Presentation on theme: "SOLCLI Meeting 22 October 2009"— Presentation transcript:
1SOLCLI Meeting 22 October 2009 The tropospheric response to idealised stratospheric forcing: its dependence on basic stateMike Blackburn(1), Joanna D. Haigh(2), Isla Simpson(2,3), Sarah Sparrow(1,2)(1) NCAS-Climate, Department of Meteorology, University of Reading, UK(2) Space and Atmospheric Physics, Imperial College London, UK(3) Department of Physics, University of Toronto, Canada.SOLCLI Meeting 22 October 2009
2OutlineTropospheric response to idealised stratospheric heating (review)Dependence on tropospheric climatological basic state equilibrium response spin-up ensembles – mechanismsRelationship to unforced annular variability
3Solar index regressions using reanalysis data Observed stratospheric temperature signalsolar max - solar minECMWF reanalyses (ERA-40)Crooks & Gray (2005)
4Circulation changes over the 11-year cycle Multiple regression analysis of NCEP/NCAR reanalysis, DJF,Weakening and poleward shift of the mid-latitude jets.Weakening and expansion of the Hadley cells.Poleward shift of the Ferrell cells.Haigh and Blackburn (2006)
5Simplified GCM - “dynamical core” model Based on University of Reading primitive equation model: (1)Spectral dynamics: T42 L15No orographyNewtonian cooling – idealised equinoctial radiative- convective equilibrium temperatures TR(lat,height) (2)Boundary layer friction (Rayleigh drag)(1) Hoskins & Simmons (1975)(2) Held & Suarez (1994)Experiments / analysis:Equilibrium response to perturbations to stratospheric TR(Haigh et al, 2005)Spin-up ensembles: 200 x 50-day run(Simpson et al, 2009)Annular variability in control run(Sparrow et al, 2009)
6The model: control climate Control run zonal windControl run temperatureRelaxation Temperature
7Idealised stratospheric heating Heating perturbations can be applied to the stratosphere by changing the relaxation temperature profileApplied 3 differentheating perturbations5K0KE5 Equatorial heating (5K)U5 Uniform heating (5K)P10 Polar heating (10K)10KHaigh et al (2005)
8Equilibrium ResponseZonal mean TemperatureE5U5P10Control zonal windE5 case gives a similar response in the troposphere to that seen over the solar cycleZonal mean zonal windE5U5P10
9Ensemble spin-up Experiments Haigh et al (2005) - Equatorial heating gave a similar tropospheric response to that seen over the solar cycleCoherent displacement of the jet and storm-trackHow does this arise?Spin-up ensemble for the equatorial heating case:200, 50-day runs5K0K4.5K0.5KSimpson et al (2009)
10Eliassen-Palm flux Flux of wave activity in latitude-height plane Conserved following eddy group velocity (assumptions)Components proportional to eddy heat + momentum fluxesE-P flux divergence quantifies eddy forcing of mean state
11Eddy-feedback processes Ensemble spin-up response to stratospheric heating distributions in an idealised model (Simpson et al, 2009)E-P Flux, days 0 to 9E-P Flux, days 20 to 29E-P Flux, days 40 to 49u, days 20 to 29u, days 40 to 49Heating: δT_refTropopause [qy] triggerRefraction feedback amplifies tropospheric anomaliesBaroclinicity feedback moves wave source11
12Refractive IndexWe can use the refractive index to see what’s causing the change in eddy propagation.Eddies should be refracted towards regions of higher refractive index.Meridional PV gradient - Depends on the vertical gradients in temperature and zonal wind and meridional zonal wind curvature.Eddy phase speedZonal wind
14E5 dependence on tropospheric basic state Decreasing baroclinicityIncreasing baroclinicityTR1TR2TR3TR4TR5Change to reference temperatureTRuClimatological zonal windE5δuE5 zonal wind responseNOTE: THERE IS 1 BLANK BOX HIDING TEXT ON THE RIGHTEquilibrium experiments with modified tropospheric reference temperatureStronger response to stratospheric forcing for lower latitude jetsIndicative of stronger eddy feedback (despite weaker eddies in control)
15NOTE: THERE ARE 2 BLANK BOXES HIDING EP-FLUX PLOTS ON THE RIGHT (CONTROL & ANOMALY)
16Dynamical Mechanisms Hypotheses Sensitivity of EP-flux propagation / refraction to basic state:- expect spin-up to vary from t=0?Sensitivity of critical latitude wave absorption (u=c or qy=0) :- different spectrum of eddy phase speeds (for climatology or spin-up)?- narrower latitude band for low-latitude jets (u/y larger)Strength of baroclinic feedback:- is low-latitude response more baroclinic ( higher eddy growth rates)?- simple metrics should verify/falsify this
18E5 spin-up dependence on climatology Trop.Strat.Vertical integralsCorrelation of eddy forcing and zonal wind response
19NOTE: THERE IS 1 BLANK BOX HIDING PLOTS ON THE RIGHT Relationship to unforced internal variabilityFind strongest response to forcing for lower latitude jetsHow is this related to the unforced internal variability?Fluctuation-Dissipation Theorem (FDT) predicts a stronger response for longer timescales of internal variabilityDue to stronger internal (eddy) feedbacks, maintaining the leading mode(s) of variability against dampingNOTE: THERE IS 1 BLANK BOX HIDING PLOTS ON THE RIGHT
