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

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)

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

AGS in Seoul, July 2001

structure of TAM

Regression of monthly NCEP anomalies on the 300 PC1

AGS in Seoul, July 2001

TAM in the zonal-mean winds

ResidualTAM

R EOF1 explains19% of total variance,significantly correlatedwith LOD (0.31)

A

GS

in S

eoul

, Jul

y 20

01

spec

tral

ch

arac

teri

stic

s

AGS in Seoul, July 2001

persistence of the TAM

(=TAM index)

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

AGS in Seoul, July 2001

simulated TAM

AGCM Linear model

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

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)

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

AGS in Seoul, July 2001

on the neutrality of the mode

Zonal-mean zonal momentum budget

close to neutrality

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

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?

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

AGS in Seoul, July 2001

convection associated with TAM

Composite OLR anomaly based on the TAM index

AGCM

NOAA

Regression of Ua on PCs for monthly 300 EOF