Spring Onset in the Northern Hemisphere: A Role for the Stratosphere?

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Spring Onset in the Northern HemisphereA Role
for the Stratosphere?

• Robert X. Black
• Brent A. McDaniel
• School of Earth and Atmospheric Sciences
• Georgia Institute of Technology, Atlanta, Georgia
• Walter A. Robinson
• Department of Atmospheric Sciences
• University of Illinois at Urbana-Champaign

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Zonal average east-west winds (u) for January
Stratosphere (polar vortex)
Troposphere
Yang Schlesinger, 1998
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Interannual Variability in Spring Onset Spring
PhenologyCross-correlation between phenodates
and late-winter NAO index

• (DOdorico et al. 2002)

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Interannual Variability in Spring Onset Lower
Tropospheric Temperature over Western
ArcticMarch/April 925 hPa geopotential height
anomalies4 Cold years
4 Warm years

• (Overland et al. 2002)

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Interannual Variability in Spring Onset
OzoneLate winter AO index vs. spring
tropospheric ozone

• (Lamarque and Hess 2004)

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Predictability Variations Winter vs. Summer

• (Newman et al. 2003)

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Predictability Variations AO/NAO Persistence
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Predictability Variations Month-to-month
persistence

• (van den Dool
• and Livezey 1983)

Anti-persistence over the North Atlantic during
Spring
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Stratosphere-Troposphere Coupling NAM
Structure Lower Troposphere
AO/NAO variations

• Stratosphere Polar
• vortex variations
• (Thompson and Wallace 2001)

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Stratosphere-Troposphere Coupling Intraseasonal
EvolutionStratospheric precursors to
Tropospheric AO/NAO events
(Baldwin and Dunkerton 2001)
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Stratosphere-Troposphere Coupling Monthly
PredictabilityNAM time series as predictor of
surface AO
(Baldwin et al. 2003)
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Climatological Trend in Stratospheric Polar Vortex
Relatively abrupt breakdown of stratospheric pola
r vortex during Spring (Stratospheric Final
Warming)
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Considerations

• Significant interannual variability in the
  timing of stratospheric final warming (SFW)
  events
• Thought experiment Composite 10 hPa zonal
  wind evolution with respect to SFW timing

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Contrast SFW Composite u with Seasonal Trend
Substantial local sharpening of u tendency field
Anomalous u both prior to and after SFW onset
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Considerations

• Significant interannual variability in the
  timing of stratospheric final warming (SFW)
  events
• Thought experiment Composite 10 hPa zonal
  wind field with respect to SFW timing plot
• Question To what extent does the stratospheric
  trend sharpening extend down to troposphere?
• Hypothesis Interannual variations in SFW
  timing provide net impact on the troposphere
  (AO/NAO)

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Composites of Winter Sudden Stratospheric Warming
Events
(Limpasuvan et al. 2004)
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Approach

• Identify SFW events based upon variation in u
  _at_ 70N (done for 10 hPa 50 hPa, respectively)
• Calculate 3-D circulation anomalies (deviations
  from seasonal trend values) for each day in a 41
  day window centered on SFW event
• Composite together 40 annual anomaly evolutions
• Events identified separately in NCEP/NCAR,
  ERA-40, and Free University Berlin datasets
• Primary dataset for compositing NCEP/NCAR
  reanalyses for 1958-1997

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Initial Assessment SFW impact upon AO/NAO
Negative AO/NAO after SFW onset
Positive AO/NAO prior to SFW onset (lag 0)
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Composite Anomaly Evolution Zonal Wind
10 day low-pass filtered data
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Composite Anomaly Evolution Zonal Wind
10 day low-pass filtered data
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Composite Circulation Anomaly Change
Zonal-mean Zonal Wind
10 day low-pass filtered data
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Composite Anomaly Evolution 50 hPa Z
10 day low-pass filtered data
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Composite Anomaly Evolution 50 hPa Z
10 day low-pass filtered data
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Composite Anomaly Evolution 1000 hPa Z
10 day low-pass filtered data
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Composite Anomaly Evolution 1000 hPa Z
10 day low-pass filtered data
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Composite Circulation Anomaly Change 1000 hPa
Z
10 day low-pass filtered data
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Composite Circulation Time Evolution
Total Zonal Wind EP Flux/wave driving
10 day low-pass filtered data
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Composite Circulation Time Evolution
Total Zonal Wind EP Flux/wave driving
10 day low-pass filtered data
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Summary

• Pronounced westerly (easterly) zonal wind
  anomalies in the high latitude stratosphere in
  the 2 weeks prior to (after) SFW events
• Reflects more rapid breakup of the stratospheric
  polar vortex compared to climatological trend
• Opposing zonal wind anomalies at low latitudes
• Stratospheric features extend downward well
  into the troposphere
• Troposphere characterized by persistent ve
  (-ve) NAO episodes prior to (following) SFW
  events

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Summary (Continued)

• Latter feature consistent with anti-persistence
  observed by van den Dool and Livezey (1983)
• Stratospheric transition is dynamically driven
  by anomalous upward flux of Rossby wave activity
  emanating from tropospheric altitudes
• SFW events provide new paradigm for studying
  stratosphere-troposphere dynamical coupling
• Better understanding simulation of SFW events
  may provide an avenue for enhancing medium
  range forecast skill during spring onset (?)