792 OCEANOGRAPHY
density is often related to the water salinity, S, and tempera-
ture, T, through an equation of state (e.g., Fofonoff, 1962).
In many oceanographic applications, the water density isexpressed in terms of a sigma-t value, s (^) t , which is a measure
of the density at atmospheric pressure, p 0. Furthermore,
since very slight variations in water density can have sig-
nificant physical, chemical and biological implications, the
sigma-t values are tabulated in the form: s (^) t , ( r 1) 10 3.
As an example, the density of sea water at 20C and salinity
35% (35 parts per thousand) at atmospheric pressure is not
written as r 1.024785 g/cm^3 , but rather as s (^) t 24.785.
The temperature distribution in the ocean, although
highly variable in space and time, is generally characterized
by a layer of relatively warm water at the surface, under-
lain by dense, colder water. Variations in deep ocean tem-
perature, that is, excluding shallow coastal areas, can exceed
25 C, with a typical range of 20C at the surface to 2C near
the bottom. These values exclude isolated aberrations such
as the very saline, 56C water found at the bottom of the Red
Sea, and the hot springs (“black smokers”) located along the
mid-ocean ridges of the eastern Pacific, where fluid having
temperatures as high as 350C seeps through cracks in the
diverging ridge axis (Edmond, 1986).
The dense bottom water in most cases originates in the
polar regions, as we shall discuss later. Typically, the tem-
perature of the surface layer is strongly dependent on surface
atmospheric conditions (i.e., air temperature, solar intensity
and wind speed). In the absence of strong vertical mixing, the
temperature in this region decreases with depth in a region
commonly referred to as the thermocline. Below the thermo-
cline, the temperature is usually significantly more uniform
with depth. Thermocline development varies in depth and
duration with latitude. In the temperate regions, the thermo-
cline is seasonal, developing only during the warmer months
of the year, and achieving an average depth on the order of
100 meters. In the tropical regions, the thermocline is a rela-
tively constant feature, with depths often exceeding 1000
meters. In both regions, periods of strong surface winds can
induce vertical turbulent mixing sufficient to break down the
thermocline and create a surface layer of uniform density
exceeding several hundred meters in depth.
The salinity of the world’s oceans is, in general, much
less variable than the temperature, typically ranging from 33
to 37 parts per thousand. Exceptions include coastal regions,
where rainfall and river runoff can add appreciable amounts
of fresh water to the nearshore waters and reduce the salinity.
In addition, tropical ocean regions tend toward higher salinity
as a result of the high evaporation (which removes only fresh
water) and low precipitation (which adds fresh water) rates
relative to temperate ocean areas. Large variations in salin-
ity can also occur in the polar regions. Ice formation, through
its extraction of very nearly fresh water from the underlying
ocean surface, results in the deposition of brine at the sur-
face and the creation of a layer of very cold, saline water.
Conversely, during ice melting, large amounts of fresh water
are introduced to the surface layer, considerably reducing the
local salinity.
OCEAN CIRCULATION
For the purposes of this discussion, we shall divide the cur-
rents that together comprise the oceanic circulation into two
components, each distinguished by the forcing mechanism
responsible for the water motion: 1) Wind-driven surface cur-
rents, and 2) Density-driven deep ocean currents. We shall
follow these discussions with a brief examination of shallow
water coastal circulation.
Wind-Driven Currents
The surface currents that describe much of the wind-driven
ocean circulation are familiar to most readers. On the east
coast of North America, the Gulf Stream carries warm water
from off the coast of Florida northward to Nova Scotia, then
running northeast and becoming the North Atlantic Current.
In the Pacific Ocean, the Kuroshio carries warm water from
a region south of Japan in a northeasterly direction, becom-
ing the North Pacific Current.
Before discussing these and other features of the wind-
driven ocean circulation, we should first examine the char-
acteristics of the atmospheric circulation. Of course, this
motion varies considerably in space and time, making any
prediction of global synoptic wind patterns an extremely dif-
ficult, if not impossible, task. We can, however, derive some
useful information by averaging the wind field observed at
MID-OCEAN RIDGE
SEDIMENTARY
STRATA
SEA
LEVEL
ASTHENOSPHERE
LITHOSPHERE
CONTINENT
CONTINENT
FIGURE 1 Typical seafloor cross-section (Sclater and Tapscott, 1979).
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