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Perceptible changes in Indian summer monsoon rainfall in relation to Indian Monsoon Index
Accepted Manuscript Perceptible changes in Indian summer monsoon rainfall in relation to Indian Monsoon Index
C.V. Naidu, A. Dharma Raju, P. Vinay Kumar, G.Ch. Satyanarayana PII: DOI: Reference:
S0921-8181(17)30034-6 doi: 10.1016/j.gloplacha.2017.08.016 GLOBAL 2632
To appear in:
Global and Planetary Change
Received date: Revised date: Accepted date:
25 January 2017 24 June 2017 28 August 2017
Please cite this article as: C.V. Naidu, A. Dharma Raju, P. Vinay Kumar, G.Ch. Satyanarayana , Perceptible changes in Indian summer monsoon rainfall in relation to Indian Monsoon Index, Global and Planetary Change (2017), doi: 10.1016/ j.gloplacha.2017.08.016
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ACCEPTED MANUSCRIPT Perceptible changes in Indian summer monsoon rainfall in relation to Indian Monsoon Index C. V. Naidu1, A. Dharma Raju2, P. Vinay Kumar1 and G. Ch. Satyanarayana3 1 Department of Meteorology and Oceanography, Andhra University, Visakhapatnam, India 2 India Meteorological Department, Hyderabad, Telangana, India Department of Atmospheric Science, KL University, Vaddeswaram, Guntur District, Andhra Pradesh, India
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Abstract
The changes in the summer monsoon rainfall over 30 meteorological subdivisions of
The relationship between the IMIs in different months
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studied for the period 1953-2012.
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India with respect to changes in circulation and the Indian Monsoon Index (IMI) have been
and whole season and the corresponding summer monsoon rainfall is studied and tested. The
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positive and negative extremes are evaluated basing on the normalized values of the
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deviations from the mean of the IMI. Composite rainfall distributions over India and the zonal wind distributions in the lower and upper troposphere of IMI’s both positive and
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negative extremes are evaluated separately and discussed. In the recent three decades of
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global warming, the negative values of IMI in July and August lead to weakening of the monsoon system over India. It is observed that the rainfall variations in the Northeast India
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are different from the rest of India except Tamil Nadu in general. Keywords: summer monsoon rainfall, Indian monsoon index, global warming.
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1. Introduction India receives 70% (about 85 cm) of its annual rainfall in the Summer Monsoon (SM) season (June-September) mainly due to the transport of moisture from Indian Ocean region
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(Naidu et al., 2011; 2015). The sea surface temperature (SST) and the low level winds
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facilitate the evaporation. The temperature gradient between the land and the ocean plays a
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vital role in transporting the moisture from the Indian Ocean to Indian mainland. Moisture transport is essential for the performance of the monsoon system. It has been observed by
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Cadet and Reverdin (1981) that 70% of the water vapor that reaches the west coast of India
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is from the Southern Hemisphere and 30% comes from the evaporation occurring over the Arabian Sea. The importance of the low level wind flow pattern in the transport of moisture
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from the oceanic area to the Indian mainland is discussed by many scientists (Ding, 2005;
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Puranik et al., 2013; Koteswaram, 1958, Joseph and Raman, 1966 and Findlater, 1969; Cadet & Desbois, 1979).
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The Somali jet stream flows intermittently from the vicinity of Mauritius over the
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northern tip of Madagascar to reach the Kenyan coast (3°S) and penetrates inland over the flat coastal strip of Kenya and lowlands of Ethiopia and Somalia and emerges out into the
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Arabian Sea near 9°N. From this point, it moves over the cold upwelling water off the Somalia coast. It then crosses the West coast of India and transports moisture to Indian subcontinent during the SM season. This cross-equatorial flow is an important part to the South as well as the East Asian SM systems (Ding, 2005; Puranik et al., 2013). The low level jet stream (Somali jet stream) is an important component of Indian SM with its core close to 850 hPa level over the Indian Ocean and south Asia (Koteswaram, 1958, Joseph and Raman, 1966 and Findlater, 1969) and brings the moisture generated by
ACCEPTED MANUSCRIPT trade winds over the South Indian Ocean and the evaporative flux from the Arabian Sea to the areas of rainfall production over south Asia. The cyclonic vorticity north of this jet in the atmospheric boundary layer is a dynamic forcing for the generation of vertical upward air motion and rainfall and for the genesis of depressions in the North Bay of Bengal (Joseph
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and Simon, 2005 and 2005a). During active SM period, the core of the jet passes eastward
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through peninsular India in between 12.5°N and 17.5°N; whereas in the break monsoon
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condition, it moves southeastward from the central Arabian Sea and by-passing India passes eastward between latitudes 2.5°N and 7.5°N (Joseph and Sijikumar, 2004). Thus in break
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monsoon spells, the large amount of moisture carried by the low level jet does not reach
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India, but goes to the west Pacific Ocean to feed the increased frequency of typhoons generating there (Joseph, 1990; Rajeevan, 1993). The presence of a strong low level jet over
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peninsular India favors the formation of monsoon depressions in the North Bay of Bengal
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(Sikka, 1977).
The Indian monsoon system is dominated by a distinct clockwise monsoon gyre
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centered in the equatorial Indian Ocean, which ties the Indian monsoon trough, Somalia jet
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and southeast winds associated with Mascarene High (Krishnamurti and Bhalme, 1976). Swapna and Ramesh Kumar (2002) have examined the role of low-level jet flow on
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SM activity for the two contrasting monsoon seasons, 1987 (poor monsoon) and 1988 (active monsoon). According to them, the magnitudes of the winds over the Arabian Sea are high when compared to the central Bay of Bengal and southern Indian Ocean. The western Arabian Sea has shown great contrast, where wind speeds are about 2 m/s higher in active monsoon than in poor monsoon. During the active SM periods, the core of the jet is directed
ACCEPTED MANUSCRIPT to the Indian subcontinent, producing heavy rainfall over India. Whereas the jet core directed south of the Indian peninsula leads to weak monsoon conditions over India. Ramesh Kumar et al.,1999 have examined an annual cycle of the moisture fluxes across different boundaries of the Arabian Sea (0° to 25°N, 45° to 75°E) for the period 1982
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to 1994. The major contribution of moisture for the Arabian Sea is from the south Indian
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Ocean and a substantial amount also comes from the western boundary, i.e. across the east
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coast of Africa. The moisture fluxes are able to depict the enhancement of moisture over the Arabian Sea from April to June. The values are thus saturated and remain constant till July.
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From August, the cross equatorial flow decreases. The maximum contribution for the
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Arabian Sea box from the western boundary is seen in July and August. The maximum loss from this box is to the Indian subcontinent and it increases almost two to three times from
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April to May/June, indicating evolution of the SM over India. There exists a sharp rise in the
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moisture flux in the Arabian Sea and this rise could be due to the cloudiness effect of sudden
over the Arabian Sea.
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increase in the zonal component of the low-level winds as well as the increase in moisture
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Joseph and Simon (2005) reported a weakening trend in the strength of the low level jet. Consistent with the weakening of low level jet, a weakening in the tropical
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easterly jet is also reported (Sathiyamoorthy, 2005; Abish et al., 2013). The low level jet generally supports the large-scale moisture and momentum transport from the ocean to the atmosphere and the consequent rainfall over the Indian mainland. The low level jet is elongated in the west-east direction over the Indian peninsula during the active phase of monsoon. Prior to the break, splitting of the low level jet takes place due to forced flow from the north; the northern branch of the low level jet disappears and southern branch passes
ACCEPTED MANUSCRIPT through the south of the Indian subcontinent taking all the moisture towards the equator. This is known as break monsoon, which was experienced during 4–5 July and 12–15 August 1995. Low level jet is a good indicator for active and break phases of Indian monsoon (Mishra et.al., 2004).
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According to Naidu et al., 2011, India experienced SM epoch (1995–2005) which is
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closely associated with weak tropical easterly jet stream. This period is very much coinciding
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with the warmest decade. The weakened easterly jet stream is ultimately due to the reduced moisture advection as associated with weaker surface westerlies, and thus rainfall.
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The decrease of SM rainfall over India, the decrease in the frequency of cyclonic
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systems over the Bay of Bengal in SM season, the decrease of vertical shear in the zonal wind over the monsoon depression prone area, the negative values of the Southern
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Oscillation Index (SOI), the weakening of tropical easterly jet in the upper troposphere and
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weakening of westerly belt in the lower troposphere are identified in the global warming era (Naidu et al., 2011a).
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According to Naidu et al., 2015, the SM rainfall has decreased over the major part of
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India during the recent three decades of the global warming era. This decrease is associated with the weakening of the westerlies in the lower troposphere, easterly jet stream in the upper
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troposphere over India and neighborhood and an increase in the frequency of the warmest years over the globe. Further, it is associated with the relaxation of the north–south temperature gradient as well as SST gradient over the Indian Ocean region and periphery. Further, a decrease in the soil moisture over India is observed in the warming environment. It seems that the global warming environment inhibits the rainfall activity over India in SM season, but enhances the extreme rainfall events. There is a dire need to take proper
ACCEPTED MANUSCRIPT care in the preservation of water resources and diverting water from flooded areas to drought areas (Naidu et al., 2015a). From the above, it is worthwhile to mention that the lower tropospheric circulation pattern which is just above the planetary boundary layer plays a vital role in the intraseasonal
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variations of the SM system over India. So, the authors have made an attempt to examine the
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relationship between the IMI which is formulated by Wang and Fan (1999) with the zonal
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winds in the lower troposphere and the rainfall activity over India in the SM season. The present study pinpoints the importance of the lower and upper tropospheric zonal winds in
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the variability of summer monsoon. The rainfall distribution point out the break or active
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monsoon conditions over India in the negative or positive extremes of IMI. 2. Data and Method of Analysis
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The rainfall amounts in the SM months, June, July, August and September over 30
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meteorological subdivisions (Figure 1) over Indian main land are considered from the website of Indian Institute of Tropical Meteorology, Pune, India (http://www.tropmet.res.in).
