Chapter 5—Part 1

Ocean Circulation/Upwelling Zones/

El Niño

Ocean Circulation

1. Eckman spiral

2. Upwelling zones

3. El Niño

• Ocean surface currents are driven by atmospheric winds

• Energy from wind transferred to water by friction

Wind

Wind moves surface water (friction)

Velocity of water decreases with depth

But

• Both the wind and the ocean currents are deflected near the atmosphere-ocean interface by a combina- tion of the Coriolis effect, friction, and gravity

• This gives rise to what is termed the Eckman Spiral - Surface wind rotated 45o relative to the

geostrophic (free troposphere) wind- Surface waters move 45o relative to the surface wind direction- Net transfer of water 90o to the surface wind

The Eckman Spiral

• Surface waters move 45o relative to surface wind• Net transfer of water 90o to surface wind direction

Eckman Pumping

Equator

Surface windSurface currentNet ocean transport

• Surface water is driven offshore by Eckman spiral• Replaced by water from below upwelling!

Geostrophic wind(Easterlies)

Consequences of upwelling

• Deep water is rich in nutrients (P, N, Fe)

• Upwelling brings nutrient-rich water to the surface ocean, fueling biological productivity (phytoplankton)

• Zooplankton eat the phytoplankton

• Fish eat both of these get good fisheries in upwelling zones

El Niño (the Christ child)

Every 3-7 yearsLast 6-18 monthsHappens near Christmas season

El Niño (the Christ child)

Every 3-7 yearsLast 6-18 monthsHappens near Christmas season

Opposite:

La Niña (the girl child)

Normally…

Warm waters pushed by trade winds towards Asia and Australia

Cold waters upwell near South America

http://www.ldeo.columbia.edu/dees/ees/climate/slides/ocean_index.html

Pacificwarmpool

Normal Conditions:

http://www.enn.com/specialreports/elnino/what.asp

• This east-west circulation pattern, which is sort of like a Hadley cell, is called the Walker Circulation

•It reinforces the normal tropical easterlies, thus helping to foster upwelling off the coast of Peru

Australia/Indonesia

South America

Normal Conditions:

El Niño Condition:

http://www.enn.com/specialreports/elnino/what.asp

NormalWalker Circulation

WeakenedWalker Circulation

South America

South America

Australia/Indonesia

Australia/Indonesia

Atmospheric Pressure:

Normal high over Tahiti, low over Darwin, Australia

El Nino low over Tahiti, high over Darwin

The shift in pressure is called the Southern Oscillation

The combined El Niño/Southern Oscillation isreferred to as ENSO

Atmospheric Pressure:Normal high over Tahiti, low over Darwin, AustraliaEl Niño low over Tahiti, high over Darwin

* Darwin

Southern Oscillation Index

[ Pdiff - Pdiffav ] SOI = 10 ----------------------- SD(Pdiff) where

Pdiff   =   (average Tahiti MSLP for the month) – (average Darwin MSLP for the month),Pdiffav   =   long term average of Pdiff for the month

in question, andSD(Pdiff)   =   long term standard deviation of Pdiff

for the month in question.

http://www.bom.gov.au/climate/glossary/soi.shtml

StrongEl Nino

La Niña

http://www.bom.gov.au/climate/current/soi2.shtml

• Is this what is caused the 2011 Texas drought?• “ENSO neutral” conditions persisted during most of 2012

• The fact that we have been in an extended La Niña may explain why the climate has not warmed as fast as expected over the past 15 years…

Total heat accumulation in sea, land, and air

http://www.skepticalscience.com/clarifying-continuation-global-warming.html

Results of El Niño

In the Pacific Region:

weaker trade windswarm waters across the Pacificrain in South America, drought in Australia

Results of El Niño

In the Pacific Region:

weaker trade windswarm waters across the Pacificrain in South America, drought in Australia

Teleconnections--shifting atmospheric patterns (global)

high rain in the US midwestmore storms in the Pacific, fewer in the Atlanticheavy rains in Californiacold winters in the Northeastern US