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Chapter 1
Introduction
In simple words, climate of a place is known as the average of the weather
components including the frequency of extreme events. As per the current practice
weather parameters over a period of at least 30 years constitutes the climate. There are
continuous interactions between the components of climate system such as atmosphere,
biosphere, cryosphere, land and ocean. Natural variation in climate can occur due to
volcanic eruption, continental drift and built up of mountains, changes in tilt angle of
earth’s orbit, sun’s radiation intensity and the slow but large scale ocean circulation etc.
However, changes in greenhouse gas (GHG) concentrations, aerosol emissions and
urbanization give rise to the anthropogenic climate change. Today, climate change is one
of the major issues on the earth system and its impact has already been realized in several
geographical locations and sectors in the society (IPCC 2007). There is a worldwide
concern about the anthropogenic climate change, particularly during the last two decades.
Most important visible changes are those in temperature, snow/ice melt and precipitation
extremes. Before understanding any scientific study on Indian climate, it is essential to
know some of the important diversities in Indian weather and climate.
1.1 Weather and climate of India
India is a south Asian country which lies to the north of the equator between 8.4o
and 37.6o north latitude and 68.7o and 97.25o east longitude. This tropical country is
surrounded by water bodies on three sides, Arabian Sea towards the West, Bay of Bengal
towards the East and Indian Ocean towards the South. India is having unique
geographical features with complex topography. Mountain range such as the Himalayas
broadens in the north and northeast. The Vindhyas separate the Indo Gangetic plain from
the Deccan Plateau. The Satpura, Aravalli and Sahyadri cover the eastern fringe of the
TERI University‐Ph.D. Thesis, 2012 1
Introduction
West Coast plains. The coasts of southern parts of India are known as Western and
Eastern Ghats. Eastern Ghats are irregularly scattered and forms the boundary of the East
Coast plains. Tibetan plateau towards the north of the foothills of Himalayas also
influences Indian summer monsoon. On the northwest part of India, Thar desert extends
from the edge of the Rann of Kachchh of Gujarat up to the frontier of Rajasthan.
Classification of climate for any region is the organization of climate information
for analysis and communication. India is a vast country with diversities in its weather and
climate. Wladimir Köppen, a German botanist and climatologist is most widely known
for the descriptive climate classification system which he first proposed in 1884. After
several modifications, world map of climatic classification by Köppen was introduced in
1936. It combines the average of annual and monthly temperature and precipitation, and
the seasonality of precipitation. According to the Köppen climate classification system,
Indian climate can be divided into six major subtypes. Desert or arid climate regions are
in the west and beyond that there is semi-arid climate. Semi-arid climate can also be
observed between Eastern and Western Ghats. Alpine tundra and glaciers are found in the
north and over the Himalayan ranges. Humid subtropical regions are in the north of
central and eastern India. Tropical wet climate regions are the Western Ghats and Island
territories. Parts of peninsular India experience tropical wet and dry climate.
In India there are four distinct seasons, pre-monsoon (April-May), summer
monsoon (June-July-August-September), post-monsoon (October-November) and winter
(December-January-February). India is a tropical country with hot weather conditions
that varies from region to region. In the pre-monsoon months vast land portion of India is
dominated by intense solar heating which leads to heat wave conditions. Similarly cold
wave conditions during winter season occur due to the intense high pressure cells and
passing of western disturbances. There is no absolute definition for heat or cold wave
events. The term is relative to the average weather condition of a region of study.
According to a study by Raghavan (1967), severe cold waves develop very often in situ
within the country itself and account for the higher incidence in certain isolated regions.
Irrespective of the intensity, severe cold wave are mostly confined to either eastern or the
TERI University‐Ph.D. Thesis, 2012 2
Introduction
western half of the Ladakh in Jammu-Kashmir. Severe heat waves are also not so far
been observed to migrate from the neighboring countries Raghavan (1966). They develop
in situ within the country itself and expand from West Pakistan to affect Northwest India.
He further concluded that available statistics do not suggest any periodicity in the
incidence of extreme temperature conditions in any region or in the country as a whole.
He considered the persistency of the waves over different subdivisions where they
dissipate or wherefrom they migrate to adjacent regions. In India more than 70% of its
population relying on agriculture directly or indirectly and thus the impact of extreme
weather events is critical. In the last two decades India has been affected by successive
extreme temperature events, monsoons flooding and droughts (De et al. 2005). Snow is
also an important component of the hydrological cycle. Major contribution of many parts
of the world snow is in the form of precipitation or total annual water supply. The
allocation of limited water resources has significant economic and policy consequences.
Eurasian, Tibetan and Himalayan snow in winter and spring may affect the Indian
summer monsoon circulation and rainfall. It may influence both river basin runoff and
climate change dynamics. It has been revealed that snow cover/depth variations during
winter in Eurasia are not only associated with monsoon rainfall in Southeast Asia but also
recognized as an effective source of freshwater flowing to the Arctic Ocean (Rogers et al.
