1. Mean annual SST

2. EQ WestEast Equatorial Divergence

3. map view 2˚N 2˚S EQ Trade Winds WestEast Ekman Transport {to right in NH} Ekman Transport {to left in SH} DIVERGENCE Equatorial Divergence

4. cross-section 2˚N2˚S XXX Trade Winds Ekman Trans. {to left in SH} Ekman Trans. {to right in NH} Equatorial Divergence

5. cross-section 2˚N2˚S DIVERGENCE XXX Trade Winds UPWELLING Ekman Trans. {to left in SH} Ekman Trans. {to right in NH} Equatorial Divergence

6. WestEast Warm Surface Layer Cold Deep Ocean Easterly Trade Winds Equatorial upwelling - E vs. W

7. WestEast Warm Surface Layer Cold Deep Ocean Easterly Trade Winds Thick Warm Surface Layer Thin Warm Surface Layer Thermocline Equatorial upwelling - E vs. W

8. WestEast Warm Surface Layer Cold Deep Ocean Easterly Trade Winds Thick Warm Surface Layer Thin Warm Surface Layer Thermocline Upwell Cold Water in East Upwell Warm Water in West Bottom of Ekman Layer Equatorial upwelling - E vs. W

9. Cold Deep Ocean Air-Sea Feedbacks Easterly Trade Winds WestEast H L Upwelling Due to Winds

10. Cold Deep Ocean Air-Sea Feedbacks Easterly Trade Winds WestEast H L Trade Winds Enhanced by Pressure Gradient Upwelling Due to Winds Upwelling Enhanced by Winds colder

11. Cold Deep Ocean Air-Sea Feedbacks Easterly Trade Winds WestEast L Trade Winds Enhanced by Pressure Gradient Upwelling Due to Winds Upwelling Enhanced by Winds Stronger H colder Trade Winds Enhanced more by stronger Pressure Gradient Upwelling Enhanced even more by Winds

13. Mean annual SST

14. Air-Sea Feedbacks WestEast H L Walker Cell Or Walker Circulation

15. A big result of Different “Pressure” zones: moisture. WE LOW PressureHIGH Pressure As air rises, it cools, water condenses, lots of rain As air sinks, it warms, lots of evaporation Warm rising air WIND Cold sinking air

16. “Normal” conditions

17. 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

18. Walker Circulation Gone, reverse flow WestEast What is an El Niño?

19. El Niño conditions

20. 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

21. “Normal” “El Niño” www.pmel.noaa.gov/tao/elnino

22. Annual Average “Normal” Short-lived deviation from “Normal”: El Niño (March 1998)

36. Walker Circulation Stronger than normal La Niña conditions

37. Normal conditions El Niño conditions

38. WestEast H L Walker Cell Or Walker Circulation Normal rainfall

39. Walker Circulation Gone, reverse flow WestEast El Niño rainfall

40. La Niña anomaly map

41. El Niño anomaly map

42. El Niño impacts - precipitation patterns