Tropical Axisymmetric Mode of Variability: the Dynamics motivation of the study –Attempt to specify and understand a principal mode in the global circulation 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. Watanabe 1, M. Kimoto 1, and F.-F. Jin 2 1: CCSR, University of Tokyo, 2: Dpt. Meteorology, University of Hawaii

principal mode in  EOF1(23%) for monthly  300, AGS in Seoul, July 2001 Tropical Axisymmetric Mode (TAM) = ‘global mode’ (Higgins et al. 2000; Bell & Halpert 2000) = ‘tropical mode’ (vonStorch 1999)

AGS in Seoul, July 2001 time series  PC1 GAM  LOD Niño3  PC GAM  LOD.41 Niño3

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 explains 19% of total variance, significantly correlated with  LOD (0.31)

AGS in Seoul, July 2001 spectral characteristics

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 AGCMLinear 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 ( X a ) and asymmetric ( X a * ) components calculate singular vectors of L

AGS in Seoul, July 2001 neutral mode Leading singular mode + associated stationary waves, v 1 +L *-1 F * (X c *,v 1 ) ・ 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 the momentum 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

convection associated with TAM Composite OLR anomaly based on the TAM index AGCM NOAA

Regression of U a on PCs for monthly  300 EOF