2001), and may thus be linked to the global thermohaline circulation (Walsh et al. 1998),
which is a major determinant of the global climate.
In India two major monsoon systems are observed, southwest/summer monsoon
from June to September and northeast/winter monsoon from October to November.
Monsoon is produced by differences between land and sea temperatures in eastern and
southern Asia. It is a seasonally varying wind system, e.g. in the Indian Ocean winds are
southwesterly with moist air and high precipitation in summer, northeasterly with dry air
and clear skies in winter. During the post-monsoon months northeast monsoon dominates
over the southern parts of Peninsular India particularly in Andhra Pradesh, Rayalaseema,
Tamilnadu and Pondicherry. The principal components of northeast monsoon system are
Siberian high pressure system, northeasterly lower tropospheric flow, wind surges carried
by the northwesterly monsoon flow along the western shores of South China Sea, the
TERI University‐Ph.D. Thesis, 2012 3
Introduction
monsoon trough located near to the north of the equator in December and south of the
equator in January and February, the west Pacific high and the subtropical jet stream of
winter. At the beginning of October, a trough of low pressure develops over the south of
Bay of Bengal and equatorial maritime air moves towards the southern India that causes
northeast monsoon rainfall (Kripalani and Kumar, 2004). They found that Indian Ocean
dipole positive phase also enhances the rainfall activity of northeast monsoon and
negative phase suppresses. Dhar and Rakecha (1983) and Singh and Sontakke (1999)
identified enhancement in northeast monsoon rainfall during El Niño events.
1.2 Indian summer monsoon main features
The weather and climate of India are dominated by the summer monsoon, which
returns with remarkable regularity in each summer and provides the rainfall needed to
sustain over a billion of people. The vastness of Indian sub-continent and the unique
configuration of the east African highlands and the Tibetan plateau mean that the Indian
summer monsoon is the most vigorous and influential of all the monsoon circulation over
the globe (Boucher 1998). It lasts from June to September and about 80% of the total
annual precipitation is received over a large part of the country except in Jammu &
Kashmir and Tamil Nadu. The economy of India is largely based on agriculture which in
turn depends on the temporal and spatial variation of rainfall especially during Indian
summer monsoon season.
Broadly, the monsoon is an atmospheric phenomenon in which there is seasonal
reversal of the mean surface wind. Southwesterly surface winds of the summer monsoon
reverses its direction to northeasterly surface winds of the winter monsoon. The principal
components of south Asian summer monsoon are the Mascarene high of the southern
hemisphere, intense low over Pakistan, monsoon trough over North-central India, Tibetan
high, Tibetan anti-cyclone, tropical easterly jet stream at the upper level, cross-equatorial
low-level jet over coastal Africa, Somali jet, southwesterly monsoon flow over the
Arabian sea monsoon disturbances and cloud cover which leads to rainfall. South Asian
TERI University‐Ph.D. Thesis, 2012 4
Introduction
monsoon is expected to be driven by seasonal variations in land–sea temperature contrast
between Asian landmass and adjacent ocean. It is well known that seasonal reversal in
wind direction is related to large scale heat sources and sinks. In the case of south Asian
summer monsoon, belt of strong convective heating is observed over the major part of the
northern India. Tibetan Plateau also acts as an elevated heat source in summer resulting
in sensible heat fluxes. Heat sink of the Asian summer monsoon resides majorly over the
Mascarene High.
Every year, onset of Indian summer monsoon occurs over Kerala, the southwest
coast of India. Onset of monsoon generally starts near to the first week of June. It is a
crucial event which indicates the beginning of rainy season in India. After that rapid
substantial and continuous progress in rainfall is noticed over Indian land that extends
towards the north. However, Indian monsoon system is highly irregular and variable at
intra-seasonal, annual, biennial and inter-annual timescales. The northward progression
of the monsoon is symptomatic of a large scale transition of a deep convection from the
equatorial to continental regions (Webster et al. 1998, Pai and Rajeevan 2007). By middle
of July, monsoon covers the whole country. Uneven distribution of rainfall can adversely
affect the agricultural sector. Variability in Indian summer monsoon can be studied on the
basis of active and break spells. Active spell leads to continuous and good amount of
rainfall whereas breaks are the discrete rainfall process that results in lower amount of
rain. Long and intense break spells can influence monsoon rainfall up to a large extent
(Gadgil and Joseph 2003). Frequent and prolonged break conditions can lead to drought
condition. Recently, Rajeevan et al. (2010) have proposed new method to define active
and break conditions. Only July and August are considered by them for active and break
spells in which the normalized anomaly of the rainfall over the monsoon core zone
exceeds 1 or is less than −1.0 respectively, provided the criterion is satisfied for at least
three consecutive days. They observed longer life span of breaks than active using
observed gridded daily data for period 1951-2007. The progress of summer monsoon
from northern Australia towards the eastern foothills of Himalayas and its reverse in the
annual cycle is an important component of the monsoon. The dates of the withdrawal of
the Indian monsoon are the reverse propagation of the last monsoon rainfall that moves
TERI University‐Ph.D. Thesis, 2012 5
Introduction
from northwest towards southeast. Withdrawal of Indian summer monsoon is more
variable than its onset. Syroka and Toumi (2004) defined withdrawal of Indian summer
monsoon in terms of wind fields at 850hPa. The retreat in monsoon follows a period of
enhanced convective activity over the Indian subcontinent and is associated with a dry
phase of the intraseasonal oscillation.
