NmedianQ
4.3 Results and Discussion .1 Rainfall analysis
4.3.5 Trends of extreme temperature indices
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.
trend of all-India TXn, it showed a decreasing trend in the Brahmaputra valley due to its significant decrease (–0.41°C/decade) during pre-monsoon season.
Table 4.15 Annual and seasonal trends (per decade) of extreme temperature indices in the Brahmaputra valley during 1971–2010
Indices Unit Winter Pre-monsoon Monsoon Post-monsoon Annual
TXx °C −0.03 0.14 0.18 0.24 0.11
TNx °C 0.21 0.24 0.18 0.25 0.19
TXn °C −0.17 −0.41 −0.11 −0.28 −0.27
TNn °C 0.23 0.25 0.14 0.39 0.32
TX90p % 5.12 0.89 1.81 4.10 0.86
TN90p % 3.97 2.41 7.74 2.29 2.72
TX10p % −0.63 0.75 −1.02 −1.88 −0.78
TN10p % −4.74 −1.94 −4.54 −2.10 −1.31
DTR °C −0.12 −0.27 −0.14 –0.08 −0.15
Trends estimated by Sen’s slope method, values rendered in bold indicate statistical significance at 95% confidence level according to the Mann–Kendall test.
Fig. 4.23 Annual trends of hottest day (a), coldest day (b), hottest night (c) and coldest night (d) temperature in the Brahmaputra valley during 1971–2010
Sen's slope: 0.11°C/decade
34 35 36 37 38 39
1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010
Temperature (°C)
(a) TXx
Sen's slope: −0.27°C/decade
5 10 15 20 25
1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010
Temperature (°C)
(b) TXn
Sen's slope: 0.19°C/decade
26 27 27 28 28 29 29
1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010
Temperature (°C)
(c) TNx
Sen's slope: 0.32°C/decade
0 2 4 6 8 10
1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010
Temperature (°C)
(d) TNn
The pre-monsoon thunderstorm activity in the Brahmaputra valley is vigorous unlike other parts of India and contributed 25% to the annual rainfall total. During the recent 30-year period (1981–2010), rainfall during this season exhibited an increasing tendency over the Brahmaputra valley (Fig 4.7). Moreover, during good monsoon years, substantial decrease in incoming solar radiation due to dense cloud cover and evaporative cooling of rainwater are responsible for lowering the surface temperature during the day (Kothawalwe et al.
2012).
4.3.5.2 Frequency indices with percentile thresholds
All the frequency indices of hot events (above 90th percentile of 1971–2000 average) showed increasing trends while that of the cold events (below 10th percentile of 1971–2000 average) showed decreasing trends in the Brahmaputra valley (Fig. 4.24).
Fig. 4.24 Annual trends of frequency indices of hot days (a), cold days (b), hot nights (c) and cold nights (d) in the Brahmaputra valley during 1971–2010
Though the magnitude of increase in the frequency of hot days was significantly higher during winter (5.12% per decade) and post-monsoon (4.10% per decade) seasons, their increase on annual basis was only 0.86% per decade due to lower frequency of hot days
Sen's slope: 0.86% per decade
0 10 20 30 40
1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010
Hot days (%)
(a) TX90p
Sen's slope: −0.78% per decade
0 5 10 15 20 25
1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010
Cold days (%)
(b) TX10p
Sen's slope: 2.7% per decade
0 10 20 30
1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010
Hot nights (%)
(c) TN90p
Sen's slope: –1.3% per decade
0 5 10 15 20
1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010
Cold nights (%)
(d) TN10p
during pre-monsoon and monsoon seasons (Table 4.15). On annual basis, increase in the frequency of hot nights (2.7% per decade) and decrease in the frequency of cold nights (1.3% per decade) were statistically significant (Table 4.15). Increase in the frequency of hot nights was highest (7.74% per decade) during monsoon and lowest (2.29% per decade) during post-monsoon seasons. On the other hand, decrease in the frequency of cold nights was highest (4.74% per decade) during winter and lowest (1.94% per decade) during pre- monsoon seasons.
The above results indicated significant changes in temperature extremes associated with observed warming in the Brahmaputra valley during last three decades (Table 4.15), especially daily minimum temperature indices i.e., the warming trend was higher for nighttime indices than for those of daytime. Similar observations in the frequency indices of annual hot and cold events were observed by Kothawale et al. (2010) and Ravekedar et al. (2012) in India; Klein Tank et al. (2006) in central and south Asia; Vincent et al. (2005) in South America; Plummer et al. (1999) in Australia to mention a few. Manton et al.
(2001) reported significant increase in the annual number of hot days and warm nights and decrease in cool days and cold nights in Southeast Asia and South Pacific during the period 1961–1998. Dash et al. (2011) also observed decreasing trend in the cold nights in northeast India.
4.3.5.3 Frequency indices with fixed thresholds
Annually, number of days above maximum temperature 35°C (hot days) increased significantly by a rate of 1.4% per decade during 1971–2010 (Table 4.15 and Fig. 4.25).
On the other hand, number of nights above 25°C (hot nights) showed significant increasing trend (10.0% per decade) while the number of nights below 10°C (cold nights) showed significant decreasing trend (4.2% per decade). Spatially, cold nights decreased at all the locations except at one station located in the central part (Tezpur) where the trend was positive. On seasonal basis, cold nights showed decreasing trends in all the four seasons, except post-monsoon and winter seasons at Tezpur.
4.3.5.4 Range index (DTR)
Annual diurnal temperature range (DTR) decreased significantly by a rate of 0.15°C/decade (Table 4.15). The lowering of DTR in the Brahmaputra valley was due to
the higher magnitudes of nighttime indices compared to their daytime counterparts. DTR decreased during all the four seasons in the valley with significant value only in the pre- monsoon and monsoon seasons. In the pre-monsoon and monsoon months, due to cloudy and humid weather, there is lowering of maximum temperatures and increase in minimum temperatures (Kothawale, 2005) narrowing down the DTR. The observed trends in DTR may be affected by large scale effects of increased GHGs and aerosols (Zhou et al. 2008).
The nighttime temperature over land had increased by about twice of the rate of daytime temperature during the past 50 years (Folland et al. 2001) and the DTR decreases was due to the evident increase of nighttime temperature.
Fig. 4.25 Annual trends of number of days with TX > 35°C (a), TX < 20°C (b), TN > 25°C (c) and TN < 10°C (d) in the Brahmaputra valley during 1971–2010