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

UPW

ELLING

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 LayerThermocline

Equatorial upwelling - E vs. W

West East

Warm Surface Layer

Cold Deep Ocean

Easterly Trade Winds

Thick Warm Surface Layer Thin Warm

Surface LayerThermocline

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

StrongerHcolder

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 ELOW Pressure HIGH Pressure

As air rises, it cools, water condenses, lots of rain

As air sinks, it warms, lots of evaporationWarm rising air

WIND

WINDCold 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