tropical axisymmetric mode of variability: the dynamics
DESCRIPTION
AGS in Seoul , July 2001. Tropical Axisymmetric Mode of Variability: the Dynamics. M. Watanabe 1 , M. Kimoto 1 , and F.-F. Jin 2 1: CCSR, University of Tokyo, 2: Dpt. Meteorology, University of Hawaii. motivation of the study - PowerPoint PPT PresentationTRANSCRIPT
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Tropical Axisymmetric Mode of Variability: the Dynamics
• motivation of the study– Attempt to specify and understand a principal mode in the global circulati
on fields ( teleconnection patterns in hemispheric fields)
• previous studies show that:– near zonally uniform pattern (superrotational flow) (e.g. Kang & Lau 1994)
– coherence with AAM (e.g. Anderson & Rosen 1983; Rosen & Salstein 1983)
– found in intraseasonal time scale (e.g. Weickmann et al. 1997)
• outline– observational data analysis
– AGCM simulation
– linear model diagnoses
AGS in Seoul , July 2001
M. Watanabe1, M. Kimoto1, and F.-F. Jin2
1: CCSR, University of Tokyo, 2: Dpt. Meteorology, University of Hawaii
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principal mode in
EOF1(23%) for monthly300, 1949-99
AGS in Seoul , July 2001
Tropical Axisymmetric Mode (TAM)= ‘global mode’ (Higgins et al. 2000; Bell & Halpert 2000) = ‘tropical mode’ (vonStorch 1999)
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A
GS
in S
eoul
, Jul
y 20
01
tim
e se
ries
P
C1
GAM
L
OD
N
iño3
P
C1
.8
3
.5
3
.4
7G
AM
.
58
.
57L
OD
.41
Niñ
o3
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AGS in Seoul, July 2001
structure of TAM
Regression of monthly NCEP anomalies on the 300 PC1
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AGS in Seoul, July 2001
TAM in the zonal-mean winds
ResidualTAM
R EOF1 explains19% of total variance,significantly correlatedwith LOD (0.31)
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A
GS
in S
eoul
, Jul
y 20
01
spec
tral
ch
arac
teri
stic
s
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AGS in Seoul, July 2001
persistence of the TAM
(=TAM index)
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AGS in Seoul, July 2001
TAM simulated by an AGCM
T42L20 CCSR/NIES AGCM, 50yr run with climatological SST
・ AGCM reproduced an overall feature of the obs. TAM・ spectrum of the coefficient is much whiter than obs.・ TAM may essentially be an internal atmospheric mode
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AGS in Seoul, July 2001
simulated TAM
AGCM Linear model
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IAMAS in Innsbruck 07/17/01
detection of zonal-mean free modes
• T21L20 steady linear model (truncated at m=5)
• zonal structure of TAM separation between zonally symmetric (Xa) and asymmetric (Xa
*) components
• calculate singular vectors of L
( ) (1) ( ) (2)
L X X F X X fL X X F X X f
c a c a
c a c a
( , )( , )
* *
* * * * *
(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
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AGS in Seoul, July 2001
neutral mode
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)
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AGS in Seoul, July 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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AGS in Seoul, July 2001
on the neutrality of the mode
Zonal-mean zonal momentum budget
close to neutrality
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AGS in Seoul, July 2001
role of the basic state vorticity
NCEP zonal-mean wind regressed on the PC1 300
Coincidence between Ua and c further suggests themomentum feedback actively working for the neutrality
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AGS in Seoul, July 2001
origin of the neutral mode
eigenmodes of the zonal-mean shallow-water eqs.
・ basic state is not crucial for the presence of the mode・ scattering on i=0, geostrophic degeneracy?
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conclusions
• Tropical Axisymmetric Mode (TAM):– tightly related to the AAM 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 origin of mode seems to come from the geostrophic degeneracy
• implication and further question– TAM may be responsible to a part of the upstream ENSO teleconnection
– need to include an interaction between dynamics and convection
AGS in Seoul, July 2001
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AGS in Seoul, July 2001
convection associated with TAM
Composite OLR anomaly based on the TAM index
AGCM
NOAA
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Regression of Ua on PCs for monthly 300 EOF