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Trends of monsoon circulation and rainfall : Role of natural variability
M RajeevanMinistry of Earth Sciences
New Delhimn.rajeevan@nic.in
Background
• With the projections of global warming, some changes are expected in monsoon circulation and rainfall.
• IPCC models generally indicate a strengthening of the monsoon due to increase in water vapour.
• The role of aerosols, especially black carbon aerosols on the Indian monsoon is uncertain.
• Observational studies suggested a weakening of the monsoon circulation and rainfall during the recent years (since 1951)
• Recent studies have attributed these changes to the changes in atmospheric constituents especially aerosols.
J Climate, 2006
Science, 2011
Nature, 2011
Monsoon circulation and rainfall trends
Science, 2011
500
600
700
800
900
1000
1100
12001
87
5
18
80
18
85
18
90
18
95
19
00
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05
20
10
Mo
nso
on
Rain
fall
(in
mm
)
Year
All India Monsoon Rainfall (in mm), 1875-2012
Trend, 1951-2012: decrease by 1.39mm/year
If we consider rainfall from 1951-2012, then there is a statistically significant decrease of all-India rainfall
-8
-6
-4
-2
0
2
4
6
8
-4
-3
-2
-1
0
1
2
3
41889
1893
1897
1901
1905
1909
1913
1917
1921
1925
1929
1933
1937
1941
1945
1949
1953
1957
1961
1965
1969
1973
1977
1981
1985
1989
1993
1997
Rain
fall (
% D
ep
art
ure
)
Central Year
31-Year Moving Average of ISMR
All India
Central India
South Peninsula
Multi-decadal variations
31-Year running means
-4
-3
-2
-1
0
1
2
3
4
19
06
19
10
19
14
19
18
19
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19
26
19
30
19
34
19
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19
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19
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19
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19
62
19
66
19
70
19
74
19
78
19
82
19
86
19
90
19
94
19
98
20
02
20
06
Year (ending)
Mo
ns
oo
n R
ain
fall
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Monsoon Rainfall
AMO
Nino 3.4
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0
19
31
19
34
19
37
19
40
19
43
19
46
19
49
19
52
19
55
19
58
19
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19
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19
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19
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19
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19
76
19
79
19
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19
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19
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19
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19
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19
97
20
00
20
03
20
06
20
09
20
12
Bre
ak D
ays
ISM
R (
% D
ep
)
Year
Multi-decadal variations of ISMR and Break Days
ISMR Break Days
-4
-3
-2
-1
0
1
2
3
4
5
6
19
05
19
08
19
11
19
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19
20
19
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19
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19
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19
98
20
01
20
04
20
07
20
10
All the years
Multi-decadal variation is not only due to extreme years only!
Only normal years
y = -0.069x + 2.249R² = 0.014
y = -0.166x + 5.259R² = 0.078
-25.0
-20.0
-15.0
-10.0
-5.0
0.0
5.0
10.0
15.0
20.0
25.0
30.0
19
53
19
55
19
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19
59
19
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19
63
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67
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20
01
20
03
20
05
20
07
20
09
20
11
ISM
R (
% D
ep
)
Year
Indian Summer monsoon rainfall ISMR(decadal removed)
ISMR (Actual)
Linear (ISMR(decadal removed))
Linear (ISMR (Actual))
If we remove the significant multi-decadal variations of ISMR, then the trend from 1951-2012 becomes statistically insignificant
Regional monsoon rainfall
Time series of LPS and Depressions
y = -0.0225x + 7.9279
R2 = 0.08680
2
4
6
8
10
12
14
16
18
201
89
1
18
95
18
99
19
03
19
07
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27
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03
20
07
Year
No
of
De
pre
ss
ion
s
0
5
10
15
20
25
30
No
of L
PS
y=-0.0863x+8.4202 R2= 0.3077
Time series of Ratio of DDs to LPS
0
0.2
0.4
0.6
0.8
1
1.21
89
1
18
95
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99
20
03
20
07
Year
Ra
tio
y=-0.668x+0.6638 R2=0.303
1951-1980 1981-2007
1951-1980 1981-2007
Impacts of enhanced CCN on the organization of convection and recent reduced counts of monsoon depressions T. N. Krishnamurti ; Andrew Martin ; Ruby Krishnamurti ; Anu Simon ; Aype Thomas ; VinayKumarProc. SPIE 8529, Remote Sensing and Modeling of the Atmosphere, Oceans, and Interactions IV, 85290E (November 29, 2012); doi:10.1117/12.979717
•This paper addresses the effects of pollution for the enhancement of cloud condensation nuclei and the resulting disruption of the organization of convection in monsoon depressions. Our specific studies make use of a high resolution mesoscale model (WRF/CHEM) to explore the impacts of the first and second aerosol indirect effects proposed by Twomey and Albrecht.
