Ocean Circulation Systems - Climate Dynamics - page 144

ocean basins. The resulting vertical pressure gradients drive water out the bot-
tom of the mixed layer.
Wind-driven upwelling and downwelling occur on smaller space scales.
Coastal upwelling occurs when winds blow directly from land to ocean, driv-
ing warm surface water away from the coast. Cooler water wells up to replace
it, obeying the constraint of conservation of mass.
Wind blowing parallel to a coast can also generate upwelling and down-
welling. For example, southerly winds off the west coast of tropical South
America (see Fig. 2.13) drive westward Ekman transport of water within the
ocean mixed layer off the coasts of Peru and Ecuador. The water mass is re-
placed by the upwelling of cool water from below as well as the cool Peru
Current ( Fig. 2.22) , and both help maintain the low temperatures in the Pacific
cold-tongue region ( Figs. 2.15 and 2.16) . Similar wind-driven upwelling occurs
off the California and West African coasts in the Northern Hemisphere and off
the west coast of Africa in the Southern Hemisphere. Regions of upwelling are
typically excellent fishing regions because the upwelling waters carry nutrients
from the deeper ocean.
Ekman dynamics can generate downwelling when the Ekman transport is
directed toward the coast, as occurs off the west coast of Alaska where the win-
tertime development of the Aleutian low (the region of low geopotential heights
in the North Pacific shown in Fig. 2.4 ) places southerly winds along the coast.
Upwelling in the open ocean along the equator is another consequence of
the Ekman dynamics. Recall that surface winds in the tropics converge near the
equator, with northeasterly flow in the Northern Hemisphere and southeasterly
flow in the Southern Hemisphere ( Fig. 2.13) . According to Eq. 8.11, these sur-
face winds drive southeasterly Ekman transport in the Northern Hemisphere
and northeasterly Ekman transport in the Southern Hemisphere. The result is
a divergence of mass within the ocean mixed layer and equatorial upwelling of
cool ocean water.
In addition to these dynamically driven vertical mixing processes related
to the Ekman dynamics and, ultimately, the frictional drag of the wind on the
ocean surface, important vertical mixing processes also are driven by density
differences.
Salt fingering is a vertical mixing process related to the physics of diffusion ,
or the transport of a quantity by random molecular motion. Heat and momen-
tum can be diffused, as well as mass such as salt. Different quantities diffuse at
different rates. When a fluid's density is determined by two different compo-
nents or properties with different rates of diffusion, double- diffusive processes
can occur and contribute to vertical mixing. Salt fingering is one example of
double- diffusive convection.
To understand the salt-ingering process, consider a tank of cold freshwater
with an embedded region of warm salty water ( Fig. 8.5) . The warm salty region
is not physically mixed or stirred into the freshwater, but both heat and salt
will diffuse into the cool freshwater. Heat diffusion takes place more quickly
than salt diffusion, so the tank will become isothermal before it attains uniform
salinity.
Now, consider the two-layer fluid system drawn in Figure 8.6a . The top fluid
has a lower density than the bottom fluid, so the system is hydrodynamically