Ocean Circulation Systems - Climate Dynamics - page 142

time scales, stabilization (weakening) of the thermohaline circulation occurred
at the end of the last glacial period when the melting of the great North Ameri-
can glaciers injected massive amounts of freshwater into the North Atlantic.
A naturally occurring transient tracer in the ocean is dissolved molecular
oxygen. Surface waters become saturated with molecular oxygen due to ab-
sorption from the atmosphere and photosynthetic activity in the euphotic zone ,
or even supersaturated when ocean surface waves break. As these young sur-
face waters move away from the atmosphere/ocean interface, that is, as the
ocean is ventilated, oxygen is consumed in the oxidation of detritus and dis-
solved organic material. As a result, the level of oxygen saturation of a sample
of ocean water is a measure of the age of the water, or how recently that water
was at the surface.
Figure 8.4a shows a north/south cross section of oxygen saturation values
in the Atlantic Ocean at 34.5°W longitude (to avoid land). Consistent with
the inferences from tritium and temperature distributions, young water with
saturation levels greater than 80% penetrates to the ocean bottom in the North
Atlantic. The water with lowest oxygen saturation levels in the Atlantic, known
as old water , is not located in the deep ocean but, rather, at a depth of about
500 m in the tropics.
The distribution of dissolved oxygen and, therefore, the large-scale circu-
lation is somewhat different in the Pacific. As seen in Figure 8.4b, the oldest
Pacific basin waters are found between 500 m and 2500 m depth in middle
northern latitudes. These are the oldest waters in the ocean, with oxygen satu-
ration levels below 20% and an estimated elapsed time since contact with
the surface of 1000 years or more. There is no deep water formation at high
northern latitudes in the Pacific due to geography, since the basin extends only
to about 60°N.
The formation of bottom water along the southern shores of Antarctica is
indicated in Figure 8.4b. The Antarctic bottom water flows north (see Fig. 2.23) ,
beneath the deep water formed by sinking in the North Atlantic and guided by
topography, with temperatures from 0°C to 1°C and salinity of about 34.7
psu. When sea ice forms and expands the ice shelves of the Weddell and Ross
Seas in winter, the salinity of the underlying water is increased because most of
the salt is not incorporated into the ice. This process is called brine exclusion or
brine rejection . The resulting increase in sea water density drives the Antarctic
bottom water formation. Another mechanism that supports the bottom water
formation is cooling of surface waters when areas of open water, known as po-
lynyas, form in the sea ice along the Antarctic coast. Katabatic winds blowing
from the continent keep polynyas open by pushing the constantly forming sea
ice to the south. Because the open surface waters have high salinity (due to brine
exclusion) and low temperatures, they sink to the ocean bottom.
8.3 VERTICAL MIXING PROCESSES
So far we have discussed two types of large-scale ocean circulation systems,
namely, the wind-driven currents of the mixed layer and the density-driven
circulation of the global ocean. The thermohaline circulation intersects the