Bimodal Behavior in the Northern Hemisphere

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Bimodal Behavior in the Northern Hemispheres
Zonal-Mean Zonal Flow and Its Possible
Association with Decadal-Scale Coupled Climate
Variability
Sergey Kravtsov Department of Mathematical
Sciences, UWM
Collaborators Michael Ghil, Andrew Robertson,
William Dewar, James McWilliams, Pavel Berloff
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Evidence for (multi)-decadal climate variability

• Positive evidence for (multi)-decadal climate
  variability

• Statistical significance? Physical processes
  ocean dynamics?

• Need for combination of observations and modeling

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Coupled GCMs vs. intermediate climate models
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Mid-Latitude Jet Stream
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North Atlantic Oscillation and Arctic Oscillation
NAO
AO
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A simple model
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Zonal-jet variability in the model
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Wind-driven (WDC) and thermohaline (THC)
circulation
Surface currents
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Circulation in an ocean model
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Oceans weather
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Eddy-resolving ocean model
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SST and NAO

• Decadal time scale detected in NAO/SST time
  series

• Is this signal real? What dynamics does it
  represent?

• AGCMs response to (small) SSTAs is weak and
  model-dependent

SST tripole pattern (Marshall et al.
2001, Journal of Climate Vol. 14, No. 7,
pp. 13991421)

• Nonlinear small SSTAs large response??

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Model geometry
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Bimodality in atmospheric model and observations
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Low-Frequency Oscillations (LFO) in the model
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LFO in observations
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Bimodality in the NH vs. SH
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20-year oscillation in observations and WDC model
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• High Ocean Energy High-
• Latitude (HL) O-Jet State

• HL ocean state A-jets
• Low-Latitude (LL) state

• O-Jet stays in HL state for
• a few years due to O-eddies

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10-year oscillation in observations and THC model
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Summary

• Mid-latitude winds may be nonlinearly sensitive
• to variations in external (to the atmosphere)
  
• forcing, which alters attraction basins of the
  two
• anomalously persistent atmospheric jet states

• Observed decadal climate variability in the
• North Atlantic region may be due, in part, to
  a
• combination of two coupled modes ocean
• eddy-driven adjustment mode and THC mode