Tropical Axisymmetric Mode of Variability in the Atmospheric Circulation UH seminar, 29 August 2001...
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Tropical Axisymmetric Mode of Variability in the Atmospheric Circulation
UH seminar, 29 August 2001
Masahiro Watanabe Department of Meteorology, SOEST, University of Hawaii
* On leave from Center for Climate System Research (CCSR), University of Tokyo
refs: Watanabe, Kimoto, and Jin (2001, submitted to JC) Watanabe, Jin, and Kimoto (2001, to be submitted to JC)
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introduction
• motivation of the study– What is the leading mode of the atmospheric circulation?
• e.g. barotropic & baroclinic instabilities, teleconnection patterns, MJO,….
– Attempt to specify and understand a principal mode in the global circulation fields ( teleconnection patterns in hemispheric fields)
• outline– observational data analysis
– AGCM simulation
– linear model diagnoses
UH seminar, 29 August 2001
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previous studies
• near zonally uniform pattern (superrotational flow) – (e.g. Kang & Lau 1994)
• coherence with atmospheric angular momentum– (e.g. Anderson & Rosen 1983; Rosen & Salstein 1983)
• found in intraseasonal time scale – (e.g. Weickmann et al. 1997)
UH seminar, 29 August 2001
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EOF1(23%) for monthly300, 1949-99
UH seminar, 29 August 2001
Tropical Axisymmetric Mode (TAM)= ‘global mode’ (Higgins et al. 2000; Bell & Halpert 2000) = ‘tropical mode’ (vonStorch 1999)
principal mode in
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UH seminar, 29 August 2001
time series
PC1 M LOD Niño3 PC1 .83 .53 .47 M .58 .57LOD .41Niño3
(defined as the TAM index)
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UH seminar, 29 August 2001
Regression of monthly NCEP anomalies on the 300 PC1
structure of TAM
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UH seminar, 29 August 2001
ResidualTAM
R EOF1 explains19% of total variance,significantly correlatedwith M (0.62) andLOD (0.31)
TAM in the zonal-mean winds
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UH seminar, 29 August 2001
spectral characteristics
NCEP
NCEP
AGCM
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UH seminar, 29 August 2001
(=TAM index)
persistence of the TAM
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UH seminar, 29 August 2001
question (1)
• The features of the TAM– high correlation with Niño3 SST index
– low-level divergence (convergence) over the maritime continent (eastern eq. Pacific)
– spectral peak around 4 yr
– persistence up to 5 mo
• Do they imply the TAM nothing more than the atmospheric response to El Niño?
• But ENSO residual fields do reveal the same variability dominating
• In essence, is TAM independent of ENSO?– How can we explore it? use AGCM !
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UH seminar, 29 August 2001
T42L20 CCSR/NIES AGCM, 50yr run with climatological SST
TAM simulated by an AGCM
CCSR/NIES AGCM
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UH seminar, 29 August 2001
simulated TAM
zonal wind
meridional wind
CCSR/NIES AGCM
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UH seminar, 29 August 2001
question (2)
• AGCM reproduced an overall feature of the observed TAM– e.g. horizontal circulation patterns, even the equatorial surface wind
• spectrum of the coefficient is much whiter than observations in addition to the absence of a peak around 4yr
• TAM may essentially be an internal atmospheric mode– What is the dynamics responsible for such a mode?
– We need to diagnose it using a simpler dynamical framework
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• Linearized multi-level PE model– derived from a dynamical core of the CCSR/NIES AGCM
– spectral truncation of T21
– vertical 20 levels
• steady version– zonally symmetric basic state obtained from the NCEP or AGCM cl
imatology (monthly or seasonal mean)
– zonal wavenumber truncated at m=5
• time integration– 3D basic state obtained from the NCEP or AGCM climatology (mo
nthly or seasonal mean)
– refs: Watanabe & Kimoto (1999, GRL)– Watanabe & Kimoto (2000, QJRMS)
UH seminar, 29 August 2001
linear baroclinic model (LBM)
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fXXFLtX
fXXFXXLtX
XXFXXL
XXXXX
PTX
aaa
caaca
caac
ac
acac
s
feedbackeddy w/ flow Zonal3.
