1. El Niño Southern Oscillation
Marine Science 320
El Niño Southern Oscillation
Equatorial upwelling chlorophyll from Seawifs during June-August 1998
Equatorial upwelling chlorophyll observed from SeaWiFS during June-August 1998 (normal conditions) 1

2. El Niño-Southern Oscillation (ENSO)
NOAA PMEL El Niño Theme Page
Southern Oscillation (atmosphere) and El Niño (ocean) processes were identified separately, and subsequently linked
Walker: noted ‘see-saw’ connection in barometer data from Tahiti and Darwin and coined the term Southern Oscillation
Jacob Bjerknes connected SST, winds and SLP with atmosphere/ocean dynamics
Sir Gilbert Walker
failure of monsoon rains (and famine) in 1899
claimed Asian monsoon linked to drought in Africa and Australia, and mild winter in Canada
Widely criticized, theory dismissed (no dynamical explanation)
Never succeeded in predicting monsoon failures
Sir Gilbert Walker: Director General of Observatories in the British colonial service in India
1899 famine in India was caused by monsoon failure
Walker found out that the seemingly random failure of the monsoons in India often coincides with low pressure over Tahiti, high pressure over Darwin, and relaxed trade winds over the Pacific.
As the first person to claim there was a connection between monsoons in India and unusually mild winters in Canada, naturally he took some grief. In a modern rendition of "the world is flat" scenario, he was publicly criticized for suggesting that climatic conditions over such widely separated regions of the globe could be linked.
Jacob Bjerknes, working on observations associated with the 1958 El Nino, developed a dynamical explanation as to why SST, winds, SLP were inter-connected

4. Typical pattern of coupled ocean/atmosphere dynamics in the equatorial pacific
What has become apparent through research over the past decade is that the phenomenon we call ENSO is an instability of the tightly coupled tropical atmosphere-ocean system.
The term ENSO is a combination of
El Niño: the episodic warming of waters along the Peruvian coast
Southern Oscillation: pattern of sea level pressure variability that is coherent over the Pacific and much of the Indian Ocean region

5. Definition of the Southern Oscillation Index
When SOI is negative: p.g. is weaker than usual corresponds to El Nino (or ENSO warm event)
Definition of the Southern Oscillation Index
The Southern Oscillation index (SOI) is classically defined as the anomaly in the difference in sea level pressure between Tahiti and Darwin.
When SOI is negative, the air pressure gradient along the equator is less than usual, and we would expect the Trade winds to be weaker than usual
Conversely, positive SOI would indicate stronger Trades.
[ Pdiff - Pdiffav ] SOI =
SD(Pdiff)
Sep 2016
Pdiff   =  (Tahiti MSLP) - (Darwin MSLP) monthly averaged Pdiffav  =  long term (years) average of Pdiff for that month SD(Pdiff)  = long term standard deviation of Pdiff
Sometimes SOI is given multiplied by 10 in which case it ranges from about –35 to about +35 (i.e. +/- 3.5 std. dev.)

6. Equatorial upwelling chlorophyll from Seawifs during June-August 1998

7. Chlorophyll from Seawifs during Sep-Nov 1997

8.
But there is more to ENSO than just variability in the equatorial winds. It has become clear that in an ENSO event the entire Pacific air-sea system …
rain bands
associated winds
wind-driven currents
and SST patterns
… all move eastward together.
It was only in the late 1960s that it was recognized that these two processes (El Niño, and SO) were linked.
Now we know that just about any upset to a component of the coupled ocean-atmosphere circulation in the tropics will cause a feedback that links all the components. (Chicken and Egg)
Understanding an ENSO event begins with understanding the evolution of the SST field.

9. Strong Trade winds
Cold upwelled water
Trades weaken in Feb/Mar
Upwelling slows and water warms
The coupling of winds and SST is apparent if we consider the seasonal cycle at the equator.
Weaker southeast Trades in the austral late summer cause February-April to be the equatorial warm season.
upwelling is weaker and sea surface temperature warms
upwelling is strongest at the end of the austral winter in Sept-Oct when the southeast Trades blow at a steady 6 m s-1
Some years, this seasonal cycle seems to get amplified and the warming of the eastern Pacific early in the year becomes dramatic and persistent:
easterlies westerlies

10. In some years, this seasonal cycle seems to get amplified and the warming of the eastern Pacific early in the year becomes dramatic and persistent:

11. December 1997
Positive anomaly means winds are more westerly (toward the east) than average
Positive wind anomaly means winds are more westerly (or less easterly, i.e. weaker Trades) and this is clearly associated with the increase in SST during the 1997/1998 ENSO warm event
Many factors influence SST
Change in wind speed
evaporative (latent) and sensible heat loss
Ekman transports advect heat laterally
produce Ekman pumping which changes deeper density field and affects temperature of water available for upwelling
alters geostrophic flow
alter the depth of directly wind mixed layer
SST itself alters cloud cover, and incoming solar radiation

15. Normal conditions El Nino conditions
The warming of eastern Pacific is not simply due to a decrease in the upwelling strength.
Warm water moves eastward form the West Pacific Warm Pool.

