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Stratospheric-tropospheric Coupling During Polar Vortex Breakdown. 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. - PowerPoint PPT Presentation
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Stratospheric-tropospheric Coupling During Polar Vortex Breakdown
Robert X. BlackBrent A. McDanielSchool of Earth and Atmospheric SciencesGeorgia Institute of Technology, Atlanta, Georgia
Walter A. Robinson Department of Atmospheric SciencesUniversity of Illinois at Urbana-Champaign
David Houghton Symposium 04/01/2006
Zonal average east-west winds (U) for JanuaryStratosphere
TroposphereSummerHemisphereCirculationWinterHemisphereCirculation[Yang & Schlesinger, 1998]StratosphericPolar VortexTroposphericJet Stream
Background & MotivationIntraseasonal variations in the wintertime stratospheric polar vortex linked to near surface variability (AO/NAO) via the Northern Annular Mode (Thompson and Wallace 1998)Mid-winter Sudden Stratospheric Warming events linked to intraseasonal NAM variability (in troposphere & stratosphere) (Taguchi 2003; Limpasuvan et al. 2004)Annual cycle of stratospheric polar vortex exhibits relatively rapid breakdown (stratospheric final warming -> SFW)However, considerable interannual variability in timing of stratospheric final warming events (Waugh et al. 1999)
David Houghton Symposium 04/01/2006
Scientific QuestionsDo interannual variations in the timing of stratospheric final warming events provide a coherent net impact upon the tropospheric circulation in comparison to climatology?If so, to what extent does this behavior resemble the canonical intraseasonal NAM behavior?
David Houghton Symposium 04/01/2006
Approach/Methodology/DataIdentify annual SFW events based on observed evolution in longitudinally-averaged east-west wind @ 70N (10;50 hPa)Assess 3-D circulation (anomalies [deviations from annual cycle/seasonal trend values]) for each day within a time window centered on SFW eventComposite together 47 annual (anomaly) evolutionsEvents identified independently in NCEP/NCAR, ERA-40, and Free University Berlin observational datasetsPrimary dataset for composite analysis: National Centers for Environmental Prediction/Nation Center for Atmospheric Research reanalysis project (Version 1) for 1958-2004
David Houghton Symposium 04/01/2006
Contrast SFW Composite [u] with Seasonal TrendSubstantial local sharpening of [u] tendency fieldAnomalous [u] both prior to and after SFW onsetQuestion: To what extent does this trend sharpening extend down to troposphere?
Composite Circulation Evolution: Zonal-mean Zonal Wind @ High Latitudes
Climatological Trend Final Warming CompositeDerived from unfiltered daily data (Units: m/s)
Composite Anomaly Evolution: Zonal-mean Zonal Wind Pressure-latitude plotting domain
Purple line: Position of Tropopause
Longitudinal-mean anomalies plotted
Evolution plotted with respect to SFW events (Lag 0)
10 day low-pass filter applied
Composite Anomaly Evolution: Zonal-mean Zonal Wind Day -30: Strong vertically coherent westerly signature
Day -10: Westerlies descend; upper level easterly anomalies
Day 0: North-South dipole forms; easterly wind anomalies dive into troposphere
Day +10: Strat signal weakens; Trop signal attains peak values
Composite Circulation Anomaly Change: Zonal-mean Zonal Wind SFW Change (w/confidence levels)
Local zonal wind reductions > 50%Canonical NAM structure*
Note structural distinctions*Thompson and Wallace 2001
Composite Evolution: High Latitude Zonal Wind
Zonal Wind Anomalies Total Zonal Wind FieldDerived from unfiltered daily data (Units: m/s)
Composite Evolution: 1000 hPa Height Anomalies
Pre-SFW Structure Post-SFW Structure
Composite Circulation Anomaly Change: 1000 hPa Geopotential Height (m) -> Near Polar Z increases
-> Regionally localized mid-latitude decreases
Canonical NAM structure
Note structural distinctions
Composite Evolution: High Latitude Dynamics
Zonal Wind Anomalies Upward E-P Flux/DrivingUnits: m/sWave Driving Units: m/s/day
High Latitude Precursor: Zonal-Mean Structure
