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EPSC 185 (3)
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El Nino Reading

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McGill University
Earth & Planetary Sciences
EPSC 185
John Stix

EPSC 185 Fall 2013 El Nino  in Peru first months of each year has warm southward current  every few years, warming starts earlier, is stronger & lasts longer (can be year or two)  warm water brings water snakes, bananas & coconuts  but shuts off deep, cold water crucial for marine life  El Nino = “The Christ Child”  named due to late December appearance  large-scale oceanic warming  affects most of tropical Pacific  counterpart = La Nina (cooling of eastern tropical Pacific)  normally accompanied by Southern Oscillation (change in atmospheric circulation)  The Basics of ENSO  clearest sign of SO = inverse relationship between surface air pressure at Darwin, Australia & Tahiti o high pressure at one site almost always concurrent with low pressure at the other o pattern reverses every few years o represents wave/see-saw of air oscillating back & forth across date line  Walker Circulation  trade winds across tropical Pacific go E  W  to complete loop air rises above W Pacific, flows back E to high altitudes & descends over E Pacific  this is connected to oceanic changes of El Nino & La Nina  trade winds affect ENSO in 2 ways:  push water toward W Pacific  allow westward-flowing water to remain near surface & gradually heat  as warm surface water collects in W Pacific, pushes down thermocline  thermocline: boundary separating well-mixed surface waters from deeper, colder waters; usually 40 m deep in E Pacific but varies from 100-200 m in W Pacific  persistent oceanic heat surrounding Indonesia & W Pacific islands leads to frequent thunderstorms & heavy rainfall  rainfall helped by upward motion of Walker circulation  distribution of sea surface temperature drives enhanced rainfall, Walker circulation & trade winds (responsible for ocean currents & distribution of sea surface temperatures)  atmosphere drives the ocean & the ocean drives the atmosphere  more rainfall during El Nino because of warm ocean temperatures  Anatomy of El Nino  El Nino & La Nina tend to alternate every 3-7 years  time from one event to the next can vary from 1-10 years  strength of the events varies greatl
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