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Lecture 13

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. Oceanography – the study of oceans.

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Lecture 13

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  1. 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

  2. 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.

  3. 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

  4. Sea surface temperature (SST)

  5. 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

  6. SST warm SST cold SST hot SST cool

  7. The major surface ocean currents (wind driven so they resemble the atmospheric wind patterns)

  8. 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

  9. Ekman transport, the Ekman spiral 100 m down

  10. Ocean upwelling, example California Ekman transport contributes to this phenomenon

  11. 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.

  12. 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

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

  14. Normal conditions, El Nino conditions

  15. Normal conditions/El Nino conditions

  16. Southern Oscillation (Normal/El Nino year)

  17. 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

  18. Global effects of El Nino Winter summer

  19. 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).

  20. Southern Oscillation Index

  21. 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

  22. 97-98 El Nino from spaceNote the buildup of warm water in the E Pac and cool water in W Pac

  23. Distribution of precip over subtrop summer continents. Note wet W of subtropical high (Florida, SE US), dry E of the subtropical high (Sahara desert)

  24. Monitoring surface wind from spacescatterometry

  25. Hurricane Mitch approaching Honduras on Oct. 27, 1998

  26. Hurricane Mitch in W Caribbean Oct 26

  27. 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

  28. 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

  29. 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

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