, University of British Columbia, Vancouver, B.C., l V6T 1W5, Canada. b Institut für Meereskunde der Universitat, Hamburg, Heimhuder Str. 71, 2000 Ham­ burg 13, FRG. Received 12/11/85, in revised form 2/4/86, accepted 7/4/86. ABSTRACT Using published material the temperature-salinity characteristics of the world's water masses are defined for the upper, intermediate and deep/abyssal regions of the ocean. The global distributions and boundaries of these water masses are mapped along with indications of their formation regions. Representative temperature-salinity relation­ ships are presented for each of the three major oceans. Oceanol. Acta, 1986, 9, 4, 383-391. RÉSUMÉ Les principales masses d'eau de l'océan mondial Les caractéristiques température-salinité des masses d'eau de l'océan mondial ont été définies en compulsant les données publiées. Pour chacune des couches, superficielle, intermédiaire et profonde/abyssale, une carte indique la répartition globale, les limites et les zones de formation de ces masses d'eau. Les diagrammes température-salinité sont présentés pour chacun des trois grands océans. Oceanol. Acta, 1986, 9, 4, 383-391. INTRODUCTION updated global water mass summary in compact form rather than review ali the regionally specifie water mas­ It is surprising that while many oceanographie observa­ ses that have been identified and studied. tions have been collected and analyzed since Sverdrup In carrying out this effort two things were immediately et al. (1942) published their familiar estimates of the apparent. First, as stated by Wright and Worthington global distributions of water masses very few new (1970): "The nomenclature of the water masses in the descriptions of these distributions have been attempted North Atlantic Ocean has become bewilderingly on a global scale. Many publications have examined confused in the past half century, although it is appa­ the details of the water masses in particular regions rent that the water masses themselves have remained but few have attempted to synthesize these regional essentially the same." studies into a comprehensive overview of the global This observation can be easily applied to other oceans distribution. As part of his treatise on TS analysis in as weil and the abundance of water masses presently the global ocean Mamayev (1975) reviewed the charac­ in use forces one to be selective and limit the description teristic limits for the global water masses. In addition to to what are, in broadest terms, the major water masses. his resulting table of global water mass characteristics The second most obvious fact is how weil Sverdrup et Mamayev ( 1975) presents global maps of intermediate al. ( 1942) did in estimating both the main water proper­ and deep water masses. Unfortunately these maps are ties, in terms of their temperature-salinity (TS) charac­ reproduced on a very small scale and it is also difficult teristics, and in determining the global distributions of to determine the criteria used to define the water mass at least the upper layer waters. This is likely one of boundaries. The same problem arises in the characteris­ the main reasons why there has been no effort to tic TS values in his table of global water masses. This improve on their earlier distribution which is widely short note is an attempt to uptake the definitions and available. Nevertheless, there are sorne important chan­ distributions of global water masses from the material ges that have come to light through the analysis of presently available. The purpose is to present this both new and old data in the form of atlases and 0399-1784/86/04 383 09/S 2.90/<e Gauthier-Villars 383 J W.J. EMERY. J. MEINCKE research papers. We draw upon these publications to Intermediate Water nor the North Atlantic Interme­ . i first define temperature and salinity ranges for each of diate Water are identified in our summary. They have the major water masses for each ocean. Then we again been replaced by the Eastern and Western Subarctic ) turn to published presentations to estimate the horizon­ Intermediate Waters and the Mediterranean Water, as tal distributions of these water masses for three vertical will be discussed later. ~ l î layers, the upper (0-500 rn), the intermediate The main upper waters of the central North Atlantic J (500-1500 rn) and the deep/abyssal (1500 rn - bot­ were separated into the Eastern North Atlantic Central 1 t tom). lt was decided to use the classical definition of Water (ENACW) and the Western North Atlantic Cen­ l vertical layers to separate the water masses since our tral Water (WNACW). These water masses and their goal was an update of existing distributions. Since most characteristics (Tab.) were selected from the maps of .t l l water masses trace their origin to the sea surface; this Worthington (1976) and Defant (1936), the