Transcript No Slide Title

Class 26:
The El Niño/Southern Oscillation Phenomenon:
An example of a complex system with
Feedback and Oscillations
NOAA’s ENSO data main page:
http://www.cdc.noaa.gov/ENSO/
ENSO animation- computer model:
http://www.cdc.noaa.gov/people/joseph.barsugl
i/anim.html
El Niño / Southern Oscillation (ENSO):
A Prime Example of Complexity in
Natural Systems
Ocean currents affect global wind patterns
Global wind patterns drive ocean currents
Part One:
Winds Drive Ocean Currents
Prevailing Winds are Modified by
Ocean Surface Temperatures:
Hot, Moist Air Rises!
Rising
Air
Sinking
Air
L
H
Part Two:
Ocean Currents Affect Winds
“Normal” Conditions
1. Sinking Air over Tropical E. Pacific
“Easter Island High”
2. Rising Air over Equatorial W. Pacific
“Indonesian Low”
3. Winds blow from high to low pressure
4. In this case, this enhances the Trade Winds
5. These push warm water toward Indonesia
6. This warmth enhances the “Indonesian Low”
“Normal” Conditions
1. Easter Island High
2. Indonesian Low
3. Winds blow from high to low
4. Enhanced Trade Winds...
5. ...push warm water westward
6. Enhanced “Indonesian Low”
Atmos.
Ocean
Normal Conditions, Cont’d
We observe:
• Cool surface waters and upwelling near Peru
•Huge fish harvest- Peru
• Tilting of sea surface -- higher on Asian side
• Tilting of thermocline--higher on American side
El Niño / Southern Oscillation (ENSO)
•Major change in both ocean and air currents
•Happens every 3(?) to 7 years
•The system oscillates back and forth
–El Niño conditions occur in some years
–La Niña conditions in some other years
–Normal conditions in the rest
•
Animation of sea surface elevation and temperature
changes:
http://www.esrl.noaa.gov/psd/people/
joseph.barsugli/anim.html
Rising
Air
Sinking
Air
El Niño Conditions
Feedback Loop:
1. Warm water in W. Pacific begins to “slosh”
eastward
2. This moves the Indonesian low eastward and
weakens Easter Island High
3. This weakens the trade winds
4. This weakens westward ocean currents
http://www.esrl.noaa.gov/psd/people/joseph.barsugli/enso_anim_jjb2.html
file:///Users/doejohnson/Documents/%20Teaching/117%20Oceans/images/EN
SO_anim_THERMOCLINE.gif
What’s the current state of ENSO?
NOAA’s ENSO data main page:
http://www.cdc.noaa.gov/ENSO/
ENSO animation- computer model:
http://www.cdc.noaa.gov/people/joseph.barsugl
i/anim.html
ENSO: Last several decades
Feedback: Interaction and complexity
in natural processes
•Two processes affect each other; change in
each one “feeds back” to affect the other
•The feedback may amplify changes (positive
feedback)
•Thus, a small change may be amplified into a
large one.....a “Vicious Circle”
Understanding Feedback:
Important in Studies of Nature
•El Niño, other aspects of weather, fish and wolf
populations, the greenhouse effect, ice ages...
•Complex behavior: Oscillations, “chaos”
•Yet, we CAN understand these phenomena if
we learn more about the feedback
•e.g., recent El Niño predictions
Local Effects of El Niño :
W. Coast of S. America
•Upwelling ceases
•No westward water movement to cause it
•Big effect on marine life -- Reductions in ...
•Nutrient supply
•Phytoplankton productivity
•Population of fish (anchovies) and fish-eating
birds.
El Niño: Global Effects
Warm Winter
Heavy
Rainfall
El Niño: Global Effects
Warm Winter
Weak Monsoon
(Drought)
Heavy Rainfall
Global Weather Changes- El Niño Years
•Droughts: SE Asia, India, Australia, Africa
•USA: Heavy Rainfall and storms:
• Gulf Coast and Southern California
•Warm Winters:
•Alaska, Western Canada, Northern USA
Last week: What part of the cyle is this?
“La Niña continued to weaken during
February 2011 as reflected by the reduced
strength of the negative surface and nearsurface temperature anomalies across
much of the equatorial Pacific Ocean .”
-NOAA Climate
Prediction Center
Other Ocean/Atmos. Oscillations
• Pacific Decadal Oscillation (recent change)
• 20 to 30 year cycle, affects temp. in the US
• Similar to ENSO, but weaker. Affects water off
west coast of USA
Other Ocean/Atmos. Oscillations
• Pacific Decadal Oscillation (recent change)
• Recently changed from warm phase to cool
phase
Other Ocean/Atmos. Oscillations
•North Atlantic Oscillation
• Similar; irregular
oscillations (several
years?)
• Affects European
Weather