Environmental Physics / Lettner
the El-Niņo (ENSO) Phenomenon
List of Contents:
- Introduction to El Niņo
- Major Contributors
i) Currents (oceanic)
i) Winds (atmospheric)
i) Ekman spiral
i) Gyre formation
i) Equatorial counter current
- The Peruvian (Humboldt) Current
- ENSO (El Niņo - Southern Oscillation)
- La Nina
- Effects of ENSO
Near the end of each year as the southern hemispherical summer is about to peak, a weak,
warm counter-current flows southward along the coasts of Ecuador and Peru, replacing the cold Peruvian current.
Centuries ago the local residents named this annual event El Niņo (span. "the child") based on Christian
theology that assigned this period of the year the name-giving Christmas season. Normally, these warm
countercurrents last for at most a few weeks when they again give way to the cold Peruvian flow.
However, every three to seven years, this countercurrent is unusually warm and strong. Accompanying
this event is a pool of warm, ocean surface water in the central and eastern Pacific.
El Niņo has made frequent appearances over the last century, with particularly severe
consequences in 1891, 1925, 1953, 1972, 1982, 1986, 1992, 1993, and 1997.
Today, scientists use the term El Niņo for these episodes of ocean warming that originally bore that name.
In order to understand the complex interaction that contributes to the phenomenon known as El Niņo, several crucial
factors have to be viewed at in more details.
Fig.1 Video of El-Niņo event '97/98 (740kB)
Forces Causing Surface Currents
The southeast trade winds move the surface water to the left of the wind and westward, forming the South Equatorial
current (fig.2 - center & bottom scan). At a higher latitude, the westerly winds push the water to the east, where
at the southern latitudes it moves almost continuously around the earth as the West Wind Drift. The tips of South
America and Africa deflect a portion of this flow northward on the east side of both South Pacific and South Atlantic
Oceans. The South Pacific gyre is therefore made up of the East Australian current, the Peruvian (or Humboldt current
as it is also known), and the South Equatorial current. The North Pacific and South Pacific gyre are formed not on either
side of 0° latitude (equator) but on either side of 5°N latitide, because the doldrum belt is displaced northward due to the
unequal heating between the northern and southern hemispheres (fig.2 - top scan).
Fig.2 Surface currents within the hydrosphere and atmosphere (365kB)
Wind blowing across the surface of the ocean create friction that sets water in motion.
That motion is a function of wind speed and, consequently, of the energy transferred to the ocean's surface
(e.g. a 15m/s wind may create only a 0.5m/s current). Winds moving across the ocean surface also raise sea
level downwind. The surface of the tropical Pacific is 50cm higher off Asia than off South America (fig.2 - top scan).
Fig.3 Sloping of sea levels (70kB)
Since winds occur as banded patterns around the earth (fig.2 / center & bottom scans, fig.10),
it might be expected that ocean currents to follow similar patterns. They do not, however, because continents,
oceanic islands, and ridges distort the expected patterns. When wind patterns are superimposed onto the South
Pacific gyre, it becomes evident that the northeast trade winds power the South Equatorial Current and that the
westerlies power the East Australia Current that feeds the Peruvian current. Since these winds move in opposite
directions, they exert a turning motion. That motion is aided by the Coriolis effect.
- Furthermore, it is known that wind stress deviates water currents considerably. This effect is
known by the Ekman spiral (fig.4). According to Ekman's hypothesis, the net mass transport in the southern
hemisphere is directed 90° to the left of the surface wind. The surface of the ocean does not match the earth's
curvature exactly. Each ocean has a mound of water at its center, which corresponds to the center of its
subtropical atmospheric gyre. These mounded centers result from the accumulation of surface waters resulting
from the Ekman transport.
- Density differences may also contribute to the existence of these bulges. The heights of two water columns of
the same mass differ if one column has warmer and/or less saline water at its surface (above the thermocline).
Lower-density water of a given ocean mass rises and causes the sea surface to slope. The cold wall of the East
Australia current and the slope of the sea surface across it illustrate density distribution. In the southern hemisphere,
denser waters, which are invariably colder, are on the outside or the right of the current flow ; warmer, less dense
waters are on the inside or to the left of the current flow.
