El Niño-Southern Oscillation

1. El Niño-Southern Oscillation AOS 101 Dis. 305

2. Global Impacts of El Niño and La Niña • Floods • Drought • Wild Fires • Hurricanes • Ecosystem and fisheries

3. Global Impacts of El Niño and La Niña • Floods • Drought • Wild Fires • Hurricanes • Ecosystem and fisheries

4. El Niño and La Niña • More than 100 years ago, the term El Niño was used to describe unusually warm waters that occasionally form along the coast of Ecuador and Peru. • Spanish for “the boy child” / “the Christ child” • Now, the term El Niño is refer to unusually warm water that occasionally forms across the tropical central and eastern Pacific. • Time between successive El Niño events: every 3-7 years

5. El Niño and La Niña • La Niña is when cooler sea surface temperatures (SSTs) are found across tropical central and eastern Pacific. • Often times a La Niña event would follow an El Niño event. • Once developed, it can last about a year and sometimes persist for 18 months.

6. How does the pressure, wind, and rainfall patterns change? http://www.cpc.noaa.gov/products/analysis_monitoring/ensocycle/meanrain.shtml

7. How does the pressure, wind, and rainfall patterns change?

8. How does the pressure, wind, and rainfall patterns change?

9. What is El Niño-Southern Oscillation (ENSO)? • There is mutual interaction between the tropical ocean and the atmosphere above it to form a coupled system. • The internal dynamics of the coupled ocean-atmosphere system determine the onset and termination of El Niño events. • During El Niño, sea level pressure is lower in the eastern Pacific and higher in the western Pacific. • During La Niña, opposite occurs. • The fluctuations in atmospheric pressure between western and eastern tropical Pacific is known as the Southern Oscillation (SO). • http://esminfo.prenhall.com/science/geoanimations/animations/26_NinoNina.html

10. How do we monitor ENSO? 1. The Southern Oscillation Index – SOI • Measure of sea level pressure difference between Tahiti and Darwin, Australia • Averaged over a 5 month period to smooth out noises caused by local weather disturbances. • During El Niño, SOI is negative, and during La Niña, it is positive.

11. 2. Sea-Surface Temperature Indices • Tropical Atmosphere Ocean (TAO) Array • It consists of a number of moored buoys distributed along the equator. • It takes both atmospheric surface meteorological and surface ocean measurements.

12. TAO buoy

13. How do we identify ENSO? • It depends on how large the value of the index is and how long that the condition last for. • One common method is to look at the NINO 3.4 Index, which is the departure in monthly SST from its long term averaged over at the NINO 3.4 region. • If the 5 months average of the NINO 3.4 index is greater than +0.4 deg C consecutively for 6 months, then El Niño is identified. (-0.4 deg C for La Niña)

14. Teleconnections • The relationship between El Niño events and climatic variations in the Equatorial Pacific region is extremely strong . The relationship outside this area is harder to prove.  • Pressure and circulation anomalies occur all over the globe every year, but some tend to recur with most or all El Niño events and are referred to as teleconnections. • El Niño impacts are strongest and most widespread when water temperatures reach the annual maximum (boreal winter). • Above normal warmth of the water, el Niño redistributes convection, which causes changes in the jet stream. • During the austral winter, El Niño impacts are weaker and confined mostly to the Southern Hemisphere Boreal Winter (Winter for NH) Austral Winter (Winter for SH)

15. Effects of ENSO in North America • During El Niño: • Winter temperature warmer, increased precipitation in CA because of the shifting of the jet stream. • Increased snowfall in the southern Rockies, below average snowfall in the upper midwest. • ENSO also changes the upper level winds, which could lower the likelihood of Atlantic hurricane hitting the US. • Jet stream shifts causing also shifts in the occurrence of severe weather. • During La Niña: • Increased precipitation in Pacific Northwest and Midwest and decreased precipitation in Alaska. • Increased chance of hurricanes.

16. References • http://iri.columbia.edu/climate/ENSO/background/basics.html#fig2 • http://www.cpc.noaa.gov/products/analysis_monitoring/ensocycle/meanrain.shtml • http://www.pmel.noaa.gov/tao/ • http://en.wikipedia.org/wiki/Effects_of_the_El_Niño-Southern_Oscillation_in_the_United_States

17. Cloud forcing AOS 101 Dis. 305

18. Cloud cover in percent - High clouds

19. Cloud cover in percent – low clouds

20. Cloud cover in percent – all clouds

21. Reduced outgoing longwave radiation

22. Reduced solar radiation

23. Net forcing

24. Cloud forcing • Clouds reflect solar radiation. • Less solar radiation gets to Earth’s surface  causing cooling. • Earth’s surface emits longwave radiation and clouds absorb and reemit radiation back towards the surface and to space  could cause warming at the surface. • http://earthobservatory.nasa.gov/Features/Clouds/

25. High Cirrus Clouds • Clouds are thin, allowing much solar radiation to get through • Low Albedo • Cloud tops are cold, they emit at colder temperatures. Less energy being radiated than the case with clear skies. • Causing net surface warming http://earthobservatory.nasa.gov/Features/Clouds/

26. Low clouds • They are thicker with higher albedo • Closer to the surface, emit at higher temperature • So they radiate at almost same intensity as the surface • Net cooling at the surface http://earthobservatory.nasa.gov/Features/Clouds/

27. Deep Convective Clouds • Thick clouds: high albedo, less solar radiation could get through • Cloud tops are cold, longwave radiation being emitted is lesser than the case with clear sky. http://earthobservatory.nasa.gov/Features/Clouds/