The Observed Climatology - Climate Dynamics - page 31

• The Antarctic Circumpolar Current, or the “West Wind Drift,” is the only
current that circles the globe. This is possible because there is no land at any
longitude between about 50°S and 60°S. The water must all be funneled
through the narrowest point, the Drake Passage, at the tip of South America.
The currents within an ocean basin can be connected into gyres , which are
roughly circular current systems that flow anticyclonically in each ocean basin.
For example, the North Pacific gyre consists of the Kuroshio, North Pacific, Cal-
ifornia, and North Equatorial Currents. The western boundary currents within
the gyres are faster, narrower, and deeper than the eastern boundary currents.
Note that the directions of the surface currents are similar to lower-
tropospheric wind directions ( Fig. 2.13) . In the tropics, off the equator, the
surface currents are easterly, and in the middle latitudes the surface currents
are westerly except close to the coasts. The boundary currents close the circu-
lation within each ocean basin. Surface winds are 5-15 m/s on average ( Fig.
2.13) , but current speeds are only a few percent of the low-level wind speed.
The Gulf Stream velocity, for example, is 15-20 cm/s in subtropical and middle
latitudes, and its velocity drops to roughly 2-4 cm/s as it turns eastward to
cross the Atlantic.
The influence of the boundary currents can be seen in sea surface tempera-
ture distributions ( Figs. 2.15 and 2.16) . For example, the Peru Current in the
eastern South Pacific Ocean basin transports cooler surface waters equator-
ward and helps maintain the cold tongue structure and, thereby, the longitu-
dinal temperature gradient that is characteristic of the tropical Pacific Ocean.
The relatively warm waters of the eastern Atlantic Ocean in northern midlati-
tudes are caused by the northward transport of warm tropical water by the
Gulf Stream.
There are similar correlations between surface currents and the surface sa-
linity distribution ( Fig. 2.19) . The western North Atlantic basin is saltier than
the eastern basin because the Gulf Stream transports salty water of Mediter-
ranean origin northward. Other examples are the freshening of the eastern
portions of the North Pacific and South Atlantic basins by the California and
Benguela Currents, respectively.
In addition to the essentially horizontal surface currents (Fig. 2.22) , the ocean
has a coherent global-scale three-dimensional circulation. While ocean surface
currents are largely driven by surface winds, the three-dimensional circulation
is driven by density differences. It is known as the thermohaline circulation be-
cause both temperature and salinity influence seawater density (see chapter 5 ).
Figure 2.23 illustrates this global circulation system, known as the great
ocean conveyor belt . Many details of the large-scale ocean circulation are not
known, since the volume of the ocean to be monitored is immense and dif-
ficult to reach, for the most part. In addition, the circulation is slow and the
time scales are very long—upward of 1000 years for the global-scale ocean
circulation—so a few decades of observations will not reveal the full scope of
the motion. (Some of the techniques used by oceanographers to piece together
a picture of the ocean's global circulation system are discussed in chapter 8. )
According to the ocean conveyor belt model ( Fig. 2.23a ), water sinks in the
North Atlantic into the deep ocean (~2-4 km). This is known as the North