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Tropical Axisymmetric Mode of Variability in the Atmospheric Circulation

UH seminar, 29 August 2001. Tropical Axisymmetric Mode of Variability in the Atmospheric Circulation. Masahiro Watanabe Department of Meteorology, SOEST, University of Hawaii * On leave from Center for Climate System Research (CCSR), University of Tokyo.

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Tropical Axisymmetric Mode of Variability in the Atmospheric Circulation

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  1. UH seminar, 29 August 2001 Tropical Axisymmetric Mode of Variability in the Atmospheric Circulation 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)

  2. UH seminar, 29 August 2001 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

  3. UH seminar, 29 August 2001 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)

  4. UH seminar, 29 August 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

  5. UH seminar, 29 August 2001 y PC1 MDLOD Niño3 y PC1 .83 .53 .47 M.58 .57 DLOD .41 Niño3 time series (defined as the TAM index)

  6. UH seminar, 29 August 2001 structure of TAM Regression of monthly NCEP anomalies on the y300 PC1

  7. UH seminar, 29 August 2001 TAM in the zonal-mean winds TAM Residual yR EOF1 explains 19% of total variance, significantly correlated with M (0.62) and DLOD (0.31)

  8. UH seminar, 29 August 2001 spectral characteristics NCEP NCEP AGCM

  9. UH seminar, 29 August 2001 persistence of the TAM (=TAM index)

  10. UH seminar, 29 August 2001 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 !

  11. UH seminar, 29 August 2001 CCSR/NIES AGCM TAM simulated by an AGCM T42L20 CCSR/NIES AGCM, 50yr run with climatological SST

  12. UH seminar, 29 August 2001 CCSR/NIES AGCM simulated TAM zonal wind meridional wind

  13. UH seminar, 29 August 2001 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

  14. UH seminar, 29 August 2001 linear baroclinic model (LBM) • 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 climatology (monthly or seasonal mean) • zonal wavenumber truncated at m=5 • time integration • 3D basic state obtained from the NCEP or AGCM climatology (monthly or seasonal mean) • refs: Watanabe & Kimoto (1999, GRL) • Watanabe & Kimoto (2000, QJRMS)

  15. UH seminar, 29 August 2001 linear operators zonal-wave coupling term stationary wave feedback

  16. UH seminar, 29 August 2001 linear operator matrices z, D, T, p ... z, D, T, p ZRM Xa F(Xa*,Xc*) m=0 m=1 m=2 ＝ Xa* F*(Xa,Xc*) PWM m=M N (N～30,000 for T21L20)

  17. UH seminar, 29 August 2001 neutral mode detection calculate singular vectors of L associated stationary wave anomalies readily obtained as u-vectors singular values v-vectors ∴ singular mode with the smallest s will have the longest persistence

  18. UH seminar, 29 August 2001 neutral mode zonal wind meridional wind

  19. UH seminar, 29 August 2001 anomalous circulation associated with the 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)

  20. zonal asymmetry UH seminar, 29 August 2001 Ua observed TAM neutral mode ・neutral mode seems consistent with the observed TAM in a considerable part except for the Pacific

  21. UH seminar, 29 August 2001 close to neutrality on the neutrality of the mode Zonal-mean zonal momentum budget

  22. UH seminar, 29 August 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

  23. UH seminar, 29 August 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, due to viscosity?

  24. Tropical Axisymmetric Mode (TAM): tightly related to the angular momentum 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 process may not be crucial for the origin of mode UH seminar, 29 August 2001 conclusions (1)

  25. UH seminar, 29 August 2001 question(3) • Neutral mode failed to reproduce the lower-tropospheric feature in the observed TAM • Why? • Interaction between convection and the dynamics?

  26. UH seminar, 29 August 2001 convection associated with TAM Composite OLR anomaly based on the TAM index NOAA AGCM

  27. UH seminar, 29 August 2001 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

  28. UH seminar, 29 August 2001 global ENSO teleconnection Regression of Z500/y300 on monthly Nino3 SSTA, 1949-99

  29. UH seminar, 29 August 2001 ENSO-forced zonal-mean flow idealized heating Q DJF NCEP composite LBM response The Ua response is independent of the Rossby wave train over the Pacific!

  30. UH seminar, 29 August 2001 zonal mean response represented by singular modes 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 u-vectors singular values v-vectors ‘projection coefficient’

  31. UH seminar, 29 August 2001 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

  32. UH seminar, 29 August 2001 El Niño heating zonal-wave coupling stationary wave response to ENSO-forced Ua T850 in winter NCEP composite for El Niño 95% significance LBM response to Q idealized heating LBM response to Q+ZW

  33. UH seminar, 29 August 2001 ‘tropical-belt’ teleconnection upslope cooling (downslope warming) due to orographic forcing(cf. Hoskins & Karoly 1981)

  34. UH seminar, 29 August 2001 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,DMI) = -0.62 r(TAM,IMR) = -0.50

  35. UH seminar, 29 August 2001 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?

  36. UH seminar, 29 August 2001 convection associated with ENSO/monsoon Composite OLR anomaly in summer, following Kawamura (1998) weak monsoon/warm event: 1979, 1983, 1987, 1991, 1992, 1993 strong monsoon/cold event : 1981, 1984, 1985, 1988, 1989, 1990

  37. UH seminar, 29 August 2001 precursors for weak summer monsoon observed composite in May OLR anomaly T300 & V850anomalies

  38. UH seminar, 29 August 2001 CCSR/NIES AGCM simulated circulation anomalies in May 10-member ensemble difference for El Niño run Vertically averaged Q T300 & V850response

  39. UH seminar, 29 August 2001 CCSR/NIES AGCM simulated monsoon precursor in May

  40. UH seminar, 29 August 2001 role of Ua in forcing the continental cooling LBM at day 25 T300 & V850 response to the AGCM heating Indian Ocean heating removed Indian Ocean heating removed & zonal mean response damped

  41. UH seminar, 29 August 2001 cooling over the Himalayan upslope temperature longitude-pressure section along 30N in May NCEP composite vertical phase tilt upslope cooling for long wave (K<Ks) CCSR AGCM LBM (day 25)

  42. UH seminar, 29 August 2001 p1 p0 l C C cooling over the Himalayan upslope w<0

  43. UH seminar, 29 August 2001 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 anomalous 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?

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