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

12/07/0112/07/0112/07/0112/07/01