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

2. IAMAS in Innsbruck 07/17/01 global ENSO teleconnection Regression of Z 500 /  300 on monthly Nino3 SSTA, 1949-99

3. IAMAS in Innsbruck 07/17/01 principal mode in  EOF1(23%) for monthly  300, 1949-99 Nino3 SSTA PC1 of  300 Corr.=0.60 Tropical Axisymmetric Mode (TAM)

4. IAMAS in Innsbruck 07/17/01 structure of TAM Regression of monthly NCEP anomalies on the  300 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 ( X a ) and asymmetric ( X a * ) components calculate singular vectors of L

7. IAMAS in Innsbruck 07/17/01 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)

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

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,IMR) = -0.50r(TAM,DMI) = -0.62

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. IAMAS in Innsbruck 07/17/01 ENSO-TAM-Monsoon: possible mechanism Composite El Nino/weak monsoon Linear response to Q Linear response to Q+ZW [m/s] [K] T 500 and V 850, March-May

13. IAMAS in Innsbruck 07/17/01 Ensemble experiment by the CCSR/NIES AGCM 10-member ensemble of the 9-mo run from Jan. 1 ENSO response = - T 300 and V 850,climatology in June NCEP reanalysis AGCM run

14. IAMAS in Innsbruck 07/17/01 Monsoon response to El Niño in the AGCM -, May T 300 & V 850 response Vertically averaged Q

15. IAMAS in Innsbruck 07/17/01 Role of the zonal flow variability C) b + 1dy damping for ZM comp. B) no Q west of 100E A) AGCM Q T 300 & V 850 linear model response

16. 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 IAMAS in Innsbruck 07/17/01

17. 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  300 Coincidence between Ua and  c 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. IAMAS in Innsbruck 07/17/01 monsoon response to El Niño in the AGCM NCEP AGCM U (40-110E,5-20N) Pr. (65-90E,10-30N)

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

22. IAMAS in Innsbruck 07/17/01 Role of the zonal flow variability C) b + ZW coupling B) zonal mean BS A) AGCM Q, Pacific T 300 & V 850 linear model response

23. IAMAS in Innsbruck 07/17/01 How El Niño excites zonal flow anomaly? day 1 Linear model time integration day 5 day 10 day 15 [m/s] T 300 U 300