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


global ENSO teleconnection

Regression of Z500/300 on monthly Nino3 SSTA, 1949-99


principal mode in

EOF1(23%) for monthly300, 1949-99

Nino3 SSTAPC1 of 300

Corr.=0.60

Tropical Axisymmetric Mode (TAM)


structure of TAM

Regression of monthly NCEP anomalies on the 300 PC1


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


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


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)


ENSO-forced zonal-mean flow

DJFidealized heating Q


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


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


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

AM

AS

in I

nnsb

ruck

07/

17/0

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

I

AM

AS

in I

nnsb

ruck

07/

17/0

1

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


Monsoon response to El Niño in the AGCM

<N> - <C> , May

T300 & V850 response

Vertically averaged Q

I

AM

AS

in I

nnsb

ruck

07/

17/0

1

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



On the neutrality of the mode

Zonal-mean zonal momentum budget

close to neutrality


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


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

I

AM

AS

in I

nnsb

ruck

07/

17/0

1

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)


Monsoon response to El Niño in the AGCM

<N> - <C> , May

I

AM

AS

in I

nnsb

ruck

07/

17/0

1

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

I

AM

AS

in I

nnsb

ruck

07/

17/0

1

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

Documents

Zonal Flow Variability Linking the ENSO/Monsoon Systems