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On the extratropical low-frequency variability (LFV) in aqua-planet simulations

On the extratropical low-frequency variability (LFV) in aqua-planet simulations. Masahiro Watanabe Faculty of Environmental Earth Science, Hokkaido University K-1 Japan. hiro@ees.hokudai.ac.jp. Group K-1: CCSR/NIES/FRCGC AGCM. Global spectral model (T42L20) Physics:

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On the extratropical low-frequency variability (LFV) in aqua-planet simulations

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  1. On the extratropical low-frequency variability (LFV) in aqua-planet simulations Masahiro Watanabe Faculty of Environmental Earth Science, Hokkaido University K-1 Japan hiro@ees.hokudai.ac.jp

  2. Group K-1: CCSR/NIES/FRCGC AGCM • Global spectral model (T42L20) • Physics: • Prognostic Arakawa-Schubert (Pan and Randall, 1998) • Prognostic cloud water for layer cloud and LSC (Le Treut & Li 1991) • Semi-Lagrangian moisture / cloud water transports (Lin & Rood 1996) • K-distribution 2-stream radiation (Nakajima et al., 1995) + max.-random cloud overlap • Mellor-Yamada level 2 turbulent closure + moist effect (Smith, 1990) • Experiments: • Control and 3KW1 extended for 2700 days (7.5yrs) • All the variables are decomposed into time-mean state, low-frequency (>10dy) and high-frequency (<10dy) components

  3. Mean state & dominant LFVs in APE runs Time-mean U & √z’2 at s=0.30 EOF1 to daily LF Ps PC autocorrelation e-folding decay ~ 11days 3KW1 17.4% e-folding decay ~ 16days Control 14.3%

  4. “annularity” of the Control annular mode Cash et al. (2002,2004, JAS) arguments • Dominant annular mode in LF Ps in an aquaplanet GCM • a. Large difference in the fractional variances between • EOFs 1 to 2D (76%) & 3D Ps (20%) • b. Individual annular mode episode & • one-point correlation maps • are much less annular • Annular mode represents a superposition of zonal phase-free, localized dipole events akin to the NAO

  5. “Annular mode” composites in Control Composite technique: ・ project daily LF Ps onto EOF pattern (0-90N) ・ an annular mode “event” is defined if the projection is significant at 99% for more than consecutive 4 days ・ 24 positive / 23 negative events during 2520 days contour: Ps shade: Y annular structure + m=5 disturbances

  6. Distinction: m=5 wave and annular modes in Y.30 EOF1 (E1) EOF2 (E2) EOF3 (E3) Ctrl E1 E2 E3 c=1.7m/s 3KW1 Coherence between the annular mode and the variance of m=5 waves r( E3, √E12 & E22) = 0.34 Not negligible!

  7. Composite zonal wind in 3KW1 composite LF [u] at mature time evolution in [u] & eddy forcing transient eddy (no filter) [u] anomaly low-frequency eddy Both low-frequency eddies and high-frequency eddies (storm track) act to excite the zonal mean wind anomaly in 3KW1, as in observed AO

  8. Summary • Low-frequency surface pressure fields • Prevailing annular variability in the EOF • Local or wavy structure in the one-point cor. maps • Low-frequency upper-level Y fields • Prevailing annular variability and m=5 QS waves well separated in the EOF • Annular variability is forced by the transient eddy momentum fluxes (+QS eddies in 3KW1) • A modest coherence between the annular mode and variance of m=5 waves Annular variability viewed as a dynamical mode arising from zonally varying mean state Less annular structure in the correlation maps and individual snapshots Is this commonly found in other models?

  9. Model intercomparison of the LFV: preliminaries Time and spatial dimensions • NICAM: too short data record • MGO: lack of time dimension in the netCDF header

  10. Annular mode in APE Control experiments Leading EOF to low-frequency SLP anomalies

  11. Annular mode in APE Control experiments Persistence s2 (%) 25 days (CCAM) 15.6 21 days (NCAR) 14.8 18 days (UKMO_b) 17.8 18 days (K1 Japan) 14.0 17 days (NSIPP) 10.8 16 days (AGU for APE) 13.9 14 days (UKMO_a) 15.3 13 days (LASG) 13.3 6 days (obs. NAO) [cf. Watanabe 2004 JC] * Model annular modes are all persistent * Is the inter-model difference significant? spread of persistence

  12. Indicative of the HF eddy driving the annular mode LF eddy variance is significantly correlated in several models! Correlation with the annular mode index 250hPa EKE 250hPa [u’v’] High-freq (<10dy) Low-freq (>10dy)

  13. Low-frequency teleconnection One-point correlation to low-frequency SLP anomalies (lon. avg.) base point

  14. Dominant wavenumber selection in baroclinic adjustment experiments Y.30Fourier amplitude Dominant scale ∝DT DT=90K DT=70K DT=50K DT=30K DT=20K Dominant wavenumber in LF anomalies Fourier spectrum for LF V250 m=5 * dominant m will change following meridional gradient in SST

