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Zonal Flow Variability Linking the ENSO/Monsoon Systems

IAMAS in Innsbruck 07/17/01. Zonal Flow Variability Linking the ENSO/Monsoon Systems. M. Watanabe 1 , F.-F. Jin 2 , and M. Kimoto 1 1: CCSR, University of Tokyo, 2: Dpt. Meteorology, University of Hawaii. Step back to the atmospheric response to El Niño

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Zonal Flow Variability Linking the ENSO/Monsoon Systems

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  1. IAMAS in Innsbruck 07/17/01 Zonal Flow Variability Linking the ENSO/Monsoon Systems M. Watanabe1, F.-F. Jin2, and M. Kimoto1 1: CCSR, University of Tokyo, 2: Dpt. Meteorology, University of Hawaii • Step back to the atmospheric response to El Niño • attempt to interpret the zonal flow variability associated with ENSO • ENSO influence to the Asian summer monsoon • Why monsoon tends to be weak following El Niño winter ? • southward shift in spring ITCZ over the western Pacific • excessive snow and soil moisture over Tibetan plateau • air-sea interaction in the Indian Ocean • enhanced northern subtropical westerly

  2. IAMAS in Innsbruck 07/17/01 global ENSO teleconnection Regression of Z500/y300 on monthly Nino3 SSTA, 1949-99

  3. IAMAS in Innsbruck 07/17/01 principal mode iny Tropical Axisymmetric Mode (TAM) EOF1(23%) for monthly y300, 1949-99 PC1 of y300 Nino3 SSTA Corr.=0.60

  4. IAMAS in Innsbruck 07/17/01 structure of TAM Regression of monthly NCEP anomalies on the y300 PC1

  5. IAMAS in Innsbruck 07/17/01 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

  6. 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

  7. IAMAS in Innsbruck 07/17/01 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)

  8. IAMAS in Innsbruck 07/17/01 ENSO-forced zonal-mean flow idealized heating Q DJF

  9. IAMAS in Innsbruck 07/17/01 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

  10. IAMAS in Innsbruck 07/17/01 ENSO-TAM-Monsoon: statistical connection Lagged correlation 3mo-avg. TAM index vs. Niño3 or IMR Composite monsoon shear index El Niño/normal TAM yrs El Niño/positive TAM yrs

  11. IAMAS in Innsbruck 07/17/01 Possibility considered: • El Niño forces TAM, monsoon forces TAM, but the TAM does nothing for the ENSO/monsoon coupling • El Niño forces TAM in winter, which affects other components of climate having longer memory that can bring weaker monsoon • El Niño forces TAM in spring, which contributes to precondition a weak summer monsoon

  12. [m/s] [m/s] [K] Composite El Nino/weak monsoon Linear response to Q+ZW Linear response to Q IAMAS in Innsbruck 07/17/01 T500 and V850, March-May ENSO-TAM-Monsoon: possible mechanism

  13. IAMAS in Innsbruck 07/17/01 T300 and V850,climatology in June 10-member ensemble of the 9-mo run from Jan. 1 ENSO response = <N> - <C> Ensemble experiment by the CCSR/NIES AGCM NCEP reanalysis AGCM <C> run

  14. IAMAS in Innsbruck 07/17/01 Vertically averaged Q T300 & V850 response Monsoon response to El Niño in the AGCM <N> - <C> , May

  15. A) AGCM Q C) b + 1dy damping for ZM comp. B) no Q west of 100E IAMAS in Innsbruck 07/17/01 T300 & V850 linear modelresponse Role of the zonal flow variability

  16. IAMAS in Innsbruck 07/17/01 Conclusions • Zonal flow variability and ENSO • dominant zonally uniform structure (tropical-wide westerly) as a part of the global ENSO teleconnection • the El Niño-forced zonal flow interpreted in terms of an excitation of the near-neutral mode (Tropical Axisymmetric Mode) • Role of the zonal flow variability in the ENSO/monsoon • upstream teleconnection induced by the coupling between the zonal flow and topography & clim. stationary waves • subtropical zonal-mean westerly anomaly accompanies a tropospheric cooling over the S.Asia and northeasterly over the N.Indian Ocean, contributing to precondition a weaker monsoon • Future issue • further AGCM experiments • interaction between dynamics and convection

  17. IAMAS in Innsbruck 07/17/01 On the neutrality of the mode Zonal-mean zonal momentum budget close to neutrality

  18. IAMAS in Innsbruck 07/17/01 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

  19. IAMAS in Innsbruck 07/17/01 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

  20. U (40-110E,5-20N) Pr. (65-90E,10-30N) IAMAS in Innsbruck 07/17/01 monsoon response to El Niño in the AGCM NCEP AGCM AGCM

  21. IAMAS in Innsbruck 07/17/01 Monsoon response to El Niño in the AGCM <N> - <C> , May

  22. A) AGCM Q, Pacific IAMAS in Innsbruck 07/17/01 T300 & V850 linear modelresponse Role of the zonal flow variability C) b + ZW coupling B) zonal mean BS

  23. Linear model time integration U300 T300 [m/s] IAMAS in Innsbruck 07/17/01 How El Niño excites zonal flow anomaly? day 15 day 10 day 1 day 5

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