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( a )
( b)
Fig. 14 a HadISST of decade (2000-2009) minus (1950-1959) showing the rapid warming of the
equatorial Indian Ocean b the difference in OLR of (2003-2009) minus (1979-1985) in Watts per square
metre
and mid-latitude upper tropospheric westerly winds interacted with each other leading to
large inter-annual variability of ISMR. Convective heat source anomalies had large east-
west oscillations inter-annually between Indian and west Pacific oceans—see Fig. 12 .
These heat sources are hypothesised to generate large amplitude wave number-6 Rossby
waves in the mid-latitude westerlies which had opposite spatial phase in years of excess
and deficient ISMR as shown by Fig. 13 which gives the correlation between ISMR and
the meridional wind of 200 hPa level of June to September using data of the period
1961-1990. This large amplitude Rossby wave was named Asia Pacific Wave by Joseph
and Srinivasan ( 1999 ). The role of this Rossby wave in monsoon variability has been
studied by Ding and Wang ( 2005 ).
Analysis of SST time series showed that during DRY (WET) epochs, SST anomalies
were negative (positive) over areas in Pacific and Atlantic oceans between latitudes 30N
and 60N and the tropics to high latitude SST gradients over these oceans were large
(small). The SST variation in the Pacific Ocean (Atlantic Ocean) is the well-known Pacific
decadal oscillation—PDO (Atlantic multidecadal oscillation—AMDO). Decadal oscilla-
tions in ISMR and PDO/AMDO had about the same period and temporal phase. Can the
large SST gradient phase of PDO/AMDO induce westerly intrusions over south Asia and
create large inter-annual variability of ISMR? Modelling studies are needed.
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