We have conducted preliminary studies including examination of the evolution of radar reflectivity (computed inversely from the model hydrometeors) for normal and enhanced CCN effects (arising from enhanced monsoon pollution).
The time lapse histories show a major disruption in the organization of convection of the monsoon depressions on the time scale of a week to ten days in these enhanced CCN scenarios.
Vertical wind shear/ horizontal wind shear
Dash et al, Current Science, 2004
It is speculated that the decrease in frequency of monsoon depressions is due to decrease in horizontal and vertical wind shears
Frequency of Monsoon Depressions and SST Gradient over BOB
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.51
91
0
19
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20
06
Year (ending)
Fre
qu
en
cy
(S
td A
no
ma
ly)
-1
-0.5
0
0.5
1
1.5
SS
T G
rad
ien
t (S
td A
no
ma
ly)
Frequency
SST Gradient
Variation of Monsoon Circulation
y = -0.0184x + 10.47
R2 = 0.2539
y = 0.043x - 24.009
R2 = 0.1669
-28
-26
-24
-22
-20
-18
-16
-14
-12
-101948
1950
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2006
2008
Year
Win
d S
peed
(m
ps)
4
5
6
7
8
9
10
11
12
Win
d S
peed
(m
ps)
U150U850Linear (U850)Linear (U150)
Reanalysis and climate trends: IPCC AR4
Chapter-3
Supplement material: Page 8
“In reality, however, observational coverage varies over time, observations are themselves prone to bias, either instrumental or through not being representative of their wider surrounding, and these observational biases can change over time. This introduces trends and low-frequency variations in analyses that are mixed with the true climatic signals, making long timescale trends over the full length of the reanalyses potentially unreliable. Better representation of trends by reanalysis systems requires progress on identifying and correcting model and observational biases, assimilating as complete as set of past observations as possible.”
Reanalysis and climatic trendsTime series of NCEP U850 and Rainfall
y = -0.0195x + 9.8684
R2 = 0.2535
y = -0.0348x + 8.5411
R2 = 0.383
2
3
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8
9
10
11
12
1948
1950
1952
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1980
1982
1984
1986
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1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
Year
U850 (
m/s
)
4
5
6
7
8
9
10
11
Rain
fall (m
m/d
ay)
U850 (5-15N,70-100E)
Rainfall (10-25N, 70-80E)
Linear (U850 (5-15N,70-
100E))Linear (Rainfall (10-25N,
70-80E))Linear (Rainfall (10-25N,
70-80E))Linear (Rainfall (10-25N,
70-80E))Linear (Rainfall (10-25N,
70-80E))
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Win
d S
pe
ed
(m
/se
c)
850 hPa Zonal Wind20th Century Reanalysis Data
-36
-34
-32
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-28
-26
-24
-22
-20
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Win
d S
pe
ed
(m
/Se
c)
150 hPa Zonal Wind20th Century Reanalysis Data
Aerosols and break monsoon
During the break monsoon periods, more aerosol loading is observed over central plains of India. This could be attributed to the circulation anomalies associated with the monsoon breaks Ravi Kiran, M Rajeevan and S Vijaya Bhasakara
Rao, 2009, Goephys. Res. Letters
Increase in Pre-monsoon Tropical Cyclone Intensity
Nature, 2011
1955-1973 1974-1992 1993-2011
Frequency of intense
TCs (severe cyclonic
storm and above) 6 4 7
Days of intense TCs 17 6 20
Rajeevan et al. 2013, Atmospheric Science Letters
Variation of vertical wind shear
Rajeevan et al. 2013, Atmospheric Science Letters
Variation of Tropical Cyclone Heat Potential
Conclusions
• It is important to understand the role of atmospheric constituents including aerosols on monsoon circulation and rainfall, both using observations and models.
• However, it is very important to consider the natural variability of monsoon before any conclusions are drawn about the impact of atmospheric constituents on monsoon.
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