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Francis P.A.,Indian National Center for Ocean Information Services | Gadgil S.,Indian Institute of Science
Journal of Earth System Science | Year: 2013

It is now well known that there is a strong association of the extremes of the Indian summer monsoon rainfall (ISMR) with the El Niño and southern oscillation (ENSO) and the Equatorial Indian Ocean Oscillation (EQUINOO), later being an east-west oscillation in convection anomaly over the equatorial Indian Ocean. So far, the index used for EQUINOO is EQWIN, which is based on the surface zonal wind over the central equatorial Indian Ocean. Since the most important attribute of EQUINOO is the oscillation in convection/precipitation, we believe that the indices based on convection or precipitation would be more appropriate. Continuous and reliable data on outgoing longwave radiation (OLR), and satellite derived precipitation are now available from 1979 onwards. Hence, in this paper, we introduce new indices for EQUINOO, based on the difference in the anomaly of OLR/precipitation between eastern and western parts of the equatorial Indian Ocean. We show that the strong association of extremes of the Indian summer monsoon with ENSO and EQUINOO is also seen when the new indices are used to represent EQUINOO. © Indian Academy of Sciences. Source


Francis P.A.,Indian National Center for Ocean Information Services | Gadgil S.,Indian Institute of Science
Journal of Earth System Science | Year: 2010

The Indian summer monsoon season of 2009 commenced with a massive deficit in all-India rainfall of 48% of the average rainfall in June. The all-India rainfall in July was close to the normal but that in August was deficit by 27%. In this paper, we first focus on June 2009, elucidating the special features and attempting to identify the factors that could have led to the large deficit in rainfall. In June 2009, the phase of the two important modes, viz., El Niño and Southern Oscillation (ENSO) and the equatorial Indian Ocean Oscillation (EQUINOO) was unfavourable. Also, the eastern equatorial Indian Ocean (EEIO) was warmer than in other years and much warmer than the Bay. In almost all the years, the opposite is true, i.e., the Bay is warmer than EEIO in June. It appears that this SST gradient gave an edge to the tropical convergence zone over the eastern equatorial Indian Ocean, in competition with the organized convection over the Bay. Thus, convection was not sustained for more than three or four days over the Bay and no northward propagations occurred. We suggest that the reversal of the sea surface temperature (SST) gradient between the Bay of Bengal and EEIO, played a critical role in the rainfall deficit over the Bay and hence the Indian region. We also suggest that suppression of convection over EEIO in association with the El Niño led to a positive phase of EQUINOO in July and hence revival of the monsoon despite the El Niño. It appears that the transition to a negative phase of EQUINOO in August and the associated large deficit in monsoon rainfall can also be attributed to the El Niño. © Indian Academy of Sciences. Source


Girishkumar M.S.,Indian National Center for Ocean Information Services | Ravichandran M.,Indian National Center for Ocean Information Services | McPhaden M.J.,National Oceanic and Atmospheric Administration
Journal of Geophysical Research: Oceans | Year: 2013

Time series measurements of temperature, salinity and surface meteorological parameters recorded at 8°N, 90°E in the southern central Bay of Bengal (BoB) from a Research Moored Array for African-Asian-Australian Monsoon Analysis and predication (RAMA) buoy are used to document temperature inversions and their influence on the mixed layer heat budget during the winters, defined as October to March, of 2006-2007 (W67) and 2007-2008 (W78). There is a marked difference in the frequency and amplitude of temperature inversion between these two winters, with variations much stronger in W78 compared to W67. The formation of temperature inversions is favored by the existence of thick barrier layers, which are also more prominent in W78 compared to W67. Inversions occur when heating in the barrier layer below the mixed layer by penetrative shortwave radiation is greater than heating of the mixed layer by net surface heat flux and horizontal advection. Our analysis further demonstrates that intraseasonal and year-to-year variability in the frequency and magnitude of temperature inversions during winter have substantial influence on mixed layer temperature through the modulation of vertical heat flux at the base of mixed layer. ©2013. American Geophysical Union. All Rights Reserved. Source


Girishkumar M.S.,Indian National Center for Ocean Information Services | Ravichandran M.,Indian National Center for Ocean Information Services | McPhaden M.J.,National Oceanic and Atmospheric Administration | Rao R.R.,Andhra University
Journal of Geophysical Research: Oceans | Year: 2011

Time series measurements of temperature and salinity recorded at 8N, 90E in the south central Bay of Bengal from a Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction buoy, along with satellite altimetry and scatterometer data, are utilized to describe the seasonal and intraseasonal variability of barrier layer thickness (BLT) during November 2006 to April 2009. The BLT shows strong seasonality with climatological minima during both March-May and August-September and maxima during December-February. Large-amplitude, intraseasonal fluctuations in BLT are observed during September 2007 to May 2008 and during September 2008 to April 2009. The observed intraseasonal variability in BLT is mainly controlled by the vertical movement of isothermal layer depth (ILD) in the presence of a shallow mixed layer. Further, the analysis shows that both ILD and BLT are modulated by vertical stretching of the upper water column associated with westward propagating intraseasonal Rossby waves in the southern bay. These waves are remotely forced by intraseasonal surface winds in the equatorial Indian Ocean. Copyright 2011 by the American Geophysical Union. Source


Shenoi S.S.C.,Indian National Center for Ocean Information Services
Indian Journal of Marine Sciences | Year: 2010

The seasonal cycle of the coastal currents along the Indian coast and their dynamics is well known. Till recently, it was believed that the seasonal signal dominates the coastal currents due the seasonal reversals of monsoonal winds. But the recent studies using satellite based altimeter and in situ measurements using moored instruments provided evidences on the importance of intra and interannual variabilities embedded in them. This article reviews the evidences available from these observations. Source

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