1. El Niño / Southern Oscillation (ENSO)
and inter-annual climate variability

3. sea level pressure and near surface winds

4. surface ocean currents

5. coastal upwelling

6. equatorial upwelling

7. what is El Niño?
The term El Niño originally used by Peruvian fishermen to describe the warm current appearing off the western coast of Peru around Christmas time.

8. Today El Niño describes the warm phase of a naturally occurring sea surface temperature oscillation in the tropical Pacific Ocean.
El Niño is often followed by or preceded by La Niña: an unusual cooling of the tropical Pacific
This oscillation is associated with the atmospheric phenomenon known as the southern oscillation.

9. El Niño happens roughly every 3-7 years, lasts months, and peaks at the end of the calendar year

10. What is the Southern Oscillation?
A seesaw shift in surface pressure at Darwin, Australia and the South Pacific Island of Tahiti. When the pressure is high at Darwin it is low at Tahiti and vice versa.
El Niño, and its sister event – La Niña – are the extreme phases of the southern oscillation, with El Niño referring to a warming of the eastern tropical Pacific, and La Niña a cooling.

11. Southern Oscillation Index (SOI)
The (SOI) measures the monthly/seasonal fluctuations in surface air pressure differences at Tahiti and Darwin (SOI = Tahiti – Darwin), and thus the SOI has a negative value during an El Niño event.

12. Walker circulation

13. trade winds blow from East to West
warm water piles up in West Pacific, driving deep atmospheric convection
cold, nutrient-rich water is brought to the surface in the East Pacific (upwelling)

14. Every few years, the trade winds weaken…
In other years the trade winds strengthen…

15. schematic of changes during an El Niño event

16. 20th – 21st century El Niño years

17. quantifying & comparing ENSO events
ENSO can be quantified using a Multivariate ENSO Index (MEI) based on the six main observed variables over the tropical Pacific. These six variables are: sea-level pressure, zonal and meridional components of the surface wind, sea surface temperature, surface air temperature, and total cloudiness fraction of the sky.

18. MEI comparison of recent El Niño events

19. ENSO event

20. Teleconnections and Global Weather Patterns
El Niño shifts the probability of droughts, floods, heat waves, and extreme weather events in large regions of the globe.
El Nino is a fascinating scientific puzzle that has occupied two generations of oceanographers and meteorologists.
What makes it compelling from a socio-economic perspective is its global impacts on weather variability and ecosystems on land and in the ocean.
This shows El Nino’s impacts on global patterns of weather variability are shown here in terms of temperature and precipitation.
Here is El Nino’s fingerprint on patterns of global weather variability for the NH winter season.
Wet and dry in the tropical Pacific as the most immediate consequence of shifting rainfall patterns.
Cool and wet in the SE, warm in the NW as a result of jet stream changes.
Severity of impacts are related to the intensity of the El Nino, which we will say more about later.
La Nina impacts are roughly opposite to those of El Nino to a first approximation.

21. Teleconnections and Global Weather Patterns
El Nino is a fascinating scientific puzzle that has occupied two generations of oceanographers and meteorologists.
What makes it compelling from a socio-economic perspective is its global impacts on weather variability and ecosystems on land and in the ocean.
This shows El Nino’s impacts on global patterns of weather variability are shown here in terms of temperature and precipitation.
Here is El Nino’s fingerprint on patterns of global weather variability for the NH winter season.
Wet and dry in the tropical Pacific as the most immediate consequence of shifting rainfall patterns.
Cool and wet in the SE, warm in the NW as a result of jet stream changes.
Severity of impacts are related to the intensity of the El Nino, which we will say more about later.
La Nina impacts are roughly opposite to those of El Nino to a first approximation.

22. Figure 22.6

23. Winter Temperature
Anomaly
(El Nino - Normal)
Winter Precipitation
Anomaly
(El Nino - Normal)

24. ENSO Observing System
• Developed during the International Tropical Ocean-Global Atmosphere (TOGA) Program ( )
• Consists of in-ocean & satellite observational component
• Provides information on key environmental parameters in real-time
Sea surface temperature
Surface winds
Upper ocean heat content
Sea level
Ocean currents
Under normal conditions, the trade winds blow from east to west and they pile up warm water heated by the sun in the western Pacific.
The thermocline, which is the sharp boundary between the warm surface layer and the cold dark deeper ocean, is lifted up in the east and pushed down in the west.
Upwelling of thermocline waters in the eastern Pacific along the equator creates an equatorial cold tongue that extends from the coast of SA to near the date line. The depressed thermocline in the west
In the atmosphere, rising motion, deep convection and heavy precipitation occur over the warmest waters in the western Pacific. There is a return flow to the east at higher elevations, and sinking of air over the cold water in the east.
Every few years, the trade winds weaken, the western Pacific warm pool migrates eastward, the thermocline flattens out, and equatorial upwelling is reduced in the eastern Pacific, and the cold tongue disappears. Deep convection and heavy rainfalls migrate eastward, bringing torrential rains to the island states of the central Pacific and drought to Australia, Indonesia and the Philippines. This is El Nino.

25. Global Impacts Social and Economic Consequences
El Niño can affect life, property, and economic vitality due to weather related hazards.
Not hard to imagine why people would be concerned about El Nino’s impacts on weather variability. Obviously, too much or too little rain can adversely affect agricultural production. Drought conditions can increase the potential wildfires in forests and grasslands. Warm winter temperatures reduce snowpack in the PNW, affecting reservoir levels, hydroelectric power generation. urban water supplies. and the ski industry. Flooding can destroy homes, roads, bridges and cause fatalities.
Global Impacts

26. flooding in northern Peru

27. Ocean Biological Productivity (Chlorophyll Concentrations)
La Niña
El Niño
Jun-Aug ‘98
Dec ‘97-Feb ‘98

28. NOAA Report to the Nation, 1995
The deep ocean is rich in nutrients because of decaying organic matter that sinks out of the well lit surface layer.
Under normal conditions in the Pacific, these nutrients are upwelled to the surface along the equator and the west coasts of they Americas, where they support the growth of one celled plants called phytoplankton. All higher trophic levels depend on this primary productivity at the base of the food chain.
During El Nino, upwelling is reduced which suppresses primary productivity. Thus, fish,sea birds and marine mammals all suffer the consequences of reduced food supply.
This results in reduced fecundity, starvation, death, or forced migration for many species.
Strong Upwelling
Weak Upwelling
NOAA Report to the Nation, 1995

29. El Niño and Peruvian Anchovy
Anchovy Catch
(millions of tons)

30. Peruvian Anchovy Catch and ENSO