BSPH U6115 Climate and Water
Coupling, El Nino Southern Oscillation (ENSO)
Take away ideas and understandings:
Meridional heat and
freshwater transfer: The ocean and
atmosphere work together to move heat and freshwater across latitudes, as
required to maintain a quasi-stationary climate pattern.
Fluxes across the sea
surface interface: Heat exchange
between ocean and atmosphere across the sea surface is a product of a number of
processes: solar radiation heats the ocean; long wave back radiation cools the
ocean; heat transfer by conduction and convection between the air and water,
generally cools the ocean; and evaporation of water from the ocean surface
cools the ocean. Any imbalance of these exchange terms over the course of a
year is made up by heat transfer by ocean currents. Evaporation from the ocean
and precipitation on the ocean surface couple the ocean and atmosphere
Mean climatology of the
tropical Pacific ocean-atmosphere system.
Changes in equatorial
Pacific ocean and atmosphere circulation associated with El Niño and La Niña
Two dynamic feedback
processes which act to intensify El Niño and La Niña events.
Why this system
oscillates and the time-scale of this oscillation.
Effects of El Niño/La
Niña on regional and global climate.
I. Ocean Atmosphere
To maintain an approximate steady state
climate the ocean and atmosphere must move excess heat from the tropics to the
heat deficit polar regions (Fig.
1). Additionally the ocean and atmosphere must move freshwater to balance
regions with excess dryness with those of excess rainfall. The movement of
freshwater in its vapor, liquid and solid state is referred to as the
In low latitudes the ocean moves more heat
poleward than does the atmosphere (Fig.
2), but at higher latitudes the atmosphere becomes the big carrier.
The ocean role in climate would be zero if
there were a impervious lid over the ocean, but there is not, across the sea
surface pass heat, water, momentum, gases and other materials (Fig. 4). The wind exerts a stress
on the sea surface that induces the Ekman transport and wind driven circulation
as will be discussed in the next lecture. Much of the direct and diffuse solar
short wave (less than 2 micros, mostly in the visible range) electromagnetic
radiation that reaches the sea surface penetrates the ocean (the ocean has a
low albedo, except when the sun is close to the horizon), heating the sea water
down to about 100 to 200 meters, depending on the water clarity. It is in this
sunlit surface layer of the ocean that the process of photosynthesis can occur.
Solar heating of the ocean on a global average is 168 watts per square meter.
The ocean transmits electromagnetic radiation
into the atmosphere in proportion to the fourth power of the sea surface
temperature (°K). This radiation is at much longer wave lengths (greater than
10 micros, in the infrared range) than that of the sun, because the ocean
surface is far cooler that the sun's surface. The net long wave radiation from
the ocean surface is surprisingly uniform over the global. Why? The infrared
radiation emitted from the ocean is quickly absorbed and re-emitted by water
vapor and carbon dioxide, and other greenhouse gases residing in the lower
atmosphere. Much of the radiation from the atmospheric gases, also in the infra
red range, is transmitted back to the ocean, reducing the net long wave
radiation heat loss of the ocean. The warmer the ocean the warmer and more
humid is the air, increasing its greenhouse abilities. Thus it is very
difficult for the ocean to transmit heat by long wave radiation into the
atmosphere, it just gets kicked back by the greenhouse gases, notably water
vapor whose maximum concentration is proportional to the air temperature. Net
back radiation cools the ocean, on a global average by 66 watts per square
When air is contact with the ocean is at a
different temperature than that the sea surface, heat transfer by conduction
takes place. The ocean is on global average about 1 or 2 degrees warmer than
the atmosphere so on average ocean heat is transferred from ocean to atmosphere
by conduction. The heated air is more buoyant than the air above it, so it
convects the ocean heat into the lower atmosphere. If the ocean were colder
than the atmosphere (which of course happens, just not quite as common as a
warmer ocean) the air in contact with the ocean cools, becoming denser and
hence more stable, more stratified. As such it does a poor job of carrying the
ocean 'cool' into the lower atmosphere. This occurs over the subtropical
upwelling regions of the ocean (Cape Verde climate) The transfer of heat
between ocean and atmosphere by conduction is more efficient when the ocean is
warmer than the air it is in contact with. On global average the oceanic heat
loss by conduction is only 24 watts per square meter.