16. El Nino - warming first occurs subsurface in the central Pacific
because the thermocline is being displaced downward.
El Nino has begun, and the warming first occurs subsurface in the central Pacific
Because the thermocline is being displaced downward.

17. Warming due to thermocline displacement occurs all across.
It is shallower in the east because the thermocline itself is shallower
Warming due to thermocline displacement occurs all across.
It is shallower in the east because the thermocline itself is shallower

18. Strong subsurface warming

19. SST now increases, apparently starting from the east, but largely due to what is happening subsurface
SST now increases, apparently starting from the east, but largely due to what is happening subsurface

20. Loss of heat from the west has cooled the WPWP at depth
Warming travels up the N.Amer west coast and spreads out into the Pacific – very slowly
The loss of heat form the west and cooled the WPWP at depth
Warming has traveled up the North American west coast and begins to spread out into the Pacific – very slowly

21. It gets worse.

23. El Nino contracts but waters are still cool to the west.
The WPWP must reset itself slowly …
El Nino contracts but waters are still cool to the west.
The WPWP must reset itself slowly – the hard part of the story.

24. What does this change in SST do to the atmosphere?

25. Positive is more rain in 1999 than 1998
ENSO conditions (SOI < 0)
Positive is more rain in 1999 than 1998
Negative means much less rain in 1999 than 1998
Top right: Two-month mean rainfall rate (mm/mon) for Jan/Feb Heavy rainfall appears over the South Pacific Convergence Zone (SPCZ), South Indian Ocean, and the South America.
Bottom: Difference in the Jan/Feb mean rainfall 1999 minus 1998.
During ENSO warm event (El Niño) in 1998 central Pacific rainfall is anomalously high, and west Pacific rainfall is anomalously low.

26. How do we monitor the subsurface equatorial Pacific in real-time so that these data can be used in ENSO forecasting?

27. NOAA vessel Ka’Imimoana

31. Technicians mount an instrument to the wire rope during a RAMA buoy deployment

32. ADCP Deep water Assessment and Reporting of Tsunamis (DART) mooring.
PMEL Scientist Sonya Noor communicates with the pCO2 system mounted on a TAO mooring, prior to its equatorial deployment
PMEL Scientists Tim Nesseth and Brian Powers assist the KA deck department during the equatorial ADCP recovery.
The KA Deck Department works hard to recover the Deep water Assessment and Reporting of Tsunamis (DART) mooring.
Maria and Doc showing how much fun it is to spool nylon during a buoy recovery.

35. Consider what would happen next if any of the following occurred:
The Trade winds weakened
Central Pacific SST warmed
Rainfall increased in the central Pacific

36. Consider what would happen next if any of the following occurred:
The Trade winds weakened …
equatorial upwelling would slow down
SST would rise in the central Pacific
atmospheric convection would occur more quickly
the region of precipitation would move to the east closing the Walker cell sooner … weakening the Trade winds in the west
There is a positive feedback that amplifies initial the Trade wind weakening

37. Consider what would happen next if any of the following occurred:
2. Central Pacific SST warmed …
atmospheric convection would occur more toward the east
the region of precipitation would move to the east closing the Walker cell sooner
weakening the Trade winds
equatorial upwelling would slow down
SST would rise in the central Pacific

38. Consider what would happen next if any of the following occurred:
3. Rainfall increased in the central Pacific …
the Walker cell would close more toward the east weakening the Trade winds
equatorial upwelling would slow down
SST would rise in the central Pacific
atmospheric convection and rainfall would occur more toward the central than western Pacific

40. The westerly wind burst causes:
There is some evidence that a trigger for ENSO warm events might be westerly wind bursts in the western equatorial Pacific.
A wind burst such as this sets in train wave motions that are characteristic of the equatorial region.
The westerly wind burst causes:
converging Ekman transports (off equator) increase sea level and …
depress the thermocline
eastward geostrophic flow converges to the east and …
diverges to the west
the pattern moves eastward
Note that the same happens for an easterly wind burst: the equatorial Kelvin waves only go east so the anomalous pattern cannot easily reset itself even if the WWB is followed by easterly wind anomalies.
The equatorial Kelvin wave speed
Explain Kelvin wave dynamics and westerly wind anomaly
is about 2.5 m/s (roughly ~200 km/day)
The observed speed is about 10 – 20% faster than this due to advection by the EUC