Fig.4 The Ekman spiral (70kB)
An interesting aspect should be mentioned at this point. Ocean currents in the Pacific do have
adiabatic blobs - similar to those well known in atmospheric science. Such blobs of warm water tend to rise,
slightly expand, and while doing so they cool off . If the surrounding water mass is still cooler, the blob will continue
to rise towards the surface as long as it is less dense than the surrounding. Although, convection of some deep
ocean currents may take many hundreds of years for circulation, the water masses that make up these blobs are
so huge and conductivities are so low that no appreciable quantities of heat are transferred to or from these blobs
over these long periods of time. Therefore these blobs are warmed or cooled adiabatically by changes in internal
pressure. Changes in adiabatic ocean convection, are probably another aspect that contribute to the extent of
As water flows down the gradient of the mounds (geostrophic flow), it begins to deflect more and
more to the left of its direction of travel because of the Coriolis effect. As the geostrophic flow is obstructed by
and at a certain stage, will be even balanced by the Coriolis force, the resultant flow is not downhill but across
or parallel to the slope (fig.5).
Fig.5 Oceanic gyre (80kB)
When currents move in a curved path rather than in a straight line, centrifugal force must be
considered. The resulting motion is called a "gradient flow". In cyclonic motion the centrifugal force augments the
Coriolis force. In anticyclonic motion the centrifugal force augments the pressure-gradient force.
In addition, the counterclockwise rotation of the earth (from E to W) compresses the currents on the western
edges of the oceans and narrows the currents there, while exerting a relaxing effect on its eastern edges (results in
an expanded current width). This narrowing effects results in a stronger western boundary current (Australian current)
and a weakly expressed eastern boundary current (Peruvian current of the coast of South America).
- Finally, between the North and South Equatorial currents, under the doldrums, there is a surface current moving
down slope west to east, the Equatorial Countercurrent (fig.3). This current helps to return surface water
accumulated against the coast of Asia by the Equatorial currents (fig.2 - top scan); it is this counter-current that
decreases in strength and triggers an El-Niņo event.
The Peruvian (Humboldt) Current:
Under normal conditions, the cold Peruvian current (an eastern boundary current) flows equatorward along the
coast of Ecuador and Peru (fig.6). It flows with a speed of 0.1 to 0.15[m/s] and a transport of only 15
As outlined above, the eastern boundary current is slow and thus not very strong. Near the coast, it is only about
200m deep, while increasing to 700m offshore. In the absence of an El Niņo, prevailing surface winds cause Ekman
transport to the left (fig.4) or away from the coast, with subsequent upwelling. This upwelling of deep, nutrient-filled
waters is the primary food source for millions of fish, particularly anchovies along the Pacific Coast of South America.
Fig.6 Southern Oscillation (95kB)
Upwelling commonly occurs in the eastern regions of the oceans. In the southern hemisphere the
winds must blow north for upwelling to occur (usually happens during the northern winter - fig.2 - center scan).
Coastal upwelling of this sort takes place, because the South-American west coast sharply drops off to considerable
depths, thus facilitating the formation of the Ekman-spiral.
As indicated in figure 8 (top left scan), the prevailing converging westward surface winds causes the water beneath
to converge as well. Where water parcels meet in a convergence, they form a slight hill, thickening the surface layer.
The mixed water is usually of higher density than the surrounding water, and consequently it sinks. Because the
Peruvian current steadily feeds its waters into the westward surge, it creates an equatorial divergence zone (fig.8 -
top right scan). In such a divergence, the surface waters move away from one another causing more deeper waters
move up to the surface. This action thins the surface layer and usually (adiabatically) lowers its temperature further.
Major El Niņo events are intimately related to large-scale atmospheric circulation. Each time an El Niņo occurs, the
barometric pressure drops over large portions of the southeastern Pacific, whereas in the western Pacific, near
Indonesia and northern Australia, the pressure rises.
Then, as a major El Niņo event comes to an end, the atmospheric pressure difference between these two regions
swings back in the opposite direction. This see-saw pattern of atmospheric pressure between the eastern and
western Pacific is known as the "Southern Oscillation".
Fig.7 Down- / upwelling (70kB)
Fig.8 Ekman promoted transport (130kB)
The El Niņo - Southern Oscillation (ENSO) Phenomenon:
As the name suggests, ENSO consists of two components. The first, mainly oceanic, is known as El Niņo
and has historically been associated with an annually weak, warm current appearing along the coast of Ecuador and Peru
around Christmas time, replacing the usually cold waters of the Peru current. The second, mainly atmospheric,
component of ENSO has been described as the Southern Oscillation (Fig.9 - Walker 1924).
