TropicalM. D. Eastin The El-Nino Southern Oscillation (ENSO) La Nina ImpactsEl Nino Impacts

TropicalM. D. Eastin Outline History Observed Structure and Evolution What Causes an El-Nino? ENSO Forecasting Global Impacts

TropicalM. D. Eastin ENSO History In the 1600s, Peruvian fisherman noticed their fish harvests failed every few years due to warmer-than-normal waters (upwelling provides nutrient-rich cold water for fish). The warming always occurred in December, so the phenomena was named El Nino, in reference to the Christ child. In 1899 the Indian Monsoon failed, leading to severe drought and famine. This lead Gilbert Walker, head of the Indian Met. Service, to search for a way to predict the monsoon. He identified a peculiar surface pressure oscillation: when the pressure is high over the maritime continent (Indonesia and Darwin), surface pressures are low over India and the central southern Pacific (Tahiti). He referred to this as the Southern Oscillation. In 1969, UCLA professor Jacob Bjerknes first recognized that El Nino and the Southern Oscillation were actually manifestations of the same physical phenomena that results from unstable interactions between the ocean and atmosphere, and referred to it as ENSO The El Nino event was the first to receive significant public and research interest → a relatively new (not well understood) phenomena Sir Gilbert Walker Jacob Bjerknes

TropicalM. D. Eastin ENSO: Observed Structure Normal Conditions or La Nina: Strong easterly winds induce upwelling of cold water in the equatorial eastern Pacific Shallow oceanic thermocline in the east Pacific (due to upwelling) Warm SSTs confined to western Pacific with a deep thermocline Low pressure and convection in west Pacific High pressure and subsidence (clear air) in east Pacific

TropicalM. D. Eastin ENSO: Observed Structure El-Nino Conditions: Weaker easterly winds result in less upwelling of cold water Warm SSTs “spread” to east Pacific (also solar heating not offset by upwelling) Increase in the east Pacific thermocline Low pressure and convection shifts to the east Pacific High pressure and subsidence shifts to the west Pacific

TropicalM. D. Eastin ENSO: Observed Structure ENSO Indices Based on observed SST anomalies (difference from the long term mean) in the equatorial Pacific in four regions (observation from TAO moored buoys) Based on surface pressure differences between Tahiti and (minus) Darwin, called the Southern Oscillation Index (SOI) Nino 1+2 Region that often first warms during the onset of an El Nino Nino 3 Largest variability in SSTs over an average ENSO cycle SOI Tahiti Darwin SOI Most highly correlated with Nino 3.4 SST Nino 3.4 Most highly correlated with eastward shift of convection Nino 4 Most highly correlated with global weather patterns

TropicalM. D. Eastin ENSO: Observed Structure NOAA’s Multivariate ENSO Index (MEI) Combines normalized anomalies of SST (in Nino3.4), surface pressures (the SOI), surface winds, surface air temperatures, and cloud fraction to obtain a “composite” view of the state of ENSO Definitions:El Nino= Standardized Departures > +1.0 La Nina= Standardized Departures < -1.0 El Nino La Nina Source:

TropicalM. D. Eastin ENSO: Observed Structure [Animation] The La-Nina Event January 1995 – December 1996

TropicalM. D. Eastin ENSO: Observed Structure [Animation] The El-Nino Event January 1997 – December 1998

TropicalM. D. Eastin What Causes El Nino? Triggering Mechanism Not well understood Deep thermocline in the western Pacific believed to be a necessary (not sufficient) condition “Westerly wind bursts” (WWBs) over a period of several days may be one trigger Most often associated with the Madden-Julian Oscillation (MJO) Atmospheric Kelvin waves also generate sustained westerly winds Twin TC’s straddling the equator can also generate sustained westerly winds Multiple sustained WWBs decrease the equatorial easterlies that induced cold upwelling Less upwelling combined with a west-east ocean current (forced by the WWBs) increases the central and eastern Pacific SSTs and lowers the thermocline depth and initiates an El Nino event Anomalous surface winds (i.e. a WWB) associated with MJO convection (centered in the box)

TropicalM. D. Eastin What Causes El Nino? Onset of the El Nino Daily Mean Surface Values in the equatorial Pacific: 1 January 1997 thru 31 December 1998 Zonal Wind Anomalies (m/s)Mean Zonal Wind (m/s)SST Anomalies (ºC) WWB Strong Easterlies WWB El Nino Weaker Easterlies