41. El Niño effects during December through February
El Niño effects during June through August

42. La Niña effects during December through February
La Niña effects during June through August

44. Present conditions: Format size 75%
Present conditions:
Format size 75%
SOI
NOAA ENSO Advisory:

45. ENSO Alert System Status: Not active
EL NIÑO/SOUTHERN OSCILLATION `
DIAGNOSTIC DISCUSSION
issued by CLIMATE PREDICTION CENTER/NCEP
8 September 2016
ENSO Alert System Status: Not active
Synopsis: ENSO-Neutral
conditions are slightly
Favored (55-60%) during
Northern Hemisphere fall
and winter
ENSO-Neutral conditions were observed over the past month, although sea surface temperatures (SSTs) were below-average over the east-central equatorial Pacific Ocean(Fig. 1)
Fig 1. Average sea surface temperature (SST) anomalies (oC) for the week centered on 31 August Anomalies are computed with respect to the base period weekly means.
NOAA ENSO Advisory:

46. ENSO Alert System Status: Not active
EL NIÑO/SOUTHERN OSCILLATION `
DIAGNOSTIC DISCUSSION
issued by CLIMATE PREDICTION CENTER/NCEP
8 September 2016
ENSO Alert System Status: Not active
Synopsis: ENSO-Neutral
conditions are slightly
Favored (55-60%) during
Northern Hemisphere fall
and winter
Niño-3.4 and Niño-3 regions remained around -0.5°C most of the month. Niño-4 and Niño 1+2 were -0.1°C and +0.3°C, respectively (Fig. 2).
Next:
upper ocean heat content …
Fig 2. Time series of area-averaged sea surface temperature (SST) anomalies (oC) in the Niño regions. Anomalies are departures from the base period pentad means.

47. ENSO Alert System Status: Not active
EL NIÑO/SOUTHERN OSCILLATION `
DIAGNOSTIC DISCUSSION
issued by CLIMATE PREDICTION CENTER/NCEP
8 September 2016
ENSO Alert System Status: Not active
Synopsis: ENSO-Neutral
conditions are slightly
Favored (55-60%) during
Northern Hemisphere fall
and winter
Fig 3. Area-averaged upper-ocean heat content anomaly (oC) in the equatorial Pacific (5oN-5oS, 180o-100oW). The heat content anomaly is computed as the departure from the base period pentad means.
Subsurface temperatures across the eastern and central Pacific remained below average (Fig. 3) and negative temperature anomalies remained weak across the western Pacific (Fig. 4). Atmospheric anomalies over the tropical Pacific Ocean largely indicated ENSO-Neutral conditions
Fig 4. Depth-longitude section of equatorial Pacific upper-ocean (0-300m) temperature anomalies (oC) centered on the pentad of 31 August The anomalies are averaged between 5oN-5oS. Anomalies are departures from the base period pentad means.

48. ENSO Alert System Status: Not active
EL NIÑO/SOUTHERN OSCILLATION `
DIAGNOSTIC DISCUSSION
issued by CLIMATE PREDICTION CENTER/NCEP
8 September 2016
ENSO Alert System Status: Not active
Synopsis: ENSO-Neutral
conditions are slightly
Favored (55-60%) during
Northern Hemisphere fall
and winter
Convection was suppressed over the western and central tropical Pacific, although less suppressed compared to last month
Positive anomalies in brown imply decreased cloudiness and precipitation.
Fig 5. Average outgoing longwave radiation (OLR) anomalies (W/m2) for the period 6-31 August OLR anomalies are computed as departures from the base period pentad means

49. The multi-model averages favor borderline Neutral-La Niña conditions (3-month average Niño-3.4 index less than or equal to -0.5°C) during the Northern Hemisphere fall, continuing into winter (Fig. 6).
However, the more recently updated model runs from the North American Multi-Model Ensemble (NMME) more strongly favor ENSO-Neutral.
Forecaster consensus prefers ENSO-Neutral, which is supported by the lack of significant anomalies in several indicators over the past month (winds, convection, subsurface temperatures).
NINO3.4 SST anomaly (oC)
Fig 6. Forecasts of sea surface temperature (SST) anomalies for the Nino 3.4 region (5oN-5oS, 120Wo-170oW). Figure updated 16 August 2016

50. Climate Prediction Center 3-month forecast
Climate Prediction Center 3-month forecast

51. (e) temperature anomaly (g)
chlorophyll
SOI
(a)
(b)
(c) (d)
Equatorial thermocline
(e) temperature anomaly
(g)
(h)
(f)