Historical Introduction
9
included in the annual reports of the Ice Patrol in the U.S. Coast Guard Bulletins, are
a reproach to the lazy clumsiness of some computer-generated mapping these days.)
4. WATER MASSES
The most formal conceptual device for describing water-property distributions is
the water mass. Helland-Hansen (1916) observed that when the temperatures and
salinities measured at individual stations were plotted against each other, the resulting
curves were similar over broad regions (and markedly different from those obtained
in widely separated regions). This fact suggested the useful concept of the “water
mass,” defined by some appropriate segment of a regional T –S curve, much as an air
mass was defined by a temperature–humidity curve.
Since subsurface temperature and salinity characteristics derive ultimately from
the air–sea exchange at the sea surface, different segments of T –S curves correspond
to different sites and mechanisms of water-mass “formation,” and are so labeled:
e.g., North Atlantic Deep Water or Antarctic Bottom Water as water masses formed
through deep sinking in high latitudes. At thermocline levels the characteristics are
imparted at lower, subtropical latitudes; Iselin (1939) emphasized that different portions of (vertical) T –S curves in the North Atlantic thermocline match closely the
late-winter T –S conditions in different regions of the sea surface there, and he suggested that the thermocline curves were formed through lateral mixing from the sea
surface along isopycnal surfaces. Because oxygen is consumed in the decay of sinking detritus, levels of oxygen concentration can give a qualitative notion of the local
“age” of a water mass, i.e., how long since the water was “renewed” by exposure
to the atmosphere. Oxygen concentration is sometimes used, in fact, as a subsidiary
diagnostic in the definition of a water mass.
Water-mass analysis provides a summary description of the property fields; by
highlighting prominent features and tracing them back to the locations where they
were generated, it helps explain them; and their distribution (similar in concept to
that of core layers) gives an impression of the patterns of prevailing flow and mixing
responsible for their spreading.
Unfortunately, the concept is somewhat amorphous. Different writers have used
different schemes of nomenclature, and when enthusiasts for fine distinctions proliferate names across the ocean (especially names reduced to letter sequences) with such
vigor that the bewildered reader needs a glossary, the enterprise founders. And some
writers have misconceived their water masses more as objective building blocks or
primordial solutions than as designations for features in continuous property fields.
The outstanding achievement of water-mass analysis was Sverdrup’s famous
Chapter XV in The Oceans (Sverdrup et al., 1942). He contrived enough names
to cover the major features of the world ocean—but not so many as to befuddle a
reader—and designed them to relate different ocean regions (e.g., “Central Water” of
the South Atlantic or the North Pacific or the Indian Ocean). He combined this with
9
included in the annual reports of the Ice Patrol in the U.S. Coast Guard Bulletins, are
a reproach to the lazy clumsiness of some computer-generated mapping these days.)
4. WATER MASSES
The most formal conceptual device for describing water-property distributions is
the water mass. Helland-Hansen (1916) observed that when the temperatures and
salinities measured at individual stations were plotted against each other, the resulting
curves were similar over broad regions (and markedly different from those obtained
in widely separated regions). This fact suggested the useful concept of the “water
mass,” defined by some appropriate segment of a regional T –S curve, much as an air
mass was defined by a temperature–humidity curve.
Since subsurface temperature and salinity characteristics derive ultimately from
the air–sea exchange at the sea surface, different segments of T –S curves correspond
to different sites and mechanisms of water-mass “formation,” and are so labeled:
e.g., North Atlantic Deep Water or Antarctic Bottom Water as water masses formed
through deep sinking in high latitudes. At thermocline levels the characteristics are
imparted at lower, subtropical latitudes; Iselin (1939) emphasized that different portions of (vertical) T –S curves in the North Atlantic thermocline match closely the
late-winter T –S conditions in different regions of the sea surface there, and he suggested that the thermocline curves were formed through lateral mixing from the sea
surface along isopycnal surfaces. Because oxygen is consumed in the decay of sinking detritus, levels of oxygen concentration can give a qualitative notion of the local
“age” of a water mass, i.e., how long since the water was “renewed” by exposure
to the atmosphere. Oxygen concentration is sometimes used, in fact, as a subsidiary
diagnostic in the definition of a water mass.
Water-mass analysis provides a summary description of the property fields; by
highlighting prominent features and tracing them back to the locations where they
were generated, it helps explain them; and their distribution (similar in concept to
that of core layers) gives an impression of the patterns of prevailing flow and mixing
responsible for their spreading.
Unfortunately, the concept is somewhat amorphous. Different writers have used
different schemes of nomenclature, and when enthusiasts for fine distinctions proliferate names across the ocean (especially names reduced to letter sequences) with such
vigor that the bewildered reader needs a glossary, the enterprise founders. And some
writers have misconceived their water masses more as objective building blocks or
primordial solutions than as designations for features in continuous property fields.
The outstanding achievement of water-mass analysis was Sverdrup’s famous
Chapter XV in The Oceans (Sverdrup et al., 1942). He contrived enough names
to cover the major features of the world ocean—but not so many as to befuddle a
reader—and designed them to relate different ocean regions (e.g., “Central Water” of
the South Atlantic or the North Pacific or the Indian Ocean). He combined this with