This oscillation is associated with large east-west shifts of mass in the tropical atmosphere between the Indian
and West Pacific and the East Pacific Ocean.
It is well known that the atmosphere influences the oceans mainly through anomalies in the stress exerted by the surface
winds, whereas the ocean in turn influences the atmosphere mainly through anomalies in the sea surface temperatures
and in the associated upward fluxes of sensible latent heat. Thus, it is to some extent arbitrary to group ENSO entirely
within the atmospheric or the oceanic branch of science.
Fig.9 Southern Oscillation (80kB)
Superimposed to the zonal-mean Hadley circulation there are also important east-west circulations
in the equatorial atmosphere (Fig.10). Near the equator, the rising, converging air from both hemispheres is associated
with the pressure zone known as the equatorial low, a region marked by abundant precipitation. Because it is the region
where trade winds converge, it is also referred to as the intertropical convergence
Besides a weakening and a zonal shift of the normal east-west Walker oscillation over the equator (fig.9), both the
north- & southbound Hadley circulations (fig.10) become stronger. This results in enhanced inflow of mass near the surface
into the ITCZ, in increased rising motions between the 10°S and 10°N latitude, and the accelerated sinking motions
in the subtropics of each hemisphere. As can be understood from the conservation of absolute angular momentum,
the boosted Hadley circulation also leads to a strengthening of the eastward flow in the subtropical jets by several
m/s (Bjerknes 1969 - see fig.10 - left scan).
Fig.10 Global circulation (95kB)
The horse latitudes of the Pacific Ocean are characterized by high air-pressure systems and low rainfall.
The equatorial region of the Indian Ocean is, at the same time, characterized by low air pressure and abundant rainfall
(fig.10 - left scan). The existing high pressure system in the eastern Pacific forces trade winds to blow toward the moist,
low-pressure system residing over Indonesia. These westward trade winds maintain the ocean surface-currents moving
towards Indonesia and cause water to pile up in the western Pacific, causing the sea level in Southeast Asia to rise as
much as 46cm higher than along the west coast of South America (fig.11).
In the months preceding an El Niņo event, the normal weather pattern brakes down. For some reason, that are not yet well
understood, the westward atmospheric pressure gradient decreases. With modern satellite technology, this reversal can
be well documented.
Fig.11 ENSO Phenomenon (105kB)
Sea surface temperatures (SST) can be easily deducted from the telemetric data in that warmer waters
produce higher mean surface levels (thermal expansion and stronger positive buoyancy), while cooler waters are significantly
lower (Topex/Poseidon remote sensing statellite data).
As the SST distribution is about to change (shortly before an ENSO-event) a concomitant faltering of the trade wind can be
observed shortly afterwards. With the decline in the trade winds, a region of weak, subtropical easterly winds begin to develop.
The ceasing westward windforce causes the westerly pushed masses of warm water to swing back towards the east. As the
cooling surface wind ceases SST rise even further. Simultaneously, warmed bottom air at the water-air interface off the
South American coast start to rise, resulting in a major updraft. In combination with the overheating surface waters, it boosts
evaporation and ultimately triggers abnormally strong storm activity that heavily affects that region.
The traditional index of the Southern Oscillation is given by the difference in surface pressure between
two stations at or near the maximum and minimum values; i.e. between Darwin and Easter Island. For example, when the
index swings deep into the negative spectrum, a strong ENSO event is in progress (fig.12). ENSO events are thus
characterized by a low negative Southern Oscillation Index (i.e. reduced westward pressure gradient over equatorial Pacific),
weaker than normal trade winds over the central Pacific, and warmer than normal SST in eastern equatorial Pacific.
Its noteworthy that in 1982/83 an exceptionally strong ENSO-event was observed with highest SST anomalies in the
eastern Pacific (see fig. 12).
Fig.12 ENSO and atmospheric pressure differences (70kB)
It seems from these observations that variations in the atmospheric pressure gradient are first followed by
changes in sea-surface wind stress, and later by changes in temperature. The factors causing a pressure drop and weakening
surface winds, are so complex that scientists are not yet able to pinpoint the actual triggering mechanism. But one thing
can be said with certainty: Winds in the lower atmosphere are the link between the pressure change associated with the
Southern Oscillation and the extensive ocean warming associated with El Niņo.
