El Niño & La Niña Southern Oscillation Index
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
First Recall: Earth s Main Wind Bands Trade Easterlies
Question: What do you think would happen, if the trade-winds slowed down a little bit? (maybe avg 5 mph less in one year?)
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
Yearly Sea surface temps in Pacific.. Is it Symmetrical?
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
Model: combined currents and temperatures over basin
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
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
Fig. 4.14.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
Equatorial Temperature Example- August 1996 Normal Thermocline = steep thermal gradient between cold water below and warm surface water above
Reprised Annual Sea Surface Temperature: can now see W. Warm pool?
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
So: What is an El Nino?
El Nino Condition: Walker Circulation Gone, reverse flow West East
Equatorial Temperature August 1996 Normal Thermocline = steep thermal gradient between cold waters below and warm surface water August 1997 El Niño
Fig. 4.14 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
La Niña Conditions Walker Circulation Stronger than normal Southern Oscillation Index very positive
Southern Oscillation Index (1952-2000) How often? SOI 50 yrs
La Nina Southern Oscillation Index (1952-2000) 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..
SST Evolution of the 1997-98 El Nino / La Nina cycle
El Nino /La Niña Conditions What are the effects?
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..)
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
Conditions during El Niño Northern US warms (bad ski year in NW) Most of US has more rain
Teleconnections Refers to global linkage of climate system: How a major change in one part of the system can have global effects.
El Niño Effects: rain / storms wet conditions throughout north america, warm conditions through canada, But also *dry indonesia, *dry south east africa
What are some practical effects?
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, 1998. 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 1997 98 El Niño atmospheric phenomenon. (Photos by Lea Suzuki and Vince Maggiora / copyright San Francisco Chronicle.)
El Niño Effects Damaging landslides during the 97-98 el nino in oakland (USGS map)
El Niño Effects Cowell s october 1998
El Niño Effects Cowell s Feb 1998: covered in wood..
El Niño Effects
El Niño Effects
El Niño Effects
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
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
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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