Spring Onset in the Northern Hemisphere: A Role for the Stratosphere? Robert X. Black Brent A....
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Transcript of Spring Onset in the Northern Hemisphere: A Role for the Stratosphere? Robert X. Black Brent A....
Spring Onset in the Northern Hemisphere:A Role for the Stratosphere?
Robert X. BlackBrent A. McDaniel
School of Earth and Atmospheric SciencesGeorgia Institute of Technology, Atlanta, Georgia
Walter A. Robinson
Department of Atmospheric SciencesUniversity of Illinois at Urbana-Champaign
Zonal average east-west winds (u) for January
Stratosphere(polar vortex)
Troposphere
[Yang & Schlesinger, 1998]
(D’Odorico et al. 2002)
Interannual Variability in Spring Onset: Spring Phenology
Cross-correlation between phenodates and late-winter NAO index
(Overland et al. 2002)
Interannual Variability in Spring Onset: Lower Tropospheric Temperature over Western Arctic
March/April 925 hPa geopotential height anomalies
4 Cold years 4 Warm years
(Lamarque and Hess 2004)
Interannual Variability in Spring Onset: Ozone
Late winter AO index vs. spring tropospheric ozone
(Newman et al. 2003)
Predictability Variations: Winter vs. Summer
Predictability Variations: AO/NAO Persistence
(van den Dool and Livezey 1983)
Predictability Variations: Month-to-month persistence
Anti-persistence over the North Atlantic during Spring
Stratosphere: Polar vortex variations
(Thompson and Wallace 2001)
Stratosphere-Troposphere Coupling: NAM Structure
Lower Troposphere: AO/NAO variations
(Baldwin and Dunkerton 2001)
Stratosphere-Troposphere Coupling: Intraseasonal Evolution
Stratospheric precursors to Tropospheric AO/NAO events
(Baldwin et al. 2003)
Stratosphere-Troposphere Coupling: Monthly Predictability
NAM time series as predictor of surface AO
Climatological Trend in Stratospheric Polar Vortex
Relatively abrupt breakdown of stratosphericpolar vortex during Spring
(Stratospheric Final Warming)
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
Substantial local sharpening of [u]
tendency field
Anomalous [u] both prior to and after SFW onset
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)
(Limpasuvan et al. 2004)
Composites of Winter Sudden Stratospheric Warming Events
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
Negative AO/NAO after SFW onset
Initial Assessment: SFW impact upon AO/NAO
Positive AO/NAO prior to SFW onset
(lag 0)
Composite Anomaly Evolution: Zonal Wind
10 day low-pass filtered data
Composite Anomaly Evolution: Zonal Wind
10 day low-pass filtered data
Composite Circulation Anomaly Change: Zonal-mean Zonal Wind
10 day low-pass filtered data
Composite Anomaly Evolution: 50 hPa Z
10 day low-pass filtered data
Composite Anomaly Evolution: 50 hPa Z
10 day low-pass filtered data
Composite Anomaly Evolution: 1000 hPa Z
10 day low-pass filtered data
Composite Anomaly Evolution: 1000 hPa Z
10 day low-pass filtered data
Composite Circulation Anomaly Change: 1000 hPa Z
10 day low-pass filtered data
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 driving
10 day low-pass filtered data
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) 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 (?)