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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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