the marine atmospheric boundary layer. These
fluxes are the ocean’s principal source of momentum and energy. Together with the redistribution
of heat by ocean currents and mixing, they define
the temperature and salinity (and thus the density)
of the ocean surface. They also determine the
oceans’ three-dimensional current structure and
thus the temperature, salinity, density and tracer
distributions from surface to seabed. Conversely,
the lower atmosphere receives most of its energy
from the ocean via these energy fluxes rather than
directly from the sun. The ocean and the atmosphere also exchange gases and chemical compounds. Our climate arises through the interplay
of these two dynamic systems.
1.2.2.1 Momentum transfer
Surface winds are the principal driver of the upper
ocean circulation through wind stress (the product
of a drag coefficient and the square of the surface
wind speed). The drag coefficient is itself a function
of wind speed, atmospheric stability and wave conditions (WGASF, 2000). To first order the pattern
(Fig. 1.2.2) is of westward stress from the easterly
trade winds at low latitudes and eastward stress
from the high-latitude westerlies. The strongest
mean wind stresses occur in the Southern Ocean.
Wind stress variations both determine and are
determined by climate variability. For example, El
Niño events occur when the equatorial Pacific
trade winds weaken and are replaced by westerly
1.2 Ocean Processes and Climate Phenomena
13
Clarke, Church and Gould
°N 80
60
40
20
0
20
40
60
°S 80
60
120°E
180°W
120
60
0
°N 80
60
40
20
0
20
40
60
°S 80
0
60
120
180°W
120°E
60
JANUARY
JULY
Latitude
Latitude
N m
–2
0.3
0.25
0.2
0.15
0.1
0.05
0.3
0.3
0.25
0.2
0.15
0.1
0.05
0.3
Fig. 1.2.2 The global mean wind stress patterns for January and July from Josey et al. (2000). Shading indicates wind
stress magnitude and arrows the direction.