Zonal Flow Variability Linking the ENSO/Monsoon Systems

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

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Com

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se t

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Lin

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[m/s

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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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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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Pr.

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IAMAS in Innsbruck 07/17/01

Monsoon response to El Niño in the AGCM

<N> - <C> , May

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[m/s

]

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U3

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