Results and Discussions
Linear Trends in CDs, TCs and Severe TCs Frequencies
Table 1 presents the seasonal and annual linear trend in the frequency of CDs, TCs and severe TCs over the NIO, BOB and AS during the period 1901-2010. Time series of significant trends (95 % confidence level) over the BOB and the AS is presented in Fig. 1. Considering the entire NIO basin, on a seasonal scale, significant decreasing trend (-0.25 and -0.17 per decade) is observed in the frequencies of CDs and TCs during the monsoon season. However, there is a significant increasing trend in severe TCs (0.07 per decade) during the post-monsoon season. In the annual scenario, there is a significant decreasing trend (-0.19 per decade) in the frequency of TCs over the NIO, and there is no trend in severe TCs. Considering the BOB alone, similar significant decreasing trends are observed for the monsoon and annual CDs and TCs. Trend values of -0.30 (-0.30) and -0.17 (-0.19) per decade are obtained for frequencies of CDs and TCs during the monsoon season (year as a whole). A significant increasing trend of 0.06 per decade is observed for severe TCs over the BOB during the post-monsoon season. For the AS, there is a significant increase in the frequency of CDs during the monsoon and the post-monsoon seasons (0.05 and 0.06 per decade), which is reflected in the trend for the year as a whole (0.12 per decade).