The largest heat loss for the ocean is
evaporation, (which links heat exchange with hydrological cycle). On global
average the heat loss by evaporation is 78 watts per square meter. Why so
large? Its because of the large heat of vaporization (or latent heat) of water,
a product of the polar bonding of the H2O molecule, as discussed in
the Ocean Stratification lecture. Approximately 570 calories are needed to
evaporate one gram of water! A gram of water is roughly one cubic centimeter,
amounts to a loss of one centimeter of water per a square centimeter of ocean
surface area. The water vapor leaving the ocean is transferred by the
atmosphere eventually condensing into water droplets forming clouds, releasing
its latent heat of vaporization in the atmosphere.
The annual heat flux between ocean and
atmosphere (Fig. 5) is formed by the
sum of all of the heat transfer process: solar and terrestrial radiation; heat
conduction and evaporation. While the ocean gains heat in low latitudes and
losses heat in high latitudes, the largest heat loss is drawn from the warm
Gulf Stream waters (Fig. 6) off the east
coast of the US during the winter, when cold dry continental air spreads over
the ocean. An equivalent pattern is found near Japan, where the Kuroshio
current is influenced by the winter winds off Asia. It is in these regions that
the atmosphere takes over as the major meridional heat transfer agent.
The annual freshwater flux between ocean and
atmosphere (Fig. 7) reflects the water
vapor content (relative humidity) of the atmosphere, resulting from the general
circulation of the atmosphere. The dry regions of the subtropics where the air
subsides along the poleward edges of the Hadley Cell; the rainy Intra Tropical
Convergence Zone (ITCZ) where the trades winds of northern and southern
hemisphere meet, forcing updrafts of air.
II. General Background about ENSO
Nino now refers to a warming of the central and eastern tropical Pacific. (Fig 1, the two largest episodes
of the 20th century)
it only meant a warming at the coast of South America
makes it so notable are the worldwide impacts on climate of El Niño (Fig 2) and it's "mirror image",
La Nina (Fig 3).
III. Impacts of 1982/83 El Nino Episode (Fig 4)
of the two largest amplitude El Nino of this century
recent (1997-98) El Niño was comparably large as well
in Australia, India, Southern Africa
in Peru, Ecuador, USA Gulf of Mexico states, & Colorado River basin
of coastal fishery in Peru (largest average annual catch of marine fish in
IV. Mean Tropical Pacific Ocean-Atmosphere Climatology
Intertropical Convergence Zone (ITCZ) is where the trade winds from the
Northern and Southern Hemispheres converge into a narrow belt close to the
equator, a result of the general Hadley circulation which dominates the
tropics and subtropics (Fig. 5,
winds have two main effects on the tropical Pacific ocean. They cause a
general westward motion of surface waters and warmest waters pile up at
the western Pacific (Fig 7).
the winds are from the NE in the northern tropics and from the SE in the
southern tropics, these winds also cause the surface waters in the eastern
tropical Pacific to diverge as a result of Ekman pumping. This divergence
causes cold, nutrient-rich subsurface water to upwell at the equator.
two processes cause the tropical Pacific to develop an E-W temperature
asymmetry: Warm in the west and cold in the east. Deep atmospheric
convection and heavy rainfall occurs in the western Pacific over the warm
water, whereas there is net atmospheric subsidence over the colder water
in the eastern Pacific (see schematic diagram of "normal"
conditions in the tropical Pacific (Fig 8)). This is the
"seesawing" of atmospheric mass described in the next section.
The rising motions in the west and descending motions in the east
establish an E-W atmospheric circulation called the Walker circulation.
and weakening of this Walker circulation play a crucial role in
reinforcing El Niño/La Niña perturbations to the mean tropical Pacific
circulation in the Pacific could thus be considered as including two
orthogonal, but coupled, components: (A) Hadley Cell transport crossing
latitude lines (Fig. 8A; towards
the equator at the surface and away from the equator in the upper
troposphere); and (B) Walker Cell transport parallel to the equator (Fig. 8B; westward at the surface
and eastward in the upper troposphere).
combined effect of the Hadley Cell, Walker Cell and rotating earth
dynamics is to cause intense convergence and vertical circulation at the
ITCZ (near 0° latitude), over both the land and the sea (Fig 8C).
of the trade winds varies in phase with the strength of the Walker Cell
circulation, and can experience large interannual variations.
