1. Indian Ocean Warm Pool by Sindu Raj Parampil Centre for Atmospheric and Oceanic Sciences Indian Institute of Science Bangalore

2. Outline of talk : § Introduction to Warm Pool § Comparisons between Western Pacific Warm Pool and Indian Ocean Warm Pool § Evolution of the Indian Ocean Warm Pool § Mechanism of formation of warm pool in the South Eastern Arabian Sea § Summary

3. Warm Pool : Sea Surface Temperature > 28°C Two regions which satisfy this criterion throughout the year are : a. equatorial western Pacific Ocean Western Pacific Warm Pool (WPWP) b. Indian Ocean Indian Ocean Warm Pool (IOWP) Warm pools have SST > threshold SST for organised convection Onset Vortices of SW Monsoon generally form over the warm pool over the south eastern Arabian Sea

4. WPWP is warmer (except in May) and larger than the IOWP

5. Area above 28°C throughout the year : WPWP ~ 10 * 10^6 km² IOWP ~ 2.8 * 10^6 km² Seasonal changes in area of the warm pool : WPWP August : 33 * 10^6 km² (maximum area) March : 24 * 10^6 km² (minimum area) IOWP February : 14 * 10^6 km² April : 24 * 10^6 km² (maximum area) September : 8 * 10^6 km² (minimum area) (Vinaychandran and Shetye, 1990)

6. Indian Ocean (solid lines) Arabian Sea; Eastern Eq. Indian Ocean & Bay of Bengal Pacific Ocean (dashed lines) Eastern Eq. Pacific Seasonal fluctuations are much greater in the IOWP !

7. comparisons show a. Seasonal cycle stronger in the IOWP than in WPWP b. Arabian Sea, Western Eq. Indian Ocean and southern Bay of Bengal have 2 warming and cooling phases annually Warming -> Mar – May (Spring) Sep – Nov (Autumn) Cooling -> Jul – Sep (SW monsoon) Dec – Jan (NE monsoon) Spring Warming > Autumn Warming South Eastern Arabian Sea (SEAS) warmest region of world ocean

8. Evolution of the IOWP spring warming -> Feb to May Core of the IOWP is in the South Western Arabian Sea (SEAS) ~ 8.3°N 72°E SST of the SEAS > 30°C by last week of May (week preceding the SW Monsoon onset) core in SEAS –> 'mini-warm pool' cooling –> southern Indian Ocean – May western Eq. Indian Ocean – Jun Arabian Sea – Jul to Sep

9. MoMo Monthly Mean SST of the Indian Ocean

10. Mechanisms leading to formation of warm pool in SEAS Wave activity plays an important role: Collapse of SW monsoon triggers downwelling coastal Kelvin waves (KW) in BoB KW force the Eastern Indian Coastal Current (EICC) and the Western Indian Coastal Current (WICC) bringing low saline BoB water to the SEAS during Dec - Feb KW along the west coast radiate downwelling Rossby Waves (RW), which spreads the low-saline water to central Arabian Sea Salinity during NE monsoon : Arabian Sea > 34.5 Bay of Bengal < 34.0

11. Monthly Mean Salinity & Surface Currents

13. Conditions favorable for increasing SST a. Downwelling -> deepens thermocline b. Near-surface low salinity layer -> stable stratification a. & b. deep and stable upper layer c. By Jan insolation increases in Northern Hemisphere - clear skies persist d. Wind field, consisting of NE Trade Winds weaken from Jan – Mar; weak clockwise circulation by Apr Radiative inputs > heat losses (by latent / sensible heat flux) low winds & stable stratification prevent vertical mixing; resulting in warming of near surface layers

14. (Shenoi et al., 1999)

15. Summary The IOWP shows prominent seasonal fluctuations in its area of extent and SST The IOWP builds up from Feb to May and collapses after the onset of the South West Monsoon in the SEAS The mini-warm pool in the SEAS with SST > 30°C, is the warmest region of world ocean during April and May (Joseph., 1990) The evolution of IOWP and its effects on onset of SW Monsoon is a classic example of coupled ocean-atmosphere system : ocean dynamics ⇨ ocean thermodynamics ⇧ ⇩ atmospheric dynamics ⇦ atmospheric thermodynamics

16. List of References : Gadgil,S, N.V. Joshi and P.V. Joseph, Ocean – Atmosphere coupling over monsoon regions, Nature, 312, 141-143, 1984 Joseph P.V., Warm Pool in the Indian Ocean and Monsoon Onset, Tropical Ocean-Atmos. News Let., 53, 1-5, 1990 Rao,R.R., and R. Sivakumar, On the possible mechanisms of the evolution of the mini-warm pool during the pre-summer monsoon season and the genesis of onset vortex in the south-eastern Arabian Sea, Q. J. R. Meteorol. Soc., 125, 787 – 809, 1999 Shenoi,S.S.C., D. Shankar and S.R. Shetye, On the Sea Surface Temperature high in the Lakshadweep Sea before the onset of southwest monsoon, J. Geophys. Res., 104, 15703 – 15712, 1999

17. Shenoi, SSC., D. Shankar, S.R. Shetye., Remote Forcing Annihilates Barrier Layer in the south eastern Arabian Sea, 31, 2004 Sengupta,D., P.K. Ray, G.S. Bhat, Spring Warming of the Eastern Arabian Sea and Bay of Bengal from buoy data, Geophys. Res. Lett., 29, 2401-2404, 2002 Vinayachandran,P.N. and S.R. Shetye, The warm pool in the Indian Ocean, Proc. Ind. Acad. Sci., 100, 165-175, 1990

18. Thank You