RESULTS AND DISCUSSION Assessing Climatic Patterns in India
3.3. Periodical Cycle of Regional Monsoon Rainfall
3.3.2. Results
ALLIN
700 800 900 1000
1875 1885 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005
5-year running mean (mm) NWIN
300 350 400 450 500 550 600 650
1875 1885 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005
5-year running mean (mm)
(a) (b)
WCIN
700 800 900 1000 1100
1875 1885 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005
5-year running mean (mm) CNEIN
800 900 1000 1100 1200
1875 1885 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005
5-year running mean (mm)
(c) (d)
NEIN
1200 1300 1400 1500 1600
1875 1885 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005
5-year running mean (mm) PENIN
500 600 700 800
1875 1885 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005
5-year running mean (mm)
(e) (f)
Figure 3.12: Variation of half decadal (5-year) nowcasted moving average of monsoon rainfall based on the rainfall data from 1871 to 2005 in various regions in India considered of (a) ALLIN, (b) NWIN, (c) WCIN, (d) CNEIN, (e)
NEIN and (f) PENIN.
It is noted in Table 3.5 that all the trends corresponding to 5- and 10-year moving averages had high significance that determine the certainty of the monsoon rainfall change. The majority of the regions were found to have been undergoing decreasing trends of monsoon rainfall that are 95 to 99% significant, as shown in Table 3.5. On the
contrary, only region PENIN shows increasing trends of monsoon rainfall, which are 85 and 99% significant and only region, NWIN has no-trend in both the cases. Table 3.5 also shows that the maximum monsoon rainfall decrease, which is 95 to 99%
significant, is in North-Eastern India (NEIN).
ALLIN
700 800 900 1000
1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
10-year running mean (mm) NWIN
300 400 500 600
1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
10-year running mean (mm)
(a) (b)
WCIN
800 900 1000 1100
1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
10-year running mean (mm) CNEIN
900 1000 1100
1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
10-year running mean (mm)
(c) (d)
NEIN
1300 1350 1400 1450 1500 1550
1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
10-year running mean (mm) PENIN
600 650 700 750
1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
10-year running mean (mm)
(e) (f)
Figure 3.13: Variation of decadal (10-year) nowcasted moving average of monsoon rainfall based on the rainfall data from 1871 to 2005 in various regions in India considered of (a) ALLIN, (b) NWIN, (c) WCIN, (d) CNEIN, (e) NEIN
and (f) PENIN.
Taking the previous results into consideration, i.e. increasing trend of meteorological overall droughts and no change in overall floods (see Figures 3.8 and 3.9) it could be concluded that the North East region remains the most vulnerable region to disasters in India during the monsoon season.
Table 3.5: Trends of 5- and 10-year moving mean monsoon rainfalls in India for the period of 1871- 2005.
5-Year 10-Year Region
Trend (mm/100years) Significance Trend (mm/100years) Significance
ALLIN - 18 95% - 14 95%
NWIN No Trend N/A No Trend N/A
WCIN - 36 99% - 30 99%
CNEIN - 20 95% - 26 99%
NEIN - 36 99% - 34 99%
PENIN + 12 85% + 19 99%
Note: +ve sign = increasing trend; -ve sign = decreasing trend.
Furthermore, West Central India (WCIN) also falls in the ‘disastrous’ region category despite its overall drought showing ‘no trend’ and floods showing decreasing trends (refer Figures 3.8 and 3.9). This is because this region has a similar result to NEIN i.e.
decrease in half decadal and decadal mean monsoon rainfall (Table 3.5), and increase in meteorological ‘moderate’ droughts and no change in ‘moderate’ floods, as shown and demonstrated previously in Figures 3.10 and 3.11.
Central North East India, on the other hand, shows slightly different results from NEIN and WCIN, with significantly decreasing trend in decadal and half decadal monsoon averages (95-99% significant) and no trend in ‘moderate’ flood and droughts (Figures 3.10 and 3.11). NWIN shows results that are also unique with both ‘moderate’ flood and droughts increasing (Figures 3.10 and 3.11), no change in overall flood and droughts (Figures 3.8 and 3.9), and no change in decadal and half decadal monsoon averages (Table 3.5).
The results above also confirm that, decadal (10-year) changes are much more versatile and statistically significant than the shorter time i.e. half decadal (5-year) changes. So, it is now clear that, although all-India mean-monsoon rainfall shows a statistically significant decreasing trend and virtually provides a comprehensive overall
generalisation of Indian monsoon rainfall, it does not at all reflect the regional variability, as seen in the result of ALLIN in Table 3.5 and Figures 3.8, 3.9, 3.10, 3.11, 3.12 and 3.13. Therefore, a region-specific study is always important and essential in order to better understand the locale climate that is vital for various local developments requiring local climatological information.
Figure 3.14 displays the 25-year percent departures of NMA and SD from the long-term mean and SD of monsoon rainfall. The most common and interesting observation for all the cases in the figure is the accompaniment of the highest variability of the monsoon rainfall with the prolonged dry spell. This is consistent with Pant and Rupa Kumar (1997) who also noticed similar outcomes for the all-India monsoon rainfall, where the data ranged from 1871 to 1995. A more region specific study and more up to date time series analysis here reaches a similar conclusion for a regional perspective. Moreover, Figure 3.14 was also helpful to depict that the number of decadal dry/wet cycles do not always occur at every region at a time. However, the results corresponding to WCIN look representative of ALLIN, as seen in Figure 3.14.
The results in section 3.1, 3.2 and 3.3 agree with the earlier analyses (Kumar et al., 2004; Sen Roy and Balling, 2004; Manton et al., 2001) which reported a decline in summer monsoon rainfall over South and Southeast Asian region and specifically in the north-eastern states of India and an increase in rainfall in peninsular India. However, the results disagree with Goswami et al. (2006) in relation to the presence of a trend for all- India monsoon rainfall. This disagreement might be caused by the methods employed to determine the trends or the difference in the data sampling period. This indicates that the selection of the test and methods of analysis significantly affect the underlying conclusions. Since this part of the study tries to detect the long-term trends by incorporating the oscillations present in the database, the methodologies adopted here would show different results to the all-India trend showed in Goswami et al. (2006).
ALLIN
-12 -8 -4 0 4 8 12
1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005
Percent departure of mean
-60 -40 -20 0 20 40 60SD Percent departure of
mean
SD NWIN
-12 -8 -4 0 4 8 12
1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005
Percent departure of mean
-60 -40 -20 0 20 40 60 Percent departure of
SD
mean SD
(a) (b)
WCIN
-12 -8 -4 0 4 8 12
1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005
Percent departure of mean
-60 -40 -20 0 20 40 60 Percent departure of
SD
mean
SD CNEIN
-12 -8 -4 0 4 8 12
1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005
Percent departure of mean
-60 -40 -20 0 20 40 60SD Percent departure of
mean SD
(c) (d)
NEIN
-12 -8 -4 0 4 8 12
1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005
Percent departure of mean
-60 -40 -20 0 20 40 60 Percent departure of
SD
mean
SD PENIN
-12 -8 -4 0 4 8 12
1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005
Percent departure of mean
-60 -40 -20 0 20 40 60SD Percent departure of
mean SD
(e) (f)
Figure 3.14: Variation of 25-year-nowcasted-moving-average of percent departure of mean and standard deviation of monsoon rainfall based on the rainfall data from 1871 to 2005 in various regions in India considered of (a) ALLIN, (b)
NWIN, (c) WCIN, (d) CNEIN, (e) NEIN and (f) PENIN.