V. General History of ENSO Research; Sir Gilbert Walker (Fig 9)
a British mathematician entered the British colonial service as director
general of observations for India
shortly after huge famine caused by drought (very weak monsoon rains)
goal was to predict the Indian Monsoon, which periodically fails to bring
sufficient rain for crops to the Indian subcontinent (modern India,
Pakistan and Bangladesh); e.g. 1899 famine.
Observatory was founded after the monsoon failure of 1877, the worst
famine in Indian history.
met service collected meteorological data from all over the world and made
correlations of that data with Monsoon rainfall, to explore whether it was
part of global phenomenon
found that many global climate variations, including Monsoon rains in
India, were correlated with the Southern Oscillation, a large scale
seesawing of atmospheric mass between the eastern and western sides of the
tropical Pacific (Fig 10)
VI. End Members of ENSO Circulation
transport of trade winds in Pacific is nearly always from east to west
than average trade winds tend to push the warm surface layer of the ocean
(upper few 100 meters) towards the western end, creating a thick warm
layer (La Nina conditions)
Nina has higher than average precipitation in Australia, India &
Indonesia (Fig 11)
trades relax pressure on surface ocean layer & it starts to move back
across Pacific from west to east, raising SST in the eastern tropical
water, including Peru (El Nino conditions), with the zone of heavy rains
shifting out over the central Pacific islands
records of mean sea level pressure (MSLP) & sea surface temperature
(SST) extend only back to the last third of the 19th century, but growth
of coral on islands (Fig 12) across
the low latitudes Pacific have retained signals in their isotopic
composition which permit some aspects of the history of ENSO to be
reconstructed over at least several centuries
VII. Mean Sea Level Pressure (MSLP) Index of ENSO
widely used index of the strength of the Southern Oscillation, the
Southern Oscillation Index (SOI) is given by the normalized difference,
SLP at Tahiti - SLP at Darwin. Barometric records at those stations go
back to the 1880's.
Darwin & Tahiti have an appreciable seasonal cycle of Mean Sea Level
Pressure (1933-1992) (Fig 13)
contrast between the two locations occurs in the southern hemisphere
autumn, while the highest gradient in MSLP is during the austral summer
positive values of SOI indicate stronger trade winds, especially in the
southern hemisphere (La Nina conditions)
most of the time, mean sea level pressure (MSLP) is relatively high in the
south central Pacific (e.g. Tahiti) and MSLP is relatively low over the
Indian Ocean and N Australia (e.g. Darwin), with a net transport of air at
low latitude from east to west -- the easterly trade winds
few years the MSLP difference between east and west weakens; consequently
the trades relax and there is often drought in India and Australia.
Monsoon rainfall correlations to SO were established by mid 1920s
1970 and 1990, there were about four La Nina episodes when high values of
SOI persisted for many months, and an equal number of El Nino episodes (Fig 14)
these episodes were NOT spaced uniformly in time with a transition period
of constant length from one to the other; thus the oscillator process
clearly is complicated
the past century (Fig 15),
approximately 25% of the years could be classified as being in each of the
two extreme ENSO states, assuming the six month mean value of SOI (June -
November) diverged from the long-term mean by more than 0.5 standard
is a tight coupling between the SO and eastern equatorial SST -- that is,
El Nino. Fig 16 is a time
series of Darwin SLP plotted in parallel with a widely used El Nino index
called NINO3. NINO3 is the SST anomaly (departure from normal) averaged
over the region 5°N to 5°S and 90°W to 150°W. (CF Fig 1 -- this is the heart of
the El Nino warm anomaly.) By chance, the units of Darwin MSLP (millibars)
and NINO3 SST (°C) have almost exactly the same range of values. NOTE: The
Darwin MSLP data have been smoothed (5-month running mean) to reduce
VIII. General History of ENSO Research; Jacob Bjerknes (Fig 17)
however, failed to make the connection between the SO and El Nino. This
link was made convincingly by the Norwegian America meteorologist, Jacob
Bjerknes. He made extensive use of data gathered during the 1957
International Geophysical Year, which happened to be a time with a strong
warm (El Nino) event.