,
model response Zonal2.
,
model wavePlanetary 1.
anomaly : y,climatolog : wave,: mean, zonal :
ln,,,
**
****
*
**
zonal-wave coupling term
stationary wave feedback
UH seminar, 29 August 2001
linear operators
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=
, D, T, ...
Xa
Xa*
m=0
m=1
m=2
m=M
, D, T,
ZRM
PWM
N(N~ 30,000 for T21L20)
F*(Xa,Xc*)
F(Xa*,Xc*)
UH seminar, 29 August 2001
linear operator matrices
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calculate singular vectors of L
associated stationary wave anomalies readily obtained as
X L F X va c* * * *( , ) 1
i
(3)
(4)
1 2 3
1 2 3
L U V
X L f
U u u u
V v v v
u fa
v
T
, , ,
T
( , , ,...),( ...),( , , ,...),
( , )
1 2 3
1
ii
i
i
u-vectorssingular valuesv-vectors
∴ singular mode withthe smallest will have the longest persistence
UH seminar, 29 August 2001
neutral mode detection
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UH seminar, 29 August 2001
neutral mode
zonal wind
meridional wind
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UH seminar, 29 August 2001
Leading singular mode + associated stationary waves, v1+L*-1F*(Xc*,v1)
・ much prevailing zonal structure in 300
・ low-level features less similar to obs./AGCM TAM・ decay time ~ dissipation timescale of the free troposphere
(< month)
anomalous circulation associated with the neutral mode
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UH seminar, 29 August 2001
zonal asymmetry
observed TAM
neutral mode
Ua
・ neutral mode seems consistent with the observed TAM in a considerable part except for the Pacific
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UH seminar, 29 August 2001
Zonal-mean zonal momentum budget
close to neutrality
on the neutrality of the mode
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UH seminar, 29 August 2001
NCEP zonal-mean wind regressed on the PC1 300
Coincidence between Ua and c further suggests themomentum feedback actively working for the neutrality
role of the basic state vorticity
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UH seminar, 29 August 2001
eigenmodes of the zonal-mean shallow-water eqs.
・ basic state is not crucial for the presence of the mode・ scattering on i=0, due to viscosity?
origin of the neutral mode
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conclusions (1)
• Tropical Axisymmetric Mode (TAM):– tightly related to the angular momentum variability and LOD
– contains a signature of El Niño (may suggest ENSO forces TAM)
• dynamics of the TAM– AGCM with climatological SST does reproduce the observed TAM
– A near-neutral mode found in the singular mode computation of the linear model is considerably similar to the observed/AGCM TAM
– The essence of the TAM can be interpreted as an internal atmospheric mode which is easily excited by forcing
– The neutrality partially arises from a positive momentum feedback in the zonal mean state (i.e. coupling between Ua and Hadley circulation) , although the process may not be crucial for the origin of mode
UH seminar, 29 August 2001
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question(3)
UH seminar, 29 August 2001
• Neutral mode failed to reproduce the lower-tropospheric feature in the observed TAM– Why?
– Interaction between convection and the dynamics?
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UH seminar, 29 August 2001
Composite OLR anomaly based on the TAM index
AGCM
NOAA
convection associated with TAM
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implication
UH seminar, 29 August 2001
• Can we interpret the change in zonal mean state during ENSO in terms of an excitation of the neutral mode?
• Role of the zonal mean flow (Ua) in:– ENSO upstream teleconnection
– ENSO-monsoon coupling
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UH seminar, 29 August 2001
Regression of Z500/300 on monthly Nino3 SSTA, 1949-99
global ENSO teleconnection
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UH seminar, 29 August 2001
DJFidealized heating Q
ENSO-forced zonal-mean flow
The Ua response is independent of the Rossby wave train over the Pacific!