Zonal Wind Anomalies Day -30 Anomaly Structure(Units: m/s)UnfilteredLow-Pass Filtered
High Latitude Precursor: Event Identification
Project Daily Anomalies on Day -30 Precursor StructureRed Line: Composite first annual crossing of +1.5 in precursor indexMulti-year index composite based upon 24 precursor episodesBlue Line: Composite first annual peak above +1.5 in precursor index
High Latitude Precursor: Composite [U] Evolution
SFW Composite Precursor-Based Composite(Units: m/s)Day 0: SFW OnsetDay -30: Precursor Onset
Composite Dynamical Evolution:
Total Zonal Wind EP Flux Anomalies
Composite Dynamical Evolution:
Total Zonal Wind EP Flux Anomalies
Composite Evolution: High Latitude Zonal Wind
Zonal Wind Anomalies Tropospheric DecelerationUnits: m/sDay -10 to 0Day +4 to +9
Composite Dynamical Evolution: Days -10 to 0
Zonal Wind Tendency E-P Flux/Wave DrivingWave Driving Units: m/s/dayUnits: m/s/day
Composite Dynamical Evolution: Days -10 to 0 [U] Tendency
Units: m/s/dayUnits: m/s/dayTotal Invert Strat PV Change
Composite Dynamical Evolution: Days +4 to +9
Zonal Wind Tendency E-P Flux/Wave DrivingWave Driving Units: m/s/dayUnits: m/s/day
Composite Dynamical Evolution: Days +4 to +9 [U] Tendency
Units: m/s/dayUnits: m/s/dayTotal Invert Strat PV Change
Summary: Structural EvolutionSFW events provide a strong organizing influence upon the large-scale circulation of the stratosphere and troposphereEnhanced (weakened) polar vortex before (after) SFW events => Reflects more rapid breakup of the stratospheric polar vortex than indicated by the climatological trend Robust tropospheric flow changes extending down to surface => NAO/AO-like phase transition near Earths surfaceBut Important structural distinctions from canonical NAM => SFW events linked to a distinct class of annular variability? (potentially missed if searching for NAM)
David Houghton Symposium 04/01/2006
Summary: Dynamical EvolutionApparent stratospheric preconditioning ~ Day -30 => strengthening and polar shift of polar vortexDistinct stratospheric decelerations dynamically driven by anomalous upward flux of (planetary scale) Rossby wave activity emanating from tropospheric altitudesConcomitant and time-lagged tropospheric decelerations => Initial direct response? Subsequently indirect?TEM dynamics dominated by low frequency, planetary scale waves dynamics in stratosphere and troposphereRobust bi-directional dynamical coupling between stratosphere and troposphere during SFW events
David Houghton Symposium 04/01/2006
ImplicationsSFW events provide new phenomenological paradigm for studying stratosphere-troposphere dynamical couplingA better understanding & simulation of SFW events may provide an avenue for enhancing medium range forecast skill during spring onset (e.g., via improved representation of the coupled troposphere-stratosphere system)Long-term trends in annual strength and persistence of the wintertime stratospheric polar vortex may have important unanticipated consequences for extratropical climate
David Houghton Symposium 04/01/2006
Composite Anomaly Evolution: 1000 hPa Z Northern Hemisphere polar stereographic
Plot longitudinally varying anomaly field
Evolution plotted with respect to SFW events (Lag 0)
Composite Anomaly Evolution: 1000 hPa Z Day -20: Anomalous Low Z @ high lats; High Z @ mid lats; Positive NAO-like
Day -10/0: Pattern weakens & opposite phase emerges
Day +5/+10: Opposite phase pattern reaches peak magnitudes
Day +10/+20: Slow weakening
Composite Anomaly Evolution: 50 hPa Z (Stratosphere) Northern Hemisphere polar stereographic (note orientation)
Plot longitudinally varying anomaly field
Evolution plotted with respect to SFW events (Lag 0)
Composite Anomaly Evolution: 50 hPa Z (Stratosphere) Day -20: Anomalous Low Z surrounds NP
Day -10: Evolves into dipole anomaly (wavenumber 1)
Day 0: Dipole moves High Z envelops NP; Low Z shifts south
Day 5: Longitudinally symmetric N-S dipole forms; weakens after
~1.5 change in Z
Composite Evolution: High Latitude Dynamics
QGPV Anomalies QGPV TendencyUnits: 10-6 s-1Units: 10-6 s-1 day-1
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