1. Multiscale processes in the tropics April 27- May 1, 2009, Banff Role of mean state and local air-sea interaction on the propagation of intraseasonal oscillations R. S. Ajayamohan Canadian Center for Climate Modelling and Analysis, Victoria, Canada Collaborators: H. Annamalai, W. J. Merryfield, J. J. Luo, J. Hafner, T. Yamagata

2. Multiscale processes in the tropics April 27- May 1, 2009, Banff Outline  Brief Introduction  Boreal summer intraseasonal oscillations (BSISO)  Role of mean-state and local air-sea interaction on BSISO propagation  Partial coupling experiments  Few Results; How crucial is local Air-Sea interaction in the simulation of intraseasonal oscillations in a CGCM ?  Summary

3. Multiscale processes in the tropics April 27- May 1, 2009, Banff Tropical intraseasonal variability (20-90 days) Summer Winter Rainfall standard deviation (mm/day)

4. Multiscale processes in the tropics April 27- May 1, 2009, Banff Motivation  Relative strengths of slowly varying boundary forcing and northward propagating MJO (Boreal Summer IntraSeasonal Oscillations ) determine the seasonal mean monsoon. [ E.g. Goswami and Ajayamohan (2001), Krishnamurthy and Shukla, 2007 ]  Recent studies highlights the importance of coupled evolution of SST, circulation and precipitation in the Indian Ocean in simulating the correct phase and amplitude of BSISO. [ E.g. Woolnough et al. (2000), Sengupta et al. (2001), Fu. et al. 2002;2003, Waliser et al 2004, Jiang. et al. 2004 ]  Large-scale modes of variability (ENSO, IOD) influences intraseasonal variability. [ Ajayamohan et al., 2008, 2009 ]

5. Multiscale processes in the tropics April 27- May 1, 2009, Banff ISO Characteristics: Evolution of BSISO. Note the clear northeastward propagation of precipitation anomalies. Ajayamohan and Goswami, 2007, JAS

6. Multiscale processes in the tropics April 27- May 1, 2009, Banff Why convection moves poleward ?  Several theories and hypothesis in the literature to explain poleward propagation. [ Review articles in Lau and Waliser, Wang and references therein ]  Cyclonic vorticity at low-levels and associated boundary layer convergence must be maximum north of convection maximum to initiate poleward propagation of BSISO.  Summer mean flow and mean boundary layer humidity is the key factor.  Large easterly vertical shear seen over monsoon region is an important factor, north of equator.  Near the equator, asymmetric specific humidity contributes to the northward shift of the convection.  Intraseasonal variation of SST. Warm (cool) SST ahead of enhanced (suppressed) convection.

7. Multiscale processes in the tropics April 27- May 1, 2009, Banff A schematic representation of the evolution and northward propagation of the meridional circulation associated with the 30-60 day mode in the meridional plane. The thin arrows indicate anomalous Hadley circulation. The thick vertical arrow indicates the location of the center of the boundary layer moisture convergence, while the thick horizontal arrow indicates the direction of poleward motion of the cloud band. The thin solid (dotted) line indicates the phase of the relative vorticity at 850 hPa (divergence at 925 hPa) with positive (negative) phase being above (below) the base line. The location of clear sky conditions is shown by the sun-like symbol. Why Convection Moves Northward ? A simple model Warm SST and Convection over EIO, intensifies TCZ. Ascending motion over EIO and descending motion and clear sky over MT. Cyclonic ζ and associated BLMC is maximum north of max: convection. Convection moves northward. After 10 days, convection reaches ~10N. Both MT and EIO under subsidence and clear sky. BLMC north of convection Active monsoon, convection over MT. Clear sky over EIO. Anticyclonic ζ and subsidence over EIO. BLMC north of convection. Convection moves to foothills of Himalayas. Clear conditions over EIO also moves northward. Decrease in subsidence, continued clear sky conditions, raises SST as net heat flux at surface becomes positive, causing convection to break-out. Convection builds up to become maximum in another 10 days. From Lau & Waliser, ISO Book