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The hilly subdivisions due to sparse rain-gauge network and island subdivisions to maintain
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contiguity have not been considered. The IMI designed by Wang and Fan (1999) is considered from the website,
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http://apdrc.soest.hawaii.edu/projects/monsoon/realtime-monidx.html.
The IMI is U850
(400E-800E, 50N-150N) minus U850 (700E-900E, 200N-300N). U850 represents the average zonal wind component at 850 hPa level in the corresponding domain. The normalized values of IMI in June, July, August, September and the whole monsoon season (June through September) for the duration 1953-2012 have been used. The relationships between the IMI in individual months and whole season and the corresponding subdivisional rainfall amounts
ACCEPTED MANUSCRIPT are studied. The positive and negative extremes are evaluated basing on the normalized values of the deviations from the mean of the IMI. If the normalized value of the IMI is greater (less) than or equal to 1.0 (-1.0), then the corresponding event is taken as a positive (negative) extreme.
Composite rainfall distributions over India and the zonal wind
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distributions in the lower and upper troposphere in positive and negative extremes of IMI are
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evaluated separately.
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The correlations are tested statistically using the student’s t-test. The statistic, “t” is given by t = r [(n-2) / (1-r2)]1/2, where r and n represent correlation coefficient and number
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of pairs of parameters respectively. The n-2 is the degrees of freedom. If the calculated
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value of “t” is equal to or greater than the table value [1.65, 1.96 and 2.58], the correlation is significant statistically at that particular level of significance [10%, 5% and 1% respectively].
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The correlation values 0.21, 0.25 and 0.32 are significant at 10%, 5% and 1% levels
3 Results and Discussion
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respectively.
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3.1 Relation between IMI and Indian SM rainfall
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In SM season, the IMI is a lateral shear at 850 hPa level between the westerlies over
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the domain (400E-800E, 50N-150N) and the easterlies over the domain (700E-900E, 200N300N). If the zonal winds of the two regions are having more strength, the lateral shear i.e., IMI will be high and responsible for the formation of more number of cyclonic systems over the Bay of Bengal. Hence, good amount of rainfall will be recorded over the central parts of India particularly. The reverse situation is also true for less intensified zonal winds of the two regions.
ACCEPTED MANUSCRIPT Naidu et al., 2015 pointed that particularly there is a decreasing trend in the summer monsoon rainfall over India during the recent 60 year period [1953–2012] [trends in the smoothed series and original series are −0.6 mm/yr and −1.1 mm/yr respectively]. The period 1953–2012 is divided into two parts [1953–1982 and 1983– 2012] for the analysis. In this
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study, the same pattern has been implemented. The means of normalized values of IMI in
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June, July, August, September and SM season for the duration 1953-1982, 1983-2012 and
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1953-2012 and the differences in the mean values (mean value for 1983-2012 minus mean value for the 1953-1982) are given in table-1.
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During 1953-2012, the means of normalized values of IMI are positive in July and
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August and negative in June and September. July and August are the prime monsoon months and contribute major amount of rainfall over India. In June, the SM system is not fully
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established over the entire Indian mainland. Monsoon takes ten days (ie., from 1st June to
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10th June) to cover the Indian mainland area south of 200N and northeastern parts of India, and 35 days more to cover the remaining part of India. Hence, good amount of rainfall over
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the northwestern parts of India and adjoining areas cannot be anticipated in June.
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By 1st September, the SM starts to retrieve from the northwestern parts of India. Hence the rainfall in September over Northwestern parts of India and surrounding region is
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not adequate. During 1953-1982, the IMI mean values are positive in July, August and September and negative in June. These positive values are responsible for active monsoon epoch during 1953-1982. But, the signs of mean values of IMI in different months for latest three decades (1983-2012) are reversed. Positive values of IMI in June lead to enhanced rainfall amounts over India in the recent three decades of global warming. Enhanced rainfall activity in June over India is discussed by Naidu et al., 2015. But negative values of IMI in
ACCEPTED MANUSCRIPT the rest of monsoon months result in decline of monsoon activity over India. The differences of the mean values of IMI (mean for 1983-2012 minus mean for 1953-1982) are positive for June, and negative for July, August, September and whole SM season. These deviations/differences may not be favorable for normal or surplus SM rainfall amounts over
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India in the global warming period i.e. recent 3 decades (Naidu et al., 2015).
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The correlations between the IMI in different months and the corresponding sub-
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divisional rainfall over India during 1953-2012 are presented here.
June: Only four subdivisions (three subdivisions in Northeast India, one in the East
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coast (Tamil Nadu)) showed negative correlations. Assam & Meghalaya showed significant Twenty four
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negative correlation. Twenty six subdivisions showed positive relations.
subdivisions out of twenty six subdivisions showed high positive relationships which are
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significant at more than 10% level. (Figure 2a).
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July: Only seven subdivisions (Assam & Meghalaya, Nagaland, Manipur, Mizoram and Tripura, Sub-Himalayan West Bengal & Sikkim and Gangetic West Bengal in Northeast
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India, Bihar and East Uttar Pradesh in Central Northeast India, and Tamil Nadu) showed
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negative relationships. The correlations over the subdivisions shown above with bold italic letters are statistically significant at more than 10% level.
Remaining twenty three
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subdivisions showed positive relationships. Twenty subdivisions out of twenty three subdivisions established significant positive relationships (Figure 2b). August: Only eight subdivisions (Assam & Meghalaya, Nagaland-ManipurMizoram & Tripura, Sub-Himalayan West Bengal & Sikkim, Gangetic West Bengal and Bihar in Northeast India and neighborhood; Rayalaseema, Tamil Nadu and South Interior Karnataka in South Peninsular India) showed negative relationships. The correlations over
ACCEPTED MANUSCRIPT the subdivisions shown above with bold italic letters are significant at more than 10% level. Remaining twenty two subdivisions showed positive relationships with rainfall. Rainfall over eighteen subdivisions out of twenty two divisions showed significant positive relationships with IMI in August (Figure 2c).
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September: Only six subdivisions (Assam & Meghalaya, Nagaland, Manipur,
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Mizoram & Tripura and Sub-Himalayan West Bengal & Sikkim in Northeast India; East
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Uttar Pradesh, Tamil Nadu and South Interior Karnataka) showed negative relationships. Two subdivisions (shown by bold italic letters) out of the six subdivisions showed significant
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negative relationships. Remaining twenty four subdivisions showed the direct relationships;
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out of which, twenty two subdivisions showed significant positive relationships (Figure 2d). SM Season: The correlations between IMI in the monsoon season and Indian
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subdivisional SM rainfall are shown in Figure 3.
Only three subdivisions (Assam &
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Meghalaya, Sub-Himalayan West Bengal, Bihar) in northeast India and surrounding showed negative correlations which are not significant. Remaining twenty seven subdivisions showed
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positive correlations; the correlations over twenty four subdivisions are significant at more
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than 10% level. The correlation pattern clearly demonstrates the importance of IMI over Indian SM rainfall activity.
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From the above results, it is worthwhile to pinpoint that the IMI in the individual months as well as whole SM season showed a profound influence on the activity of rainfall over major part of India. The relationship over major part of India is positive and highly significant. Extreme events: The IMI is very much related with monsoon rainfall over the subdivisions, East Rajasthan, West Madhya Pradesh, Gujarat, Vidarbha and Chhattisgarh
ACCEPTED MANUSCRIPT and significantly with all India SM rainfall. The years having the normalized values of IMI in SM season (June through September) less than or equal to -1.0 are 2009, 1987, 1972, 2012, 1965, 1966, 1974, 1997 and 1979. The years having normalized values greater than or equal to 1.0 are 1980, 1973, 2005, 1983, 1954, 1978, 1994, 1958, 1956, 1975, 1970, 1961 and
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1959. The normalized values of the IMI, the corresponding years and the all-India SM
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rainfall amounts are shown in the table (Table 2). The area-weighted average SM rainfall of
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the 30 meteorological subdivisions of India constitutes all-India SM rainfall. In Gujarat, the composite rainfall is 620.1 mm and 1134.1 mm for negative and
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positive extreme events respectively. East Rajasthan also experienced high amounts of
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rainfall (745.3 mm) in extreme positive events when compared to extreme negative events (480.2 mm). West Madhya Pradesh experienced 704.4 mm rainfall in negative extremes and
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1017.4 mm in the positive extremes. Vidarbha also reported high rainfall (1099.2 mm) in While in
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the positive extremes when compared to the negative extremes (767.1 mm).
Chhattisgarh, the difference between the composite rainfall amounts in positive extremes
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(1321.4 mm) and negative extremes (932.5 mm) is also high. Further, all-India SM rainfall
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enhances in positive extreme events. The difference of the composite rainfall amounts in negative extremes (730.5 mm) and positive extremes (930.3 mm) is 199.8 mm.
