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On the extratropical low-frequency variability (LFV) in aqua-planet simulations. Masahiro Watanabe Faculty of Environmental Earth Science, Hokkaido University K-1 Japan. [email protected]. Group K-1: CCSR/NIES/FRCGC AGCM. Global spectral model (T42L20) Physics: - PowerPoint PPT Presentation
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APE workshop at Univ. Reading, 20-22 April 2005
On the extratropical low-frequency variability (LFV) in aqua-planet simulations
Masahiro Watanabe Faculty of Environmental Earth Science,
Hokkaido University
K-1 Japan
APE workshop at Univ. Reading, 20-22 April 2005
• 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 199
6) – 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, lo
w-frequency (>10dy) and high-frequency (<10dy) components
Group K-1: CCSR/NIES/FRCGC AGCM
APE workshop at Univ. Reading, 20-22 April 2005
Mean state & dominant LFVs in APE runs
3KW1
Control
EOF1 to daily LF Ps PC autocorrelation
17.4%
14.3%
Time-mean
U & √z’2 at =0.30
e-folding decay ~ 11days
e-folding decay ~ 16days
APE workshop at Univ. Reading, 20-22 April 2005
“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
APE workshop at Univ. Reading, 20-22 April 2005
“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
annular structure + m=5 disturbances
contour: Psshade:
APE workshop at Univ. Reading, 20-22 April 2005
Distinction: m=5 wave and annular modes in
E1 E2 E3Ctrl
3KW1
EOF1 (E1) EOF2 (E2) EOF3 (E3)
Coherence between the annular mode and the variance of m=5 waves
r( E3, √E12 & E2
2) = 0.34
Not negligible!
c=1.7m/s
APE workshop at Univ. Reading, 20-22 April 2005
Composite zonal wind in 3KW1
transient eddy (no filter)
low-frequency eddy
[u] anomaly
composite LF [u] at mature time evolution in [u] & eddy forcing
Both low-frequency eddies and high-frequency eddies (storm track) act to excitethe zonal mean wind anomaly in 3KW1, as in observed AO
APE workshop at Univ. Reading, 20-22 April 2005
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 fields– Prevailing annular variability and m=5 QS waves well sep
arated in the EOF– Annular variability is forced by the transient eddy momen
tum fluxes (+QS eddies in 3KW1)
– A modest coherence between the annular mode and variance of m=5 waves
Less annular structure in the correlation maps and individual snapshots
Is this commonly found in other models?
Annular variability viewed as a dynamical mode arising from zonally varying mean state
APE workshop at Univ. Reading, 20-22 April 2005
Model intercomparison of the LFV: preliminariesModel intercomparison of the LFV: preliminaries
AGU CCAM K1Japan LASG
Hemispheric resolution (x,y)
120x30 180x46 128x32 128x54
Data length (dy) 730 730 5040 730
NCAR NSIPP UKMO_a UKMO_b
Hemispheric resolution (x,y)
128x32 96x30 192x73 192x73
Data length (dy) 2190 730 360 360
Time and spatial dimensions
• NICAM: too short data record• MGO: lack of time dimension in the netCDF header
APE workshop at Univ. Reading, 20-22 April 2005
Annular mode in APE Control experimentsAnnular mode in APE Control experimentsLeading EOF to low-frequency SLP anomalies
APE workshop at Univ. Reading, 20-22 April 2005
Annular mode in APE Control experimentsAnnular mode in APE Control experiments
Persistence
25 days (CCAM) 15.621 days (NCAR) 14.818 days (UKMO_b) 17.818 days (K1 Japan) 14.017 days (NSIPP) 10.816 days (AGU for APE) 13.914 days (UKMO_a) 15.313 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
APE workshop at Univ. Reading, 20-22 April 2005
250hPa EKE 250hPa [u’v’]
High-freq(<10dy)
Low-freq(>10dy)
Indicative of the HF eddy driving the annular mode
LF eddy variance is significantly correlated in several models!
