Characteristics and trend analysis of absolute and relative temperature extremes indices and related indices of Kolkata

Abstract

An attempt has been made, in this paper, to understand the characteristics and trends of temperatures of Kolkata, India, by using the temperature extremes, as recommended by Expert Team on Climate Change Detection and Indices (ETCCDI), and other relevant indices. For the purpose, daily maximum, minimum, and mean temperatures (1969–2012) of Alipore and Dum Dum observatories have been used. Both parametric (linear regression test) and non-parametric (Mann–Kendall test) have been done to detect the change, and Sen’s slope estimator has been used to establish the degree of change. To detect homogeneity of the dataset, four homogeneous tests have been applied. Long-term (1969–2012) trends of extreme temperature indices like TXa, TXn, TMa, TNx, TNa, and TNn of monthly, seasonal, and annual time steps significantly detect a positively increasing trend. The rate is found to be the maximum at Dum Dum. Long-term trends of TXa, TNx, TNa, TNn, and TMa indicate an average positive temperature change of 0.30 °C/decade to 0.62 °C/decade, and in extreme cases over 0.65 °C/decade. Results also point out that in the monsoon and the post-monsoon seasons most of the trends are noticeable. In most of the cases, rates of positive temperature trends have been lower in recent time period (series II, 1985–2012), compared to the past time period (series I, 1969–1995). Distribution of the relative temperature indices also points out presence of asymmetric distribution between warm nights and cold days, and cold nights. Around three times more positive increase of warm nights has been noticed at Dum Dum station during 1971–1980 and 2001–2010.