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Next, the composite rainfall distributions during SM season in negative and positive extremes of IMI are evaluated to examine the spatial distribution of the rainfall (Figure 4). Their differences (rainfall distribution for positive extreme minus rainfall distribution for the negative extreme) are also shown in Figure 4. Twenty five subdivisions showed positive differences. The rainfall is abnormally increased in positive events in some subdivisions. The differences are high over Konkan and Goa (631 mm), Gujarat (514 mm) and Coastal
ACCEPTED MANUSCRIPT Karnataka (433 mm). The rainfall amounts in positive events are enhanced by 313 mm to 389 mm over five subdivisions (region comprising Chhattisgarh, East Madhya Pradesh, Vidarbha and West Madhya Pradesh, and the west coast subdivision (Kerala)). Over nine subdivisions (Punjab, Haryana, Chandigarh and Delhi, East Uttar Pradesh, West Uttar Pradesh, East
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the rainfall differences range from 203 mm to 287 mm.
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Rajasthan (over north India), Saurashtra, Kutch & Diu, Jharkhand, Marathwada, Telangana),
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The differences are about 104 mm – 156 mm over Madhya Maharashtra, North Interior Karnataka, Orissa, Coastal Andhra Pradesh and West Rajasthan. The differences
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are 47 mm – 70 mm over South Interior Karnataka, Rayalaseema and Gangetic West Bengal.
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Only five subdivisions showed reduced rainfall in extreme positive events. They are Tamil Nadu and Pondicherry, Bihar, Nagaland, Manipur, Mizoram & Tripura, Sub-Himalayan
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West Bengal & Sikkim and Assam & Meghalaya (Northeast India and adjoining area). The
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difference over Coastal Karnataka is negligible when compared to its mean amounts. Here Northeast India is in phase with Tamil Nadu but not with the rest of India. Northeast India
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constitutes a separate homogeneous region. In the break monsoon situation, Northeast India
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and Tamil Nadu can experience good amount of rainfall. This situation inhibits the rainfall activity over the rest of India. Generally in SM season, the rain-shadow effect due to
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presence of Western Ghats results in less amounts of rainfall over Tamil Nadu. At the time of onset of SM over south peninsular India, the coastal stations of Tamil Nadu do not show any rise in rainfall activity (Subaramayya et al., 1988; Subbaramayya and Naidu, 1992). 3.2 Zonal wind distributions in positive and negative extremes of IMI 850 hPa: The composite zonal wind pattern at 850 hPa in SM season for positive and negative extremes of IMI and their differences (zonal wind in positive extremes minus zonal wind in
ACCEPTED MANUSCRIPT negative extremes) are shown in Figure 5. Particularly westerlies associated with low level jet stream (Somali jet) in the positive extremes are intensified (10-16 m/s) when compared to those in the negative extremes (8-12 m/s). Also the wind shear between northern latitudes and southern latitudes of the selected domain is dominant in the positive extremes. The
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differences between the above two patterns (Positive extreme wind pattern minus negative
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extreme wind pattern) clearly indicate the presence of positive values particularly south of
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200N and negative values in the northern region from this latitude. This type of pattern clearly indicates that the strong (weak) low level zonal winds in the positive (negative)
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extreme events facilitate more (less) moisture transport from both the Arabian Sea and the
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Bay of Bengal to the Indian mainland.
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150 hPa: The composite zonal wind pattern at 150 hPa in SM season for positive and negative extremes of IMI and their differences (zonal winds in positive extremes minus zonal
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winds in negative extremes) are shown in Figure 6. The width of easterly winds is more in
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the positive extremes of IMI when compared to that in the negative extremes. The zero line (almost coinciding with subtropical ridge position) is situated around 27.50N in the positive
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and 250N in the negative extremes. In extreme positive events, the subtropical ridge is shifted
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to northern latitudes when compared to the negative extremes. Also the strength of the easterlies is more (maximum is about -30 m/s) in the positive extremes when compared to that in the negative extremes (-25 m/s). The differences in the wind pattern (wind pattern in positive IMI extreme minus wind pattern in negative IMI extreme) for the above two extremes pinpoint the presence of negative values over the entire region except for the small regions over northwestern parts of the selected domain. This pattern clearly indicates the presence of more intensified tropical
ACCEPTED MANUSCRIPT easterly jet stream over southern latitudes and less intensified westerly winds over northern parts of India in the positive extreme of the IMI. The positive IMI extreme contributes good amounts of rainfall over India and high amounts of energy to atmospheric system due to condensation process. This results in more north-south temperature gradient and hence high
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intensified easterly jet over South Peninsular India and neighborhood. Reverse is true for
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negative extremes.
Wind Shear: The distribution of the vertical shear in the zonal wind for the levels
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850 hPa and 150 hPa for positive and negative extremes of IMI and their differences (distribution in the positive minus distribution in the negative) are shown in Figure 7. In the
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positive extremes, the areas to the south and north of 290-300N show negative and positive
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values respectively. But in the negative extremes, the zero line shifts to 270-280N. The magnitude of negative shear (when compared to that of positive shear) is more in both
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extremes. The magnitude of the negative shear is more in the positive extremes when
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compared to the negative extremes. But the magnitude of positive shear is slightly less over northern most latitudes of the selected domain in positive extremes when compared to the
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negative extremes.
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According to Naidu et. al. (2011),
the vertical wind shear in the zonal wind
component (zonal wind at 100 hPa minus zonal wind at 850 hPa) for the domain (5°–22°N and 80°–100°E) shows decreasing tendency (0.1532 m/s per year), which is significant at 0.1% level, for the period, 1950–2005. Particularly it has low values in global warming era. The past studies indicate that the baroclinic instability is responsible for the generation of the monsoon depressions (Shukla, 1977, 1978; Mishra and Salvekar, 1980; Moorti and Arakawa, 1985; Aravequia et al., 1995). They considered the baroclinic instability of the zonal wind
ACCEPTED MANUSCRIPT with easterly shear associated with the tropical easterly jet. They showed that higher (lower) wind shear leads to higher (lower) growth rates of the depressions with or without the cumulus heating. Srinivasa Rao et al., 2004 quoted the above works and concluded their results that an increase (decrease) of the easterly shear in the earlier (latter) part of their study
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period leads to monsoon depressions with higher/lower growth rates; disturbances with weak
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growth rates may not sustain the frictional dissipation and may not develop. They presume
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that this may explain the decrease in the number of monsoon cyclonic systems when the
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shears are weak and increase when the shears are strong. 4. Conclusions
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It is well established fact that the Indian SM rainfall oscillates in association with the strength of the low level winds in the Arabian Sea and the Bay of Bengal. These winds
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facilitate the transport of moisture from the ocean to hot Indian main land and are responsible
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for convective activity. This results in the rainfall activity over India. The IMI values established good direct relationships with the subdivisional SM
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rainfall over India. The IMI is the lateral shear of the zonal wind component between the
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northern and the southern latitudes of the Indian SM regime. The results pinpoint that the large shear leads to active SM conditions over India.
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During 1953-1982, the IMI mean values are positive in July, August and September and negative in June. These positive values are responsible for good monsoon epoch during 1953-1982. The signs of mean values in different months of IMI are reversed in the period 1983-2012. Positive values of IMI in June lead to enhanced rainfall amounts over India in the recent three decades of global warming. July and August months are prime monsoon months. In the same period, the negative values of IMI in these two months result in
ACCEPTED MANUSCRIPT weakening of the monsoon system over India and hence poor amounts of rainfall are reported. The presence of more intensified easterly jet stream over southern latitudes and less intensified westerly winds over northern parts of India in the upper troposphere is a feature of
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the positive extreme of IMI. Reverse is true for the negative extreme of IMI.
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The rainfall distributions in the negative and positive extremes of the IMI reveal that
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Northeast India is in phase with Tamil Nadu but not with the rest of India. Northeast India constitutes a separate homogeneous region. In break monsoon situation, Northeast India and
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Tamil Nadu can experience good amount of rainfall. The condition favorable for Northeast
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India and Tamil Nadu inhibits the activity of rainfall over the rest of India. Acknowledgments
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One of the authors, P.Vinay Kumar wishes to thank the DST, Government of India
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for providing a research fellowship [DST/INSPIRE Fellowship/ IF160113]. The authors are thankful to the IITM, Pune, India and NCEP/NCAR Reanalysis team for providing data and
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also to DST for providing the computer facility under FIST programme to our department .
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suggestions.
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The authors are thankful to the editor and the reviewers for their constructive comments and
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Joseph, P.V., Simon, A., 2005a. Weakening trend of monsoon Low Level Jet Stream through India 1950–2003. CLIVAR Exchanges 10 (3), 27–30.
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Joseph, P. V., Raman P.L., 1966. Existence of low-level westerly jet-stream over peninsular India during July, Indian J. Meteorol. Phys., 17, 407–410. Koteswaram, P., 1958. The easterly jet stream in the tropics. Tellus 10, 43–57. Krishnamurti, T. N., Bhalme H. N., 1976. Oscillations of monsoon system. Part I: Observational aspects. J. Atmos. Sci. 45, 1937–1954. Mishra, A. K., Gnanaseelan C., Seetaramayya, P., 2004. A study of rainfall along the west coast of India in relation to low level jet and air–sea interactions over the Arabian Sea. Curr. Sci. 87 (4),475-485.
ACCEPTED MANUSCRIPT Mishra, S.K., Salvekar, P.S., 1980. Role of baroclinic instability in the development of monsoon disturbances. J. Atmos. Sci. 37, 383–394. Moorthi, S., Arakawa A., 1985. Baroclinic instability with cumulus heating. J. Atmos. Sci. 42, 2007-2031.