sections of •' vertical separation is not going to hold everywhere and Fuglister (1960), the volume census of Wright and it may in general have been more appropriate to sepa­ Worthington (1970), the TS curves of Emery and rate these water masses in terms of density. Dewar (1982), the study by Harvey (1982) and the A natural problem encountered in constructing our recent analysis of Pollard and Pu (1985). Here Wright table and diagrams was the disparate nature of the and Worthington ( 1970) point out that the Western data coverage in the northern and southern hemisphe­ North Atlantic Central Water (WNACW), with a tem­ res. This is most acute in the South Pacifie where few perature range from 7.0-20.ooc and salinities from publications are available that treat the general water 35.0-36. 7, really corresponds to the North Atlantic properties over this large, unsampled area. Here we Central Water identified by Sverdrup et al. (1942). The were forced to use the few sections that were available differentiation between Western and Eastern Central to estimate our distributions. Fortunately, there are waters expresses the physically different characteristics considerably more data and correspondingly more of the eastern and western portions of the northern publications concerned with the South Atlantic. subtropical gyre circulation. In the western part, which Nevertheless, it may be that the greater complexity is dominated by the intense recirculation of the Gulf in the North Atlantic, indicated for the water mass Stream System, relevant isopycnals are generally deeper distribution (especially in the upper waters), reflects the than in the eastern basin. Thus the Western North greater degree to which the North Atlantic bas been Atlantic Central Water (WNACW) is in doser contact studied compared to the South Atlantic. Also the with the fresher underlying subpolar intermediate water Antarctic or Southern Ocean, bas been the focus of (the Western .Atlantic Subarctic Intermediate Water, recent studies which greatly improved our knowledge W ASIW) while in the northeast win ter convection is of the water mass distribution in this important area. able to directly transfer salt from the surface layer into Finally, the Indian Ocean has been weil summarized the central water resulting in salinities for the Eastern in the atlas by Wyrtki (1971) which provides a good North Atlantic Central Water (ENACW) being on the description of the water properties. average 0.1-0.2 higher. The higher salinity may also reflect the influence of the Eastern Atlantic Subarctic Intermediate Water (EASIW), which is saltier due to WA TER MASSES AND THEIR TEMPERA TURE­ its proximity to Mediterranean Water (MW). SALINITY RANGES To the south the North Atlantic Central Waters transi­ tion into the South Atlantic Central Water (SACW) at The major water masses and their TS property ranges about 15°N (Tomczak, 1984a and b). The TS are presented here in the Table for the upper, interme­ characteristics of the SACW were taken from the maps diate and deep/abyssal waters. The surface waters of of Defant (1936) and the sections of Wüst (1935) and the Antarctic or Southern Ocean are treated separately Fuglister (1960). The resulting temperature range is 5.0 in this table. to 18.0°C with a salinity range of 34.3 to 35.8. This water mass is very similar to that identified for the South Atlantic upper layer by Sverdrup et al. (1942). ATLANTIC OCEAN Five intermediate waters were identified in the Atlantic. Four can be identified as salinity minima, due to their Four dominant upper waters were identified for the formation in subpolar regions, and one signifies its Atlantic as shown in the Table. Farthest North the presence as a salinity maximum due to its formation Atlantic Subarctic Upper Water (ASUW) was identi­ through high evaporation in a marginal sea. Among fied by Wright and Worthington (1970) in terms of the fresh intermediate waters, coming from the north, source water for what they identified as Subarctic the Arctic Intermediate Water (AIW) plays a special Intermediate Water and North Atlantic Intermediate rote since in the region of the Greenland-Scotland sill Water (in our discussion ali water mass names will be it becomes a major constituent of the Denmark Strait capitalized white only those we have Iisted in our sum­ and lceland-Scotland overflowes thus becoming a mary table will receive an acronym title as weil). Taking source for North Atlantic Deep Water (NADW). The the values from these two components along with a TS two intermediate waters formed south of the diagram for the North Atlantic Intermediate Water by Greenland-Scotland sill show significant salinity diffe­ Wüst (1935) we arrived at the TS ranges of 0.0-4.0°C rences between the western and eastern basins.
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