Indian Ocean Warm Pool

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



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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
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Warm Pool Sea Surface Temperature gt 28C
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 gt threshold SST for organised

convection Onset Vortices of SW Monsoon
generally form over the warm pool over the
south eastern Arabian Sea
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WPWP is warmer (except in May)
and larger than the IOWP
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Area above 28C throughout the year WPWP 10
106 km² IOWP 2.8 106 km² Seasonal
changes in area of the warm pool WPWP August
33 106 km² (maximum
area) March 24 106 km²
(minimum area) IOWP February 14 106
km² April 24 106 km²
(maximum area) September 8 106 km²
(minimum area) (Vinaychandran and Shetye,
1990)
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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 !
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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
-gt Mar May (Spring)
Sep Nov (Autumn) Cooling -gt
Jul Sep (SW monsoon)
Dec Jan (NE monsoon)
Spring Warming gt Autumn Warming
South
Eastern Arabian Sea (SEAS) warmest region
of world ocean

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Evolution of the IOWP spring warming -gt Feb to
May Core of the IOWP is in the South Western
Arabian Sea (SEAS) 8.3N 72E SST of the
SEAS gt 30C by last week of May (week preceding
the SW Monsoon onset) core in SEAS gt
'mini-warm pool' cooling gt southern
Indian Ocean May western Eq.
Indian Ocean Jun Arabian Sea
Jul to Sep
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Mo
Monthly Mean SST of the Indian Ocean
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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 gt 34.5 Bay of
Bengal lt 34.0

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Monthly Mean Salinity Surface Currents
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(No Transcript)
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Conditions favorable for increasing SST a.
Downwelling -gt deepens thermocline b.
Near-surface low salinity layer -gt 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 gt heat losses (by latent /
sensible heat flux) low winds stable
stratification prevent vertical mixing
resulting in warming of near surface layers
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(Shenoi et al., 1999)
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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 gt
30C, 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
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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
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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
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Thank You