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Spring Onset in the Northern Hemisphere: A 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

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spring onset in the northern hemisphere a role for the stratosphere

Spring Onset in the Northern Hemisphere:A 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

zonal average east west winds u for january
Zonal average east-west winds (u) for January

Stratosphere

(polar vortex)

Troposphere

[Yang & Schlesinger, 1998]

slide3

Interannual Variability in Spring Onset: Spring PhenologyCross-correlation between phenodates and late-winter NAO index

(D’Odorico et al. 2002)

overland et al 2002

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)

interannual variability in spring onset ozone late winter ao index vs spring tropospheric ozone

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

(Lamarque and Hess 2004)

predictability variations month to month persistence

Predictability Variations: Month-to-month persistence

Anti-persistence over the North Atlantic during Spring

(van den Dool

and Livezey 1983)

stratosphere troposphere coupling nam structure lower troposphere ao nao variations

Stratosphere-Troposphere Coupling: NAM StructureLower Troposphere: AO/NAO variations

Stratosphere: Polar

vortex variations

(Thompson and Wallace 2001)

slide10
Stratosphere-Troposphere Coupling: Intraseasonal EvolutionStratospheric precursors to Tropospheric AO/NAO events

(Baldwin and Dunkerton 2001)

stratosphere troposphere coupling monthly predictability nam time series as predictor of surface ao
Stratosphere-Troposphere Coupling: Monthly PredictabilityNAM time series as predictor of surface AO

(Baldwin et al. 2003)

climatological trend in stratospheric polar vortex

Climatological Trend in Stratospheric Polar Vortex

Relatively abrupt breakdown of

stratospheric

polar vortex

during Spring

(Stratospheric

Final Warming)

considerations
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
contrast sfw composite u with seasonal trend

Contrast SFW Composite [u] with Seasonal Trend

Substantial local sharpening of [u] tendency field

Anomalous [u] both prior to and after SFW onset

considerations15
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)
approach
Approach
  • Identify SFW events based upon variation in [u] @ 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
initial assessment sfw impact upon ao nao

Initial Assessment: SFW impact upon AO/NAO

Negative AO/NAO after SFW onset

Positive AO/NAO prior to SFW onset (lag 0)

composite anomaly evolution zonal wind
Composite Anomaly Evolution: Zonal Wind

10 day low-pass filtered data

composite anomaly evolution zonal wind20
Composite Anomaly Evolution: Zonal Wind

10 day low-pass filtered data

composite anomaly evolution 50 hpa z
Composite Anomaly Evolution: 50 hPa Z

10 day low-pass filtered data

composite anomaly evolution 50 hpa z23
Composite Anomaly Evolution: 50 hPa Z

10 day low-pass filtered data

composite anomaly evolution 1000 hpa z
Composite Anomaly Evolution: 1000 hPa Z

10 day low-pass filtered data

composite anomaly evolution 1000 hpa z25
Composite Anomaly Evolution: 1000 hPa Z

10 day low-pass filtered data

composite circulation time evolution total zonal wind ep flux wave driving
Composite Circulation Time Evolution:Total Zonal Wind EP Flux/wave driving

10 day low-pass filtered data

composite circulation time evolution total zonal wind ep flux wave driving28
Composite Circulation Time Evolution:Total Zonal Wind EP Flux/wave driving

10 day low-pass filtered data

summary
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
summary continued
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 (?)