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El Niño
The
key reason that the delta still receives water is that flows of
desert rivers are inherantly variable, and when there is excess,
not all can be captured. In the case of the Colorado River, the
El Niño Southern Oscillation (ENSO) events appear to be the
main source of excess water. The El Niño Southern Oscillation
(ENSO) event begins when a low-frequency, oceanic Rossby wave (resulting
from the rotation of the Earth) sets off from the western boundary
of the Pacific. The reflected wave pushes down
the thermocline that exists in the west-central Pacific; this produces
warm sea surface temperatures (SST) and thereby reduces the efficiency
of upwelling to cool the surface. Winds blow towards the warmer
water (in the west) and really start the "bathtub sloshing"
prior to the El Niño event.
In normal (non-El Niño) years, the trade
winds blow towards the west across the tropical Pacific and are
responsible for increased warm surface water in the west Pacific;
SST is 8 C warmer in the west Pacific Ocean, with cooler temperatures
of the west coast of South America due to upwelling from the deep,
cold, nutrient-rich water with diverse lifeforms. At 110 W longitude,
a thermocline of cool water (17 C) is about 50 m from the surface.
The onset and termination of El Niño result
from interactions of the coupled ocean-atmosphere system, specifically,
the unstable air-sea interaction and planetary scale oceanic waves. In El Niño years, the trade winds weaken
over the central Pacific Ocean and the piled-up water in the west
sloshes back east, carrying the warm water with it. Also moving
east, heat and moisture rises into the atmosphere, distorting the
jet stream path and shifting weather patterns globally. This leads
to a depression in the thermocline in the East Pacific, a depression
in elevation in the west Pacific, and a general warming of the surface
layers in the east and central equatorial waters. At 110 W longitude,
the thermocline of cool water (17 C) is 150 m from the surface.
When the further the cool water is from the surface, then unusually
warm water can appear near the surface; this phenomenon is where
El Niño gets its "warm event" association. The depth of the thermocline reduces the efficiency
of the upwelling and decreases the supply of nutrient rich cold
water to the surface. There is a corresponding
decrease in primary productivity and effect on higher trophic levels. When eastern SST becomes warm, the east-to
west temperature contrast is small, so the trade winds weaken even
further, leading to a complete collapse with essentially flat conditions
across the entire equatorial Pacific.
La
Niña the "cold event" is characterized by strengthening
of the trade winds and cold SST in the equatorial Pacific levels. These events occur irregularly at intervals of 2-7 years, although
the average is about 3-4 years and they typically last 12-18 months.
La Niña follows El Niño most of the time, but El Niño
is more frequent, stronger. The most severe effects are found close
to the equator. The El Niño Southern Oscillation (ENSO) is
the breakdown of the surface transport or trade winds that drive
warm surface water to the western Pacific and subsequently cause
cold, bottom waters to rise/upwell along the coast of Peru.
In 1990, the metropolitan water district of southern
California took twice (over 1-maf./yr.) their water allowance from
the river because Arizona was not using its share (Carrier, 1991).
In fact, from 1980 until now, Arizona has had considerably more
wet years than in the past due to ENSO. The 1980's
and 1990's featured a very active ENSO cycle, with five El Nino
episodes (1982-83, 1986-87, 1991-1993, 1994-95, and 1997-98) and
three La El Nina episodes (1984/85, 1988/89, 1995-96) occurring
during the period. This period also featured two of the strongest
El Nino episodes of the century (1982-83 and 1997-98), as well as
two consecutive periods of El Nino conditions during 1991-1995 without
an intervening cold episode.
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