8. Multiscale processes in the tropics April 27- May 1, 2009, Banff Role of local Air-Sea Interaction on BSISO propagation Recent studies emphasize the crucial role of air-sea interactions in defining the observed phase structure of BSISO. Warm (cool) SST ahead of enhanced (suppressed) convection with a time lag of 7- 10 days. Positive SST anomaly can account for enhanced moisture perturbation through enhancing evaporation and result in BLMC north of active phase of convection. SEIO is a very important region where BSISO amplification and re-initiation takes place [Fu and Wang (2004), Wang et al. (2006)]

9. Multiscale processes in the tropics April 27- May 1, 2009, Banff CMAP Rainfall & Reynolds SSTA, Ajayamohan et al., 2008 SST leads precipitation TRMM Rainfall & TMI SSTA, Wang et al., 2006 SST leads 

10. Multiscale processes in the tropics April 27- May 1, 2009, Banff Incoherent propagation of BSISO precipitation during positive IOD years  CMAP Intraseasonal Variance during contrasting IOD years Role large scale SST mode of variability on BSISO propagation Ajayamohan et al., 2008, 2009

11. Multiscale processes in the tropics April 27- May 1, 2009, Banff 大気海洋結合循環モデル（ SINTEX-F1 CGCM ） Every 2 hrs T106L19 2.2 OCEAN: OPA8.2 ORCAR2 Grid 2 0 X1.5 0 Eq-0.5 Level 31 Earth Simulator Non-flux adjustment 5 ATMOSPHERE: ECHAM4 T106 L19 EU-Japan Collaboration （日欧協力） 海洋 大気 カップラー Coupler OASIS 2.4 T106L19 2  x 0.5  2 , L31 No flux correction, no sea ice model

12. Multiscale processes in the tropics April 27- May 1, 2009, Banff SINTEX-F1 JJAS Climatology and BSISO Variance CGCMsNo:CGCMsNo: gfdl_cm2_12miroc3_2_medres14 ingv_echam43inmcm3_015 mpi_echam54miroc3_2_hires16 mri_cgcm2_3_2a5ncar_pcm117 iap_fgoals1_0_g6csiro_mk3_518 gfdl_cm2_07csiro_mk3_019 cccma_cgcm3_t478ncar_ccsm3_020 cccma_cgcm3_1_t639ipsl_cm421 miub_echo_g10giss_model_e_r22 cnrm_cm311giss_model_e_h23 giss_aom12bcc_cm124 bccr_bcm213SINTEX25

13. Multiscale processes in the tropics April 27- May 1, 2009, Banff Regressed precipitation wrt to a base time series at EIO.

14. Multiscale processes in the tropics April 27- May 1, 2009, Banff Regressed SSTA wrt to a base time series at EIO.

15. Multiscale processes in the tropics April 27- May 1, 2009, Banff Simulation of BSISO Characteristics by SINTEX-F1 Regressed filtered anomalies of precipitation (mm.day −1 ) averaged over the domain mentioned above. Regression is calculated with respect to a base region 70-90E;12-22N. SINTEX-F1 CMAP

16. Multiscale processes in the tropics April 27- May 1, 2009, Banff JJAS SST anomalies at southeast IO  Summer monsoon is punctuated by vigorous intraseasonal oscillations in the form of active and break spells. These oscillations influence the seasonal mean monsoon.  Influence of IOD on poleward propagation of boreal summer intraseasonal oscillations (BSISO) is studied using observations and a 220- year simulation of a coupled ocean-atmospheric model.

