796 OCEANOGRAPHY
explanation given here, with the onset and subsidence depen-
dent on many factors in addition to the surface wind magnitude
and direction, including temperature variations and turbulent
mixing (Send et al., 1987). The phenomenon of downwelling
is induced by the opposite forcing scenario, with the wind
blowing along a coast with the shoreline to the right (left) in
the northern (southern) hemisphere. Downwelling is charac-
terized by a transport of nearshore surface waters downward
and in the offshore direction.
Upwelling is of great concern to ocean scientists because
of its potentially significant impact on climate and water
quality. The offshore bottom water brought to the nearshore
region is often nutrient-rich. This input of nutrients can have
either beneficial or detrimental effects, providing sufficient
food sources to support a large fisheries population (e.g., the
Peruvian coastline) or promoting algal growth and deoxygen-
ation in the coastal waters, as seen in the 1976 shellfish die-
off along the coast of New Jersey (Swanson and Sindermann,
1979). The transport of relatively cold bottom water to the
shallow nearshore region can also alter the heat exchange
between the atmosphere and the ocean, with potentially sig-
nificant consequences for the regional climate.
In our introduction, we alluded to the 1982–1983 El Nino
event as evidence for the ocean’s influence on climate varia-
tions. Generally speaking, an El Nino event is characterized
by a warming of the coastal waters off the coasts of Peru
and Ecuador. Under normal circumstances, these waters are
held relatively cool by the upwelling activity typical of this
region of the eastern Pacific. The atmospheric circulation of
this area is characterized by what is commonly referred to as
the Southern Osciollation: the seasonal shift in atmospheric
surface pressure between the Australian Indian Ocean region
and the southeastern Pacific. The Southern Oscillation has
been shown to influence surface pressure, temperature and
rainfall variations over much of the earth (Barnett, 1985).
It has been postulated by many scientists that the disastrous
effects of severe El Nino events (e.g., abnormal rainfall
variations on a global scale) are due to the alternation of
the Southern Oscillation by the nearshore surface warming
along Peru and Ecuador. Although evidence of a direct con-
nection between El Nino and alternations in the Southern
Oscillation is by no means complete (Deser and Wallace,
1987), there is little doubt that the warming and cooling of
ocean surface waters has a significant impact on atmospheric
circulation, potentially on a global scale.Density-Driven Deep Ocean CurrentsThe wind-driven ocean circulation, because of its forcing
from an applied shear stress at the water surface, does not
persist at great depth, as shown in our solution indicating an
exponential decrease in velocity with depth. Using this solu-
tion, we can define an “Ekman Depth”, or depth of influence
of the surface wind stress as: D e p (2A v /f) ½.
Depending on the magnitude of the turbulent eddy vis-
cosity, A v , and the latitude of interest, the Ekman Depth
can vary from order of 100 meters to 1000 meters. Since
the average depth of the world’s oceans is approximately4000 meters, one may be led to believe that the waters of
the deep ocean remain motionless. However, quite the con-
trary is true. Measurements indicate that the oxygen content
of the ocean’s deep waters is much higher than would be
expected for a motionless water volume, thereby indicating
that motion does indeed occur at great depth. As alluded to
earlier, water masses can be characterized by their tempera-
ture and salinity. This T/S structure can be employed to track
specific water volumes from point of origin to final destina-
tion. Using this methodology, scientists have, over the last
200 years, identified several large scale, deep ocean flows.
The initial question in addressing the deep ocean transport
of water masses must necessarily concern the identification
of point of origin of specific water types. The two dominant
water masses associated with deep ocean transport are North
Atlantic Deep Water and Antarctic Bottom Water.
The first water type we will consider is Antarctic Bottom
Water. This water is formed primarily on the south-west con-
tinental shelf of Antarctica (i.e., the Weddell Sea). During
ice formation at the water surface, brine is expelled from
the sea water, creating a layer of cold, highly saline water
immediately below the ice. Because of its significantly
higher density, this water sinks at the shelf break (the steep
slope connecting the continental shelf with the deep ocean),
and mixes with the relatively warm, saline deep water trans-
ported by the North Atlantic Deep Water current (discussed
later). This mixture is more dense than either of the con-
stituent water types, and therefore sinks toward the ocean
bottom, becoming Antarctic Bottom Water. This water then
moves in two directions, northward to the North Atlantic,
and eastward around the Antarctic continent.
The second water type, North Atlantic Deep Water, is
formed in the Norwegian Sea, which is separated from the
Atlantic Ocean by a submarine ridge running from Greenland
to Europe. We know that at the lower latitudes near the equa-
tor, the surface waters undergo considerably more heating and
evaporation than those of the mid and high latitudes, where sur-
face cooling and precipitation dominate. As one would expect,
therefore, the surface waters of the lower latitudes are consider-
ably warmer and more saline than those of the higher latitudes.
The wind-driven Gulf Stream carries a large volume of this
warm, saline water from the lower latitudes northward, some
of it eventually being transported into the Norwegian Sea. This
transport is sufficiently fast that the water remains highly saline
relative to the surrounding North Atlantic water. As this saline
water is cooled at the surface, it becomes increasingly dense,
finally sinking to the bottom layers of the Norwegian Sea. This
water then flows back into the North Atlantic over the sub-
marine ridge mentioned earlier. Since the water depth at the
ridge is only 800 meters at its deepest location, much shallower
than the equilibrium depth of the very dense bottom water, this
water flows over the ridge and down toward the bottom of
the North Atlantic, falling thousands of meters much like an
underwater waterfall. The water mixes with the surrounding
water during its vertical flow, resulting in a very dense water
mass, although not as dense as the Antarctic Bottom Water.
The North Atlantic Deep Water therefore flows in a southerly
direction, on top of the northward flowing Antarctic BottomC015_001_r03.indd 796C015_001_r03.indd 796 11/18/2005 11:10:05 AM11/18/2005 11:10:05 AM