realized that unusual events separated by half the circumference of the
planet -- El Nino & the Southern Oscillation -- could be linked
together as parts of a huge coupled phenomena -- ENSO-- involving both the
ocean and atmosphere
IX. General Description of ENSO Processes: Why is There an El Nino State?
did more than establish the empirical connection between EN and SO. He
also provided an hypothesis about the mechanism of ENSO that underlies our
key is to appreciate how odd the "normal" state of the Pacific
is. Fig 18 shows climatological
SSTs in the tropical Pacific for December. Note how cold the eastern
equatorial ocean is, despite the strong solar heating. (By contrast, the
El Nino state (Fig 19) is more
what one would expect from solar heating alone. Note that the anomalies in
Fig 1 are the difference
between the full SST fields in Figs 18 and 19). Cold water is present in
the eastern tropical Pacific because:
- The easterly trades drive westward currents,
bringing the cold waters of the Peru Current from the South American
- The Coriolis force turns westward surface
currents poleward in both hemispheres -- this is the Ekman transport.
Consequently, surface waters diverge from the equator (Ekman divergence)
and the poleward currents must be fed by waters upwelling from below.
These waters are denser and colder than the surface waters they replace.
- The tropical ocean may usefully be viewed as a 2
layer fluid, with an upper warm water layer separated from a lower cold
water layer by a sharp thermocline (the temperature change of 10°C
between the upper and lower layers may take place in as little as 100m).
The trade winds push the warm upper layer waters poleward as well as
westward, pulling the thermocline to the surface in the east (Fig 20, top). As a result the waters
upwelled there are even colder than they would be if the upper layer
waters were more evenly distributed with longitude.
3 of the above effects contribute, but the upwelling ones are the most
are due to the easterly winds. Bjerknes realized that the easterly winds
are in turn due to the temperature contrast along the equator: the surface
air flows from cold (high pressure) to warm (low pressure), the strength
of the temperature contrast controlling the strength of the winds. It does
this via the pressure gradient force -- the difference in MSLP between the
eastern and western Pacific (Note the direct relation to the SO). Higher
pressure characterizes colder air over colder water.
there is a positive feedback: stronger SST gradients across the Pacific
tropics lead to stronger easterly winds lead to stronger SST gradients.
(Bjerknes termed it a "chain reaction.")
Nino is the opposite state (Fig 20,
bottom): Suppose the waters in the east warm somehow. Then the trades
will weaken. The thermocline will relax, so the upwelled water will not be
as cold as before. (If the winds in the east are part of the weakening
then the strength of the Ekman divergence -- the equatorial upwelling --
will also weaken.) so the east-west SST gradient will weaken further and
the trades will weaken further and so on. ANOTHER POSITIVE FEEDBACK.
when the trades relax and the warm water layer starts to move back across
Pacific from west to east, it raises SST in the eastern tropical water,
including Peru (El Nino conditions), with the zone of heavy rains shifting
out from the western Pacific over the central Pacific islands.
thus explained the existence of a cold (high SOI) state and a warm (low
SOI) state, but could not explain the transition from one to the other. He
couldn't explain why one state (e.g. east cold) or the other (east warm)
would not just stabilize and remain forever.
in January SST between the two extreme ENSO episodes of this century (Jan
1983 [El Nino] minus Jan 1956 [La Nina]) illustrates that the eastern
tropical Pacific experiences the largest shift in SST for any large area
of the ocean (Fig 21), being about
4°C warmer during El Nino conditions
spatially averaged SST indices for the eastern Pacific have been developed
to reflect the state of ENSO as expressed in the surface ocean temperature
region is now the most commonly used SST region for ENSO (Fig 22), but other such as the Wright
Index have been used historically in the literature.