ISMR has large spatial and temporal variations and there are several occasions
when some parts of the country receive heavy rainfall while at the same time some other
parts have serious rainfall deficiency (Mooley and Parthasarathy 1984; Parthasarathy et.
al. 1995; Dash et al. 2002). Parthasarathy et al. (1995) calculated and analyzed summer
monsoon rainfall in India as a whole. Statistical analysis of ISMR over different regions
by Dash et al. (2002) shows a large spatial variations from region to region. They
observed minimum and maximum variations in rainfall over Northeast and Northwest
India respectively.
The study of changes in the spatial and temporal distribution of rainfall in India
has great relevance in the context of planning policy formulation especially in the context
of global warming. Also, the distribution of ISMR has a large variability and therefore it
is always a challenging task for scientists to predict and detect extremes either using
observational data or model simulated data. Simulation of rainfall and its variability by a
global model at different time scale is difficult due to coarse resolution of the model. The
global models are unable to capture extremes of rainfall with good confidence. Now-a-
days, the regional models are being used worldwide by scientists for rainfall simulation
due to their comparative high resolution and better physics. Detection of rainfall extremes
using regional model simulation during ISM season would be helpful to assess the
damage due to extremes of rainfall.
A number of studies on Indian summer monsoon are made by different
researchers with the help of GCMs. Under Monsoon Numerical Experimentation Group
(MONEG), a set of seasonal integrations was carried out using a number of GCMs all
over the world (WMO 1992 & 1993). Gadgil and Sajani (1998) extensively examined the
results of 30 GCMs during the period from January, 1979 to December, 1988 under
TERI University‐Ph.D. Thesis, 2012 6
Introduction
Atmospheric Model Intercomparison Project (AMIP) and inferred the short comings of
dynamical models in simulating ISMR. Most of the GCMs poorly simulate the rainfall
along the West Coast, North Bay of Bengal and North East India with large bias in mean
monsoon rainfall (Kripalani et al. 2007; Rajeevan and Nanjundiah 2009). The effects of
regional forcing or non-linear steep topography of Himalayas and Western Ghats may not
be fully captured by the GCMs because of their coarser resolutions. In order to resolve
regional features at finer scale, performance of a high resolution GCM at 20km horizontal
resolution was observed by Rajendran and Kitoh (2008). After analysis of present and
future climate both at 10 years time scale, they indicate better performance in simulation
of ISMR from higher resolution GCM. However, such high resolution GCMs require a
large number of good quality computational resources.
Today, regional models are increasingly used by scientists all over the world to
better resolve the weather systems of high resolutions. Bhaskaran et al. (1996) simulated
the Indian summer monsoon for four years using a regional climate model with a
horizontal resolution of 50 km nested with global atmospheric GCM. Their study showed
that regional model derived precipitation is higher by 20% than GCM due to stronger
vertical motions arising from finer horizontal resolution. Using regional Eta model of
National Centers for Environmental Prediction, nested in the GCM of Center for Ocean-
Land-Atmosphere (COLA), Ji and Vernekar (1997) verify its performance for two
contrasting summer monsoons in 1987 and 1988. Their comparative studies showed that
Eta simulations were closer to the observations than GCM over India and southeast
China. Their comparative studies showed that for 1987, the Eta model simulates deficient
summer monsoon rainfall over northern and peninsular India and the Indonesian region
and excess rainfall over southeast China, Burma and the sub-Himalayan region compared
to 1988. Due to increase in the availability of computational resources, now-a-days
longer simulations are performed by the scientists. Rupa Kumar et al. (2006) used a high
resolution regional climate model PRECIS (Providing Regional Climates for Impacts
Studies) which was developed by Hadley Centre for Climate Prediction and Research to
study the climate change scenarios for present (1961–1990) and a future period (2071–
2100) and observed significant improvement in representation of the spatial pattern of
TERI University‐Ph.D. Thesis, 2012 7
Introduction
ISMR compared to that simulated by the GCM. The skills of seasonal precipitation
predictions can also studied using multi-model ensemble (Kar et al. 2006; Sahai et al.
2008). Mesoscale models are also used to study the characteristics of Indian monsoon
(Dudhia 1989; Bhaskar Rao et al. 2004).
The regional climate model (RegCM) of Abdus Salam International Centre for
Theoretical Physics (ICTP) has been successfully used by several researchers to examine
atmospheric circulation features at different temporal and spatial scales. It is increasingly
used to examine the circulation and precipitation patterns (Chow et al. 2006; Dash et al.
