Extratropical Rossby Wave breaking: a cause for Indian summer monsoon failure

1. ExtratropicalRossby Wave breaking: a cause for Indian summer monsoon failure Mihir Kumar Dash*, Dhrubajyoti Samanta* and P. C. Pandey** * Indian Institute of Technology, Kharagpur * Indian Institute of Technology, Bhubaneswar

2. Indian Institute of Technology, Kharagpur Outline • Introduction • Data and Methodology • Rossby wave breaking and circulation over the West Asia • Convective inhibition over the Indian subcontinent • Summary

3. Indian Institute of Technology, Kharagpur Introduction • Westerly Jet or wave guide • High Pressure regions Major 200-hPa circulation anomalies accompanied with climatological zonal wind (shading >15m) from June to September . The dotted line indicates the zero zonal wind line in 200-hPa Adopted from Ding Q. 2008

4. Indian Institute of Technology, Kharagpur • Low over WCI • Blocking high over south eastern china Krishnan et al. 2009

5. Indian Institute of Technology, Kharagpur Western Asia Dry Air Incursion 16th July 2002 18 June 2009 4th August 2009 30th August 2001 Ten-day back trajectories from central India. Dry spell for trajectories terminating at 850 (green) and 700 (red) hPa (from Krishnamurti et al, 2010)

6. Indian Institute of Technology, Kharagpur Rossby Wave Breaking Overturning of PV event at 350-K PV Anticyclonic (b) Cyclonic. contoured every 2 potential vorticityunits. From Strong and Magnusdottir, 2009

7. Data and Methodology • Daily wind fields (u, v, w) and temperature fields at eleven different pressure levels at a resolution of 2.5° × 2.5° from NCEP-NCAR reanalysis • Geopotential height from NCEP –NCAR reanalysis • Precipitation from TRMM 𝛁T + 𝛚

8. Indian Institute of Technology, Kharagpur Climatology of Isentropic Potential Vorticity and horizontal wind at 350K Level

9. Indian Institute of Technology, Kharagpur Sample cases of Rossby Wave Breaking and intensification of circulation over the western India

11. Potential Vorticity and Wind Field anomalies at 350K Over Asia and Africa

12. Geopotential Height and Temperature anomalies at 500hPa Over Asia and Africa

13. Indian Institute of Technology, Kharagpur

14. Meridional Cross-section of Specific humidity over Central India

15. Thermodynamics of the upper-troposphere

16. Thermodynamics of the upper-troposphere

17. Hadley Circulation during Breaking condition

18. Summary Descending of dry air from western and central Asia forming a inversion layer over Indian sub-continent Suppression of convection over the Indian –subcontinent and Drought Rossby Wave Breaking over the Western/Western Central Asia Formation of blocking high over Western Central Asia Modification of Hadley circulation over the Indian Subcontinent

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