Ocean Circulation Systems - Climate Dynamics - page 141

as undercurrents . These currents are sometimes described as jets because they
tend to be narrow (~200 m) and fast (~1.5 m/s).
8.2 THE DENSITY-DRIVEN CIRCULATION:
THE THERMOHALINE CIRCULATION
Away from the surface, the thermohaline circulation (Fig. 2.23) is driven by
density differences related to temperature (Fig. 2.17) and salinity (Fig. 2.21)
distributions. The circulation is slow and time scales are long—1000 years and
longer for the global-scale ocean circulation.
The word's oceans are vast and mostly far from human habitation, making
them difficult to observe. To supplement direct measurements, transient trac-
ers are used to infer large-scale circulation in the ocean. These are substances
that are introduced at the ocean surface, transferred downward, and circulated
around the globe in a process known as ocean ventilation . Some of these trac-
ers are of human origin and others are naturally occurring.
The testing of atomic bombs in the earth's upper atmosphere began in the
1950s and persisted until the Test Ban Treaty of 1963 prohibited the deto-
nation of nuclear weapons in outer space, underwater, or in the atmosphere.
These detonations produced an anthropogenic source of tritium ( 3 H), an iso-
tope of hydrogen with a half-life of 12.45 years, that is much larger than the
natural source due to the bombardment of molecular hydrogen by cosmic rays.
Because tritium is chemically and biologically nonreactive in the atmosphere
and oceans, and does not modify the circulation, it serves as a passive tracer of
the circulation.
In the early 1970s, oceanographic cruises organized to measure tritium dis-
tributions found that 3 H had penetrated to greater depths at higher latitudes.
Whereas the tritium was confined above about 500 m in the tropics and sub-
tropics, it had mixed down to about 2 km in middle latitudes of the Northern
Hemisphere. In the North Atlantic Ocean, high tritium levels were found down
to the seafloor at about 4 km. Note that this tritium distribution is similar to
the thermal structure, since North Atlantic surface waters are at about the
same temperature as Atlantic deep waters ( Fig. 2.17) . Both distributions are
evidence of the North Atlantic deep water formation discussed in section 2.2.
The Gulf Stream carries the warm, high-salinity water of the subtropical
Atlantic ( Fig. 2.19) to higher latitudes. The North Atlantic Ocean basin opens
into the Labrador Sea and the Arctic Ocean, so the high-salinity flow continues
north as the North Atlantic Drift ( Fig. 2.22) . Exposure to the cold, dry atmo-
sphere at high latitudes drives strong longwave, sensible, and latent heat fluxes
from the surface water ( Figs. 5.11a and 5.12) . The longwave back radiation is
small due to low levels of moisture in the Arctic atmosphere ( Fig. 5.11b) , so the
waters cool rapidly to 1.0°C-2.5°C. This strong cooling, combined with high
salinity, produces high density and sinking.
Changes in the North Atlantic deep water formation influence the large-scale
thermohaline circulation system. Decadal-scale variations in sinking rates, tem-
perature, and salinity are documented, so it is clear that the system changes on
time scales comparable to those of anthropogenic climate change. On longer