2006b; Abiodun et al. 2008; Gao et al. 2008; Davis et al. 2009; Ratnam et al. 2009),
regional climate change (Mearns et al. 1995; Pal et al. 2004; Diffenbaugh et al. 2005;
Giorgi and Coppola 2007; Im et al. 2008, 2010) and seasonal climate variability
(Rauscher et al. 2006; Seth et al. 2007) over several parts of the world. Rauscher et al.
(2006) performed the downscaling technique using RegCM3 over tropical and sub–
tropical South America for two test seasons during El Niño and La Niña years. Their
studies show that NCEP/NCAR reanalysis and GCM simulations driven with RegCM3 is
able to replicate the distribution of daily rainfall intensity in most of the regions. Chow et
al. (2006) applied some convection suppression criteria to MIT–Emanuel cumulus
parameterization scheme. Their study shows some significant improvement in RegCM3
to simulate Asian summer monsoon precipitation, particularly the precipitation over
southeastern China and the Mei–yu rain band over the East Asia region. Davis et al.
(2009) customized their study for the precipitation processes over the tropical regions of
eastern Africa and the Indian Ocean using RegCM3 with all the existing convective
schemes for determining the most realistic spatial distribution of rainfall and partitioning
of convective/stratiform rainfall. Their results suggest that the convective schemes of
Grell with Arakawa–Schubert (AS) and Fritch–Chappel (FC) closures scheme
underpredicted the rainfall rates over the land, while over the ocean FC overestimates and
AS underestimates the convective rainfall MIT–Emanuel scheme provided the most
realistic spatial distribution of convective rainfall despite the tendency for overestimating
total rainfall. Sylla et al. (2009) examined the present day integrations (1981–2000) using
RegCM3 with both NCEP/NCAR reanalysis data and output from a coupled
TERI University‐Ph.D. Thesis, 2012 8
Introduction
atmospheric–ocean general circulation model (AOGCM) nested over West Africa.
Spatial distributions of RegCM3 simulations are shown to be realistic when compared
with observations and also offered some improvements compared to the AOGCM driving
fields. The relationship between rainfall changes and monsoon dynamics in multidecadal
experiments over West Africa was performed by Sylla et al. (2010) using the RegCM3 at
40km resolution for the recent past (1981–2000) and for the late 20th century (2081–
2100) climate conditions under increased greenhouse gas forcing (A1B scenario) driven
by the global climate model European Center/Hamburg 5 (ECHAM5). With respect to
the recent past climate, late 20th century scenarios show drier conditions over the Sahel
and wetter conditions over the orographic areas. Sahel drying may be associated with
changes in the wind circulation pattern and is similar to the conditions found in the late
twentieth century observed drought over the region.
Using RegCM, relatively few researchers have focused on Indian sub–continent
(Liu et al. 2004; Shekhar and Dash 2005; Landman et al. 2005; Dash et al. 2006b; Singh
et al. 2007; Ratnam et al. 2009 and Ashfaq et al. 2009). Sensitivity experiments using
RegCM and Tibetan snow as one of the boundary condition were conducted by Liu et al.
2004 and Shekhar and Dash 2005. Liu et al. 2004 demonstrated the effect of anomalous
snow cover over the Tibetan plateau with the South Asian summer monsoon by
numerical simulations using the RegCM2. They found that the heavier snow cover over
the plateau can reduce the intensity of the South Asian summer monsoon and rainfall to
some extent, but the influence is only obvious in early summer and almost disappear in
later stages. Shekhar and Dash 2005 tested RegCM3 to study the effect of Tibetan
snowfall in the month of April on the Indian summer monsoon circulation and associated
seasonal rainfall. Their sensitivity experiment shows that Tibetan snow results in weak
lower level monsoon westerlies and upper level easterlies therefore the Indian summer
monsoon rainfall reduced over entire India and its five homogenous zones. RegCM3
ability over the southwestern Indian Ocean to reproduce observed cyclones and their
land–falling tracks was performed by Landman et al. 2005. Their results show that the
regional model can produce cyclone–like vortices and their tracks up to some extent.
Singh et al. 2007 investigate the impact of Indian Ocean sea–surface temperature
TERI University‐Ph.D. Thesis, 2012 9
Introduction
anomaly on ISMR using RegCM3. They observed that regional warming of SST over the
Indian Ocean enhanced the monsoon precipitation mainly over south and west Peninsular
India, Indian Ocean and reduced precipitation over northeast India. Ratnam et al. (2009)
coupled the RegCM3 with Regional Ocean Modeling System (ROMS) and show more
realistic spatial and temporal distribution of ISMR compared to the uncoupled
atmosphere–only model.
Over the Indian domain, performance of RegCM driven with reanalysis data at
their boundaries was demonstrated in few studies (Dash et al. 2006b; Ashfaq et al. 2009).
