on the extratropical low-frequency variability (lfv) in aqua-planet simulations

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

[email protected]


• 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


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


“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


“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:


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


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


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


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


Annular mode in APE Control experimentsAnnular mode in APE Control experimentsLeading EOF to low-frequency SLP anomalies


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


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


Low-frequency teleconnectionLow-frequency teleconnection

One-point correlation to low-frequency SLP anomalies (lon. avg.)

base point


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


Dominant wavenumber in LF anomaliesDominant wavenumber in LF anomalies

Fourier spectrum for LF V250

m=5

Corr. m=5 LF EKE with annular mode


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


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

PDF


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


Projection onto neutral modes

daily LF 0.30 trajectory in the v1-v3 phase space


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


Observed mean state & the dominant LFV

Data source: NCEP/NCAR reanalysis 1948-2002

Climatological jet and storm tracks NAO in 10dy low-pass SLP


“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


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


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


・ 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


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？


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


Distinction: m=5 wave and annular modes

E1 E2 E3

E*1 E*2[E]1

W0


Distinction: m=5 wave and annular modes

E1 E2 E3

E*1 E*2[E]1

W1


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？

on the extratropical low-frequency variability (lfv) in aqua-planet simulations

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