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Naidu, C.V., Muni Krishna, K., Ramalingeswara Rao, S., Bhanu Kumar, O.S.R.U., Durgalakshmi, K., Ramakrishna, S.S.V.S., 2011. Variations of Indian summer monsoon rainfall induce the weakening of easterly jet stream in the warming environment?, Glob. Planet. Chang. 75, 21–30.
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Naidu, C.V., Durgalakshmi, K., Satyanarayana, G.Ch., Malleswararao, L., Ramakrishna, S.S.V.S., Jaddu Rama Mohan, Nagaratna, K., 2011a. An observational evidence of climate change during global warming era?, Glob. Planet. Chang. 79, 11–19.
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Naidu, C.V., Dharma Raju, A., Satyanarayana, G.Ch., Vinay Kumar, P., Chiranjeevi, G., P. Suchitra 2015a. An observational evidence of decrease in Indian summer monsoon rainfall in the recent three decades of global warming era. Glob. Planet. Chang. 127, 91–102.
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Naidu, C.V., Dharma Raju, A., Satyanarayana, G.Ch., Vinay Kumar, P., Chiranjeevi, G., P. Suchitra 2015. An observational evidence of decrease in Indian summer monsoon rainfall in the recent three decades of global warming era. Glob. Planet. Chang. 127, 91–102.
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Puranik, S. S., Sinha Ray, K. C., Sen, P. N., Kumar, P. P., 2013. An index for predicting the onset of monsoon over Kerala. Curr. Sci. 2013, 105(7), 954–961.
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Rajeevan, M., 1993. Inter-relationship between NW Pacific typhoon activity and Indian summer monsoon on inter-annual and intra-seasonal time-scales. Mausam 44, 109–111.
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Ramesh Kumar, M. R., Shenoi, S. S. C., Schluessel, P., 1999. On the role of the cross equatorial flow on summer monsoon rainfall over India using NCEP/NCAR reanalysis data. Meteorol. Atmos. Phys., 1999, 70, 201–213.
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Sathiyamoorthy, V., 2005. Large scale reduction in the size of the Tropical Easterly Jet. Geophysical Research Letters 32 (14): n/a-n/a. doi: 10.1029/2005gl022956. Shukla, J., 1977. Barotropic–baroclinic instability of mean zonal wind during summer monsoon. Pure Appl. Geophys. 115, 1449–1462. Shukla, J., 1978. CISK barotropic–baroclinic instability and the growth of monsoon depressions. J. Atmos. Sci. 35, 495–508. Sikka, D.R., 1977. Some aspects of the life history, structure and movement of monsoon depressions. PAGEOPH 115, 1501–1529.
ACCEPTED MANUSCRIPT Srinivasa Rao, B.R., Bhaskar Rao, D.V., Brahmananda Rao, V., 2004. Decreasing trend in the strength of tropical easterly jet during the Asian summer monsoon season and number of tropical cyclonic systems over Bay of Bengal. Geophys. Res. Lett. 31. doi:10.1029/2004GL019817. Subbaramayya, I., Naidu, C.V., 1992. Spatial variations and trends in the Indian monsoon rainfall. International J. Clim. 12, 597-609, 1992.
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Subbaramayya, I., Vivekananda Babu, S., Naidu, C.V., 1988. A note on the normal dates of onset of summer monsoon over south peninsular India, Met. Mag. (U.K.), 117, 371-377.
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Swapna, P., Ramesh Kumar, M. R., 2002. Role of low-level flow on the summer monsoon rainfall over the Indian subcontinent during two contrasting monsoon years. J. Indian Geophys. Union, 2002, 6, 123–137.
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Wang, B., Fan, Z., 1999. Choice of South Asian summer monsoon indices. Bull. Amer. Meteor. Soc., 80, 629-638.
ACCEPTED MANUSCRIPT Figure Captions: Figure 1: Meteorological subdivisions of India Figure 2: The correlations between the Indian Monsoon Index in different months and the corresponding subdivisional rainfall ( a: June, b: July, c: August and d: September)
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Figure 3: The correlations between the Indian Monsoon Index in summer monsoon season and the corresponding subdivisional summer monsoon rainfall over India
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Figure 4: Composite Summer Monsoon Rainfall for (a) Positive extreme events of IMI, (b) Negative extreme events of IMI and (c) their differences
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Figure 5: Composite zonal wind pattern at 850hPa for summer monsoon season in positive (upper level left panel) and negative (upper level right panel) extreme years of IMI and their differences (Positive extreme minus Negative Extreme) (lower panel)
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Figure 6: Composite zonal wind pattern at 150hPa for summer monsoon season in positive (upper level left panel) and negative (upper level right panel) extreme years of IMI and their differences (Positive extreme minus Negative Extreme) (lower panel)
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Figure 7: Composite zonal wind shear pattern for summer monsoon season in the layer 150hPa- 850hPa in positive (upper level left panel) and negative (upper level right panel) extreme years of IMI and their differences (Positive extreme minus Negative Extreme) (lower panel)
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Means of the normalized values of Indian Monsoon Index (IMI)
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Table-2: Summer Monsoon Rainfall for all-India in the extreme events of the Indian Monsoon Index (IMI). NV indicates Normalized value of IMI.
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Figure 1: Meteorological subdivisions of India
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b
June Rainfall and Indian Monsoon Index (1953-2012) 0.32 0.38
July Rainfall and Indian Monsoon Index (1953-2012) 0.34 0.35
-0.11
-0.54
0.42 0.15
0.52
0.5
0.42
0.35
0.61 0.56
0.55 0.46
0.58 0.39
0.52
0.51 0.35 0.53
0.14 0.50
0.27
0.02
-0.03
-0.07
0.32
0.37
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0.43
0.43
0.44 0.09
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-0.12
0.39 0.18
0.48
0.28
0.41
-0.02
0.44
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0.42
-0.37
0.34
0.34 0.56
0.61
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0.17
September Rainfall and Indian Monsoon Index (1953-2012)
d
August Rainfall and Indian Monsoon Index (1953-2012)
0.42 0.53 0.53
0.35
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0.55
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0.41 0.56 0.22
-0.09 0.27
0.45 0.13
0.67
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Figure 2: The correlations between the Indian Monsoon Index in different months and the corresponding subdivisional rainfall ( a: June, b: July, c: August and d: September)
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JJAS Rainfall and Indian Monsoon Index (1953-2012) 0.41 0.49
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Figure 3: The correlations between the Indian Monsoon Index in summer monsoon season and the corresponding subdivisional summer monsoon rainfall over India
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a
Negative Extreme Rainfall (mm) events of Indian summer monsoon index for JJAS(1953-2012)
b
Positive Extreme Rainfall (mm) events of Indian summer monsoon index for JJAS(1953-2012)
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611.1 546 835.4
342
1837.6 937.6
212.4
1393.2 1233.5
1127.4 1167 1247 1321 1181.4 1099.2
1134 607 2657
366 2026
511
855.5
687.2
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501.7
1721.2
311.3
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2058
465
414
1789
461.3 571.6
2223
1272.7
599.5
543.9
606
1563.4
546.7
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820 667
965.8
924.3 1098 704.4 875.1 932.5 1039 767.2
620
1017
662.5
480.2
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949.6
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316.4 745.3
1994.3
568
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c
Figure 4: Composite Summer Monsoon Rainfall for (a) Positive extreme events of IMI, (b) Negative extreme events of IMI and (c) their differences
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Means of the normalized values of Indian Monsoon Index (IMI)
June
July
August
September
1953-2012
-0.0049
0.0186
0.053183
-0.01813
0.019533
1953-1982 (A)
-0.0838
0.167633
0.258067
0.132467
0.1988
0.074
-0.13043
-0.1517
0.1578
-0.29807
-0.40977
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(B) minus (A)
-0.16873
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1983-2012 (B)
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Duration
Monsoon season
-0.3012
-0.15973 -0.35853
ACCEPTED MANUSCRIPT Table-2: Summer Monsoon Rainfall for all-India in the extreme events of the Indian Monsoon Index (IMI). NV indicates Normalized value of IMI.
-1.8
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Rainfall in mm 882.6 913.2 856.8 955.6 885.2 909.2 952.7 889.1 983.1 962.5 939.7 1020.1 944 930.3
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Mean
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-2.6 -2.1 -2.1 -1.9 -1.9 -1.4 -1.4 -1.2 -1.2
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2009 1987 1972 2012 1965 1966 1974 1997 1979
Positive Extremes Rainfall Year NV in mm 1.0 667.6 1980 1.0 697 1973 1.0 2005 652.8 1.1 1983 780.7 1.2 709.2 1954 1.2 739.9 1978 1.7 1994 747.9 1.4 1958 871.4 1.4 707.7 1956 1.5 1975 1.5 1970 1.7 1961 2.0 1959 1.3 Mean 730.5
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Negative Extremes Year NV
ACCEPTED MANUSCRIPT Highlights In the recent three decades, the negative values of IMI in July and August lead to weakening of Indian monsoon system. The positive relationship between IMI and the monsoon rainfall over major part of
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India is highly significant.
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The rainfall variations in the Northeast India and Tamil Nadu are different from the
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rest of India.
More intensified easterly jet stream over India is a feature of the positive extreme of
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IMI.
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Strong easterlies over North Bay of Bengal facilitate the active monsoon conditions
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over India.
