AtmosphereOcean Variability

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Atmosphere-Ocean Variability
Lecture 10
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Modes of Variability

• Climate System has many modes of variability
• Some modes are induced by external forcing from
  Sun (diurnal, seasonal, Milankovitch cycle)
• Some modes are internal to the system and result
  from complex and non-linear interactions between
  the systems fluids (atmosphere and ocean) over a
  variety of time scales

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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
• Strongest oscillations are
• 20 - 50 tropical waves
• El Nino Southern Oscillation - ENSO
• North Atlantic Oscillation - NAO
• Pacific Decadal Oscillation - PDO
• Arctic Oscillation - AO

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El Niño Southern Oscillation ENSO
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1997- 98 El Niño
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Not all El Niño are alike

• Timing
• intensity
• extent

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El Niño Impacts
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El Niño Mechanisms
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Typical Atmospheric Circulation Walker Cell
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Normal and El Niño Years
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El Niño Teleconnections
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El Niño Theory Delayed Oscillator
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Radiative Forcing of ENSO

• Major cloud redistribution that occurs during El
  Nino does impact the atmospheric and surface
  radiative forcing
• How does this impact relate to climate change is
  still a question

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Connections to Climate Change

• Hypothesis In a warmer world , it is suggested
  that El Nino will occur more often and will be
  more powerful than present ones.
• Models diverge on whether this hypothesis is
  correct or not
• Some data sets (e.g, corals) suggest that strong
  El Nino have occurred in the past before
  anthropogenic warming

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North Atlantic Oscillation

• Dominant mode of climate variability in North
  Atlantic region
• Large-scale seesaw of atmospheric mass between
  subtropical high and polar low
• NAO index varies but also has a tendency to
  remain in one phase for a long time

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Positive Phase of NAO

• subtropical high pressure
• Centers and deep Icelandic low
• increased pressure gradient
• results in more and stronger
• East-West winter storms
• Warm and wet winters in Europe
• dry and cold winter in N Canada
• and Greenland
• - Mild and wet conditions in E US

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NAO Index
NAO Index Anomalous difference between polar
and sub-tropic high during the winter season
Dec - March
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NAO and Climate Change
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Pacific Decadal Oscillation - PDO

• Long-lived El Nino like pattern of variability
• PDO events persist 20 -30 years
• Climatic fingerprints more visible in the North
  Pacific North American sector with only secondary
  signatures in the tropics
• Causes of PDO are not known

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Pacific Decadal Oscillation - PDO
Warm phase
Cold phase
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PDO
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Negative Phase of NAO

• - weak subtropical high and
• Icelandic low
• reduced pressure gradient
• results in fewer and weaker
• East-West winter storms
• Moist air into Mediterranean
• and cold air over N. Europe
• More cold air outbreaks and
• Snowy conditions in E US
• Mild winter temperature in
• Greenland