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Role of mean state and local air-sea interaction on the propagation of intraseasonal oscillations

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. Outline.

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Role of mean state and local air-sea interaction on the propagation of intraseasonal oscillations

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  1. 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. 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. Tropical intraseasonal variability (20-90 days) Summer Winter Rainfall standard deviation (mm/day)

  4. 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. ISO Characteristics:Evolution of BSISO. Note the clear northeastward propagation of precipitation anomalies. Ajayamohan and Goswami, 2007, JAS

  6. 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. 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. 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. From Lau & Waliser, ISO Book

  8. 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. SST leads precipitation TRMM Rainfall & TMI SSTA, Wang et al., 2006 CMAP Rainfall & Reynolds SSTA, Ajayamohan et al., 2008 SST leads 

  10. Role large scale SST mode of variability on BSISO propagation Incoherent propagation of BSISO precipitation during positive IOD years • CMAP Intraseasonal Variance during contrasting IOD years Ajayamohan et al., 2008, 2009

  11. 大気海洋結合循環モデル（ SINTEX-F1 CGCM） EU-Japan Collaboration　（日欧協力） OCEAN: OPA8.2 ORCAR2 Grid 20X1.50 Eq-0.5 Level 31 ATMOSPHERE: ECHAM4 T106 L19 大気 海洋 T106L19 5 Non-flux adjustment T106L19 2 x 0.52, L31 Every 2 hrs Coupler OASIS 2.4 No flux correction, no sea ice model 2.2 カップラー Earth Simulator

  12. SINTEX-F1 JJAS Climatology and BSISO Variance

  13. Regressed precipitation wrt to a base time series at EIO.

  14. Regressed SSTA wrt to a base time series at EIO.

  15. Simulation of BSISO Characteristics by SINTEX-F1 SINTEX-F1 CMAP 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.

  16. 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. CMAP SINTEX-F1 Regressed filtered anomalies of precipitation (mm.day−1) averaged over 70-95E. Positive IOD years are associated with disorganized or incoherent poleward propagation.

  18. Mean-State Changes Shading: JJAS SSTA Vectors: Divergent component of vertically integrated (1000hPa to 300 hPa) moisture transport anomalies.

  19. Changes in Mean- State SINTEX-F1 SPH averaged over 70-95E U850-U200 averaged over 40-95E + ve IOD yrs - ve IOD yrs All yrs

  20. Arrows : convection Contours : vorticity BLMC is north of maximum convection in all phases leading to coherent propagation.

  21. Arrows : convection Contours : vorticity BLMC is not always north of maximum convection leading to incoherent propagation.

  22. SST-Precipitation Lead-Lag Correlations for contrasting IOD Years Observation SINTEX-F1

  23. How crucial is the SST-Precipitation lead relationship in simulating BSISO propagation in this CGCM? • 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)

  24. Contours: SST Shaded: PRCP

  25. Contours: 850hPa Divergence Shaded: PRCP

  26. 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.

  27. 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

  28. SINTEX-F1 Negative IOD Years Positive IOD Years

  29. 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.

  30. Negative IOD Years Positive IOD Years

  31. Why convection moves northward ? • 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 3oN 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.

  32. Composite wavelet spectrum of precipitation anomalies averaged over 70-95E;10S-Eq (SINTEX-F1)

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