X. Why Does ENSO State Tend to Oscillate?
short answer is "equatorial ocean dynamics" (something that was
poorly understood in the late 60s when Bjerknes did his ENSO work.)
key observations were made in the 1970s by Klaus Wyrtki, an oceanographer
at the University of Hawaii. Wyrtki had a network of tide gauges in the
tropical Pacific which gave records of sea level. In the tropics, monthly
average sea level is an excellent substitute for the monthly average depth
of the thermocline -- that is, for the thickness of the upper ocean warm
layer. Wyrtki showed that an El Nino event is associated (preceded, in
fact) by a transfer of warm water from west to east. The top panel of Fig 23 is a reworking of a figure
showing increased sea level between Oct. 1975 and Oct. 1976 from a 1979
paper of Wyrtki's. The bottom panel shows that west to east movement of
warm water also occurred between 1981 and 1982. The next figure (Fig 24) shows a sequence of sea level
(thermocline depth) maps for 1975-6. Initially sea level is low in the
east and high in the west, but by April 1976 sea level in the east is
already high. This precedes the warmest SST anomalies there (Sept.-Nov.)
by a number of months.
is this transfer of warm water to the east that is thought to trigger a
what triggers the movement of waters to the east? Think of the tropical
Pacific as a huge tub, with the waters sloshing back and forth. In the
cold phase the warm waters are low in the east, so they must be high somewhere
else. This is because water is conserved and because warm water is very
nearly conserved: there is some heat exchange with the atmosphere, but
from the ocean's point of view it doesn't amount to much. (From the
atmosphere's point of view its quite a lot -- it is just this
rearrangement of the atmospheric heating that sets off the worldwide
climate anomalies associated with El Nino.)
"somewhere else" that the water level is high is primarily the
western tropical Pacific. Eventually this water will return to the east
and set off the next warm event. Most immediately, it pushes down the
thermocline and raises the temperature of the upwelled waters in the
eastern Pacific. The Bjerknes positive feedback takes over: the trade
winds weaken and still more warm water flows east and SSTs warm. The main
center of atmospheric convection shifts eastward, disrupting the world's
"normal" weather patterns.
eastward sloshing of warm surface waters overshoots equilibrium. Since
there is now more warm water in the east, there is less in the west.
Eventually this message (the raised thermocline signal ) is transmitted
back to the east and the warm event starts to weaken, to be replaced in
turn by a normal to cold phase. And so on, forever (or at least thousands
of years, judging from the observational record).
is one more wrinkle in the story to point out: part of what makes the
oscillation possible is an asymmetry between eastward and westward motions
in the ocean (Fig 25, 2-D Animation). Along the equator
there is a relatively fast eastward (and only eastward) motion called an
equatorial Kelvin wave. Peaking somewhat off the equator are westward
motions called Rossby waves. These carry the message of the high (say)
thermocline in the west, westward to the boundary of the ocean
(Philippines, New Guinea, Australia) where they are reflected eastward in
the equatorial Kelvin wave. This delay is needed for the oscillation --
without it one would have the amplification in place that Bjerknes
is a 3-D animation the tropical
Pacific as it cycles through an El Niño then La Niña event. The surface
shown is sea-level (in cm) and the surface is colored according to the SST
anomalies associated with each event.
Philander (1998) "Is the Temperature Rising?: The Uncertain Science
of Global Warming" Chapter 9, pp. 143-157.
M.H. (ed.) et al. "Teleconnections Linking Worldwide Climate
Anomalies." Chapter 2, pp. 13-42. (more advanced).
by Steve Zebiak, The Sciences, March/April 1989.
by Cane et al, (1994) Science, 370, 204-205.
NOAA, El Nino and Climate Prediction Reports to the Nation,
Spring 1994, No. 3.
Mike (2001) Late Victorian Holocausts, El Nino Famines and the Making of
the Third World. Verso, London & New York.
- Sources of ENSO information, much of it current:
- Zebiak and Cane, diverse papers.
- Chen et al. (1995) Science, V269 (Sept 22, 95),
- El Nino and
- CPC - Data: Current
Monthly Atmospheric and SST Index Values.
of El Niño and Benefits of El Niño Prediction.
- Science, 251 (Feb 8
- Environmental Science and Technology. v25 (Feb '91) 210-212.
- Science, 248 (Apr 6
- New York Times. APR 5
- Oceanus, V34 (Spring
- Hays, J., Water:
the vital fluid, April 1996.
lecture text by Mark Cane,
Peter deMenocal, Arnold Gordon and Jim Simpson.