Uncertainty in the Identification of Indian Summer Monsoon Onset

Thursday, 18 December 2014
Sahana A S and Subimal Ghosh, Indian Institute of Technology Bombay, Mumbai, India
Variabilities in the onset of Indian Summer Monsoon Rainfall (ISMR) can set an extensive imbalance in the life and economy of India as it has significant influence on the crop planning and agricultural output value. Onset is generally perceived as the beginning of rainfall along the western coast of India and hence many of the onset identification methods are based on precipitation. But any variation in the initiation of rainfall is actually caused by the changes in the large scale circulations and the pressure and temperature gradients. Therefore several indices based on large scale circulations are developed to determine the onset and are found to be resilient to bogus onsets unlike the precipitation based methods. Onset dates determined using indices or criteria based on different physical mechanisms may not be consistent with one another. Therefore, here we attempt to assess the uncertainty associated with the identification of onset dates using different methods and data. We find onset dates using moisture flux based Hydrological Onset and Withdrawal Index (HOWI), zonal wind based Onset Circulation Index (OCI) and tropospheric temperature gradient (∆ TT) based index. We observe that a huge uncertainty is associated with the onset dates identified using the three indices and different reanalysis data sets. The study reveals that the underlying physical mechanisms behind each of the selected indices influence the onset of monsoon to different degrees. We find that while the tropospheric temperature gradient has significant influence on the monsoon circulation, it shows little association with the moisture availability over the Arabian Sea region during the pre monsoon period, which is one of the reasons for the uncertainty in onset identification.