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Mean annual SST
EQ
West East
Equatorial Divergence
map view
2˚N
2˚S
EQ Trade Winds
West East
Ekman Transport{to right in NH}
Ekman Transport{to left in SH}
DIVERGENCE
Equatorial Divergence
cross-section
2˚N2˚S X XXTrade Winds
Ekman Trans.
{to left in SH}
Ekman Trans.
{to right in NH}
Equatorial Divergence
cross-section
2˚N2˚S
DIVERGENCE
X XXTrade Winds
UP
WE
LL
ING
Ekman Trans.
{to left in SH}
Ekman Trans.
{to right in NH}
Equatorial Divergence
West East
Warm Surface Layer
Cold Deep Ocean
Easterly Trade Winds
Equatorial upwelling - E vs. W
West East
Warm Surface Layer
Cold Deep Ocean
Easterly Trade Winds
Thick Warm Surface Layer
Thin Warm Surface Layer
Thermocline
Equatorial upwelling - E vs. W
West East
Warm Surface Layer
Cold Deep Ocean
Easterly Trade Winds
Thick Warm Surface Layer
Thin Warm Surface Layer
Thermocline
Upwell Cold Water in East
Upwell Warm Water in West
Bottom of Ekman Layer
Equatorial upwelling - E vs. W
Cold Deep Ocean
Air-Sea Feedbacks
Easterly Trade Winds
West East
HL
Upwelling Due toWinds
Cold Deep Ocean
Air-Sea Feedbacks
Easterly Trade Winds
West East
HLTrade Winds Enhanced by Pressure Gradient
Upwelling Due toWindsUpwelling Enhanced by Winds
colder
Cold Deep Ocean
Air-Sea Feedbacks
Easterly Trade Winds
West East
LTrade Winds Enhanced by Pressure Gradient
Upwelling Due toWindsUpwelling Enhanced by Winds
Stronger
Hcolder
Trade Winds Enhanced more by strongerPressure Gradient
Upwelling Enhanced even more by Winds
Mean annual SST
Air-Sea Feedbacks
West East
HL
Walker CellOr Walker Circulation
A big result of Different “Pressure” zones: moisture.
W E
LOW Pressure HIGH Pressure
As air rises, it cools, water condenses, lots of rain
As air sinks, it warms, lots of evaporationWarm rising air
WIND
WIND
Cold sinking air
“Normal” conditions
• Walker Circulation Strong– Trades blow E to W– Thermocline shallow in E– Upwelling of cold water in E– Warm water piles up in W– E to W temperature gradient reinforces
trades
“Normal” conditions
Walker CirculationGone, reverse flow
West East
What is an El Niño?
El Niño conditions
• Walker Circulation Weak or Gone– Trades very weak or reversed– Thermocline deep in E– Upwelling of warm water in E– Sea level rises in E– E to W temperature gradient reduced or
gone
El Niño conditions
“Normal” “El Niño”
www.pmel.noaa.gov/tao/elnino
Annual Average “Normal”
Short-lived deviation from “Normal”: El Niño (March 1998)
• Walker Circulation Stronger than normal
La Niña conditions
Normal conditions
El Niño conditions
West East
HL
Walker CellOr Walker Circulation
Normal rainfall
Walker CirculationGone, reverse flow
West East
El Niño rainfall
La Niña anomaly map
El Niño anomaly map
El Niño impacts - precipitation patterns
