Indian Ocean Warm Pool

by

Sindu Raj Parampil

Centre for Atmospheric and Oceanic Sciences Indian Institute of Science

Bangalore

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

Warm Pool : Sea Surface Temperature > 28°C

Two regions which satisfy this criterion throughoutthe 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

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

Area above 28°C throughout the year :

WPWP ~ 10 * 10^6 km²IOWP ~ 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)

IOWPFebruary : 14 * 10^6 km²April : 24 * 10^6 km² (maximum area) September : 8 * 10^6 km² (minimum area)

(Vinaychandran and Shetye, 1990)

Indian Ocean (solid lines) Arabian Sea; Eastern Eq. Indian Ocean & Bay of Bengal Pacific Ocean (dashed lines)Eastern Eq. Pacific

Seasonal fluctuations are much greaterin the IOWP !

comparisons show

a. Seasonal cycle stronger in the IOWP than in WPWP

b. Arabian Sea, Western Eq. Indian Ocean and southernBay of Bengal have 2 warming and cooling phasesannually 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

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

Mo

Monthly Mean SST of the Indian Ocean

Mechanisms leading to formation of warm pool in SEAS

Wave activity plays an important role:

Collapse of SW monsoon triggers downwelling coastalKelvin waves (KW) in BoB

KW force the Eastern Indian Coastal Current (EICC) andthe 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

Monthly Mean Salinity & Surface Currents

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

(Shenoi et al., 1999)

Summary

The IOWP shows prominent seasonal fluctuations inits area of extent and SST

The IOWP builds up from Feb to May and collapses afterthe 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

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 MonsoonOnset, Tropical Ocean-Atmos. News Let., 53, 1-5, 1990

Rao,R.R., and R. Sivakumar, On the possible mechanismsof the evolution of the mini-warm pool during the pre-summer monsoon season and the genesis of onset vortexin 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 beforethe onset of southwest monsoon, J. Geophys. Res., 104,15703 – 15712, 1999

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

Thank You