Contrasting Summer Monsoon Cold Pools South of Indian Peninsula Presented at

1. Contrasting Summer Monsoon Cold Pools South of Indian Peninsula Presented at ROMS/TOMS Asia-Pacific Workshop-2009, Sydney Institute of Marine Sciences, Sydney, Australia on 31st March 2009 By Francis P. A. and M. Ravichandran MOG, INCOIS, Hyderabad

2. What is Summer Monsoon Cold Pool? Average TMI SST for July-August 2002 Summer Monsoon Cold Pool The mechanism proposed for the formation of the SMC is the coastal upwelling due to strong monsoon wind. ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

3. TMI SST Patterns for July/August 2002 and 2003 Intensity of SMC varies both in interannual and intraseasonal time scales ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

4. TMI SST Difference Patterns (2002-2003) JULY AUGUST The variation in the intensity of SMC is attributed to the variation in the strength of upwelling due to the variation of forcing ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

5. Difference in the wind filed (July 2002-July2003) This difference is too small to produce such a difference in the cooling ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

6. OLR Anomaly patterns for July 2002 and July 2003 July 2003 July 2002 • The convection pattern in July 2002 and July 2003 are opposite over the equatorial Indian Ocean • Such a scenario was present in may occasions, and it is known as Equatorial Indian Ocean Oscillation (EQUINOO) [S. Gadgil, P. N. Vinayachandran, P. A. Francis and S. Gadgil, GRL, 2004] ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

7. Hypothesis • The signature of such an oscillation is seen in many atmospheric and oceanic parameters • When it is coupled with the ocean, then it is referred as Indian Ocean Dipole mode[Saji et al 1999, Webster et al 1999] • The differential heating of the atmospheric column due to anomalous convection associated with the EQUINOO leads to anomalous easterlies/westerlies over the equator Difference in the wind filed over the tropical Indian Ocean (July 2003-July2002) ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

8. Hypothesis • The zonal wind along the equatorial Indian Ocean was dramatically different in the boreal summer months of 2002 and 2003 Zonal wind averaged over 70-90E, 2.5oS-2.5oN ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

9. Strong easterlies along the equatorial Indian Ocean trigger upwelling Kelvin waves, which propagate to the east and downwelling Rossby waves which propagate to the west Downwelling upwelling Downwelling easterlies EQ upwelling EQ Downwelling westerlies upwelling The off-equatorial maxima of downwelling (upwelling) can deepen (shoalen) the isothermal layers and decrease (increase) the efficiency of surface cooling ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

10. We show, with the help of ROMS simulations, that it is this downwelling that prevented the summer monsoon cooling (south of peninsnular India) in July 2003. • Further, by August 2003, the zonal wind along the equator became westerly. • This lead to the triggering of downwelling Kelvin waves and upwelling Rossby waves in the equatorial Indian Ocean which in turn lead to intensification of SMC • Hence, in addition to the local forcing, the interannual and intraseasonal variation in the intensity of summer monsoon cold pool is related to the processes over the equatorial Indian Ocean. ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

11. A supporting evidence of this hypothesis has come from the difference in SSH patterns between July 2002 and July 2003 Difference in SSH anomaly (TOPEX) patterns (July2002- July2003) ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

12. This hypothesis needs to be tested with a “GOOD” Ocean General Circulation Model, which can simulate not only the coastal features, but also the basin scale features • It is essential to make sure that the equatorial wind forcing influence the subsurface thermal structure in the SMC region • We use ROMS for confirming our hypothesis ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

13. Model set up Domain: 30-160E, 30S-30N Horizontal resolution: 0.25 x 0.25 degree Vertical levels :40 Theta_s :10 Theta_b :0 Hc :100m Spin-up : 10yrs Interannual : from 2000 onwards • Major CPP options chosen: • Bulk parameterization • EMINUSP computed • Prescribed net long wave radiation • Eastern and southern boundaries are relaxed to climatology • Mellor-Yamada mixing scheme • Salinity correction with a time scale of 15 days ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

14. Forced with Wind fields : QSCAT Air temperature : NCEP-Reanalysis Relative humidity : NCEP-Reanalysis Precipitation : CMAP Net longwave Radiation : NCEP-Reanalysis Net shortwave Radiation : NCEP-Reanalysis Sea level pressure : NCEP-Reanalysis All forcing fields except precipitation are daily mean precipitation field is monthly mean. ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

15. Validation Temperature profiles at 90oE, 1.5oS compared with PMEL-RAMA buoy profiles ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

16. Validation Temperature Zonal Current Timeseries of Temperature and current at 1.5m depth at 90oE, 1.5oS ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

17. Simulated summer monsoon cold pools in July/August 2002/2003 ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

18. Difference Patterns of surface temperature (2002-2003) for July/August ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

19. Difference in the dynamic height (zeta) anomaly between Jul2002 and Jul2003 ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

20. Difference in vertical velocity (2002-2003) at 65m in July and August ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

21. 8 20 30 43 60 90 150 Evolution of the difference in the vertical structure of temperature in the cold pool region (4-8oN, 75-83oE) July August ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

22. Conclusions: • The observed intraseasonal/interannual variation in the intensity of the summer monsoon cold pool is closely linked to the positive/negative phase of the equatorial Indian Ocean oscillation (EQUINOO) • The easterlies along the central equatorial Indian Ocean (CEIO) during a positive EQUINOO trigger upwelling Kelvin waves to the east and downwelling Rossby waves to the west. The off-equatorial maxima of downwelling prohibit effective surface cooling • It is essential to consider the basin wide processes in order understand the coastal processes in the Indian Ocean domain. Hence any GCM for the coastal forecasting system in the Indian Ocean should have a domain, at least covering the entire basin. ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

23. Thank You

24. Important References: • Rao et al 2006, 2007(GRL) • Shankar et al 2008, (J. Earth Sys. Sci.) • Gadgil et al 2003, 2007 (Cur. Sci.) • Gadgil et al 2004 (GRL) • Saji et al 1999, (Nature) • Webster et al 1999, (Nature) ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009