C.V. Naidu, A. Dharma Raju, P. Vinay Kumar, G.Ch. Satyanarayana PII: DOI: Reference:
S0921-8181(17)30034-6 doi: 10.1016/j.gloplacha.2017.08.016 GLOBAL 2632
To appear in:
Global and Planetary Change
Received date: Revised date: Accepted date:
25 January 2017 24 June 2017 28 August 2017
Please cite this article as: C.V. Naidu, A. Dharma Raju, P. Vinay Kumar, G.Ch. Satyanarayana , Perceptible changes in Indian summer monsoon rainfall in relation to Indian Monsoon Index, Global and Planetary Change (2017), doi: 10.1016/ j.gloplacha.2017.08.016
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ACCEPTED MANUSCRIPT Perceptible changes in Indian summer monsoon rainfall in relation to Indian Monsoon Index C. V. Naidu1, A. Dharma Raju2, P. Vinay Kumar1 and G. Ch. Satyanarayana3 1 Department of Meteorology and Oceanography, Andhra University, Visakhapatnam, India 2 India Meteorological Department, Hyderabad, Telangana, India Department of Atmospheric Science, KL University, Vaddeswaram, Guntur District, Andhra Pradesh, India
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Abstract
The changes in the summer monsoon rainfall over 30 meteorological subdivisions of
The relationship between the IMIs in different months
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studied for the period 1953-2012.
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India with respect to changes in circulation and the Indian Monsoon Index (IMI) have been
and whole season and the corresponding summer monsoon rainfall is studied and tested. The
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positive and negative extremes are evaluated basing on the normalized values of the
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deviations from the mean of the IMI. Composite rainfall distributions over India and the zonal wind distributions in the lower and upper troposphere of IMI’s both positive and
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negative extremes are evaluated separately and discussed. In the recent three decades of
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global warming, the negative values of IMI in July and August lead to weakening of the monsoon system over India. It is observed that the rainfall variations in the Northeast India
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are different from the rest of India except Tamil Nadu in general. Keywords: summer monsoon rainfall, Indian monsoon index, global warming.
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1. Introduction India receives 70% (about 85 cm) of its annual rainfall in the Summer Monsoon (SM) season (June-September) mainly due to the transport of moisture from Indian Ocean region
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(Naidu et al., 2011; 2015). The sea surface temperature (SST) and the low level winds
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facilitate the evaporation. The temperature gradient between the land and the ocean plays a
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vital role in transporting the moisture from the Indian Ocean to Indian mainland. Moisture transport is essential for the performance of the monsoon system. It has been observed by
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Cadet and Reverdin (1981) that 70% of the water vapor that reaches the west coast of India
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is from the Southern Hemisphere and 30% comes from the evaporation occurring over the Arabian Sea. The importance of the low level wind flow pattern in the transport of moisture
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from the oceanic area to the Indian mainland is discussed by many scientists (Ding, 2005;
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Puranik et al., 2013; Koteswaram, 1958, Joseph and Raman, 1966 and Findlater, 1969; Cadet & Desbois, 1979).
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The Somali jet stream flows intermittently from the vicinity of Mauritius over the
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northern tip of Madagascar to reach the Kenyan coast (3°S) and penetrates inland over the flat coastal strip of Kenya and lowlands of Ethiopia and Somalia and emerges out into the
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Arabian Sea near 9°N. From this point, it moves over the cold upwelling water off the Somalia coast. It then crosses the West coast of India and transports moisture to Indian subcontinent during the SM season. This cross-equatorial flow is an important part to the South as well as the East Asian SM systems (Ding, 2005; Puranik et al., 2013). The low level jet stream (Somali jet stream) is an important component of Indian SM with its core close to 850 hPa level over the Indian Ocean and south Asia (Koteswaram, 1958, Joseph and Raman, 1966 and Findlater, 1969) and brings the moisture generated by
ACCEPTED MANUSCRIPT trade winds over the South Indian Ocean and the evaporative flux from the Arabian Sea to the areas of rainfall production over south Asia. The cyclonic vorticity north of this jet in the atmospheric boundary layer is a dynamic forcing for the generation of vertical upward air motion and rainfall and for the genesis of depressions in the North Bay of Bengal (Joseph
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and Simon, 2005 and 2005a). During active SM period, the core of the jet passes eastward
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through peninsular India in between 12.5°N and 17.5°N; whereas in the break monsoon
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condition, it moves southeastward from the central Arabian Sea and by-passing India passes eastward between latitudes 2.5°N and 7.5°N (Joseph and Sijikumar, 2004). Thus in break
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monsoon spells, the large amount of moisture carried by the low level jet does not reach
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India, but goes to the west Pacific Ocean to feed the increased frequency of typhoons generating there (Joseph, 1990; Rajeevan, 1993). The presence of a strong low level jet over
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peninsular India favors the formation of monsoon depressions in the North Bay of Bengal
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(Sikka, 1977).
The Indian monsoon system is dominated by a distinct clockwise monsoon gyre
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centered in the equatorial Indian Ocean, which ties the Indian monsoon trough, Somalia jet
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and southeast winds associated with Mascarene High (Krishnamurti and Bhalme, 1976). Swapna and Ramesh Kumar (2002) have examined the role of low-level jet flow on
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SM activity for the two contrasting monsoon seasons, 1987 (poor monsoon) and 1988 (active monsoon). According to them, the magnitudes of the winds over the Arabian Sea are high when compared to the central Bay of Bengal and southern Indian Ocean. The western Arabian Sea has shown great contrast, where wind speeds are about 2 m/s higher in active monsoon than in poor monsoon. During the active SM periods, the core of the jet is directed
ACCEPTED MANUSCRIPT to the Indian subcontinent, producing heavy rainfall over India. Whereas the jet core directed south of the Indian peninsula leads to weak monsoon conditions over India. Ramesh Kumar et al.,1999 have examined an annual cycle of the moisture fluxes across different boundaries of the Arabian Sea (0° to 25°N, 45° to 75°E) for the period 1982
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to 1994. The major contribution of moisture for the Arabian Sea is from the south Indian
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Ocean and a substantial amount also comes from the western boundary, i.e. across the east
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coast of Africa. The moisture fluxes are able to depict the enhancement of moisture over the Arabian Sea from April to June. The values are thus saturated and remain constant till July.
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From August, the cross equatorial flow decreases. The maximum contribution for the
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Arabian Sea box from the western boundary is seen in July and August. The maximum loss from this box is to the Indian subcontinent and it increases almost two to three times from
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April to May/June, indicating evolution of the SM over India. There exists a sharp rise in the
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moisture flux in the Arabian Sea and this rise could be due to the cloudiness effect of sudden
over the Arabian Sea.
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increase in the zonal component of the low-level winds as well as the increase in moisture
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Joseph and Simon (2005) reported a weakening trend in the strength of the low level jet. Consistent with the weakening of low level jet, a weakening in the tropical
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easterly jet is also reported (Sathiyamoorthy, 2005; Abish et al., 2013). The low level jet generally supports the large-scale moisture and momentum transport from the ocean to the atmosphere and the consequent rainfall over the Indian mainland. The low level jet is elongated in the west-east direction over the Indian peninsula during the active phase of monsoon. Prior to the break, splitting of the low level jet takes place due to forced flow from the north; the northern branch of the low level jet disappears and southern branch passes
ACCEPTED MANUSCRIPT through the south of the Indian subcontinent taking all the moisture towards the equator. This is known as break monsoon, which was experienced during 4–5 July and 12–15 August 1995. Low level jet is a good indicator for active and break phases of Indian monsoon (Mishra et.al., 2004).
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According to Naidu et al., 2011, India experienced SM epoch (1995–2005) which is
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closely associated with weak tropical easterly jet stream. This period is very much coinciding
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with the warmest decade. The weakened easterly jet stream is ultimately due to the reduced moisture advection as associated with weaker surface westerlies, and thus rainfall.
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The decrease of SM rainfall over India, the decrease in the frequency of cyclonic
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systems over the Bay of Bengal in SM season, the decrease of vertical shear in the zonal wind over the monsoon depression prone area, the negative values of the Southern
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Oscillation Index (SOI), the weakening of tropical easterly jet in the upper troposphere and
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weakening of westerly belt in the lower troposphere are identified in the global warming era (Naidu et al., 2011a).
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According to Naidu et al., 2015, the SM rainfall has decreased over the major part of
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India during the recent three decades of the global warming era. This decrease is associated with the weakening of the westerlies in the lower troposphere, easterly jet stream in the upper
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troposphere over India and neighborhood and an increase in the frequency of the warmest years over the globe. Further, it is associated with the relaxation of the north–south temperature gradient as well as SST gradient over the Indian Ocean region and periphery. Further, a decrease in the soil moisture over India is observed in the warming environment. It seems that the global warming environment inhibits the rainfall activity over India in SM season, but enhances the extreme rainfall events. There is a dire need to take proper
ACCEPTED MANUSCRIPT care in the preservation of water resources and diverting water from flooded areas to drought areas (Naidu et al., 2015a). From the above, it is worthwhile to mention that the lower tropospheric circulation pattern which is just above the planetary boundary layer plays a vital role in the intraseasonal
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variations of the SM system over India. So, the authors have made an attempt to examine the
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relationship between the IMI which is formulated by Wang and Fan (1999) with the zonal
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winds in the lower troposphere and the rainfall activity over India in the SM season. The present study pinpoints the importance of the lower and upper tropospheric zonal winds in
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the variability of summer monsoon. The rainfall distribution point out the break or active
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monsoon conditions over India in the negative or positive extremes of IMI. 2. Data and Method of Analysis
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The rainfall amounts in the SM months, June, July, August and September over 30
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meteorological subdivisions (Figure 1) over Indian main land are considered from the website of Indian Institute of Tropical Meteorology, Pune, India (http://www.tropmet.res.in).