Once an ENSO event has started, the reversal of pressure gradients causes the the surface trade winds and equatorial
currents to change direction. Warmer water flowing from west to east causes local sea level rise and prevents upwelling
along the west coast of North and South America - see fig.11 - lower scan.
In a broader picture, ENSO events not only destabilize the flow of energy along the equator but also the flow of energy
from the tropics to the poles, thus altering global weather patterns.
La Nina - the anti-ENSO Phenomenon:
If, on the other hand, the surface trade winds strengthen and with it the east-west slopes and along with it the east-west
oceanic temperature gradients (fig.6), the resulting weather pattern leads to an anti-El Niņo, that is often referred to as La
Niņa. Such events are characterized by a high positive Southern Oscillation Index (i.e. an increased westward pressure
gradient over the equatorial Pacific), stronger surface trade winds over the central Pacific, and cooler SSTs in the eastern
equatorial Pacific. Such a weather pattern, on the other hand, is associated with increased cyclone activity in the western
Pacific, off shore of eastern Australia, the Phillipines, and the western Atlantic region.
As can be seen in fig.12, strong El Niņo years, are usually followed by several years of unusually cool and persistant La
Niņa years as the systems swings back in a dynamic fashion.
Fig.13 La Nina (110kB)
Effects of El Niņo:
The onset of El Niņo is marked by abnormal weather patterns that drastically affect the economies of Ecuador and Peru.
The abnormally strong winds originating from the west push masses of warm surface water from the equatorial region against
the South-American coast, and are ultimately deflected towards Mexico, Peru, and Ecuador, creating an area of warm water
thousands of kilometers in length (fig.14). The mixed layers deepen, and the deeper cold waters are burried underneath.
The sun warms the surface layer still further, thus enhancing the effect. The thermocline falls, and along with it the pool
of nutrient rich water. In an immediate effect, this warm blob of water blocks off the upwelling of colder, nutrient rich water
driving anchovies into starvation (fig.15). In addition to the torpedoed effect of the local fishing industry, these fish do no
longer support large population of fish-feeding birds, whose droppings (guano) are mined for fertilizer. With the disappearance
of anchovies and other marine organisms, predators like seabirds, further up the food-chain, experience a drastic decline in
nutritonal resources. El Niņo accounted for severe drops in the seabird populations in 1957 and 1965. In a long-lasting ENSO
event, the dissolved seawater oxygen content becomes depleted. This favours production of foul-smelling hydrogen-sulfide
and other gases, blackening the "lead paint" on ships and producing other discoloring effects (Callao Painter).
Yet at the same time, some inland areas that are normally arid receive an uncommon abundance of rain (fig.16). Here
pastures and cotton fields have yields far above the norm. These climatic fluctuations have been known for years,
but were always considered local phenomena. Today, we know that El Niņo is part of the global circulation and
affects the weather patterns far beyond Peru or Ecuador.
Fig.14 an ENSO event (105kB)
Video of Thermocline anomalies of the '97/98 El-Niņo (600kB)
Fig.15 Anchovy catch (70kB)
Perhaps the most important aspect of an ENSO event is the change in the precipitation patterns over the globe (fig.16).
The figure shows a composite map of the regions of abnormally wet (blue) and abnormally dry (red) conditions associated
with a typical ENSO event. Within each region, the approximate period of extreme conditions was determined during the
24-month period starting with the July month preceding the El Niņo event designated by Jul(-), continuing through the June
month following the event, designated by Jun(+). The index (0) behind the month refers to the year of El Niņo.
As a result of ENSO, monsoons in India, Southeast Asia, and Indonesia are omitted and storms in the eastern Pacific
region become an almost daily event.
Fig.16 ENSO-related anomalies (100kB)
The temperatures in the free atmosphere are also profoundly affected by events in the equatorial
Pacific Ocean. A zonal mean cross section of the warm-cold temperature difference on the order of +0.5 to 1°C over
the entire tropical troposphere and values on the order of -0.5°C or less in mid-latitudes were observed. So apparently
convection and upper-level release of latent heat must be very effective in distributing the heat vertically in the tropics.
Keeping in mind that Hadley circulation rotates these masses of air into both the northern and southern subtropical
regions, it becomes quite evident why these areas are affected most (fig.10 - left scan).
