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 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 stratospheric polar 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 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

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 (?)