  15. Dominant wavenumber in LF anomalies Fourier spectrum for LF V250 Corr. m=5 LF EKE with annular mode m=5

  16. Model intercomparison of the LFV in APE • Preliminary summary • All the Control experiments show dominant annular variability in low-frequency fields • Persistence of 2 weeks to 1 month • High-frequency eddy driving • Some models reveal a coherence between the annular mode and variance of dominant, m=5 wave • For further intercomparison • How the annular mode is generated/maintained in models without coherence with m=5 waves? • Is there systematic relationship between time-mean states and behavior of annular modes? • Analysis to the 3KW experiments

  17. Mode of non-zonal time mean state How we think example: stochastically forced a point mass in a potential wall Near-neutral singular vectors of the linear dynamical operator pointing an axis along which LF anomaly has the largest fluctuation PDF

  18. Y0.30 v1 v2 v3 Large barotropic energy conversion near the centers of v2 and v3 Neutral modes ・ T21L11m6 LBM (Watanabe and Kimoto 2000, QJRMS) ・ 3KW1 time-mean basic state ・ damping follows v.diff.coef. evaluated with M-Y2.0 in 3KW1 (system is stable)

  19. Projection onto neutral modes daily LF Y0.30 trajectory in the v1-v3 phase space

  20. Neutral mode dynamics for the LFV in 3KW1 • Preliminary summary • Annular mode tends to appear along the axis spanned by the leading 2 neutral vectors A certain part of the annular variability in 3KW1 may be explained without transient eddy forcing • Observational counterparts? Neutral singular mode (v-vector) Least-damped eigenmode Obs. AO T21L11 LBM Refs: Kimoto et al. (2001, GRL); Watanabe & Jin (2004, JC)

  21. Observed mean state & the dominant LFV Climatological jet and storm tracks NAO in 10dy low-pass SLP Data source: NCEP/NCAR reanalysis 1948-2002

  22. “Annular mode” composites in W1 Composite technique: ・ daily LF Ps を EOF1へ投影 (0-90N) ・ 99%で有意な相関が4日 以上続いたら一つの persistent eventと定義 ・ W1では 24 positive / 23 negative “events” を同定 ・ W1 positive composites from Lag -10 to Lag +10 ・ annular structure + m=5 disturbances ・ enhancement near the jet exit region

  23. Neutral singular modes arising from observed climatological state (DJF) Neutral singular mode (v-vector) Least-damped eigenmode Obs. AO T21L11 LBM Refs: Kimoto et al. (2001, GRL); Watanabe & Jin (2004, JC)

  24. Y0.30 W1 composite 10dy “hindcast” 10dy “forecast” r=0.61 r=0.59 anomaly correlation Linear anomaly prediction ・ T42L20 LBM ・ identical twin type prediction ・ initial: W1 composite LF anomalies ・ “hindcast”: LF diabatic heating & transient eddy (heat & vorticity) forcing

  25. Annular modeはホントウにLF stateの中で 予測しやすい偏差場か？ ― Monte Carlo test Annular modeの自己相関 Annular mode forecast Random 100 LF states Forecasts for random 100 LF states Linear anomaly prediction ・ different lead time (10, 8, 6, 4, 2 days) Annular modeの自己相関 “forecast” “hindcast” Correlation with Lag-0 Y0.30 Projection with Lag-0 Y0.30 ・ Anomaly correlationはforcingの有無であまり変わらない ・ 振幅は”hindcast”で増大　⇒　forcingはpreferred structureを強めるように働く　

  26. “hindcast” “forecast” Linear anomaly prediction Neutral mode and projection in W0 強い投影は”purely zonal” modeへのみ (波数5擾乱の起源は中立モードでは説明できない) ….. にもかかわらず Annular modeに関する10dy linear predictionの成績はW1よりもよい ⇒　投影のあるneutral modeは東西一様なのだから、このscoreはzonal成分についてのみ？

  27. W1ではeddyもzonal meanと同程度に予測できるが、W0ではzonal meanのみ 　⇒　中立モードへの投影と整合的 What is predictable? Correlation with Lag-0 Y0.30 , 10dy “forecast” W1 W0 実線：　total anomaly、　　破線：　eddy (zonal meanからのずれ） anomaly

  28. Distinction: m=5 wave and annular modes W0 E1 E2 E3 [E]1 E*1 E*2

  29. Distinction: m=5 wave and annular modes W1 E1 E2 E3 [E]1 E*1 E*2

  30. Singular values (s-1) CSST W0 W1 現実に近い(大きく曲がった気候場)ではよりsingular! 　⇒　長周期変動における潜在構造の役割も大きい？

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