Correlation with the annular mode indexCorrelation with the annular mode index
APE workshop at Univ. Reading, 20-22 April 2005
Low-frequency teleconnectionLow-frequency teleconnection
One-point correlation to low-frequency SLP anomalies (lon. avg.)
base point
APE workshop at Univ. Reading, 20-22 April 2005
Dominant wavenumber in LF anomaliesDominant wavenumber in LF anomalies
Fourier spectrum for LF V250
m=5
* dominant m will change following meridional gradient in SST
Dominant wavenumber selection Dominant wavenumber selection in baroclinic adjustment experimentsin baroclinic adjustment experiments
T=50K
T=20K
T=90K
Dominant scale ∝T
T=30K
T=70K
Fourier amplitude
APE workshop at Univ. Reading, 20-22 April 2005
Dominant wavenumber in LF anomaliesDominant wavenumber in LF anomalies
Fourier spectrum for LF V250
m=5
Corr. m=5 LF EKE with annular mode
APE workshop at Univ. Reading, 20-22 April 2005
Model intercomparison of the LFV in APEModel 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
APE workshop at Univ. Reading, 20-22 April 2005
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 operatorpointing an axis along which LF anomaly has the largest fluctuation
APE workshop at Univ. Reading, 20-22 April 2005
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)
0.30
v1 v3v2
Large barotropic energy conversion near the centers of v2 and v3
APE workshop at Univ. Reading, 20-22 April 2005
Projection onto neutral modes
daily LF 0.30 trajectory in the v1-v3 phase space
APE workshop at Univ. Reading, 20-22 April 2005
Neutral mode dynamics for the LFV in 3KW1Neutral 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?
Obs. AONeutral singular mode (v-vector)
Least-damped eigenmode
Refs: Kimoto et al. (2001, GRL); Watanabe & Jin (2004, JC)
T21L11 LBM
APE workshop at Univ. Reading, 20-22 April 2005
Observed mean state & the dominant LFV
Data source: NCEP/NCAR reanalysis 1948-2002
Climatological jet and storm tracks NAO in 10dy low-pass SLP
APE workshop at Univ. Reading, 20-22 April 2005
“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
APE workshop at Univ. Reading, 20-22 April 2005
Neutral singular modes arising from observed climatological state (DJF)
Obs. AONeutral singular mode (v-vector)
Least-damped eigenmode
Refs: Kimoto et al. (2001, GRL); Watanabe & Jin (2004, JC)
T21L11 LBM
APE workshop at Univ. Reading, 20-22 April 2005
Linear anomaly prediction
r=0.61 r=0.59
anomaly correlation
W1 composite
0.30
10dy “hindcast” 10dy “forecast”
・ T42L20 LBM・ identical twin type prediction・ initial: W1 composite LF anomalies・ “ hindcast”: LF diabatic heating & transient eddy (heat & vorticity) forcing
APE workshop at Univ. Reading, 20-22 April 2005
・ 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
Linear anomaly prediction・ different lead time (10, 8, 6, 4, 2 days)
“hindcast” “forecast”
Correlation with Lag-0 0.30
Projection with Lag-0 0.30
Annular mode の自己相関
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
APE workshop at Univ. Reading, 20-22 April 2005
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
“hindcast” “forecast”
Linear anomaly prediction
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?
APE workshop at Univ. Reading, 20-22 April 2005
What is predictable?, 10dy “forecast”Correlation with Lag-0 0.30
実線: total anomaly 、 破線: eddy (zonal mean からのずれ) anomaly
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
W1 W0
APE workshop at Univ. Reading, 20-22 April 2005
Distinction: m=5 wave and annular modes
E1 E2 E3
E*1 E*2[E]1
W0
APE workshop at Univ. Reading, 20-22 April 2005
Distinction: m=5 wave and annular modes
E1 E2 E3
E*1 E*2[E]1
W1
APE workshop at Univ. Reading, 20-22 April 2005
Singular values ()
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?