Unit VI Coupled Systems El Ni o and the Southern Oscillation ENSO Coupled System SOI Southern Oscillation Index Walker Circulation B Walker Circulation Characteristics of the normal condition Strong easterly trades Concentration of warm water in W Pacific Surface water in W Pacific thick Sea level in the W Pacific is high Atmospheric pressure over Australasia is low rainy over Tahiti it is high dry Characteristics of the normal condition Off Peru off shore winds produce coastal upwelling productive fisheries Atmosphere over S America around Peru high pressure dry Easterly trades off the coast of Peru weaken around Christmas time Fishermen traditionally call this time of year El Ni o In some years the winds weaken upwelling ceases but does not come back for a year or more This is what we now call El Ni o The Warm Pool sloshes back across the Pacific Kelvin Wave Corilois Effect keeps the wave intact across the Pacific Ocean until it hits the west coast of South America How does El Ni o influence weather Direct within the tropical Pacific region Indirect global outside the tropical Pacific El Ni o La Ni a and normal Consequences of El Ni o Direct Rain in Peru Collapse of Peruvian fisheries Drought in Australia Indonesia Rain in Tahiti Weakened Asian Monsoon Consequences of El Ni o Indirect Through the ocean Warm Kelvin Wave diverted N and S when impacts South America Sends anomalously warm water along the coast of western US and southern S America Water mass changes influence atmospheric jet stream Rainfall transfer of energy through release of latent heat ENSO through time Does the frequency and amplitude of ENSO remain constant Might it change under a warming planet Looking at ENSO past tropical corals ENSO through time ENSO is a pervasive feature of the climate system Present under many different climate states However ENSO exhibits large variations in frequency and amplitude much larger than during the historic record ENSO in a warmer world might be very different than today