29
Descriptive Oceanography
south, of Arctic intermediate waters and the flow, north, of Antarctic intermediate waters.
The Pacific subarctic waters (S = 33.5 to 34.5 and t = 2 to 4°C) are more extensive than the
Atlantic subarctic waters. They are formed at the subarctic convergence (western Pacific)
between the more saline Kuroshio Extension and the cold Oyashio Current waters.
The Antarctic intermediate waters in the Pacific (t = 2.2°C and S = 33.8) form at the
Antarctic convergence. It is limited in its northern extension by the Pacific equatorial
waters. In the Atlantic, the equatorial waters are not as clearly defined, and the intermediate waters go across the equator. The North Pacific intermediate waters are delineated by
a salinity minimum at 800 m in the west and 300 m in the east. These waters appear to
circulate in a clockwise gyre similar to surface waters. The high O 2 values indicate that
the waters are replenished with surface waters. Reid (1973) has pointed out, however, that
the density of these waters differs from those at the surface. He thus concluded that these
intermediate waters attain their properties below the surface.
The deep Pacific waters (2000 m to the bottom) have temperatures of 1.1 to 2.2°C and
salinity of 34.65 to 34.75. The salinity increases or remains constant with depth, unlike the
Atlantic, where a salinity maximum is observed. The reason for the uniformity is that no
deep water is formed in the North Pacific; it is a sink for North Atlantic (and other) deep
waters. In the South Pacific, bottom waters are supplied from the Antarctic circumpolar
waters. The temperature of these bottom waters increases from 0.9°C in the Antarctic to
1.5°C in the North Pacific. This is thought to be due to heat flow from the interior of the
earth. The deep and bottom water movements in the Pacific are very slow, and only a little
exchange occurs between the South and North Pacific. The deep waters of the Pacific and
Indian Oceans have similar properties (t = 1.5°C and S = 34.7) and form the largest water
mass of the world oceans, called oceanic or common water. It is largely a mixture formed
from North Atlantic deep water and Antarctic bottom waters. As will be discussed, the
deep Pacific waters have much lower O 2 and higher nutrients than deep Atlantic waters.
This is due to the waters being older and due to the high productivity of some of the surface waters. The Pacific has no source of high- salinity water like the Mediterranean in the
Atlantic and the Red Sea in the Indian Ocean.
The T- S diagrams for the major water masses in the Pacific are shown in Figure 1.37.
The water masses are more complicated than in the Atlantic because of the size of the
Pacific. The upper water masses below the surface have a variety of properties. Between
100 to 800 m, the North and South Pacific waters and the Pacific equatorial water masses
dominate. The equatorial water extends over the entire ocean and has a very uniform T- S
relationship. The surface waters are separated from the equatorial waters by a strong thermocline that inhibits the vertical transfer of waters. The depth of the discontinuity layer
decreases from 150 to 200 m in the west to 50 m or less in the east. The layer sometimes
reaches the surface near the American coast and is called the Costa Rican thermal dome.
The equatorial undercurrent in the Pacific, called the Cromwell Current, has been studied for some time. Its rediscovery by Cromwell, Montgomery, and Stroup in 1954 caused
a great deal of excitement because it had not been predicted by theory. The current was
discovered by tracking drogues that were released at different depths. The submerged
current was observed as a narrow undercurrent above the thermocline. The technique was
similar to the methods used by Buchanan in the nineteenth century to study the Atlantic
undercurrent. The velocity of the eastward flow reaches speeds as high as 120 cm s –1 , and
transport is about 40 Sv. The South Equatorial Current flows above the undercurrent
and separates it from the surface countercurrents to the north and south. It should be
pointed out that the vertical circulation in the equatorial Pacific is similar to the Atlantic.
Descriptive Oceanography
south, of Arctic intermediate waters and the flow, north, of Antarctic intermediate waters.
The Pacific subarctic waters (S = 33.5 to 34.5 and t = 2 to 4°C) are more extensive than the
Atlantic subarctic waters. They are formed at the subarctic convergence (western Pacific)
between the more saline Kuroshio Extension and the cold Oyashio Current waters.
The Antarctic intermediate waters in the Pacific (t = 2.2°C and S = 33.8) form at the
Antarctic convergence. It is limited in its northern extension by the Pacific equatorial
waters. In the Atlantic, the equatorial waters are not as clearly defined, and the intermediate waters go across the equator. The North Pacific intermediate waters are delineated by
a salinity minimum at 800 m in the west and 300 m in the east. These waters appear to
circulate in a clockwise gyre similar to surface waters. The high O 2 values indicate that
the waters are replenished with surface waters. Reid (1973) has pointed out, however, that
the density of these waters differs from those at the surface. He thus concluded that these
intermediate waters attain their properties below the surface.
The deep Pacific waters (2000 m to the bottom) have temperatures of 1.1 to 2.2°C and
salinity of 34.65 to 34.75. The salinity increases or remains constant with depth, unlike the
Atlantic, where a salinity maximum is observed. The reason for the uniformity is that no
deep water is formed in the North Pacific; it is a sink for North Atlantic (and other) deep
waters. In the South Pacific, bottom waters are supplied from the Antarctic circumpolar
waters. The temperature of these bottom waters increases from 0.9°C in the Antarctic to
1.5°C in the North Pacific. This is thought to be due to heat flow from the interior of the
earth. The deep and bottom water movements in the Pacific are very slow, and only a little
exchange occurs between the South and North Pacific. The deep waters of the Pacific and
Indian Oceans have similar properties (t = 1.5°C and S = 34.7) and form the largest water
mass of the world oceans, called oceanic or common water. It is largely a mixture formed
from North Atlantic deep water and Antarctic bottom waters. As will be discussed, the
deep Pacific waters have much lower O 2 and higher nutrients than deep Atlantic waters.
This is due to the waters being older and due to the high productivity of some of the surface waters. The Pacific has no source of high- salinity water like the Mediterranean in the
Atlantic and the Red Sea in the Indian Ocean.
The T- S diagrams for the major water masses in the Pacific are shown in Figure 1.37.
The water masses are more complicated than in the Atlantic because of the size of the
Pacific. The upper water masses below the surface have a variety of properties. Between
100 to 800 m, the North and South Pacific waters and the Pacific equatorial water masses
dominate. The equatorial water extends over the entire ocean and has a very uniform T- S
relationship. The surface waters are separated from the equatorial waters by a strong thermocline that inhibits the vertical transfer of waters. The depth of the discontinuity layer
decreases from 150 to 200 m in the west to 50 m or less in the east. The layer sometimes
reaches the surface near the American coast and is called the Costa Rican thermal dome.
The equatorial undercurrent in the Pacific, called the Cromwell Current, has been studied for some time. Its rediscovery by Cromwell, Montgomery, and Stroup in 1954 caused
a great deal of excitement because it had not been predicted by theory. The current was
discovered by tracking drogues that were released at different depths. The submerged
current was observed as a narrow undercurrent above the thermocline. The technique was
similar to the methods used by Buchanan in the nineteenth century to study the Atlantic
undercurrent. The velocity of the eastward flow reaches speeds as high as 120 cm s –1 , and
transport is about 40 Sv. The South Equatorial Current flows above the undercurrent
and separates it from the surface countercurrents to the north and south. It should be
pointed out that the vertical circulation in the equatorial Pacific is similar to the Atlantic.