El Niño / Southern Oscillation (ENSO) and inter-annual climate variability
seasonal cycle what is “normal”? monthly average conditions through a calendar year
sea level pressure and surface winds
surface ocean currents
coastal upwelling
equatorial upwelling
every year is different inter-annual variability of the seasonal cycle El Niño/Southern Oscillation (ENSO) is the most important coupled ocean-atmosphere phenomenon to cause climate variability on inter-annual time scales.
what is El Niño? The term El Niño originally used by Peruvian fishermen to describe the warm current appearing off the western coast of Peru around Christmas time.
Today El Niño describes the warm phase of a naturally occurring sea surface temperature oscillation in the tropical Pacific Ocean. This oscillation is associated with the atmospheric phenomenon known as the southern oscillation.
what is the Southern Oscillation? A seesaw shift in surface pressure at Darwin, Australia and the South Pacific Island of Tahiti. When the pressure is high at Darwin it is low at Tahiti and vice versa. El Niño, and its sister event – La Niña – are the extreme phases of the southern oscillation, with El Niño referring to a warming of the eastern tropical Pacific, and La Niña a cooling.
Southern Oscillation Index (SOI) The (SOI) measures the monthly/seasonal fluctuations in surface air pressure differences at Tahiti and Darwin (SOI = Tahiti – Darwin), and thus the SOI has a negative value during an El Niño event.
Southern Oscillation Index (SOI) The (SOI) measures the monthly/seasonal fluctuations in surface air pressure differences at Tahiti and Darwin (SOI = Tahiti – Darwin), and thus the SOI has a negative value during an El Niño event.
Walker circulation
trade winds blow from East to West warm water piles up in West Pacific, driving deep atmospheric convection cold, nutrient-rich water is brought to the surface in the East Pacific ( upwelling )
Every few years, the trade winds weaken… In other years the trade winds strengthen…
schematic of changes during an El Niño event
quantifying & comparing ENSO events ENSO can be quantified using a Multivariate ENSO Index (MEI) based on the six main observed variables over the tropical Pacific. These six variables are: sea-level pressure, zonal and meridional components of the surface wind, sea surface temperature, surface air temperature, and total cloudiness fraction of the sky.
MEI comparison of recent El Niño events
MEI comparison of recent La Niña events
Western Pacific “warm pool” Upwelling zones El Niño happens roughly every 3-7 years, lasts 12-18 months, and peaks at the end of the calendar year El Niño is often followed by or preceded by La Niña: an unusual cooling of the tropical Pacific
1997-1998 ENSO event
Teleconnections and Global Weather Patterns El Niño shifts the probability of droughts, floods, heat waves, and extreme weather events in large regions of the globe.
Teleconnections and Global Weather Patterns
Social and Economic Consequences El Niño can affect life, property, and economic vitality due to weather related hazards.
Ocean Biological Productivity (Chlorophyll Concentrations) La Niña El Niño Jun-Aug ‘98 Dec ‘97-Feb ‘98
• Strong Upwelling • Weak Upwelling NOAA Report to the Nation, 1995
El Niño and Peruvian Anchovy 1972-73 1997-98 Anchovy Catch (millions of tons)
Peruvian Anchovy Catch and ENSO
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