& La Niña Southern Oscillation Index

Transcription

1 El Niño & La Niña Southern Oscillation Index

2 Today: first of the natural changes Lectures 1) El Nino/La Nina: year- decadal changes in climate system 2) Next: Short term natural changes (centuries to millenia) 3) Next: Long term natural change: ice ages to history of planet

3 First Recall: Earth s Main Wind Bands Trade Easterlies

4 Question: What do you think would happen, if the trade-winds slowed down a little bit? (maybe avg 5 mph less in one year?)

5 El Niño / La Niña Oscillation A story of: Climate feedbacks How relatively small changes in the Air/Sea climate system can have much larger effects

6 Yearly Sea surface temps in Pacific.. Is it Symmetrical?

7 Equatorial Divergent Upwelling 2 N EQ 2 S divergent upwelling Winds push water to right in NH Trade Winds Winds push water to left in SH West East

8 Model: combined currents and temperatures over basin

9 Bottom of layer influenced by winds EQ Equatorial Upwelling Differences between West and East Upwell Warm Water in West Thick Warm Surface Layer Easterly Trade Winds Thin Warm Surface Layer Warm Surface Layer Thermocline Upwell Cold Water in East But.there s more to this story Cold Deep Ocean West East

10 L Air-Sea Feedbacks! Trade Winds Enhanced Trade more Winds by stronger Enhanced Easterly by Pressure Trade Gradient Winds Stronger H H colder Upwelling Due Enhanced to Winds by even Winds more by Winds Cold Deep Ocean West East

11 Fig a SUMMARY OF NORMAL CONDITIONS: Walker Circulation Strong Trades blow E to W Thermocline shallow in E Upwelling of cold water in E E - W temperature gradient reinforces trades Southern Oscillation Index positive

12 Equatorial Temperature Example- August 1996 Normal Thermocline = steep thermal gradient between cold water below and warm surface water above

13 Reprised Annual Sea Surface Temperature: can now see W. Warm pool?

14 Equatorial cross-section (across the Pacific) Walker Cell Or Walker Circulation Top of troposphere L H Darwin, Australia = Normally Low Pressure Tahiti = Normally High Pressure Southern Oscillation West Index is East = Pressure at Tahiti minus Pressure at Darwin

15 So: What is an El Nino?

16 El Nino Condition: Walker Circulation Gone, reverse flow West East

17 Equatorial Temperature August 1996 Normal Thermocline = steep thermal gradient between cold waters below and warm surface water August 1997 El Niño

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19 Fig SUMMARY OF EL NIÑO CONDITIONS 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 Southern Oscillation Index negative

20 La Niña Conditions Walker Circulation Stronger than normal Southern Oscillation Index very positive

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22 Southern Oscillation Index ( ) How often? SOI 50 yrs

23 La Nina Southern Oscillation Index ( ) How often? SOI 50 yrs El Nino * Average frequency: 3-5 years (sometimes as little as 2, as much as 7..) La Nina does not always follow.. Is it a story of perturbation, or perpetual cycle? Debated..

24 SST Evolution of the El Nino / La Nina cycle

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38 El Nino /La Niña Conditions What are the effects?

39 Normal Winter El Niño Winter Disrupting the walker circulation and hadley cells affects the pressure centers that sit over the pacific..these pressure centers dictate storm tracks (Jet stream shifts..)

40 El Niño Effects N. America: Low pressure in the gulf of Alaska sets up a persistent storm track - brings more moisture and more storms from Pacific to much of north America

41 Conditions during El Niño Northern US warms (bad ski year in NW) Most of US has more rain

42 Teleconnections Refers to global linkage of climate system: How a major change in one part of the system can have global effects.

43 El Niño Effects: rain / storms wet conditions throughout north america, warm conditions through canada, But also *dry indonesia, *dry south east africa

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45 What are some practical effects?

46 El Niño Effects A huge wave breaks over the seawall at Fort Point under the Golden Gate Bridge and crashes onto a parked car on February 1, Throughout the following week, high winds and heavy rains combined with abnormally high tides to wreak havoc in the San Francisco Bay region. Inset photo shows a worker hauling sandbags through floodwaters in Sausalito, north of the Golden Gate Bridge, on February 7. U.S. Geological Survey scientists have shown that these extreme conditions were the direct result of the El Niño atmospheric phenomenon. (Photos by Lea Suzuki and Vince Maggiora / copyright San Francisco Chronicle.)

47 El Niño Effects Damaging landslides during the el nino in oakland (USGS map)

48 El Niño Effects Cowell s october 1998

49 El Niño Effects Cowell s Feb 1998: covered in wood..

50 El Niño Effects

51 El Niño Effects

52 El Niño Effects

53 La Niña Effects N. Hemisphere winter during la nina- opposite. dry and warm in much of the US, wet pacific north west, very wet western equatorial pacific

54 La Niña Effects N. America La Nina: high pressure strengthens over the north pacific, get two variable jet streams funneling cold air into canada and dry cool conditions much of the US, wet pacific north west

55 Interconnections El Nino/ La Nina cycles demonstrate the interconnected nature of the Atm/ Ocean system How a perturbation in one element, can create a far- reaching cascade of effects, across the planet..

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