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The hilly subdivisions due to sparse rain-gauge network and island subdivisions to maintain
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contiguity have not been considered. The IMI designed by Wang and Fan (1999) is considered from the website,
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http://apdrc.soest.hawaii.edu/projects/monsoon/realtime-monidx.html.
The IMI is U850
(400E-800E, 50N-150N) minus U850 (700E-900E, 200N-300N). U850 represents the average zonal wind component at 850 hPa level in the corresponding domain. The normalized values of IMI in June, July, August, September and the whole monsoon season (June through September) for the duration 1953-2012 have been used. The relationships between the IMI in individual months and whole season and the corresponding subdivisional rainfall amounts
ACCEPTED MANUSCRIPT are studied. The positive and negative extremes are evaluated basing on the normalized values of the deviations from the mean of the IMI. If the normalized value of the IMI is greater (less) than or equal to 1.0 (-1.0), then the corresponding event is taken as a positive (negative) extreme.
Composite rainfall distributions over India and the zonal wind
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distributions in the lower and upper troposphere in positive and negative extremes of IMI are
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evaluated separately.
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The correlations are tested statistically using the student’s t-test. The statistic, “t” is given by t = r [(n-2) / (1-r2)]1/2, where r and n represent correlation coefficient and number
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of pairs of parameters respectively. The n-2 is the degrees of freedom. If the calculated
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value of “t” is equal to or greater than the table value [1.65, 1.96 and 2.58], the correlation is significant statistically at that particular level of significance [10%, 5% and 1% respectively].
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The correlation values 0.21, 0.25 and 0.32 are significant at 10%, 5% and 1% levels
3 Results and Discussion
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respectively.
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3.1 Relation between IMI and Indian SM rainfall
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In SM season, the IMI is a lateral shear at 850 hPa level between the westerlies over
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the domain (400E-800E, 50N-150N) and the easterlies over the domain (700E-900E, 200N300N). If the zonal winds of the two regions are having more strength, the lateral shear i.e., IMI will be high and responsible for the formation of more number of cyclonic systems over the Bay of Bengal. Hence, good amount of rainfall will be recorded over the central parts of India particularly. The reverse situation is also true for less intensified zonal winds of the two regions.
ACCEPTED MANUSCRIPT Naidu et al., 2015 pointed that particularly there is a decreasing trend in the summer monsoon rainfall over India during the recent 60 year period [1953–2012] [trends in the smoothed series and original series are −0.6 mm/yr and −1.1 mm/yr respectively]. The period 1953–2012 is divided into two parts [1953–1982 and 1983– 2012] for the analysis. In this
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study, the same pattern has been implemented. The means of normalized values of IMI in
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June, July, August, September and SM season for the duration 1953-1982, 1983-2012 and
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1953-2012 and the differences in the mean values (mean value for 1983-2012 minus mean value for the 1953-1982) are given in table-1.
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During 1953-2012, the means of normalized values of IMI are positive in July and
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August and negative in June and September. July and August are the prime monsoon months and contribute major amount of rainfall over India. In June, the SM system is not fully
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established over the entire Indian mainland. Monsoon takes ten days (ie., from 1st June to
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10th June) to cover the Indian mainland area south of 200N and northeastern parts of India, and 35 days more to cover the remaining part of India. Hence, good amount of rainfall over
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the northwestern parts of India and adjoining areas cannot be anticipated in June.
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By 1st September, the SM starts to retrieve from the northwestern parts of India. Hence the rainfall in September over Northwestern parts of India and surrounding region is
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not adequate. During 1953-1982, the IMI mean values are positive in July, August and September and negative in June. These positive values are responsible for active monsoon epoch during 1953-1982. But, the signs of mean values of IMI in different months for latest three decades (1983-2012) are reversed. Positive values of IMI in June lead to enhanced rainfall amounts over India in the recent three decades of global warming. Enhanced rainfall activity in June over India is discussed by Naidu et al., 2015. But negative values of IMI in
ACCEPTED MANUSCRIPT the rest of monsoon months result in decline of monsoon activity over India. The differences of the mean values of IMI (mean for 1983-2012 minus mean for 1953-1982) are positive for June, and negative for July, August, September and whole SM season. These deviations/differences may not be favorable for normal or surplus SM rainfall amounts over
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India in the global warming period i.e. recent 3 decades (Naidu et al., 2015).
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The correlations between the IMI in different months and the corresponding sub-
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divisional rainfall over India during 1953-2012 are presented here.
June: Only four subdivisions (three subdivisions in Northeast India, one in the East
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coast (Tamil Nadu)) showed negative correlations. Assam & Meghalaya showed significant Twenty four
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negative correlation. Twenty six subdivisions showed positive relations.
subdivisions out of twenty six subdivisions showed high positive relationships which are
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significant at more than 10% level. (Figure 2a).
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July: Only seven subdivisions (Assam & Meghalaya, Nagaland, Manipur, Mizoram and Tripura, Sub-Himalayan West Bengal & Sikkim and Gangetic West Bengal in Northeast
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India, Bihar and East Uttar Pradesh in Central Northeast India, and Tamil Nadu) showed
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negative relationships. The correlations over the subdivisions shown above with bold italic letters are statistically significant at more than 10% level.
Remaining twenty three
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subdivisions showed positive relationships. Twenty subdivisions out of twenty three subdivisions established significant positive relationships (Figure 2b). August: Only eight subdivisions (Assam & Meghalaya, Nagaland-ManipurMizoram & Tripura, Sub-Himalayan West Bengal & Sikkim, Gangetic West Bengal and Bihar in Northeast India and neighborhood; Rayalaseema, Tamil Nadu and South Interior Karnataka in South Peninsular India) showed negative relationships. The correlations over
ACCEPTED MANUSCRIPT the subdivisions shown above with bold italic letters are significant at more than 10% level. Remaining twenty two subdivisions showed positive relationships with rainfall. Rainfall over eighteen subdivisions out of twenty two divisions showed significant positive relationships with IMI in August (Figure 2c).
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September: Only six subdivisions (Assam & Meghalaya, Nagaland, Manipur,
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Mizoram & Tripura and Sub-Himalayan West Bengal & Sikkim in Northeast India; East
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Uttar Pradesh, Tamil Nadu and South Interior Karnataka) showed negative relationships. Two subdivisions (shown by bold italic letters) out of the six subdivisions showed significant
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negative relationships. Remaining twenty four subdivisions showed the direct relationships;
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out of which, twenty two subdivisions showed significant positive relationships (Figure 2d). SM Season: The correlations between IMI in the monsoon season and Indian
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subdivisional SM rainfall are shown in Figure 3.
Only three subdivisions (Assam &
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Meghalaya, Sub-Himalayan West Bengal, Bihar) in northeast India and surrounding showed negative correlations which are not significant. Remaining twenty seven subdivisions showed
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positive correlations; the correlations over twenty four subdivisions are significant at more
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than 10% level. The correlation pattern clearly demonstrates the importance of IMI over Indian SM rainfall activity.
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From the above results, it is worthwhile to pinpoint that the IMI in the individual months as well as whole SM season showed a profound influence on the activity of rainfall over major part of India. The relationship over major part of India is positive and highly significant. Extreme events: The IMI is very much related with monsoon rainfall over the subdivisions, East Rajasthan, West Madhya Pradesh, Gujarat, Vidarbha and Chhattisgarh
ACCEPTED MANUSCRIPT and significantly with all India SM rainfall. The years having the normalized values of IMI in SM season (June through September) less than or equal to -1.0 are 2009, 1987, 1972, 2012, 1965, 1966, 1974, 1997 and 1979. The years having normalized values greater than or equal to 1.0 are 1980, 1973, 2005, 1983, 1954, 1978, 1994, 1958, 1956, 1975, 1970, 1961 and
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1959. The normalized values of the IMI, the corresponding years and the all-India SM
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rainfall amounts are shown in the table (Table 2). The area-weighted average SM rainfall of
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the 30 meteorological subdivisions of India constitutes all-India SM rainfall. In Gujarat, the composite rainfall is 620.1 mm and 1134.1 mm for negative and
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positive extreme events respectively. East Rajasthan also experienced high amounts of
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rainfall (745.3 mm) in extreme positive events when compared to extreme negative events (480.2 mm). West Madhya Pradesh experienced 704.4 mm rainfall in negative extremes and
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1017.4 mm in the positive extremes. Vidarbha also reported high rainfall (1099.2 mm) in While in
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the positive extremes when compared to the negative extremes (767.1 mm).
Chhattisgarh, the difference between the composite rainfall amounts in positive extremes
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(1321.4 mm) and negative extremes (932.5 mm) is also high. Further, all-India SM rainfall
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enhances in positive extreme events. The difference of the composite rainfall amounts in negative extremes (730.5 mm) and positive extremes (930.3 mm) is 199.8 mm.
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Next, the composite rainfall distributions during SM season in negative and positive extremes of IMI are evaluated to examine the spatial distribution of the rainfall (Figure 4). Their differences (rainfall distribution for positive extreme minus rainfall distribution for the negative extreme) are also shown in Figure 4. Twenty five subdivisions showed positive differences. The rainfall is abnormally increased in positive events in some subdivisions. The differences are high over Konkan and Goa (631 mm), Gujarat (514 mm) and Coastal
ACCEPTED MANUSCRIPT Karnataka (433 mm). The rainfall amounts in positive events are enhanced by 313 mm to 389 mm over five subdivisions (region comprising Chhattisgarh, East Madhya Pradesh, Vidarbha and West Madhya Pradesh, and the west coast subdivision (Kerala)). Over nine subdivisions (Punjab, Haryana, Chandigarh and Delhi, East Uttar Pradesh, West Uttar Pradesh, East
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the rainfall differences range from 203 mm to 287 mm.
