The ocean, its role in the climate system

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Lecture 13

• The ocean, its role in the climate system
• Coupled climate modes
• ENSO El Nino, La Nina
• (LATER (North Atlantic Oscillation (NAO), more
  recently referred to as the AO (AAO), NAM (SAM)))
  
• Tropical cyclones

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Oceanography the study of oceans

• They are a source of atmospheric water vapor and
  other trace gases
• They exchange energy with the atmosphere
• They transport heat poleward
• It takes approximately two weeks for all the
  water in the atmosphere to recycle. The oceans
  provide the majority of water for precipitation.

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Surface fluxes of energy and trace gases

• The rate of heat and moisture transfer depends on
  temperature/moisture difference as well as wind
  speed.
• Warm SST and high wind are favorable to large
  heat exchanges between atmosphere and ocean
• US west coast vs. coast of N. Europe, cool SST
  vs. warm SST Affects climate profoundly

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Sea surface temperature (SST)
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Things to note about distribution of SSTs

• SSTs off west coasts in midlatitudes and
  subtropics are cool
• SSTs off east coasts in midlatitudes are warm
• SSTs off east coasts in high latitudes are cold

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SST warm
SST cold
SST hot
SST cool
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The major surface ocean currents (wind driven so
they resemble the atmospheric wind patterns)
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The North Atlantic gyre (N equatorial current,
Gulf stream, N Atl current, Canary current)
atmospheric winds
Force balance friction from atm wind acting on
ocean and Coriolis force
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Ekman transport, the Ekman spiral
100 m down
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Ocean upwelling, example California Ekman
transport contributes to this phenomenon
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Effects of upwelling

• Upwelling brings cold water, rich in nutrients up
  to the surface
• Effects both SSTs and life in the ocean
• During an El Nino, upwelling ceases off the coast
  of Peru. This results in warm SSTs and dearth of
  life including fish.

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El Nino periodic warming of the equatorial
Pacific between S America and the dateline

• The warming first appears off the coast of S
  America around Christmas and lasts for several
  months.
• Occur periodically every 2-7 year
• Normal cold SST in east, steady trades, warm
  water piles up in the eq west Pacific
• El Nino weak trade winds, surface eq current
  toward the east replacing the cold sfc water off
  the coast of S America

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• The height of the ocean sfc drops near Indonesia
  and rises in the E Pacific, forcing the
  thermocline down near S America preventing
  upwelling

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Normal conditions, El Nino conditions
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Normal conditions/El Nino conditions
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Southern Oscillation (Normal/El Nino year)
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Global effects of El Nino

• The W Pacifc below normal precipitation
• Precipitation moves over E Pacific w. warm SST
• Global impacts most noticable in winter
• Warm in Canada and E Asia
• Wet is S US and coastal S America
• Dry in W equatorial Pacific, wet in E Pacifi
• In summer, dry in monsoon regions

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Global effects of El Nino
Winter
summer
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Southern Oscillation

• Seesaw in atmospheric pressure between eastern
  equatorial Pacific and Indonesia-Australia.
• When pressure is high over the Pacifc it is low
  over the the eastern Indian Ocean, and vice versa
• Measure sfc pressure in Tahiti in the W and
  Darwin in the E, Southern Oscillation Index (SOI).

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Southern Oscillation Index
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La Nina

• Cooler than normal SST in E Pacific
• Intense trade winds drive warm ocean surface
  waters toward the west, while increasing cold
  water upwelling off American coast
• During La Nina it is drier than normal in the
  Southwest in late summer through winter, in
  Central plains in fall and in SE in late fall and
  early winter

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97-98 El Nino from spaceNote the buildup of warm
water in the E Pac and cool water in W Pac
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Distribution of precip over subtrop summer
continents. Note wet W of subtropical high
(Florida, SE US), dry E of the subtropical high
(Sahara desert)
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Monitoring surface wind from spacescatterometry
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Hurricane Mitch approaching Honduras on Oct. 27,
1998
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Hurricane Mitch in W Caribbean Oct 26
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Tropical cyclones

• What are they?
• How and where do they form?
• How are they structured?
• What is the life cycle?
• Tropical cyclone tracks
• How do they cause destruction?
• Observing and forecasting them

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Tropical cyclones

• In an average year
• 5-6 form over N Atlantic and Gulf of Mexico
• nine form in E Pacific off of Mexico
• sixteen typhoons form in W Pacific
• Can be very distructive
• Galveston, TX, in 1900 killed 8000
• Bangladesh in 1970, killed 300,000
• Mitch in 1998, horrific deaths in C Am

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Structure

• The eye, clear of clouds (8 km or more)
• eye wall narrow circular rotating region of
  intense thunderstorms
• Spiral rain bands
• Depend on warm water, warm air