Geoscience Reference
In-Depth Information
Warm, moist air
causes low pressure
and heavy rainfall
Cool, dry air with
high pressure
and little rainfall
Warm air rises,
causing low
pressure and
heavy rainfall
Southeast trade winds
L
Descending air causes
high pressure and
warm, dry weather
Accumulation
of warm water
South Equatorial
Current
H
Upwelling of
nutrient-rich water
(Humboldt Current)
(a) Normal climatic conditions
Weak tradewinds
H
L
Warm water
flows eastward
to South America
Upwelling
blocked by
warm water
(b) El Niño conditions
Figure 6.35 Atmospheric circulation and surface water flow in the Equatorial Pacific. (a) Normal conditions occur when
strong easterly flow pushes warm water into the western Pacific. (b) An El Niño occurs when easterly flow weakens, allowing warm
water to collect along the South American coast. Note the relationship between precipitation and the location of pressure systems.
this increased salinity, the surface waters gradually cool as they
continue northward.
Due to decreasing temperature and high salinity, the water
density increases and the water sinks in the northern Atlantic in
Figure 6.34. In other words, the sinking water becomes a down-
welling current . This downward motion is enhanced by the de-
velopment of sea ice at high latitudes, which pushes most salt
into the water below the ice and thus further increases the salin-
ity of the water and its density compared to the water around it.
The resulting downwelling current subsequently flows at great
depths to the southern part of the Atlantic basin. From there, it
flows to the east between Antarctica and Australia to the Pacific
Ocean, where it then flows northward toward Alaska. As the
water moves into the tropical regions of the Pacific, it warms
and becomes an upwelling current that moves back to the
surface. This pattern is repeated in several places around the
world's oceans. To give you an idea of the time involved for
ocean currents like this to move, research indicates that it takes
about 2000 years for water to flow from Great Britain in the
northern Atlantic Ocean to the southern Pacific Ocean!
erly trade winds that flow from strong subtropical high-pressure
systems to the ITCZ (Figure 6.35).
When the easterly pattern is in place, the flow of water
away from the South American coast allows cold water from
the deep-flowing Humboldt Current (which flows northward
from Antarctica along the coast of Chile) to upwell (rise to
the surface) in the eastern Pacific near the Equator. Due to this
upwelling, the overlying air is cool and descending, which, in
turn, results in little evaporation of moisture from the ocean and,
consequently, little precipitation. You can see the effect of this
low precipitation in the lack of vegetation along the west coast
of South America in Figure 6.18. As the water flows westward
along the Equator, however, it warms because of the high Sun
angle. By the time the current reaches the western Pacific, the
water is quite warm, which results in overlying (atmospheric)
low pressure and heavy rains in that part of the basin.
For some as yet unexplained reason, the “normal” cir-
culatory pattern in the Pacific Ocean reverses every three to
eight years. The apparent cause of this reversal is that the nor-
mally strong tropical easterlies weaken, allowing a distinct
westerly flow to develop in surface ocean waters. This op-
posing pattern is called El Niño, derived from the expression
Corriente del Niño , or the “Current of the Christ Child,” used
by Peruvian fishermen to describe the influx of warm waters
into the eastern Pacific Ocean. This reversal of surface flow
usually occurs around Christmastime (hence the name) and is
particularly noticeable to fishermen because the warmer wa-
ters are relatively sterile when compared to the nutrient-rich
upwelling current (Humboldt Current) that normally supports
superb fishing. In addition to the depleted fish industry, an
El Niño brings intense storms to the normally dry parts of
El Niño
Like many other Earth processes that can depart from the nor-
mal pattern, the oceanic circulatory system sometimes func-
tions in unusual ways. One of the most important anomolies
in oceanic circulation is the El Niño phenomenon, which is a
reversal of the “normal” flow in the tropical Pacific Ocean. Dur-
ing most years, the dominant current in the tropical Pacific is
easterly along the South Equatorial Current (see Figure 6.32).
As you might suspect, this easterly flow is caused by the east-
 
 
Search WWH ::




Custom Search