17. Multiscale processes in the tropics April 27- May 1, 2009, Banff Regressed filtered anomalies of precipitation (mm.day −1 ) averaged over 70-95E. Positive IOD years are associated with disorganized or incoherent poleward propagation. CMAP SINTEX-F1

18. Multiscale processes in the tropics April 27- May 1, 2009, Banff Shading: JJAS SSTA Vectors: Divergent component of vertically integrated (1000hPa to 300 hPa) moisture transport anomalies. Mean-State Changes

19. Multiscale processes in the tropics April 27- May 1, 2009, Banff + ve IOD yrs - ve IOD yrs All yrs SINTEX-F1 SPH averaged over 70-95E U 850- U 200 averaged over 40-95E Changes in Mean- State

20. Multiscale processes in the tropics April 27- May 1, 2009, Banff Arrows : convection Contours : vorticity BLMC is north of maximum convection in all phases leading to coherent propagation.

21. Multiscale processes in the tropics April 27- May 1, 2009, Banff Arrows : convection Contours : vorticity BLMC is not always north of maximum convection leading to incoherent propagation.

22. Multiscale processes in the tropics April 27- May 1, 2009, Banff SST-Precipitation Lead-Lag Correlations for contrasting IOD Years Observation SINTEX-F1

23. Multiscale processes in the tropics April 27- May 1, 2009, Banff Partial Coupling: atmosphere sees specified SSTs instead of interactive SSTs in specific regions In these experiments the specified SSTs consist of model climatological SSTs obtained from a fully coupled control run (  SSTA = 0 ) How crucial is the SST-Precipitation lead relationship in simulating BSISO propagation in this CGCM?

24. Multiscale processes in the tropics April 27- May 1, 2009, Banff Contours: SST Shaded: PRCP

25. Multiscale processes in the tropics April 27- May 1, 2009, Banff Contours: 850hPa Divergence Shaded: PRCP

26. Multiscale processes in the tropics April 27- May 1, 2009, Banff

27. Multiscale processes in the tropics April 27- May 1, 2009, Banff Conclusions Both mean state and local air-sea interaction seems to play a role in BSISO propagation enabling boundary layer convergence to be ahead of convection. In the sensitivity experiments, preliminary results suggest that local air-sea interaction provides a modest amplification of BSISO.

28. Multiscale processes in the tropics April 27- May 1, 2009, Banff Regressed filtered precipitation anomalies averaged over 70-95E as a function of latitude and time lag from a 200 year simulation of a coupled ocean-atmosphere model. [SINTEX-F1] Ajayamohan, Rao, Luo and Yamagata, 2008

29. Multiscale processes in the tropics April 27- May 1, 2009, Banff SINTEX-F1 Negative IOD Years Positive IOD Years

30. Multiscale processes in the tropics April 27- May 1, 2009, Banff IOD is defined as a dipole mode in SST anomalies in Indian Ocean coupled to zonal winds and convection. [ Saji et al. (1999) ; Yamagata et al. (2004 Review) ] Coupled ocean-atmosphere Phenomenon with cool (warm) SST anomalies in southeastern IO with warm (cool) SST anomalies in western IO. Impact on seasonal and interannual climate variations.

31. Multiscale processes in the tropics April 27- May 1, 2009, Banff Negative IOD Years Positive IOD Years

32. Multiscale processes in the tropics April 27- May 1, 2009, Banff At any time, cyclonic (anticyclonic) vorticity at 850hPa is to the north of negative (positive) precipitation anomalies. The cyclonic (anticyclonic) vorticity at 850hPa is associated with convergence (divergence) of moisture in the boundary layer. The atmospheric circulation driven by the diabatic heating associated with the zonally oriented cloud band in the presence of background mean flow with easterly vertical shear produces a cyclonic vorticity with a maximum about 3 o N of the center of the cloud band. Cyclonic vorticity drives frictional convergence in the planetary boundary layer and leads to higher moisture convergence north of the cloud band. Meridional gradient of the mean boundary layer moisture also helps in making moisture convergence larger to the north of the cloud band. This leads to convection center to move northward. Why convection moves northward ?

33. Multiscale processes in the tropics April 27- May 1, 2009, Banff

34. Multiscale processes in the tropics April 27- May 1, 2009, Banff Composite wavelet spectrum of precipitation anomalies averaged over 70-95E;10S-Eq (SINTEX-F1)