ENSO_2010

ENSO_2010 - Tropical Circulation in a Global Context Mean...

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Unformatted text preview: Tropical Circulation in a Global Context Mean surface pressure and winds during DJF Westerlies Westerlies Westerlies W e s te r lie s S E t r a d e s S E t r a d e s N E t r a d e s N E t r a d e s S E t r a d e s S E t r a d e s N E t r a d e s N E t r a d e s N E t r a d e s N E t r a d e s E a s t e r l i e s E a s t e r l i e s E a s t e r l i e s E a s t e r l i e s N E t r a d e s N E t r a d e s El Niño Southern Oscillation ENSO 2 ENSO - Mode of Variability of Atmosphere-Ocean System • Climate System has many modes of variability • Some modes are induced by external forcing from Sun (diurnal, seasonal, Milankovitch cycle – relate to changes in orbital parameters of the Earth) • Some modes are internal to the system and result from complex and non-linear interactions between the system’s fluids (atmosphere and ocean) over a variety of time scales 3 Internal Climate System Oscillations • Climate system has its own internal oscillations independent of external forcing • Modes of internal variability (Oscillations) vary over a variety of time scales • Some of the strongest oscillations are: – 20 – 100 days tropical waves (hold your curiosity for a few more classes) – El Nino Southern Oscillation - ENSO – North Atlantic Oscillation NAO – Pacific Decadal Oscillation PDO – Arctic Oscillation AO – Antarctic Oscillation AAO 4 High signal in high High signal in high latitudes but can affect latitudes but can affect low latitudes too low latitudes too What is El Niño: Tropical Pacific Sea Surface Temperature Anomalies 5 History of El Niño • Locally recognized in precolonial times • Sir Gilbert Walker (1924) – While working for the Indian Meteorological Service, he noticed pressure anomalies – Attempted to relate anomalies to the failure of the Indian Monsoon, but failed • H.P. Berlage (1950s): Develops Southern Oscillation Index (SOI) • 1972-1973: Major El Niño causes collapse of Peruvian fishing • 1983: Major El Niño causes flooding in Southern California • 1998: Strongest El Niño of 20 th century Pressure Anomalies From Caviedes, C., 2001, El Niño in History Pg. 8 6 Sea Level Pressure at Darwin Negative correlation: Sea Level at S Pacific High 7 Tahiti Darwin The Southern Oscillation Index - SOI • Calculated as the pressure differences between Tahiti and Darwin (Tahiti-Darwin) Negative SOI is an indicator of El Niño Sea Surface Temperature at equator and 110 o W 8 T D 9 10 11 The Walker Circulation 12 Typical Atmospheric Circulation Walker Cell 13 14 Thermocline: Depth at which the rate of decrease of temperature with increase of depth is the largest. It separates the zone between the mixed layer above, much influenced by atmospheric fluxes, and the deep ocean. In the tropics, the thermocline is shallow on average, as in the eastern Pacific (50m), or deeper as in the western part (160-200m). Overview of Atmosphere-Ocean System during El Niño • Increased SST in the Eastern Pacific • Weakened Easterlies...
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ENSO_2010 - Tropical Circulation in a Global Context Mean...

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