21Annular variability in TR3 control Evidence for 2 types of natural variability: poleward propagating anomalies – short timescale persistent stationary anomalies – long timescalePersistent behaviour dominates for lower latitude jetsPropagating behaviour dominates for higher latitude jets
22NOTE: THERE IS 1 BLANK BOX HIDING TEXT ON THE RIGHT ConclusionsPreviously identified eddy feedbacks responsible for the tropospheric response to idealised stratospheric heatingLarge variation of response magnitude to climatological basic stateSeveral possible dynamical mechanismsResponse variation consistent with timescale of unforced variability (FDT) poleward propagating anomalies – short timescale – weak response persistent stationary anomalies – long timescale – strong responseFuture WorkAnalyse dynamics of forcing response & spin-up (mechanisms)Dynamics of unforced variability – separate & characterise 2 typesExtended stratosphere; mechanical forcing (Alice Verweyen PhD)NOTE: THERE IS 1 BLANK BOX HIDING TEXT ON THE RIGHT
23SOLCLI Meeting 22 October 2009 - Thank you -SOLCLI Meeting 22 October 2009
30Sarah Sparrow1,2, Mike Blackburn2 and Joanna Haigh1 Modes of Annular Variability in the Atmosphere and Eddy-Zonal Flow InteractionsSarah Sparrow1,2, Mike Blackburn2 and Joanna Haigh11. Imperial College London, UK2. National Centre for Atmospheric Science, University of Reading, UKMOCA-09 M06 Theoretical Advances in Dynamics 20 July 2009v.6
31Leading Modes of Variability Control RunEOF 1 (51.25%)EOF 2 (18.62%)Height →Latitude (equator to pole) →EOF1 represents a latitudinal shift of the mean jet.EOF2 represents a strengthening (weakening) and narrowing (broadening) of the jet.Both of these patterns are needed to describe a smooth latitudinal migration of the jet.
32Phase Space Trajectories UnfilteredPeriods Longer than 30 DaysLow Pass FilterPeriods Shorter than 30 DaysHigh Pass FilterPC1 →PC2 →At low frequencies circulation is anticlockwise with a timescale of 82 ± 27 days.At high frequencies circulation is clockwise with a timescale of 8.0 ± 0.3 days.
33Phase Space View of Momentum Budget PC1 →PC2 →Low PassHigh PassEddies change behaviour at high and low frequencies and jet migration changes direction.At low frequencies it is unclear what drives the poleward migration.
34Empirical Mode Decomposition (EMD): Spectra EMD is a technique for analysing different timescales in non-linear and non-stationary data.Resulting time-series are similar to band-pass filtered data.For a given mode a similar frequency band is sampled for both PC1 and PC2.Period (Days) →Amplitude (ms-1) →Zonal Wind PC1Zonal Wind PC2
36Transformed Eulerian Mean Momentum Budget –+ωHigh Frequencies:Eddies drive equatorward migration.Eddies out of phase with winds near the surface.Intermediate Frequencies:Eddies drive poleward migration.Residual circulation drives jet migration at lower levels.Eddies in phase with the winds near the surface.
38Phase angle lagged correlation –+ωPhase Space Angle Lag →Mode 2Mode 4240 hPa967 hPaCorrelation →Consideration of the phase lag between the zonal wind anomalies and .F at low levels, together with each mode’s circulation timescale, shows that the EP-flux source responds to low level baroclinicity with a lag of 2-4 days for all modes.Low frequencies: almost in phase, small .F lag.High frequencies: almost out of phase.
39Refractive Index and EP-flux (single composite) High FrequencyLow FrequencyEddy propagation responds to current zonal wind anomalies.Resulting upper level EP-flux divergence forces further zonal wind changes.Eddies propagate towards high refractive indexRefractive index anomalies determined by wind anomaliesLarger effect near critical lines phase offset
40Eddy feedback processes HeightHigh FrequencyLatitudeLatitudeLatitudeLatitudeEddy source lags baroclinicity (zonal wind anomalies) by 2-4 daysRefractive Index determined by wind anomaliesEddies propagate towards high refractive indexResulting EP-flux divergence drives zonal wind changes (phase offset)LatitudeHeightLow Frequency
41ConclusionsAnnular variability at different timescales in a Newtonian forced AGCM:Equatorward migration of anomalies at high frequenciesPoleward migration at low frequenciesFor all timescales the jet migration is driven by the eddies at upper levels and conveyed to lower levels by the residual circulation.Evidence for two feedback processes:Eddy source responds to low-level baroclinicity, with lag 2-4 days:High frequency flow is so strongly eddy driven that wind anomalies almost out of phase with wave source.Low frequency wind anomalies and eddy source are almost in phase.Wind anomalies dominate refractive index, leading to positive eddy feedback via EP-flux divergence.Direction of propagation from relative phases of wave source/sink and wave refraction.