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$BBj!!L\!'(BA role of SST over the Indian Ocean in influencing the intraseasonal variability of the Indian summer monsoon

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A role of SST over the Indian Ocean in influencing the intraseasonal variability of the Indian summer monsoon (Roxy Mathew) $BH/I=MW;](B :

 
    Intraseasonal variability (ISV, 10-60 days) of sea surface temperature
 (SST) over the north Indian Ocean and its influence on precipitation
 variability over the Indian subcontinent are examined using satellite
 data for the years 1998 to 2002. Western Ghats (WG) in the southwest
 and the Ganges-Mahanadi Basin (GB) in the northeast of Indian
 subcontinent are observed to be the regions of maximum precipitation
 with large standard deviations. Active (break) phases of precipitation
 occur in these regions by the northward propagation of positive
 (negative) precipitation anomalies over the Arabian Sea and the Bay of
 Bengal. Latitude-time plots during the active phase of the WG region
 shows that the positive SST anomalies over the Arabian Sea formed by
 suppressed surface latent heat flux and downward shortwave radiation
 flux anomalies lead the positive precipitation anomalies. Surface air
 temperature anomalies slightly follow the SST anomalies and then
 destabilize the lower atmosphere between 1000hPa and 700hPa. These
 results indicate that, in the northward propagating dynamical surface
 convergence, underlying SST anomalies tend to form a favorable
 condition for convective activity and may sustain enhanced
 precipitation over the convergence region, indicative of an
 ocean-to-atmosphere effect. This results in enhanced precipitation
 anomalies over the WG region that move further northward and merge with
 the northward propagating precipitation anomalies from the Bay of
 Bengal, enhancing the active phase of the GB region. 

    The ocean-to-atmosphere effect observed above is examined using the
 recently available reanalysis dataset Japanese 25 reanalysis (JRA-25)
 and two widely used reanalysis, ECMWF reanalysis (ERA-40) and National
 Centers for Environmental Prediction reanalysis (NCEP II) for a longer
 time span, from 1979 to 2001. JRA-25 is found to portray the ISV of
 precipitation over the Indian subcontinent and the ocean-to-atmosphere
 effect over the Indian Ocean. The interannual variability (IAV) of the
 ISV of the Indian summer monsoon is investigated using JRA-25. The IAV
 of ISV of positive SST anomalies over Arabian Sea is found to be
 positively correlated with positive precipitation anomalies with a
 significant correlation of 0.65. Coherent northward propagation of SST,
 $B&$&H(Be, cloud water content and precipitation anomalies are observed
 over the Arabian Sea when the monsoon is strong over the WG
 region. However, coherent northward propagation of the anomalies is
 absent when the monsoon is weak over the WG region. The study shows
 that during strong (weak) monsoon years, the northeasterlies are
 stronger (weaker) over the Arabian Sea due to the weakening
 (strengthening) of the branches of a southwesterly low level jet (LLJ)
 at 850 hPa. This result in strong positive SST anomalies over most of
 the Arabian Sea during a strong monsoon year and weak positive SST
 anomalies isolated to the east of Arabian Sea during a weak monsoon
 year. These contrasting anomalous characteristics results in strong
 (weak) monsoon years over the WG region on interannual timescales.
  
    The present study suggests that positive SST anomalies lead to
 destabilization of the lower atmosphere enhancing the northward
 propagating precipitation anomalies over the Indian Ocean. Stronger
 positive SST anomalies with a wider zonal distribution over Arabian Sea
 is observed to result in a coherent ocean-to-atmosphere effect and a
 strong monsoon over WG region while reverse is the case for a weak
 monsoon. 

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 $BJ?MN$NCfAX$O>:29!&MOB8;@AG8:>/$N82Cx$J%H%l%s%I$r<($7$F$$$k(B(Nakanowatari et 
 al., 2007)$B!#(B 
  
 --$BB@J?MN$H$N3$?e8r49(B$B!'CfAX!J(B500-750m$B!K$G%I%j%U%H$7$F$$$?%U%m!<%H$O$[$H$s$I$,(B 
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$BO"Mm@h(B

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mail-to:toyota$B!w(Blowtem.hokudai.ac.jp / Tel: 011-706-7431