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Rajasthan (over north India), Saurashtra, Kutch & Diu, Jharkhand, Marathwada, Telangana),
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The differences are about 104 mm – 156 mm over Madhya Maharashtra, North Interior Karnataka, Orissa, Coastal Andhra Pradesh and West Rajasthan. The differences
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are 47 mm – 70 mm over South Interior Karnataka, Rayalaseema and Gangetic West Bengal.
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Only five subdivisions showed reduced rainfall in extreme positive events. They are Tamil Nadu and Pondicherry, Bihar, Nagaland, Manipur, Mizoram & Tripura, Sub-Himalayan
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West Bengal & Sikkim and Assam & Meghalaya (Northeast India and adjoining area). The
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difference over Coastal Karnataka is negligible when compared to its mean amounts. Here Northeast India is in phase with Tamil Nadu but not with the rest of India. Northeast India
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constitutes a separate homogeneous region. In the break monsoon situation, Northeast India
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and Tamil Nadu can experience good amount of rainfall. This situation inhibits the rainfall activity over the rest of India. Generally in SM season, the rain-shadow effect due to
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presence of Western Ghats results in less amounts of rainfall over Tamil Nadu. At the time of onset of SM over south peninsular India, the coastal stations of Tamil Nadu do not show any rise in rainfall activity (Subaramayya et al., 1988; Subbaramayya and Naidu, 1992). 3.2 Zonal wind distributions in positive and negative extremes of IMI 850 hPa: The composite zonal wind pattern at 850 hPa in SM season for positive and negative extremes of IMI and their differences (zonal wind in positive extremes minus zonal wind in
ACCEPTED MANUSCRIPT negative extremes) are shown in Figure 5. Particularly westerlies associated with low level jet stream (Somali jet) in the positive extremes are intensified (10-16 m/s) when compared to those in the negative extremes (8-12 m/s). Also the wind shear between northern latitudes and southern latitudes of the selected domain is dominant in the positive extremes. The
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differences between the above two patterns (Positive extreme wind pattern minus negative
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extreme wind pattern) clearly indicate the presence of positive values particularly south of
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200N and negative values in the northern region from this latitude. This type of pattern clearly indicates that the strong (weak) low level zonal winds in the positive (negative)
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extreme events facilitate more (less) moisture transport from both the Arabian Sea and the
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Bay of Bengal to the Indian mainland.
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150 hPa: The composite zonal wind pattern at 150 hPa in SM season for positive and negative extremes of IMI and their differences (zonal winds in positive extremes minus zonal
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winds in negative extremes) are shown in Figure 6. The width of easterly winds is more in
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the positive extremes of IMI when compared to that in the negative extremes. The zero line (almost coinciding with subtropical ridge position) is situated around 27.50N in the positive
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and 250N in the negative extremes. In extreme positive events, the subtropical ridge is shifted
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to northern latitudes when compared to the negative extremes. Also the strength of the easterlies is more (maximum is about -30 m/s) in the positive extremes when compared to that in the negative extremes (-25 m/s). The differences in the wind pattern (wind pattern in positive IMI extreme minus wind pattern in negative IMI extreme) for the above two extremes pinpoint the presence of negative values over the entire region except for the small regions over northwestern parts of the selected domain. This pattern clearly indicates the presence of more intensified tropical
ACCEPTED MANUSCRIPT easterly jet stream over southern latitudes and less intensified westerly winds over northern parts of India in the positive extreme of the IMI. The positive IMI extreme contributes good amounts of rainfall over India and high amounts of energy to atmospheric system due to condensation process. This results in more north-south temperature gradient and hence high
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intensified easterly jet over South Peninsular India and neighborhood. Reverse is true for
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negative extremes.
Wind Shear: The distribution of the vertical shear in the zonal wind for the levels
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850 hPa and 150 hPa for positive and negative extremes of IMI and their differences (distribution in the positive minus distribution in the negative) are shown in Figure 7. In the
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positive extremes, the areas to the south and north of 290-300N show negative and positive
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values respectively. But in the negative extremes, the zero line shifts to 270-280N. The magnitude of negative shear (when compared to that of positive shear) is more in both
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extremes. The magnitude of the negative shear is more in the positive extremes when
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compared to the negative extremes. But the magnitude of positive shear is slightly less over northern most latitudes of the selected domain in positive extremes when compared to the
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negative extremes.
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According to Naidu et. al. (2011),
the vertical wind shear in the zonal wind
component (zonal wind at 100 hPa minus zonal wind at 850 hPa) for the domain (5°–22°N and 80°–100°E) shows decreasing tendency (0.1532 m/s per year), which is significant at 0.1% level, for the period, 1950–2005. Particularly it has low values in global warming era. The past studies indicate that the baroclinic instability is responsible for the generation of the monsoon depressions (Shukla, 1977, 1978; Mishra and Salvekar, 1980; Moorti and Arakawa, 1985; Aravequia et al., 1995). They considered the baroclinic instability of the zonal wind
ACCEPTED MANUSCRIPT with easterly shear associated with the tropical easterly jet. They showed that higher (lower) wind shear leads to higher (lower) growth rates of the depressions with or without the cumulus heating. Srinivasa Rao et al., 2004 quoted the above works and concluded their results that an increase (decrease) of the easterly shear in the earlier (latter) part of their study
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period leads to monsoon depressions with higher/lower growth rates; disturbances with weak
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growth rates may not sustain the frictional dissipation and may not develop. They presume
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that this may explain the decrease in the number of monsoon cyclonic systems when the
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shears are weak and increase when the shears are strong. 4. Conclusions
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It is well established fact that the Indian SM rainfall oscillates in association with the strength of the low level winds in the Arabian Sea and the Bay of Bengal. These winds
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facilitate the transport of moisture from the ocean to hot Indian main land and are responsible
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for convective activity. This results in the rainfall activity over India. The IMI values established good direct relationships with the subdivisional SM
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rainfall over India. The IMI is the lateral shear of the zonal wind component between the
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northern and the southern latitudes of the Indian SM regime. The results pinpoint that the large shear leads to active SM conditions over India.
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During 1953-1982, the IMI mean values are positive in July, August and September and negative in June. These positive values are responsible for good monsoon epoch during 1953-1982. The signs of mean values in different months of IMI are reversed in the period 1983-2012. Positive values of IMI in June lead to enhanced rainfall amounts over India in the recent three decades of global warming. July and August months are prime monsoon months. In the same period, the negative values of IMI in these two months result in
ACCEPTED MANUSCRIPT weakening of the monsoon system over India and hence poor amounts of rainfall are reported. The presence of more intensified easterly jet stream over southern latitudes and less intensified westerly winds over northern parts of India in the upper troposphere is a feature of
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the positive extreme of IMI. Reverse is true for the negative extreme of IMI.
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The rainfall distributions in the negative and positive extremes of the IMI reveal that
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Northeast India is in phase with Tamil Nadu but not with the rest of India. Northeast India constitutes a separate homogeneous region. In break monsoon situation, Northeast India and
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Tamil Nadu can experience good amount of rainfall. The condition favorable for Northeast
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India and Tamil Nadu inhibits the activity of rainfall over the rest of India. Acknowledgments
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One of the authors, P.Vinay Kumar wishes to thank the DST, Government of India
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for providing a research fellowship [DST/INSPIRE Fellowship/ IF160113]. The authors are thankful to the IITM, Pune, India and NCEP/NCAR Reanalysis team for providing data and
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also to DST for providing the computer facility under FIST programme to our department .
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suggestions.
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The authors are thankful to the editor and the reviewers for their constructive comments and
ACCEPTED MANUSCRIPT References Aravequia, J. A., Rao, V.B., Bonatti, J. P., 1995. The Role of Moist Baroclinic Instability in the Growth and Structure of Monsoon Depressions. J. Atmos. Sci. 52, 4393-4401. Abish, B., Joseph, P.V., Johannessen, O.M., 2013. Weakening Trend of the Tropical Easterly Jet Stream of the Boreal Summer Monsoon Season 1950–2009. J. Clim. 26, 9408-9414. doi:10.1175/jcli-d-13-00440.1.
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Cadet, D., Reverdin, G., 1981. Water vapour transport over the Indian Ocean during summer 1975. Tellus 33, 476–487.
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Cadet, D., Desbois, M., 1979. Low-level air flow over the western Indian Ocean as seen from METEOSAT. Nature 278, 538 – 540.
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Joseph, P.V., 1990. Monsoon variability in relation to equa-torial trough activity over Indian and west Pacific oceans. Mausam.41 (2), 291–296.
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Joseph, P.V., Simon, A., 2005a. Weakening trend of monsoon Low Level Jet Stream through India 1950–2003. CLIVAR Exchanges 10 (3), 27–30.
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Joseph, P. V., Raman P.L., 1966. Existence of low-level westerly jet-stream over peninsular India during July, Indian J. Meteorol. Phys., 17, 407–410. Koteswaram, P., 1958. The easterly jet stream in the tropics. Tellus 10, 43–57. Krishnamurti, T. N., Bhalme H. N., 1976. Oscillations of monsoon system. Part I: Observational aspects. J. Atmos. Sci. 45, 1937–1954. Mishra, A. K., Gnanaseelan C., Seetaramayya, P., 2004. A study of rainfall along the west coast of India in relation to low level jet and air–sea interactions over the Arabian Sea. Curr. Sci. 87 (4),475-485.