When El Niņo began in late 1991, forecasts at the National Weather Service predicted that the
pool of warm water over the Pacific would displace the paths of both the subtropical and polar jet streams, which
steer weather systems across North America and Europe. As predicted, the subtropical jet brought rain to the Gulf
Coast, where Texas received more than twice its normal December precipitation. Furthermore, the polar jet pumped
warm air far north into the continent. As a result, winter temperatures from western Canada to Lake Superior were
3 to 7°C above normal.
The water temperatures in the eastern Pacific dropped in late 1992, signaling the end of El Niņo. However, rather
than giving way to relative cold, El Niņo strengthened somewhat in 1993. This event may have contributed to the
extensive Midwest flooding in the summer of 1993. Storm after storm rolled over the country's midsection, causing
floods that left people dead and caused losses exceeding U$10×
Fig.17 Coral bleaching (140kB)
The effects of El Niņo are highly variable depending in part on the temperatures and size of the
warm pools. During El Niņo, an area may experience flooding, only to be hit by drought during the next event.
Nevertheless, some locals appear to be affected more consistently. In particular, during most El Niņos,
warmer-than-normal winters occured in the northern US and Canada. In addition, normally arid sections of the
eastern US and Europe experienced wet conditions.
By contrast, drought conditions are generally observed in Australia and the Philippines. Interestingly, during
El Niņo years, fewer Atlantic hurricanes are recorded than during non-El Niņo years.
The strongest El Niņo on record occurred in 1982/83, and was blamed for weather extremes of
a variety of types in many parts of the world. Heavy rains and flooding during this event plagued normally dry
portions of Ecuador, Peru, Bolivia, and Brazil. Some locations that usually receive only 10-13cm of rain each
year had as much as 350cm of precipitation. At the same time, severe drought beset Australia, Indonesia, the
Philippines, southern India, Sri Lanka, Hawaii, and Mexico. Huge crop losses, property damage, and much
human suffering were recorded, while large areas of the tourist based Seychelles and Maledivian tropical reefs,
along with huge sections of the central-eastern Pacific reefs experienced extensive bleaching (fig.17).
Farther north, one of the warmest winters on record was followed by one of the wettest springs for much of
the United States. The ferocious storms that struck the California coast brought unprecedented beach erosion,
landslides, and floods. Heavy snow in the Sierra Nevada and the mountains of Utah and Colorado led to mudflows
and flooding in Utah, Nevada, and along the Colorado River in the spring of 1983. Nor was the Gulf of Mexico
spared. Unusual rains brought floods to the Gulf states and Cuba.
Fig.18 Teleconnections (55kB)
Elevated surface temperatures caused unusual migration-like movements of sea animals (fig.18).
Penguins and other sea birds moved south along the coast of South America; bluefin tuna were caught off British
Columbia (CDN); salmon moved further northward; barracuda appeared off the Oregon coast; small pelagic
crustaceans washed up on San Diego beaches; and many sea birds did not return to their traditional breeding grounds.
Through the course of this particular El Niņo, a number of records were set for rainfall, drought, and temperature,
as well as unseasonal and particularly devastating hurricanes (Thaiti) and significantly suppressed hurricane activity
in the Atlantic.
Based on the result presented above and because of its obvious socio-economic importance, an intense effort is
presently be made to study the ENSO phenomenon. It is clear that an explanation of what happens during ENSO
must include a detailed understanding of the complex feedback processes between the oceans and the atmosphere.
In this highly interactive system it may prove very difficult to determine cause and effect.
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Ingmanson D.E., Wallace W.J.,1995; Oceanography 5th ed.; Wadsworth Publishing Co.; Belmont CA - USA
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Related sites on the WWW:
Satellite Images and Weather:
Climate Change Research Center: http://www.grg.sr.unh.edu:80/ccrc/
BBC-World - El Niņo, Climate Change
Eureka El Niņo & Air Pollution
Stratospheric Ozone - EPA: http://www.epa.gov/docs/ozone
El Niņo theme page: http://www.pmel.noaa.gov/toga-tao/el-nino/home.html
National Hurricane Center: http://www.nhc.noaa.gov/index.html
NASA reference page regarding La Nina: http://www.jpl.nasa.gov/elnino/990629.html
Mass Mortality of Reef Corals: http://www.abc.net.au/science/coral/story.htm