Modes of Climate Variability

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Modes of Climate Variability

• ENSO El Nino Southern Oscillation
• And
• NAO North Atlantic Oscillation

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East Pacific SST Anomalies
El Nino Conditions
La Nina Conditions
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Normal Conditions
El Nino Conditions
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Normal Pacific Conditions

• Walker Circulation East-West Atmospheric Cell
• High pressure and sinking in east Pacific
• Low pressure and rising (and precipitation) in
  west Pacific
• Easterlies help drive SEC to the west, causing
  warm water to pile up in the western Pacific
  (Pacific Warm Pool)

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Normal Pacific Conditions

• Coastal winds along South America produce strong
  upwelling
• This is associated with a strong thermocline
  slope across the basin, with the thermocline deep
  in the west and shallow in the east
• This supports rich fisheries off South America

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ENSO

• ENSO This is an abbreviation for El-Nino
  Southern Oscillation
• Named indicates that this phenomenon is related
  to coupled atmospheric-ocean process, the
  atmospheric Southern Oscillation and the oceanic
  El Nino process
• Since this process starts in September (in most
  years when it occurs) and reaches the Peru coast
  by December (Christmas), the local named it El
  Nino, which is Spanish for Christ Child

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ENSO

• Atmospheric Processes (details of which are still
  under study) can cause bursts of westerly winds
  to occur at times along the equator
• This weakens (or even reverses) the Walker
  circulation

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ENSO

• Without the trade winds (or with them weakened),
  the warm pool is no longer held against the
  western side of the basin and sloshes back to
  the central Pacific
• This also initiates Kelvin waves, that travel
  eastwards across the tropical Pacific
• These Kelvin waves strengthen the EUC

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ENSO

• The strengthened EUC leads to the thermocline
  slope being flattened
• This depresses the thermocline off Peru, warming
  up the ocean in this region and reducing the
  upwelling
• This means less nutrients reach the surface and
  cause a collapse of the local fisheries

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Normal
El Nino
La Nina
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ENSO

• Waves that reflect off South America
• 1) Travel north and south along the coastlines of
  North and South America, bring warmer water to
  higher latitudes (as far north as British
  Columbia)
• 2) Also produce waves that reflect back across
  the tropical Pacific, that eventually help end
  the ENSO event

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El Niño /La Niña
Changes in SST? Convective displacement
? Changes in high level winds ? Changes in
global precipitation and temperatures
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Indonesia Fires due to ENSO, 1997
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Malaria and ENSO
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ENSO Frequency

• Historical SST records shows El Ninos have been
  occurring from hundred of years, but the
  frequency may be increasing
• On average they occur every 2-10 years
• Over the last 30 years, maybe every 2-5 years
• Variability represented by the ENSO index
• Positive ? El Nino conditions
• Negative ? La Nina conditions

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ENSO Time series
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ENSO Predictability
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North Atlantic Oscillation (NAO)
The NAO, a component of the Arctic Oscillation
(AO), is the sea level pressure (SLP) difference
between Ponta Delgada, Azores and Stykkisholmur,
Iceland (Hurrell 1995).
Data Images D. Stephenson
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