RegCM3 has been successfully integrated by Dash et al. (2006b) to simulate the salient
features of summer monsoon circulation with a horizontal resolution of 55km over a
South Asia domain for period April–September of the years 1993 to 1996. Their study
shows that, the Grell convection precipitation scheme has performed better than other
available convection schemes in simulating both summer monsoon circulation and
rainfall. Their study also indicates that RegCM3 can be effectively used to study the
monsoon processes over the south Asia region. Using RegCM3, Ashfaq et al. 2009 show
the simulated dynamical features of the summer monsoon are comparable with reanalysis
data. They have used a high resolution nested climate modeling system to investigate the
response of South Asian summer monsoon dynamics to anthropogenic increases in
greenhouse gas concentrations. Further they found that there is an overall suppression of
summer precipitation, a delay monsoon onset and an increase in the occurrence of
monsoon break periods.
Next to the sea surface temperature (SST), snow cover/depth is the most
important surface condition to affect the Indian summer monsoon rainfall (ISMR). Along
with the temperature of snow surface, sensible heat flux and latent heat flux are the
important factors that define surface energy balance. Albedo and related sensible heat
flux are associated with snow cover while snow depth significantly affects both sensible
and latent heat flux. There is weakening of sensible heat flux due to high albedo from
large areal extent of snow cover that leads to reflect more solar radiation while moisture
content of soil from snow melt utilises some amount of solar energy in evaporation
TERI University‐Ph.D. Thesis, 2012 10
Introduction
(latent heat flux) process. Thus, when there is more snow cover/depth, relatively a small
part of solar energy is utilised in heating continental landmass during summer prior to
monsoon. The sensible heat flux at the higher altitude like in the case of the Tibetan
Plateau is one of the important factors in land–sea heating contrast. It is further enhanced
due to the latent heat released in the troposphere. Excessive snowfall during the winter
delays melting of snow in spring and consequently furthers delay in buildup of land–sea
temperature contrast which drives the monsoon. The relationship between Indian summer
monsoon and snow cover/depth has been studied extensively by several scientists in the
past based on observed data as well as using numerical models. The following paragraphs
give brief review of these studies.
1.3 Observational studies on snow-monsoon relationship
Long back Blanford (1884) based on the study of weather reports suggested
negative influence of Himalayan snow on the summer monsoon of India and Burma. Luo
and Yanai (1984), He et al. (1987) and Murakami (1987) summarised that the high
elevated Tibetan Plateau acts as a heat source in summer and heat sink in winter, in
addition to its acting as a mechanical barrier that affects flow pattern around it. Further, in
summer a temperature gradient develops in the upper troposphere between Tibetan
Plateau and equatorial Pacific which is further associated with the onset of South Asian
summer monsoon (He et al. 1987; Li and Yanai 1996; Shaman and Tziperman 2005;
Yanai and Wu 2006). Zhang et al. (2004) observed interdecadal increase in spring snow
depth in Tibetan Plateau after late 1970s in his study of the period 1962–1993. This
excessive snowfall is concurrent with excessive precipitation over northern India,
northwestern China and Western Asia and with drought over Central Asia during March–
April. The increase of the snow depth is also related to the excessive summer rainfall
over the Yangtze River valley and the drought conditions in southeastern and
northeastern China and the Indochina peninsula.
The negative relationship between Tibetan snow and ISMR has been extended to
that between Eurasian snow cover/depth and ISMR. Studies of Hahn and Shukla (1976),
TERI University‐Ph.D. Thesis, 2012 11
Introduction
Dey and Bhanu Kumar (1982), Dickson (1984), Barnett et al. (1989) and Sankar Rao et
al. (1998) based on observational data have inferred the negative relationship between
Eurasian snow cover and ISMR. Bamzai and Shukla (1999) and Kripalani and Kulkarni
(1999) reexamined the relationship and found significant inverse correlation for West
Eurasian region while positive correlation for East Eurasian region. Bamzai and Shukla
(1999) after analyzing areal extent of snow concluded that winter and spring snow covers
of southern Eurasia and the Himalayas have high interannual variability and are poorly
correlated with the subsequent monsoon rainfall. They used satellite-derived snow cover
data for 22 years (1973–1994) and studied the frequency of occurrence of snow at grid
points over Eurasia and correlated December, January, February and March mean snow
cover anomalies for four regions with the subsequent ISMR. Using Historical Soviet
Daily Snow Depth (HSDSD) version-1 dataset, Kripalani and Kulkarni (1999) studied the
monthly climatology and variability of snow depth and its interaction with ISMR. Instead
of taking Eurasia as whole as in earlier studies, they converted the monthly snow depth
data for 284 stations into 70 uniform blocks and correlated it with ISMR time series.
They conjectured the existence of a mid-latitude long wave pattern with an anomalous
ridge (trough) over the Eastern Siberia during the winter prior to a strong (weak)
monsoon. They also identified reversal in correlations between pre-monsoon and post-
monsoon months over West and East Eurasia. This reversal in correlation could be
associated with mid-latitude circulation pattern and support the theory proposed by
Meehl (1997) that monsoon plays an active part in the tropospheric biennial oscillation.
