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Tropical Axisymmetric Mode of Variability: the Dynamics

AGS in Seoul , July 2001. Tropical Axisymmetric Mode of Variability: the Dynamics. M. Watanabe 1 , M. Kimoto 1 , and F.-F. Jin 2 1: CCSR, University of Tokyo, 2: Dpt. Meteorology, University of Hawaii. motivation of the study

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Tropical Axisymmetric Mode of Variability: the Dynamics

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  1. AGS in Seoul , July 2001 Tropical Axisymmetric Mode of Variability: the Dynamics M. Watanabe1, M. Kimoto1, and F.-F. Jin2 1: CCSR, University of Tokyo, 2: Dpt. Meteorology, University of Hawaii • 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

  2. AGS in Seoul , July 2001 principal mode iny Tropical Axisymmetric Mode (TAM) = ‘global mode’ (Higgins et al. 2000; Bell & Halpert 2000) = ‘tropical mode’ (vonStorch 1999) EOF1(23%) for monthly y300, 1949-99

  3. y PC1 GAM DLOD Niño3 y PC1 .83 .53 .47 GAM .58 .57 DLOD .41 Niño3 AGS in Seoul, July 2001 time series

  4. AGS in Seoul, July 2001 structure of TAM Regression of monthly NCEP anomalies on the y300 PC1

  5. AGS in Seoul, July 2001 TAM in the zonal-mean winds TAM Residual yR EOF1 explains 19% of total variance, significantly correlated with DLOD (0.31)

  6. AGS in Seoul, July 2001 spectral characteristics

  7. AGS in Seoul, July 2001 persistence of the TAM (=TAM index)

  8. 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

  9. AGS in Seoul, July 2001 simulated TAM AGCM Linear model

  10. 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

  11. AGS in Seoul, July 2001 neutral mode Leading singular mode + associated stationary waves, v1+L*-1F*(Xc*,v1) ・much prevailing zonal structure in y300 ・low-level features less similar to obs./AGCM TAM ・decay time ～　dissipation timescale of the free troposphere (< month)

  12. zonal asymmetry AGS in Seoul, July 2001 Ua observed TAM neutral mode ・neutral mode seems consistent with the observed TAM in a considerable part except for the Pacific

  13. AGS in Seoul, July 2001 on the neutrality of the mode Zonal-mean zonal momentum budget close to neutrality

  14. AGS in Seoul, July 2001 role of the basic state vorticity NCEP zonal-mean wind regressed on the PC1 y300 Coincidence between Ua and zc further suggests the momentum feedback actively working for the neutrality

  15. AGS in Seoul, July 2001 origin of the neutral mode eigenmodes of the zonal-mean shallow-water eqs. ・basic state z is not crucial for the presence of the mode ・scattering on si=0, geostrophic degeneracy?

  16. AGS in Seoul, July 2001 conclusions • Tropical Axisymmetric Mode (TAM): • tightly related to the AAM variability and DLOD • 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

  17. AGS in Seoul, July 2001 convection associated with TAM Composite OLR anomaly based on the TAM index NOAA AGCM

  18. Regression of Ua on PCs for monthly y300 EOF

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