TropicalM. D. Eastin What Causes El Nino? An Oceanic Component The WWBs acting alone would lead to a gradual eastward progression of SST anomalies (which is observed but the signal is weak) In contrast, observations show a pronounced rapid “emergence” of warm SST anomalies in the equatorial east Pacific (along the Peruvian coast in the Nino1+2 region) What causes this rapid emergence? Delayed Oscillator Theory is one explanation for this rapid emergence Atmospheric WWBs generate equatorial Rossby and Kelvin waves in the ocean Oceanic waves propagate along the density contrast of the thermocline Oceanic Rossby waves:Move westward at slow speeds Induce upwelling (decreases the thermocline depth) Effectively cool the ocean mixed layer and SSTs Oceanic Kelvin waves:Move eastward very rapidly (much faster than Rossby waves) Induce downwelling (increases the thermocline depth) Effectively warm the ocean mixed layer and SSTs This theory also provides an explanation for the ENSO oscillation every 4-5 years

TropicalM. D. Eastin What Causes El Nino? The Delayed Oscillator in a Simple Ocean Model Initial Time Forcing from single WWB Thermocline Depth Upwelling Rossby Waves Downwelling Kelvin Wave Thermocline Depth 25 days Kelvin Wave reflects and becomes a Rossby wave Rossby Wave reflects becomes a Kelvin wave 50 days 75 days 100 days 175 days 225 days Multiple reflections can lead to La Nina onset

TropicalM. D. Eastin What Causes La Nina? Onset of the La Nina Daily Mean Surface Values in the equatorial Pacific: 1 January 1997 thru 31 December 1998 Zonal Wind Anomalies (m/s)Mean Zonal Wind (m/s)SST Anomalies (ºC) Lack of Strong WWB Strong Easterlies La Nina Weaker Easterlies Lack of Strong WWB

TropicalM. D. Eastin ENSO: Global Impacts Global Impacts ENSO variability alters convection in tropical Pacific This convective variability produces zonal anomalies in the Walker and Hadley Circulations, which, in turn, influences mid-latitude synoptic-wave patterns and alters the global weather Anomalous synoptic wave patterns lead to warmer/colder and wetter/drier conditions La Nina Impacts (Winter)El Nino Impacts (Winter)

TropicalM. D. Eastin ENSO: U.S. Impacts El Nino: Summer TemperaturesEl Nino: Winter Temperatures

TropicalM. D. Eastin ENSO: U.S. Impacts El Nino: Summer RainfallEl Nino: Winter Rainfall

TropicalM. D. Eastin ENSO: U.S. Impacts La Nina: Summer TemperaturesLa Nina: Winter Temperatures

TropicalM. D. Eastin ENSO: U.S. Impacts La Nina: Summer RainfallLa Nina: Winter Rainfall

TropicalM. D. Eastin ENSO Forecasting Forecast Models All models forecast SSTs in the equatorial Pacific (most often for the Nino3.4 region) Statistical models Employ simple multiple regression techniques based on ENSO indices Based on evolution of previous ENSO events (and historical records) Quality of forecasts reliant on quality of historical data Cannot forecast “record” events No physical interpretation possible Dynamical Models Most are complex coupled atmosphere-ocean models Initialization requires 3-D observations of ocean and atmosphere (data sparse region) Small scale features are parameterized Can forecast record events (not bound by past events)

TropicalM. D. Eastin ENSO Forecasting Source:

TropicalM. D. Eastin The El-Nino Southern Oscillation (ENSO) Summary: History (basic timeline and rise to prominence) Oceanic and atmospheric structure/flows during El Nino and La Nina ENSO Indices (defining parameter, differences, and uses) Causes of El Nino Westerly Winds Bursts (definition, origin, impact/forcing) Delayed Oscillator Theory (role of waves, explain rapid onset) Global impacts of ENSO Impact of ENSO in the U.S. ENSO Forecasting (difference in model types)

TropicalM. D. Eastin References Climate Diagnostic Center’s (CDCs) Interactive Plotting and Analysis Webage ( ) Kindle, J. C., and P. A. Phoebus, 1995: The ocean response to perational westerly wind bursts during the El Nino. J. Geophysical. Res., 100, Knaff, J. A., and C. W. Landsea, 1997: An El Nino-Southern Oscillation Climatology and Persistence (CLIPER) Forecasting Scheme. Wea. Forecasting, 12, McPhaden, M. J., 2004: Evolution of the 2002/3 El Nino. Bull. Amer. Meteor. Soc., 85,