Data and Methodology
The primary data used for the analysis are IMD’s Cyclone eAtlas (IMD 2008,
2011). Time series analysis of frequency of tropical disturbances is carried out based on intensity stratification into (a) CDs, (b) TCs and (c) severe TCs for the pre-monsoon, monsoon and post-monsoon seasons for the NIO as well as for the BOB and the AS separately. Long-term linear trend for each of these categories is determined based on the data of 1901—2010 in the inter-annual and decadal scales and tested for significance using Mann—Kendall trend test. Trends significant at the 95 % confidence level are discussed. In the view of less reliability of the data during the pre-satellite era (Mohapatra et al. 2012b) when observing and monitoring tools were inadequate, trends are also analysed separately using data of the last five decades (1961—2010) of satellite era.
It is of great scientific interest to understand the atmospheric features (vorticity and windshear) associated with the trends in CDs/TCs/severe TCs frequency despite increasing SST and ocean thermal energy. Using the NCEP data and Mann—Kendall test, the trend in the vertical shear of the horizontal wind between 850 and 200 hPa levels and low-level relative vorticity (at 850 hPa level) on monthly and seasonal scales for the period 1951—2010 is also examined. To examine the ENSO influence, correlation coefficients (CC) of SST anomaly over Nino 3.4 (5°N—5°S, 170°W—120° W) region (https://climatedataguide.ucar.edu/climate-data/nino-sst-indices) with frequency of (a) CDs, (b) TCs and (c) severe TCs over the NIO in both antecedent and concurrent modes for pre-monsoon, monsoon and post-monsoon seasons are computed and analysed. The ENSO index is taken from the climate data sets of Climate Prediction Centre, NOAA (www.esrl.noaa.gov/psd/data/climateindices/).
IMD’s monsoon seasonal rainfall series for all-India and various homogeneous sub-regions of India available in IMD website (www.imd.gov.in/) as well as the monthly rainfall data for various meteorological sub-divisions of India available in IITM website (www.tropmet.res.in) are used. The analysis is based on (i) calculation of CC between rainfall over all-India/homogeneous regions/meteorological sub-divisions and the CDs frequency during the monsoon and post-monsoon seasons of 1901—2010 and (ii) 30-year sliding CCs for the same period to find out the secular variations in their relationships.
The results are presented and discussed in Sect. 3 and broad conclusions drawn are presented in Sect. 4.