NmedianQ
4.3 Results and Discussion .1 Rainfall analysis
4.3.4 Temperature analysis .1 Descriptive statistics
4.3.4.3 Seasonal and annual temperature trends .1 Maximum temperature
Annual mean maximum temperature increased significantly by a rate of 0.077°C/decade during 1951–2010 in the Brahmaputra valley (Table 4.14). Figure 4.22a exhibits an above normal epoch from 1966 to 1981 followed by a below normal epoch from 1982 to 1992 and again an above normal epoch since 1993. The rate of increase in maximum temperature was 3 times higher during the recent 30-year normal period (NT2) compared to the period 1951–1980 (NT1) (Fig 4.22b and Table 4.13). On seasonal basis, statistically significant increasing trends were observed during monsoon (0.113°C/decade) and post- monsoon (0.188°C/decade) seasons. On the other hand, maximum temperature during pre- monsoon season showed negative trend (–0.030°C/decade) due to its significant decrease
in the month of April (Table 4.14). The magnitude of changes in annual as well as seasonal maximum temperature was significantly higher during NT2 compared to NT1 (Table 4.14). The trend rate of maximum temperature during NT2 in different seasons was almost identical and the rate varied between 0.23°C to 0.28°C/decade with statistically significant value only during monsoon season.
Table 4.14 Trends (°C/decade) of annual and seasonal temperature in the Brahmaputra valley during trend periods of 1951–2010 (EP), 1951–1980 (NT1) and 1981–2010 (NT2)
Sl No Variable Annual Pre-monsoon Monsoon Post-monsoon Winter 1951–2010 (EP)
1 Tmax 0.077 −0.030 0.113 0.188 0.052
2 Tmin 0.090 0.063 0.080 0.124 0.131
3 Tmean 0.088 0.005 0.094 0.159 0.106
1951–1980 (NT1)
1 Tmax 0.084 0.133 0.133 0.091 −0.136
2 Tmin −0.119 −0.090 −0.096 −0.082 −0.240
3 Tmean –0.033 −0.033 0.012 −0.033 −0.167
1981–2010 (NT2)
1 Tmax 0.250 0.262 0.233 0.244 0.282
2 Tmin 0.363 0.333 0.226 0.526 0.373
3 Tmean 0.313 0.285 0.219 0.353 0.300
Trends estimated by Sen‘s slope method, values rendered in bold indicate statistical significance at 95% confidence level according to the Mann–Kendall test.
4.3.4.3.2 Minimum temperature
The annual mean minimum temperature increased significantly by a rate of 0.090°C per decade during 1951–2010. Two distinct phases of temperature change was observed during the study period: a falling trend (–0.119°C/decade) during NT1 followed by an abrupt rising trend (0.363°C/decade) during NT2 (Fig 4.22b). During NT1, seasonal and annual minimum temperature exhibited decreasing trends with significant values only for the annual trend (Table 4.14) On the other hand, annual and seasonal minimum temperature during NT2 showed sharp increasing trends and all trends were statistically significant.
Among different seasons, the magnitude of trend was highest in post-monsoon (0.536°C/decade) followed by winter (0.375°C/decade) and lowest in monsoon season
during NT2. Station level trends showed annual as well as seasonal mean minimum temperature to increase significantly at four locations of the valley during 1951–2010.
4.3.4.3.3 Mean temperature
A statistically significant positive trend (0.088°C/decade) in the mean air temperature during 1951–2010 was observed with an accelerated warming (0.313°C/decade) during 1981–2010 (Table 4.14). Figure 4.22c, which displays the mean air temperature anomaly relative to the 1961–90 average along with its 9-point binomial filtered series, showed consisted warming tendencies of mean temperature in the Brahmaputra valley since 1993.
The air temperature in all the seasons tended to increase with a wide range of slope values.
Increasing tendency of post-monsoon (0.159°C/decade), (0.106°C/decade) and winter was statistically significant. The recent 30-year period (NT2) showed statistically significant increasing trend of mean temperature during all the seasons in contrast to the previous 30- year period (1951–1980) which exhibited negative trends during all the seasons except monsoon (Table 4.14) The magnitude of increase was the highest in post-monsoon (0.353°C/decade) followed by winter (0.300°C/decade) and the lowest in monsoon (0.219°C/decade) season.
The increase in annual minimum temperature was 1.5 times higher than that of the maximum temperature during 1981–2010 in the Brahmaputra valley. A lower change in increase in maximum temperature than that minimum temperature may be a sign of presence of solar dimming (Wild et al. 2007). Moreover, less amount of precipitation coupled with enhanced green house gas (GHG) contents in the atmosphere have rendered the post-monsoon and winter significantly warmer particularly in the last three decades (1981–2010). During post-monsoon and winter, maximum temperature increased by 0.244°C/decade and 0.282°C/decade respectively (Table 4.14) whereas increase in minimum temperature in the two seasons was much higher (0.536°C/decade and 0.375°C/decade). Kothawale and Rupa Kumar (2005) found that the all India mean annual surface temperature had increased by 0.05°C/decade for the period 1901–2003 and the rise was steeper during the last three decades (0.22°C/decade). This indicates that the rate of increase in mean annual temperature in the Brahmaputra valley was 1.6 times higher than the all-India average during the last three decades (1981–2010).
Fig. 4.22 Annual maximum (a), minimum (b) and mean (c) temperature anomaly relative to 1961-1990 mean along with its 9-point binomial filtered series during trend periods 1951–2010 (ET), 1951–1980 (NT1) and 1981–2010 (NT2)
-1.0 -0.5 0.0 0.5 1.0 1.5
1951 1954 1957 1960 1963 1966 1969 1972 1975 1978 1981 1984 1987 1990 1993 1996 1999 2002 2005 2008
Anomaly (°C)
(a)Tmax
Original series Filtered series Linear trend (ET) Linear trend (NT1) Linear trend (NT2)
-1.0 -0.5 0.0 0.5 1.0 1.5
1951 1954 1957 1960 1963 1966 1969 1972 1975 1978 1981 1984 1987 1990 1993 1996 1999 2002 2005 2008
Anomaly (°C)
(b)Tmin
Original series Filtered series Linear trend (ET) Linear trend (NT1) Linear trend (NT2)
-1.0 -0.5 0.0 0.5 1.0 1.5
1951 1954 1957 1960 1963 1966 1969 1972 1975 1978 1981 1984 1987 1990 1993 1996 1999 2002 2005 2008
Anomaly (°C)
(c) Tmean
Original series Filtered series Linear trend (ET) Linear trend (NT1) Linear trend (NT2)
Changes in monthly minimum temperature during 1951–1980 (NT1) were highly different from that of maximum temperature changes. During this period, minimum temperature showed negative trend in nine months in contrast to only two months in case of maximum temperature. During NT2 (1981–2010), on the contrary, the direction of changes in monthly minimum temperatures were similar with that of maximum temperature.
However, changes in monthly maximum temperatures were less significant compared to that of minimum temperature. This result disagrees with Pal and Al-Tabbaa (2010b) who reported that the changes in monthly minimum temperatures were less significant than maximum temperature changes in NE India during 1901–2003. They also reported statistically significant decreasing trends in minimum temperature during the months from May to August in NE India, which contradicts with the positive and statistically significant trends observed for said months during 1951–2010 in this study. This may be due to the length of data period considered in trend analysis.