ACCEPTED MANUSCRIPT Mishra, S.K., Salvekar, P.S., 1980. Role of baroclinic instability in the development of monsoon disturbances. J. Atmos. Sci. 37, 383–394. Moorthi, S., Arakawa A., 1985. Baroclinic instability with cumulus heating. J. Atmos. Sci. 42, 2007-2031.
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Naidu, C.V., Muni Krishna, K., Ramalingeswara Rao, S., Bhanu Kumar, O.S.R.U., Durgalakshmi, K., Ramakrishna, S.S.V.S., 2011. Variations of Indian summer monsoon rainfall induce the weakening of easterly jet stream in the warming environment?, Glob. Planet. Chang. 75, 21–30.
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Naidu, C.V., Dharma Raju, A., Satyanarayana, G.Ch., Vinay Kumar, P., Chiranjeevi, G., P. Suchitra 2015. An observational evidence of decrease in Indian summer monsoon rainfall in the recent three decades of global warming era. Glob. Planet. Chang. 127, 91–102.
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Puranik, S. S., Sinha Ray, K. C., Sen, P. N., Kumar, P. P., 2013. An index for predicting the onset of monsoon over Kerala. Curr. Sci. 2013, 105(7), 954–961.
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Rajeevan, M., 1993. Inter-relationship between NW Pacific typhoon activity and Indian summer monsoon on inter-annual and intra-seasonal time-scales. Mausam 44, 109–111.
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Ramesh Kumar, M. R., Shenoi, S. S. C., Schluessel, P., 1999. On the role of the cross equatorial flow on summer monsoon rainfall over India using NCEP/NCAR reanalysis data. Meteorol. Atmos. Phys., 1999, 70, 201–213.
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Sathiyamoorthy, V., 2005. Large scale reduction in the size of the Tropical Easterly Jet. Geophysical Research Letters 32 (14): n/a-n/a. doi: 10.1029/2005gl022956. Shukla, J., 1977. Barotropic–baroclinic instability of mean zonal wind during summer monsoon. Pure Appl. Geophys. 115, 1449–1462. Shukla, J., 1978. CISK barotropic–baroclinic instability and the growth of monsoon depressions. J. Atmos. Sci. 35, 495–508. Sikka, D.R., 1977. Some aspects of the life history, structure and movement of monsoon depressions. PAGEOPH 115, 1501–1529.
ACCEPTED MANUSCRIPT Srinivasa Rao, B.R., Bhaskar Rao, D.V., Brahmananda Rao, V., 2004. Decreasing trend in the strength of tropical easterly jet during the Asian summer monsoon season and number of tropical cyclonic systems over Bay of Bengal. Geophys. Res. Lett. 31. doi:10.1029/2004GL019817. Subbaramayya, I., Naidu, C.V., 1992. Spatial variations and trends in the Indian monsoon rainfall. International J. Clim. 12, 597-609, 1992.
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ACCEPTED MANUSCRIPT Figure Captions: Figure 1: Meteorological subdivisions of India Figure 2: The correlations between the Indian Monsoon Index in different months and the corresponding subdivisional rainfall ( a: June, b: July, c: August and d: September)
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Figure 3: The correlations between the Indian Monsoon Index in summer monsoon season and the corresponding subdivisional summer monsoon rainfall over India
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Figure 4: Composite Summer Monsoon Rainfall for (a) Positive extreme events of IMI, (b) Negative extreme events of IMI and (c) their differences
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Figure 5: Composite zonal wind pattern at 850hPa for summer monsoon season in positive (upper level left panel) and negative (upper level right panel) extreme years of IMI and their differences (Positive extreme minus Negative Extreme) (lower panel)
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Figure 6: Composite zonal wind pattern at 150hPa for summer monsoon season in positive (upper level left panel) and negative (upper level right panel) extreme years of IMI and their differences (Positive extreme minus Negative Extreme) (lower panel)
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Figure 7: Composite zonal wind shear pattern for summer monsoon season in the layer 150hPa- 850hPa in positive (upper level left panel) and negative (upper level right panel) extreme years of IMI and their differences (Positive extreme minus Negative Extreme) (lower panel)
ACCEPTED MANUSCRIPT Table Captions: Table-1:
Means of the normalized values of Indian Monsoon Index (IMI)
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AN
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Table-2: Summer Monsoon Rainfall for all-India in the extreme events of the Indian Monsoon Index (IMI). NV indicates Normalized value of IMI.
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Figure 1: Meteorological subdivisions of India
ACCEPTED MANUSCRIPT
b
June Rainfall and Indian Monsoon Index (1953-2012) 0.32 0.38
July Rainfall and Indian Monsoon Index (1953-2012) 0.34 0.35
-0.11
-0.54
0.42 0.15
0.52
0.5
0.42
0.35
0.61 0.56
0.55 0.46
0.58 0.39
0.52
0.51 0.35 0.53
0.14 0.50
0.27
0.02
-0.03
-0.07
0.32
0.37
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0.43
0.43
0.44 0.09
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-0.12
0.39 0.18
0.48
0.28
0.41
-0.02
0.44
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0.42
-0.37
0.34
0.34 0.56
0.61
-0.31
-0.09
0.54
-0.15
0.4
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-0.24
0.47
0.46
0.31
0.42 0.33
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0.24
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a
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c 0.39 0.37
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0.45
0.20
0.48
-0.34
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0.53
0.43 0.46
-0.35
0.33
0.47
0.17
September Rainfall and Indian Monsoon Index (1953-2012)
d
August Rainfall and Indian Monsoon Index (1953-2012)
0.42 0.53 0.53
0.35
-0.36
0.0 -0.21
0.66
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0.49
0.11
0.6
0.51
-0.27
0.09
0.66
0.36 0.48 0.73
0.44
0.35
-0.28
0.23 0.3 0.51
0.56 0.70 0.46
0.52
0.28
0.33 -0.02
-0.09
-0.27
-0.03
0.55
-0.23
0.41 0.56 0.22
-0.09 0.27
0.45 0.13
0.67
0.5
-0.12
-0.17
Figure 2: The correlations between the Indian Monsoon Index in different months and the corresponding subdivisional rainfall ( a: June, b: July, c: August and d: September)
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JJAS Rainfall and Indian Monsoon Index (1953-2012) 0.41 0.49
-0.2 0.48
0.43
-0.04
0.52 0.54 0.63
0.21
0.53 0.28 0.51
0.5 0.3
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0.1
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0.41
0.28
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0.49
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0.49
0.41
0.11
0.02
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0.55
0.47
0.32
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0.67
-0.14
0.36
0.6
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Figure 3: The correlations between the Indian Monsoon Index in summer monsoon season and the corresponding subdivisional summer monsoon rainfall over India
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a
Negative Extreme Rainfall (mm) events of Indian summer monsoon index for JJAS(1953-2012)
b
Positive Extreme Rainfall (mm) events of Indian summer monsoon index for JJAS(1953-2012)
387
611.1 546 835.4
342
1837.6 937.6
212.4
1393.2 1233.5
1127.4 1167 1247 1321 1181.4 1099.2
1134 607 2657
366 2026
511
855.5
687.2
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299.2
501.7
1721.2
311.3
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AN
2058
465
414
1789
461.3 571.6
2223
1272.7
599.5
543.9
606
1563.4
546.7
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820 667
965.8
924.3 1098 704.4 875.1 932.5 1039 767.2
620
1017
662.5
480.2
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949.6
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316.4 745.3
1994.3
568
AC
CE
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ED
c
Figure 4: Composite Summer Monsoon Rainfall for (a) Positive extreme events of IMI, (b) Negative extreme events of IMI and (c) their differences
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AN
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ACCEPTED MANUSCRIPT Table-1:
Means of the normalized values of Indian Monsoon Index (IMI)
June
July
August
September
1953-2012
-0.0049
0.0186
0.053183
-0.01813
0.019533
1953-1982 (A)
-0.0838
0.167633
0.258067
0.132467
0.1988
0.074
-0.13043
-0.1517
0.1578
-0.29807
-0.40977
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M
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(B) minus (A)
-0.16873
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1983-2012 (B)
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Duration
Monsoon season
-0.3012
-0.15973 -0.35853
ACCEPTED MANUSCRIPT Table-2: Summer Monsoon Rainfall for all-India in the extreme events of the Indian Monsoon Index (IMI). NV indicates Normalized value of IMI.
-1.8
AC
CE
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Rainfall in mm 882.6 913.2 856.8 955.6 885.2 909.2 952.7 889.1 983.1 962.5 939.7 1020.1 944 930.3
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Mean
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-2.6 -2.1 -2.1 -1.9 -1.9 -1.4 -1.4 -1.2 -1.2
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2009 1987 1972 2012 1965 1966 1974 1997 1979
Positive Extremes Rainfall Year NV in mm 1.0 667.6 1980 1.0 697 1973 1.0 2005 652.8 1.1 1983 780.7 1.2 709.2 1954 1.2 739.9 1978 1.7 1994 747.9 1.4 1958 871.4 1.4 707.7 1956 1.5 1975 1.5 1970 1.7 1961 2.0 1959 1.3 Mean 730.5
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Negative Extremes Year NV
ACCEPTED MANUSCRIPT Highlights In the recent three decades, the negative values of IMI in July and August lead to weakening of Indian monsoon system. The positive relationship between IMI and the monsoon rainfall over major part of
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India is highly significant.
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The rainfall variations in the Northeast India and Tamil Nadu are different from the
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rest of India.
More intensified easterly jet stream over India is a feature of the positive extreme of
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IMI.
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Strong easterlies over North Bay of Bengal facilitate the active monsoon conditions
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over India.