NCEP composite LBM response
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calculate singular vectors of L (zonal-mean dynamical operator)
The phase and amplitude of each mode depend not only on the singular value, but on the projection of u-vector onto forcing
(3)
(4)
1 2 3
1 2 3
L U V
X L f
U u u u
V v v v
u fa
v
T
, , ,
T
( , , ,...),( ...),( , , ,...),
( , )
1 2 3
1
ii
i
i
u-vectorssingular valuesv-vectors
UH seminar, 29 August 2001
zonal mean response represented by singular modes
‘projection coefficient’
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UH seminar, 29 August 2001
・ a large part of the forced zonal wind is reproducible with two singular modes・ different optimal heating profiles for the neutral mode (~ TAM) & a second (baroclinic) mode
optimal thermal forcing
reconstruction by singular modes
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( ) (1) ( ) (2)
L X X F X X fL X X F X X f
c a c a
c a c a
( , )( , )
* *
* * * * *
zonal-wave coupling
El Niño heating
UH seminar, 29 August 2001
stationary wave response to ENSO-forced Ua
T850 in winterNCEP composite for El Niño
LBM response to Q
95% significance
idealized heating
LBM response to Q+ZW
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UH seminar, 29 August 2001
‘tropical-belt’ teleconnection
upslope cooling (downslope warming) due to orographic forcing (cf. Hoskins & Karoly 1981)
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UH seminar, 29 August 2001
time series of : TAM index (JJA avg.), all-India monsoon rainfall (IMR),
Webster & Yang ‘s dynamical monsoon index
r(TAM,IMR) = -0.50r(TAM,DMI) = -0.62
Relationship between TAM and summer monsoon
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question (4)
UH seminar, 29 August 2001
• There is an argument that change in the subtropical jet associated with El Niño is involved in the coupled ENSO-monsoon system.– (e.g. Nigam 1994; Ju & Slingo 1995)
• The TAM index indeed shows a significant correlation with indices of the Asian summer monsoon variability
• Does the anomalous zonal-mean flow forced by El Niños (whatever the mechanism) plays any role in the ENSO-monsoon coupling?
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UH seminar, 29 August 2001
Composite OLR anomaly in summer, following Kawamura (1998)
convection associated with ENSO/monsoon
weak monsoon/warm event: 1979, 1983, 1987, 1991, 1992, 1993strong monsoon/cold event : 1981, 1984, 1985, 1988, 1989, 1990
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UH seminar, 29 August 2001
precursors for weak summer monsoon
observed composite in May
OLR anomaly
T300 & V850 anomalies
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UH seminar, 29 August 2001
simulated circulation anomalies in May
10-member ensemble difference for El Niño run
Vertically averaged Q
T300 & V850 response
CCSR/NIES AGCM
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UH seminar, 29 August 2001
simulated monsoon precursor in May
CCSR/NIES AGCM
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UH seminar, 29 August 2001
role of Ua in forcing the continental cooling
T300 & V850 response
to the AGCM heating
LBM at day 25
Indian Ocean heating removed
Indian Ocean heating removed& zonal mean response damped
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UH seminar, 29 August 2001
NCEP composite
cooling over the Himalayan upslope
CCSR AGCM
LBM (day 25)
temperature longitude-pressure section along 30N in May
vertical phase tilt
upslope cooling forlong wave (K<Ks)
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UH seminar, 29 August 2001
cooling over the Himalayan upslope
Cp1
p0
C
<0
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conclusions (2)
• zonal flow anomaly (Ua) during ENSO– subtropical westerlies and equatorial easterly anomaly
– the anomalous zonal-mean flow is plausibly independent of the Rossby wave train over the Pacific
– may be an indication of neutral modes excited by the El Niño heating
• role of zonal flow anomaly: tropical-belt teleconnection– Ua-induced teleconnection seems to explain how and why the anomalo
us circulation occurs in the upstream region of El Niño
– tropical-belt teleconnection may further play an active role in the ENSO-monsoon coupling such that it helps to precondition the weak monsoon during El Niño-like condition
• further question: how ENSO forces the zonal mean flow anomaly?– what is the role of stationary wave feedback on to the zonal mean?
UH seminar, 29 August 2001