1. Normal State of the Pacific
Wind stress
Warm pool
Fig. 18.
Sea surface temperature, SST

2. El Niño State of Pacific
Due weakening of the wind:
Warm pool shifts eastward
Fig. 19.

3. Largest El Niños of 20th century
SST Anomaly from the mean state
El Niño: warm event
La Niña: cold event
Fig. 1. El Nino now refers to a warming ot the central and eastern tropical Pacific. This figure shows the SST anomalies of the two largest episodes of the 20th century.

4. Fig. 5.

5. Ocean Response to Atmospheric Circulation
Warm pool
Fig. 7.
Winds cause a general westward motion of tropical surface waters, causing the warmest waters to “pile up” at the western Pacific (western Pacific warm pool)

6. Hadley and Walker winds vary in phase:
when Hadley cell is strong, so is the walker circulation
Fig. 8A

7. Walker circulation
Fig. 8

8. Fig The trade winds push the warm upper layer waters poleward as well as westward, pulling the thermocline to the surface in the east. As a result the waters upwelled there are even colder thatn they would be if the upper layer waters were more evenly distributed with longitude. El Nino is the opposite state. 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 so the E-W SST gradient will weaken further and the trades will weaken in response. A positive feedback situation.

9. Cool SST
Cold phase
warm SST
Warm Phase
Fig. 11.

10. Fig. 12. Instrumental records of mean sea level pressure (MSLP) and SST extend only back to the last third of the 19th century, but growth of coral on islands across the low latitude 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.

11. El Niño
La Niña
Once developed, El Niños are known to shift temperature and precipitation patterns in many different regions of the world. These shifts, although varying somewhat from one El Niño to the next, are fairly consistent in the regions shaded on the map below.

12. Fig. 4.

13. Walker Circulation
British mathematician, director general of observations for India (formed after monsoon failure of worst famine in Indian history)
Arrived in 1904, shortly after huge famine caused by drought
Goal to predict Indian Monsoon
Found that many global climate variations, including Monsoon rains in India, were correlated with the Southern Oscillation
Fig. 9.

14. Fig. 10

15. Fig. 13. Both Darwin and Tahiti have an nappreciable seasonal cycle of MSLP.

16. Fig Over the past century 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-Nov.) diverged from the long-term mean by more than 0.5 stdev

17. Fig Between 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. Note that these episodes were NOT spaced uniformly in time with a transition period of constant length from one to the other. Over the past century approximately 25% of the years could be classified as being in each of the two extreme ENSO states

18. index called NINO3. NINO3 is the SST anomaly averaged over the region
Fig. 16.
Time series of Darwin SLP plotted in parallel with a widely used El Niño
index called NINO3. NINO3 is the SST anomaly averaged over the region
5N to 5S and 90W to 150W (heart of the warm anomaly). By chance
the units (Darwin millibars, NINO3 SST ºC) have almost exactly the same
range of values.

19. Connecting SO and El Niño (ENSO)
The link between SO and El Niño was made convincingly by Bjerknes
Made extensive use of data gathered during 1957 (strong El Niño year)
Realized that unusual events separated by half the circumference of Earth could be linked together as parts of a huge coupled phenomenon ENSO– involving both the ocean and the atmospher.
Fig. 17.

20. General Description of ENSO Processes
Bjerknes established the empirical connection between EN and SO. He also provided a hypothesis about the mechanism that underlies our present understanding
The key is to appreciate how odd the “normal” state is
Warm pool due to the easterly winds, but the easterly winds are also due to temperature contrasts along the equator. Thus there are positive feedbacks to reinforce both El Niño and La Niña phases

21. Why does ENSO State tend to Oscillate?
Equatorial ocean dynamics
Key observations by Klaus Wyrtki in 1970’s
El Nino is preceded by a transfer of warm water from west to east
This transfer is thought to trigger a warm event
What triggers the movement of water?
In cold phase, waters cold (and low) in east, warm (and high) in west
Warm water from west sloshes back and overshoots equilibrium- positive feedbacks mentioned before set of El Niño conditions

22. Why does ENSO State tend to Oscillate?
The 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 west 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& coral records).
The oscillation is made possible due to the asymmetry between eastward and westward oceanic motions (see 2-D Animation in notes).
Along the equator there is a relatively fast eastward motion called an equatorial Kelvin wave. Peaking somewhat off the equator are westward motions called Rossby waves. Time scale: <1 yr for the Kelvin waves to shift the warm pool eastward [once trades relax]; 2 yrs for the Rossby waves to return the warm pool to the west, to await another wind relax.
Fig Top panel is a reworking of a figure showing increased sea level between Oct 1975 and Oct 1976 from a 1979 paper of Wyrtki. The bottom panel shows that west to east movement of warm water also occurred between 1981 and 1982.

23. 1. Equilibrium: The westward wind stress on the sea surface balances the east-west tilt of sea level with the warm pool in the west. In fact the warm pool thickness and Walker circulation may just get stronger with time, a consequence of the zonal SST gradient.
2. Then, some random (?) event in the western Pacific: a reduction in the westward wind stress, maybe even eastward wind, initiates a surge of warm water towards the east along the Equator.
3. The piece of the sea surface tilt is no longer in equilibrium, and it begins sloshing eastward, as a Kelvin Wave.
4. Even if the wind returns to normal the Kelvin Wave continues eastward, as its leading edge is always ‘out-of-wack’ with the local wind stress.
5. Eventually it reaches the eastern Pacific, warming the sea surface, deepening the thermocline, leading to weakening of the Walker Circulation! An El Niño sets in.
6. But the Kelvin Wave continues hitting the eastern boundary in ~1 year from the relaxed westward wind event, induces poleward propagate coastal Kelvin Waves. Rossby waves carry the warm water back to the west at latitudes off the Equator, say 5°N and S. They reach the western boundary in ~3 years. Eventually leading to an La Niña event.
7. So: once there is enough warm water pile in the west, the system is set up for Kelvin Wave generation, as soon as the westward winds relax.
8. Time frame for oscillation: 3 to 5 years.
Segment of
weaker westward wind
wind
Segment of
weaker westward wind
wind
Kelvin Wave
wind
Kelvin Wave
Rossby Waves
~ 3 years
EAST
WEST
Equator
~ 1year
Rossby Waves

24. Rossby
Fig. 24. A sequence of sea level (thermocline depth) maps from Initially sea level is low in the east and high in the west, by by April 1976 sea level in the east is already high. This precedes the warmest SST anomalies there (Sept-Nov) by several months.
Kelvin
Rossby

25. Here 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.
3-D animation of the tropical Pacific as it cycles through El Nino then La Nina. The surface shown is sea level (cm) and the surface is colored according to the SST anomalies associated with each event.

26. ENSO Prediction Wednesday, 16 Nov lecture by: Chet Ropelewski <chet@iri.columbia.edu>