1. AIR/SEA INTERACTION El Nino

4. OBJECTIVES To Understand what El Nino is and how it develops
To understand why we should study El Nino or ENSO
To understand how ENSO events are defined
To understand the global effects of ENSO
To understand NAO

5. OUTLINE What is El Nino? SOI vs. JMA/Nino 3.4
Importance of ENSO Events
Effects of ENSO Events
NAO

6. What is El Nino ?
A disruption of the ocean-atmosphere system in the tropical Pacific
Normally, northeasterly trade winds combined with upwelling off of the coast of Peru keep waters cool in the central and eastern Pacific
During El Nino, trade winds relax in the western Pacific leading to warmer waters in the central and eastern Pacific

7. Normal Conditions

9. El Nino Conditions

11. La Nina Conditions

16. DEFINING ENSO Events
SOI
JMA/ Nino 3.4 (ONI)

19. SST vs. SOI Index

22. An Example of the Evolution of Equatorial Pacific SST Departures (oC)
October February 2009, negative sea surface temperature (SST) anomalies dominated across the central and east-central equatorial Pacific Ocean
February- April 2009, negative SST anomalies weakened across the equatorial Pacific Ocean
Since the beginning of June 2009, positive SST anomalies have been at least +0.5°C across the equatorial Pacific.
Time
Longitude

24. Rossby Waves
Also known as planetary waves, owe their origin to the shape and rotation of the Earth
Travel from East to West
Slow moving - the speed varies with latitude and increases equatorward, but is of the order of just a few cm/s (or a few km/day)

26. Influence of Rossby Waves

27. Equatorial Kelvin Waves
Special gravity waves excited by an abrupt change in the overlying wind field
Travel west to east
Fast moving - speeds of approximately m/s (250 km/day) at the surface
Maximum amplitude at the Equator

28. Equatorial Kelvin Wave

29. Influence of Kelvin Waves
Warmest water is usually
m below the surface

30. Rossby and Kelvin Waves

31. Ending Warm ENSO Events
Rossby waves generated by the relaxation of the trade winds in the central Pacific (red oval) travel west to the Indonesian archipelago and are reflected back as Kelvin waves.
Now, the Kelvin waves bring cold water back towards the surface (in the blue region), initiating the cooling that shuts down the warm ENSO event and re-strengthens the trade winds.

34. Why study El Nino? Weather impacts human lives
California flooding
Storm damage
Loss of life
Major impacts on the economy
Agriculture
Construction
Energy distribution
Outdoor recreation
Retail
U.S. economic impacts of the 97/98 El Nino - $25 Billion !!
Both gains and losses
Department store sales up 15% during mild Midwest winter
Snow equipment sales down 35% during mild Midwest winter
Skiing was up in the west…but down in the Midwest

35. Effects of El Nino

38. North Atlantic Oscillation

39. The North Atlantic Oscillation
The NAO is the dominant mode of winter climate variability in the North Atlantic region ranging from central North America to Europe and much into Northern Asia.
The NAO is a large scale see-saw in atmospheric mass between the subtropical high and the polar low.
The corresponding index varies from year to year, but also exhibits a tendency to remain in one phase for intervals lasting several years.

40. The Positive NAO Index
Stronger than usual subtropical high pressure center and a deeper than normal Icelandic low
The increased pressure difference results in more and stronger winter storms crossing the Atlantic Ocean on a more northerly track (enhanced westerlies).
This results in warm and wet winters in Europe and colder and drier winters in northern Canada and Greenland
The eastern US experiences mild and wet winter conditions

41. The Negative NAO Index
A weak subtropical high and a weak Icelandic low.
The reduced pressure gradient results in fewer and weaker winter storms crossing on a more west-east pathway.
Brings moist air into the Mediterranean and cold air to northern Europe
The US east coast experiences more cold air outbreaks and hence snowy weather conditions.
Greenland, however, will have milder winter temperatures

42. The NAO Index
Look at the graph above, do you see any recognizable trends?