the 1901–2015 period (0.09 decade
−1 ; see Fig. 7.1a and
Table 7.2) as well as for the post-1950 period (0.16 decade
−1 ). While the frequency of LPS does not change significantly in the last 100 years, there is a rise in the duration
of LPS (Jadhav and Munot 2009). The long-term (1901–
2015) trend in the frequency of lows shows a significant
increase (+0.38 decade
−1 , see Fig. 7.1b and Table 7.2), with
a pronounced rise (+1.01 decade
−1 ) during the post-1950
period. In contrast, trends in the frequency of depressions
during the 1901–2015 period show a decline of −0.11 deccade
−1 (Fig. 7.1c and Table 7.2), which gets prominently
significant (−0.69 decade
−1 ) during 1951–2015. This
decreasing (increasing) trend in depressions (lows) may also
suggest compensating effect as evidenced in the insignificant
trends in LPS frequency (Table 7.2). Similar to depressions,
the frequency of cyclonic storms (see Fig. 7.1d) also shows
a decreasing trend which is significant in both periods, i.e.,
−0.17 decade
−1 for 1901–2015 and −0.15 decade
−1 for
1951–2015. The long-term trends in the LPS frequency has
been investigated in several earlier studies (Rajeevan et al.
2000; Patwardhan and Bhalme 2001; Rajendra Kumar and
Dash 2001; Mandke and Bhide 2003; Jadhav and Munot
2009; Ajayamohan et al. 2010; Prajeesh et al. 2013; Vishnu
et al. 2016; Mohapatra et al. 2017). Though the data periods
of these studies are different, major conclusions are, moreover, the same. However, the trends in the frequency of lows
and depressions are not significant during the recent three
decades (1986–2015).
Also, note that periods associated with frequent lows
apparently coincide with periods of fewer depressions which
clearly suggest that the frequency of lows and depressions
also exhibits significant inter-decadal variations (Fig. 7.1b,
c). Earlier studies showed that the frequency of lows and
depressions displays an epochal behavior on inter-decadal
time scale. For example, Rajendra Kumar and Dash (2001)
examined the inter-decadal variations in lows and depressions based on long-term observations (i.e., for 110 years,
1889–1998) and to understand their relationship with the
8
10
12
14
16
18
20
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year
0
2
4
6
0
3
6
9
12
0
3
6
9
12
15
18
a)Total
b) Lows
c) Depressions
d) Cyclonic Storms
Frequency
Fig. 7.1 Low-pressure systems
(LPS) forming over the Indian
region (Bay of Bengal, Arabian
Sea, and Indian land) during the
Summer monsoon season (JJAS)
for the period 1901–2015. The
data sources for depressions are
from the cyclone eAtlas archived
by the IMD (http://www.
rmcchennaieatlas.tn.nic.in), and
data sources for lows include
published documentations
(Mooley and Shukla 1987; Sikka
2006) and from the Journal of
Mausam published by the Indian
Meteorological Society. In a LPS,
i.e., total frequency of lows,
depressions and cyclonic storms,
b lows, c depressions and
d cyclonic storms (including both
cyclonic storms as well as severe
cyclonic storms)
7 Synoptic Scale Systems
147
−1 ; see Fig. 7.1a and
Table 7.2) as well as for the post-1950 period (0.16 decade
−1 ). While the frequency of LPS does not change significantly in the last 100 years, there is a rise in the duration
of LPS (Jadhav and Munot 2009). The long-term (1901–
2015) trend in the frequency of lows shows a significant
increase (+0.38 decade
−1 , see Fig. 7.1b and Table 7.2), with
a pronounced rise (+1.01 decade
−1 ) during the post-1950
period. In contrast, trends in the frequency of depressions
during the 1901–2015 period show a decline of −0.11 deccade
−1 (Fig. 7.1c and Table 7.2), which gets prominently
significant (−0.69 decade
−1 ) during 1951–2015. This
decreasing (increasing) trend in depressions (lows) may also
suggest compensating effect as evidenced in the insignificant
trends in LPS frequency (Table 7.2). Similar to depressions,
the frequency of cyclonic storms (see Fig. 7.1d) also shows
a decreasing trend which is significant in both periods, i.e.,
−0.17 decade
−1 for 1901–2015 and −0.15 decade
−1 for
1951–2015. The long-term trends in the LPS frequency has
been investigated in several earlier studies (Rajeevan et al.
2000; Patwardhan and Bhalme 2001; Rajendra Kumar and
Dash 2001; Mandke and Bhide 2003; Jadhav and Munot
2009; Ajayamohan et al. 2010; Prajeesh et al. 2013; Vishnu
et al. 2016; Mohapatra et al. 2017). Though the data periods
of these studies are different, major conclusions are, moreover, the same. However, the trends in the frequency of lows
and depressions are not significant during the recent three
decades (1986–2015).
Also, note that periods associated with frequent lows
apparently coincide with periods of fewer depressions which
clearly suggest that the frequency of lows and depressions
also exhibits significant inter-decadal variations (Fig. 7.1b,
c). Earlier studies showed that the frequency of lows and
depressions displays an epochal behavior on inter-decadal
time scale. For example, Rajendra Kumar and Dash (2001)
examined the inter-decadal variations in lows and depressions based on long-term observations (i.e., for 110 years,
1889–1998) and to understand their relationship with the
8
10
12
14
16
18
20
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year
0
2
4
6
0
3
6
9
12
0
3
6
9
12
15
18
a)Total
b) Lows
c) Depressions
d) Cyclonic Storms
Frequency
Fig. 7.1 Low-pressure systems
(LPS) forming over the Indian
region (Bay of Bengal, Arabian
Sea, and Indian land) during the
Summer monsoon season (JJAS)
for the period 1901–2015. The
data sources for depressions are
from the cyclone eAtlas archived
by the IMD (http://www.
rmcchennaieatlas.tn.nic.in), and
data sources for lows include
published documentations
(Mooley and Shukla 1987; Sikka
2006) and from the Journal of
Mausam published by the Indian
Meteorological Society. In a LPS,
i.e., total frequency of lows,
depressions and cyclonic storms,
b lows, c depressions and
d cyclonic storms (including both
cyclonic storms as well as severe
cyclonic storms)
7 Synoptic Scale Systems
147