He further inferred that snow cover anomaly may be an artifact of the mid-latitude
circulation pattern associated with convective heating anomalies, rather than an
independent forcing. The physical mechanism behind the snow-monsoon relationship has
also been examined by several other authors (Liu and Yanai 2002; Ueda et al. 2003;
Yasunari 2006). Liu and Yanai (2002) have examined the large scale mid-latitude
circulation pattern associated with Eurasian snow cover/depth and Asian summer
monsoon. They observed northerly wind anomaly during the high snow year leading to
weaker monsoon over East Asia. During heavy spring snow cover in northwestern
Eurasia, the cooling centre of the cyclonic anomaly in the lower troposphere leads to a
TERI University‐Ph.D. Thesis, 2012 12
Introduction
Rossby-wave-train-like circulation response. That makes atmospheric disturbances to
propagate from Europe to Asia. Ueda et al. (2003) based on northern Eurasia spring snow
cover variations and its relation with atmospheric circulation mechanism identified that
retreat of snow in spring is controlled by winter-spring circulation anomalies along with
northward warm advection anomalies. Fasullo (2004) using satellite observations shows
that the areas with larger variability in snow cover in southwestern Asia, Himalayan and
Tibetan Plateau and Eurasia as a whole have weak correlation with all India rainfall.
However, during weak El Niño southern oscillation (ENSO) years this negative
correlation is strong in southwestern Asia, Himalayan and Tibetan regions. During an El
Niño event, the perturbation to the development of troposphere temperature anomalies
over the southern Eurasia is observed in a study by Xavier et al. (2007). This remote
climatic phenomenon over the Eurasia region may also influence ISMR.
Dash et al. (2004b) examined the number of days with different snow depths and
found those with 5–50 cm in the months of winter and spring in West Eurasia (East
Eurasia) having significant negative (positive) correlation with ISMR. Further using
HSDSD-II data set, Dash et al. (2005) statistically examined the empirical relationship
between the anomalies in winter/spring snow depth over west (25–70oE, 35–65oN) and
east (70–140oE, 35–65oN) Eurasia and ISMR for the period 1951–1994. Their results
show that 57% of heavy snow events and 24% of light snow events over West Eurasia are
followed by deficient and excess ISMR, respectively. They concluded that large scale
changes in mid latitude circulation pattern arising due to West Eurasian snow anomaly
could be used as indicators of weak/strong monsoon circulation and deficient/excess
ISMR. Ye et al. (2005) used HSDSD-II data set along with monthly gridded global
precipitation and statistically analyzed the connection between the early snow onset dates
over northern Eurasia and the following year’s summer monsoon over Southeast Asia.
They undertook and reported that when the onset of snow in the northeastern Siberia is
earlier than normal, there is more snow cover during the early season, more moisture
coverage, higher prevalence of southwesterly monsoon winds and late monsoon
withdrawal over Southeast Asia.
TERI University‐Ph.D. Thesis, 2012 13
Introduction
1.4 Modeling studies on snow-monsoon relationship
Zwiers (1993) and Vernekar et al. (1995) using global climate models and Dash et
al. (2006a) using regional climate model RegCM3 demonstrated the negative influence of
Tibetan snow depth in spring on the next summer monsoon rainfall. Sensitivity studies
over Eurasia with general circulation models (GCMs) such as those of Barnett et al.
(1989) and Vernekar et al. (1995) have shown that when large, spatially coherent,
positive snow anomalies are imposed in winter/spring, the monsoon circulation in the
following summer is weaker than average in Southeast Asia. Vernekar et al. (1995), after
conducting model experiments concluded that February snowfall results deficient rainfall
in the following Indian summer monsoon months. Dash et al. (2006b) used observed
Eurasian snow depth values as one of the boundary conditions in a spectral GCM and
integrated the model for 6 months to examine the influence of Eurasian snow depth on
the monsoon circulation. They designed the model experiments for contrasting years of
snow depth values of April over Eurasia followed by contrasting ISMR years. The
model-simulated mean monsoon circulation features for high and low snow depth years
were compared with the corresponding years of NCEP/NCAR reanalyzed fields. In their
study, the evolution of weak/strong monsoon circulation from the mid-latitude circulation
in response to high/low Eurasian snow depth during summer monsoon was indicated.
Turner and Slingo (2010) using a coupled climate model HadCM3 reexamined the
negative relationship to monsoon rainfall exists from both northern West Eurasia and
Himalayan/Tibetan Plateau in the absence of ENSO conditions. They demonstrated that
in this model forcing from Himalayan region dominates and reduce the heating of the
troposphere over the Tibetan Plateau. Using historical coupled ocean–atmosphere
simulations of the coupled model intercomparison project3 (CMIP3) database and
observations of snow cover and snow depth data, Peings and Douville (2010) further re-
examined the snow-monsoon relationship. They found the East–West dipole pattern of
snow cover anomalies over Eurasia using observational data which is not confirmed by
model simulations. Some models which indicate strongest snow-monsoon relationship
also show an unrealistic impact of ENSO on both winter snow cover and summer
monsoon.
