Zonal Flow Variability Linking the ENSO/Monsoon Systems
• Step back to the atmospheric response to El Niño– attempt to interpret the zonal flow variability associated with ENSO
• ENSO influence to the Asian summer monsoon– Why monsoon tends to be weak following El Niño winter ?
• southward shift in spring ITCZ over the western Pacific• excessive snow and soil moisture over Tibetan plateau• air-sea interaction in the Indian Ocean• enhanced northern subtropical westerly
IAMAS in Innsbruck 07/17/01
M. Watanabe1, F.-F. Jin2, and M. Kimoto1
1: CCSR, University of Tokyo, 2: Dpt. Meteorology, University of Hawaii
IAMAS in Innsbruck 07/17/01
global ENSO teleconnection
Regression of Z500/300 on monthly Nino3 SSTA, 1949-99
IAMAS in Innsbruck 07/17/01
principal mode in
EOF1(23%) for monthly300, 1949-99
Nino3 SSTAPC1 of 300
Corr.=0.60
Tropical Axisymmetric Mode (TAM)
IAMAS in Innsbruck 07/17/01
structure of TAM
Regression of monthly NCEP anomalies on the 300 PC1
IAMAS in Innsbruck 07/17/01
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
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
IAMAS in Innsbruck 07/17/01
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)
IAMAS in Innsbruck 07/17/01
ENSO-forced zonal-mean flow
DJFidealized heating Q
IAMAS in Innsbruck 07/17/01
Relationship between TAM and summer monsoon
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
IAMAS in Innsbruck 07/17/01
ENSO-TAM-Monsoon: statistical connection
Lagged correlation3mo-avg. TAM index vs. Niño3 or IMR Composite monsoon shear index
El Niño/normal TAM yrs
El Niño/positive TAM yrs
IAMAS in Innsbruck 07/17/01
Possibility considered:
• El Niño forces TAM, monsoon forces TAM, but the TAM does nothing for the ENSO/monsoon coupling
• El Niño forces TAM in winter, which affects other components of climate having longer memory that can bring weaker monsoon
• El Niño forces TAM in spring, which contributes to precondition a weak summer monsoon
I
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1
EN
SO
-TA
M-M
on
so
on
: p
oss
ible
me
ch
an
ism
Com
posi
te E
l Nin
o/w
eak
mon
soon
Lin
ear
resp
on
se t
o Q
Lin
ear
resp
on
se t
o Q
+Z
W[m
/s]
[m/s
][K
]
T5
00 a
nd V
85
0 ,
Mar
ch-M
ay
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En
sem
ble
ex
pe
rim
en
t b
y t
he
CC
SR
/NIE
S A
GC
M
10-
me
mb
er e
nse
mb
le o
f th
e 9
-mo
ru
n fr
om
Ja
n. 1
EN
SO
re
spo
nse
= <
N>
- <
C>
T 300
and
V85
0 ,c
limat
olog
y in
Jun
e
NC
EP
rean
alys
is
AG
CM
<C
> r
un
IAMAS in Innsbruck 07/17/01
Monsoon response to El Niño in the AGCM
<N> - <C> , May
T300 & V850 response
Vertically averaged Q
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Ro
le o
f th
e zo
nal
flo
w v
aria
bil
ity
C)
b +
1d
y d
amp
ing
fo
r Z
M c
om
p.
B)
no
Q w
est
of
100E
A)
AG
CM
Q
T3
00 &
V8
50
lin
ea
r m
od
el r
es
po
ns
e
Conclusions
• Zonal flow variability and ENSO– dominant zonally uniform structure (tropical-wide westerly) as a part
of the global ENSO teleconnection– the El Niño-forced zonal flow interpreted in terms of an excitation of
the near-neutral mode (Tropical Axisymmetric Mode)
• Role of the zonal flow variability in the ENSO/monsoon– upstream teleconnection induced by the coupling between the zonal
flow and topography & clim. stationary waves– subtropical zonal-mean westerly anomaly accompanies a tropospher
ic cooling over the S.Asia and northeasterly over the N.Indian Ocean, contributing to precondition a weaker monsoon
• Future issue– further AGCM experiments – interaction between dynamics and convection
IAMAS in Innsbruck 07/17/01
IAMAS in Innsbruck 07/17/01
On the neutrality of the mode
Zonal-mean zonal momentum budget
close to neutrality
IAMAS in Innsbruck 07/17/01
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
IAMAS in Innsbruck 07/17/01
Reconstruction by singular modes
・ 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
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mo
ns
oo
n r
es
po
ns
e to
El N
iño
in t
he
AG
CM
NC
EP
AG
CM
AG
CMU
(4
0-11
0E
,5-2
0N)
Pr.
(6
5-90
E,1
0-3
0N)
IAMAS in Innsbruck 07/17/01
Monsoon response to El Niño in the AGCM
<N> - <C> , May
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Ro
le o
f th
e zo
nal
flo
w v
aria
bil
ity
C)
b +
ZW
co
up
lin
g
B)
zon
al m
ean
BS
A)
AG
CM
Q,
Pac
ificT3
00 &
V8
50
lin
ea
r m
od
el r
es
po
ns
e
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Ho
w E
l N
iño
exc
ites
zo
nal
flo
w a
no
mal
y?
day
1
Lin
ear
mo
del
tim
e in
teg
rati
on
day
5
day
10
day
15
[m/s
]
T3
00
U3
00
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