tropical axisymmetric mode of variability in the atmospheric circulation

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)

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


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)


EOF1(23%) for monthly300, 1949-99


Tropical Axisymmetric Mode (TAM)= ‘global mode’ (Higgins et al. 2000; Bell & Halpert 2000) = ‘tropical mode’ (vonStorch 1999)

principal mode in


time series

PC1 M LOD Niño3 PC1 .83 .53 .47 M .58 .57LOD .41Niño3

(defined as the TAM index)


Regression of monthly NCEP anomalies on the 300 PC1

structure of TAM


ResidualTAM

R EOF1 explains19% of total variance,significantly correlatedwith M (0.62) andLOD (0.31)

TAM in the zonal-mean winds


spectral characteristics

NCEP

NCEP

AGCM


(=TAM index)

persistence of the TAM


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 !


T42L20 CCSR/NIES AGCM, 50yr run with climatological SST

TAM simulated by an AGCM

CCSR/NIES AGCM


simulated TAM

zonal wind

meridional wind

CCSR/NIES AGCM


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

• 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)


linear baroclinic model (LBM)

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


linear operators

＝

, 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*)


linear operator matrices

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


neutral mode detection


neutral mode

zonal wind

meridional wind


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


zonal asymmetry

observed TAM

neutral mode

Ua

・ neutral mode seems consistent with the observed TAM in a considerable part except for the Pacific


Zonal-mean zonal momentum budget

close to neutrality

on the neutrality of the mode


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


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

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


question(3)


• Neutral mode failed to reproduce the lower-tropospheric feature in the observed TAM– Why?

– Interaction between convection and the dynamics?


Composite OLR anomaly based on the TAM index

AGCM

NOAA

convection associated with TAM

implication


• 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


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

global ENSO teleconnection


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

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


zonal mean response represented by singular modes

‘projection coefficient’


・ 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

( ) (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


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


‘tropical-belt’ teleconnection

upslope cooling (downslope warming) due to orographic forcing (cf. Hoskins & Karoly 1981)


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

question (4)


• 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?


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


precursors for weak summer monsoon

observed composite in May

OLR anomaly

T300 & V850 anomalies


simulated circulation anomalies in May

10-member ensemble difference for El Niño run

Vertically averaged Q

T300 & V850 response

CCSR/NIES AGCM


simulated monsoon precursor in May

CCSR/NIES AGCM


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


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)


cooling over the Himalayan upslope

Cp1

p0

C

<0

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?


tropical axisymmetric mode of variability in the atmospheric circulation

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