TERI University‐Ph.D. Thesis, 2012 14
Introduction
1.5 Climate change evidences in India
There are a number of different types of extreme weather events that include
temperature extremes such as heat waves and cold waves, rainfall extremes like intense
rainfall, drought, intra-seasonal and inter-annual monsoon variability, cyclones, wind
storms, snow storms and other events like fog and snowfall. These events can be defined
on the basis of their frequency of occurrences, persistence and magnitude at spatial and
temporal scales and their widespread impacts on society. Statistical techniques can be
used for the analysis of extremes and their trends. Scientific explanation can be given
behind the causes of extreme events and their association with circulation and physical
processes. IPCC reports projected that there is 90-99% chance of rise in the intensity of
maximum surface air temperature and precipitation. The World Meteorological
Organisation (WMO) and Indian Meteorological Organisation (IMD) latest reports on
climate change have also concluded the most anticipated effects of climate change are
possible increase in the intensity and frequency of extremes especially temperature and
rainfall. These extreme weather events are not mutually exclusive. Loss of life in India
due to the extreme weather events is frequently high. The recent occurrences of extreme
weather events in India and their unusual intensities and duration are matters of concern
for scientists and society. These events include extremes such as rainfall events leading
to flooding in Mumbai during the last couple of monsoon seasons also severe heat waves
in the summers in Orissa in 1998, 1999 and 2000 and Andhra Pradesh in 2003. These
events resulted in considerable loss of lives and property.
Several scientists have examined extreme temperature events in India which
include those conducted by Raghavan (1966, 1967), De and Mukhopadhyay (1998), Pai
et al. (2004) and De et al. (2004, 2005). They have used available meteorological
measurements at several places in the country. Pai et al. (2004) found significant increase
in the frequency, persistence and spatial coverage of extreme temperature events in the
decade 1991-2000 compared to the two earlier decades 1971-1980. The effect of
urbanization in fifteen cities in India during the second half of the last century was
examined by Prakasa Rao et al. (2004) in terms of changes in the respective
TERI University‐Ph.D. Thesis, 2012 15
Introduction
meteorological parameters. They concluded that the frequency of occurrence of summer
time maximum temperature more than 35oC has decreasing trend in north India and
increasing trend in south India. They also inferred that winter time minimum temperature
less than 10oC has increasing trend in northern Indian cities. However, their results are
not statistically significant for all the cities considered. Dash et al. (2007) and Dash and
Hunt (2007) based on observational data have examined the changes in the characteristics
of surface air temperatures during the last century over seven homogeneous regions in
India. Dash et al. (2007) identified relatively maximum increase in the daily maximum
temperature in the West Coast as compared to other homogeneous regions. They have
further highlighted the heat wave conditions that occurred at seven stations in the East
Coast of India during the period 19 May to 10 June in the year 2003. Their study has
identified four stations in the East Coast of India where the maximum temperatures
crossed their respective hundred year maximum values by about 1oC or so. Similar
unusual severe heat waves occurred in a large part of India in second half of May 1998
and affected millions. Maximum numbers of heat wave conditions were reported between
the years 1980 and 1998. These occurrences of heat wave conditions were comparatively
higher than those in the previous decade 1979-1988 (De and Mukhopadhyay, 1998). Cold
wave conditions observed in the hilly regions in the north India and adjoining plains are
usually influenced by the weather systems called the Western Disturbances. These
systems are transient winter disturbances in the mid latitude westerlies which often have
weak frontal characteristics. De et al. (2005) based on observations from various sources
have inferred that the occurrence of cold wave conditions in the last century was
maximum in the Jammu & Kashmir region followed by Rajasthan and Uttar Pradesh.
Results of Pai et al. (2004) further show that cold wave conditions were most experienced
in the west Madhya Pradesh in the decade 1971-1980, in Jammu & Kashmir in 1981-
1990 and in Punjab in 1991-2000.
Extreme temperature events can be categorized under different types depending
on their intensity and duration. Usually heat/cold wave conditions with large intensity
spanning over a number of days get noticed due to their widespread impacts on society in
terms of loss of life. However, there are also other categories of warm/cold exceedences
TERI University‐Ph.D. Thesis, 2012 16
Introduction
which may affect sectors such as health and agriculture. Abrupt and frequent temperature
changes may give rise to more vectors and diseases. Working hours may also be reduced
due to heat stress (Kjellstrom et al. 2009). Small changes in temperature may adversely
affect the growth of crops and hence agricultural products to a large extent (Attri and
Rathore 2003; Peng et al. 2004). Hence, it is essential to categorize the changes in day
and night temperatures depending on their intensity and duration and then critically
examine those at regional as well as national levels.
