Physical Oceanography: An Overview
2.10 The Thermohaline Circulation
Poleward of perhaps 30" to 35"N or S , the wind-driven surface currents begin to give up rather than to receive net thermal energy from the atmosphere via the processes of evaporation, radiation, and conduction, thereby warm- ing the air and cooling the surface layers of the water. By the time these cur- rents have made their way to the polar regions, they have lost much heat and have greatly reduced their temperature and have thereby increased their density. In the regions near the Norwegian and Greenland Seas in the North- ern Hemisphere, and again in the Antarctic Circumpolar Current near that continent's borders, the surface temperatures have dropped to the vicinity of 0.5" to - 2.0"C (cf. Fig. 2.18). The densities have increased to values where the surface waters become heavier than the subsurface waters, and sinking and convective overturning occur. In the Norwegian Sea, this sinking water flows slowly south along the bottom, crossing over the relatively shallow un- derwater sills connecting Greenland, Iceland, and the British Isles, where it undergoes a certain amount of mixing with warmer water above and there- by raises its temperature slightly. From there it continues to flow south along the bottom in a very indistinct, density-driven circulation that creeps at ve- locities too slow to be directly measured. This water mass is called North Atlantic Deep Water (NADW) and is shown schematically in Fig. 2.22 as extending as far south as 45" to 50"s. Such flows are termed thermohaline circulations and can usually be identified by the distributions of certain water type properties, particularly by their dissolved oxygen (DO) and salinity- temperature-depth (STD) correlations. The bottom temperature of NADW is near 2°C and salinity 34.95 psu (practical salinity units, formerly parts per thousand, or Yoo).
In the Antarctic near the sea ice boundaries, especially in the Weddell Sea, a similar sinking process occurs, now aided and abetted by the formation of the ice, which rejects much salt upon change of phase from liquid to crys- talline and which leaves behind water that is more saline, and thus more dense because of both decreased temperature and increased salt content. This wa- ter also sinks to the bottom to become Antarctic Bottom Water (AABW).
While not initially as dense as NADW because of its lower surface salinity, it undergoes no appreciable mixing across sills as does the North Atlantic Deep Water, and thus it maintains its low temperature and high density. It apparently underflows the NADW and extends as far north as the equator in the Atlantic (see Fig. 2.22). In the Pacific, the analogous northward flow may be traced all the way to 40" to 50"N.
A third, somewhat different process contributes to another deep water mass globally, the Antarctic Intermediate Water (AAIW). In a region near the edge of the Antarctic Current termed the Antarctic Convergence, the surface waters flow together and are forced downward due t o both increased density and the requirement for continuity of flow, there to migrate north at intermedi- ate depths near 1 km and overlie the other water masses. This region is a source of intermediate water in all three major oceans.
The picture of the thermohaline circulation as a deep, broad, and weak flow extending over much of the oceans is probably not altogether correct, since the dynamics of these flows must obey the same laws as their surface counterparts, although with quite different magnitudes. Analytical and nu- merical models suggest a distinctly altered picture from Fig. 2.22; in partic- ular, the deep flow away from the polar regions must occur as a western boundary current in each basin, as suggested in Fig. 2.23. The return flow is thought to be eastward and poleward everywhere and is much more dif- fuse, but in theory cannot cross the equator. Also a large eastward flow at depth accompanies the surface portion of the Antarctic Circumpolar Cur- rent. The models suggest a total deep western boundary transport of about 10 Sv. However, the picture is relatively uncertain and the subject remains an active research area.
One other less important but nevertheless interesting thermohaline flow occurs in the Atlantic at the mouth of the Mediterranean. This sea, which because of low rainfall, small riverine input, and limited communication with the open ocean, has a high salinity-about 39.1 psu in its eastern portion.
The dense, salty water makes its way out of the Strait of Gibraltar at about the depth of the sill (450 to 500 m) and protrudes into the Atlantic at depths near 1000 to 1200 m (Fig. 2.24). It can be identified across the entire width of the Atlantic via its properties (see Fig. 2.25) and is denoted in Fig. 2.22 as "MED." The high salinity water gradually spreads and mixes through- out the basin, and is thought t o be the reason that the North Atlantic is,
‘0 W
m -0 c a
9
5 a 0 v) D C m r r 0 z W
C r, .-
v) C c
2! a L
D C m W
0 5
u)0 c
[Adapted from Stommel, H., Deep Sea Research (1958).]
46 Principles of Ocean Physics
Fig. 2.24 Vertical cross section of the Atlantic and the Mediterranean, illustrating the high salinity tongue of water flowing over the sill at the Strait of Gibraltar and sinking to depths of order 1000 m. [Adapted from Schott, G., Geographie des Atlan- tischen Ozeans (1942).]
on the average, saltier than the other oceans: a mean surface salinity of ap- proximately 36.5 psu, compared with perhaps 35.0 psu for the Pacific and Indian Oceans.
As these multiple interleaving flows make their way on a slow but ponder- ous scale, they gradually mix to form a composite water mass constituting perhaps 40% of the volume of the sea.
The sinking motion in the limited areas of the polar regions must be com- pensated by rising motions elsewhere, and it is currently thought that these may take place over relatively large areas of the ocean. The cold, deep water is very slowly mixed upward into the warmer, shallow water, thereby in con- siderable degree establishing the permanent thermocline. The upward veloc- ities are not at all measurable, but the numerical models give estimates of vertical velocities of perhaps m s K 1 (about 3 m yr-'). As a conse- quence, the residence time of deep waters-i.e., the time for them to sink, flow, and be mixed upward again-may be as great as 1000 years.
This thermohaline circulation also explains the oceanic thermocline in a qualitative way. If there were no deep water formation in cold regions, the ocean would gradually warm up throughout its vertical extent due to the heat sources cited earlier and, under the influence of mixing and diffusion, be- come more or less isothermal in depth. The presence of cold waters with clear- ly identifiable properties attests to the continued renewal of the deep via sinking in cold regions, offset by widespread upward flow elsewhere at a rate
Fig. 2.25 Spreading of high-salinity Mediterranean water across the Atlantic at in- termediate depths. [Adapted from Worthington, L. V., and W. R. Wright, North Atlan- tic Ocean Atlas (1970).]
great enough t o balance the downward diffusion of heat and thereby main- tain the vertical temperature gradients.
This sinking of cold water also aids in the removal of heat from the tropics by providing a conduit for surface waters, their thermal free energy now ex- hausted to their surroundings, t o return t o the subsurface system and thus complete the cycle started in the equatorial regions under the forcing of winds that the currents themselves have played a major role in establishing.