1. NATS 101 Lecture 21 Atmo-Ocean Interactions El Niño-Southern Oscillation

2. Summary Global Circulation
Differential Heating Between Tropics and Poles
Three Cells: Hadley-Ferrel-Polar
Mid-Latitude Westerlies
Patterns shift slightly with seasons
Precipitation
Major Deserts occur under Sub-Tropical High
Mid-latitude storms occur along Polar Front

3. Triple Cell (~Real World)
Equator-to-Pole temperature difference and rotation of Earth produce 3 circulation cells
Hadley Cell (Thermally Direct)
Ferrel Cell (Indirect: Forced by Hadley & Polar)
Polar Cell (Thermally Direct)
Shift with Seasons
Hadley
Ferrel
Polar
Equator
Pole

4. Global Circulation - Precipitation
Ahrens Fig 13-2

5. Prevailing Winds

6. Ocean Currents
Drag from wind exerts a force on the ocean surface in the same direction as the wind.
Currents of upper ocean are due to wind.
Tend to flow in the direction of prevailing wind.
Poleward currents are warm; equatorward currents are cold.
Thus, oceans transport heat from the tropics to the poles, about the same amount of heat as the wind.

7. Ocean Currents of World
Ahrens Fig. 7.24

8. Summer SST Along West Coast
Sea surface temperatures (SST) along West Coast are quite cold during summer, especially off Northern California.
Due to upwelling of cold, nutrient rich water by prevailing N winds.
Prevailing Winds
Coastal Surface Water
Ahrens Fig 7.24

9. Ekman Spiral
Ahrens, Older Ed.
Surface water moves 45 to the right of prevailing wind. Subsurface water moves at angles greater than 45 Net transport of surface layer is 90 to the right. Coriolis force is responsible for the rightward rotation.

10. Upwelling from Alongshore Winds
Ahrens Fig 7.25
Wind pushes surface water southward. Coriolis force deflects water to the right. Cold water from below rises to surface. Fog persists over the cold water.

11. El Nino 3.4
Upwelling Regions
weather.unisys.com

12. El Nino-Southern Oscillation (ENSO)
An important atmospheric-ocean feedback
Normal conditions in tropical Pacific:
-Warm SST, low SLP, and T-storms in W Pacific
-Strong subtropical highs in E Pacific
-Easterly winds and cool upwelling water along equator in East Pacific
-Prevailing southerly winds off of Peru produce cold upwelling and excellent fishing

13. El Niño-Southern Oscillation (ENSO)
Every few years (4-5 years):
-Equatorial Central Pacific warms
-Low SLP, T-storms shift to Central and East Pacific (the Southern Oscillation)
-Trades and southerly winds off Peru weaken
-Upwelling ceases off Peru, warming leads to massive kill off of fish. Typically occurs around Christmas (an El Niño event)
Alters global patterns of wind, temp and rain

14. Walker Circulation
Pushes water westward
Darwin
Tahiti
Aguado & Burt, p230
Walker Circulation oscillates with a quasi-period of every few years. Oscillation is very evident in SLP records for Darwin and Tahiti.

15. Southern Oscillation Index (SOI)
SST
SLP

23. El Nino Precipitation Extremes

24. La Nina Precipitation Extremes

25. El Nino Precipitation Extremes

26. La Nina Precipitation Extremes

27. ENSO-Atmosphere Feedback
Ocean temperature pattern
Ocean currents, upwelling
SLP, winds, storms, etc.
By observing SST’s and surface winds in equatorial Pacific, we are able to forecast ENSO events with considerable skill. ENSO forecasts lead to skillful seasonal forecasts for the US several months in advance (e.g winter).

28. Summary Major Ocean Currents Driven by prevailing wind
Upwelling Regions
Occurs along west coasts of continents
Cold water rises from below to surface
Nutrient rich, excellent fishing regions

29. Summary El Nino-Southern Oscillation Occurs every few years
Central equatorial Pacific warms
Low SLP, T-storms move with warm water
Upwelling weakens along Peru coast
Can be predicted up to one-year in advance
Modulates global patterns of wind, temp, rain

30. Assignments Next Class - Quiz #4 Following Lecture Topic - Air Masses
Reading - Ahrens pg
Problems - 8.1, 8.11