It is a known fact that precipitation is affected by the strength of the monsoonal
flows and amount of water vapor transported. In an experiment based on quadrupling of
CO2 using a GCM, Knutson and Manabe (1995) found that the monsoonal flow and the
tropical large-scale circulation weaken in the warming atmosphere. There is emerging
consensus that in a warmer atmosphere, the effect of enhanced moisture convergence in a
warmer atmosphere dominates over weakening of the monsoon circulation resulting in
increased monsoonal precipitation (Douville et al. 2000; Giorgi et al. 2001a, b;
Stephenson et al. 2001). Douville et al. (2000) find a significant spread in the summer
monsoon precipitation anomalies despite a general weakening of the monsoon circulation
and conclude that the changes in atmospheric water content, precipitation and land
surface hydrology under greenhouse forcing could be more important than the increase in
the land-sea thermal gradient for the future evolution of monsoon precipitation.
Stephenson et al. (2001) propose that the consequences of climate change could manifest
in different ways in the physical and dynamical components of monsoon circulation.
Kang et al. (2002) commented the lack of scientific shortcoming on mean monsoon
climate and its variation on different time scale restricted them to draw any meaningful
conclusions. The monsoon flow through Peninsular India in the lower troposphere is
dominated by the low level jet stream (Joseph and Sijikumar 2004). Rao et al. (2004)
found strong decreasing trend in the tropical easterly jet strength during the Asian
summer monsoon seasons in recent years. Weakening of monsoon circulation in terms of
the decrease in its horizontal and vertical wind shears is observed in a study by Dash et
al. (2004a). Goswami et al. (2006) observed insignificant change in the mean monsoon
rainfall in India. However, they observed significant increase in the frequency and
TERI University‐Ph.D. Thesis, 2012 17
Introduction
magnitude of extreme rain events and decrease in moderate events over central India.
Dash et al (2009) further suggested weakening of the summer monsoon circulation over
India in the study on changes in the frequencies of different categories of rain events.
This hypothesis of a weakening of monsoon circulation is supported by significant
reduction in the 850hPa wind fields in the National Centers for Environmental Prediction
(NCEP)/National Center for Atmospheric Research (NCAR) reanalyzed data. Further,
they indicate significant rise in short and dry spells whereas long spells show decreasing
trend. Ashfaq et al. (2009) used a high resolution climate model to study the monsoon
pattern in changing climate and shows the overall weakening of the summer monsoon
precipitation over South Asia.
It is reported in the IPCC AR4 (2007) that the pattern and magnitude of ISMR
will be changed under warmer climate that is based on simulations of IPCC models under
different forced scenarios. The most emission scenarios suggest that future changes in
regional climate are still likely to be dominated by increasing greenhouse gas forcing
rather than changes in sulphate and absorbing aerosols, at least over the South Asian
region (IPCC 2007). The variability of monsoon circulation under changing content of
carbon dioxide is studied by Degtyarev (2008) using numerical model and analyzed
monsoon circulation indices calculated from the zonal-wind speeds simulated in the
upper and lower troposphere and model precipitation rates. The skill of predicting the
seasonal mean monsoon rainfall by almost all the global climate models remains very
small. The most emission scenarios suggest that future changes in regional climate are
still likely to be dominated by increasing greenhouse gas forcing rather than the changes
in sulphate and absorbing aerosols, at least over the South Asian region. Number of
scenarios generated by Atmosphere-Ocean General Circulation Models (AOGCMs)
under IPCC can be used to investigate the potential consequences of climate change and
weather for the environment and society over the global. Further studies are needed for
understanding changes in monsoon circulation pattern that could lead to increase
vulnerability due to the impact of climate change. The skill of predicting the seasonal
mean monsoon rainfall by almost all the global climate models remains very small. In
warming climate, variations of the Himalayan/Tibetan/Eurasian snow and ground surface
TERI University‐Ph.D. Thesis, 2012 18
Introduction
temperature and links with IMSR are not clear, although some progress has been made in
analyzing snow depth and surface temperature over the western Himalaya under a
doubling CO2 scenario (Parth Sarthi et al. 2011).
1.4 Background and scope of this thesis
Indian summer monsoon has its own characteristics of evolution such as onset,
active, break and withdrawal phases which have been studied extensively. However, the
evolution of Eurasian snow is yet to be examined. Further, it is interesting to explore the
characteristics of evolution of snow over the different regions of Eurasia and their
relationship with the evolution characteristics of summer monsoon. In this thesis, a
detailed examination has been done on the starting and the ending dates of snowfall over
different regions of Eurasia and attempts have been made to explore any relationship with
onset of ISMR. It is also essential to know the detailed temporal and spatial distributions
of snow depth over Eurasia and precipitation over Indian landmass in order to make use
of these findings in long range monsoon prediction.
The regional climate model of the ICTP has been successfully used by several
researchers such as those by Dash et al. (2006b), Giorgi et al. (2007) and Im et al. (2010)
to examine atmospheric circulation features of different temporal and spatial scales. In
the earlier study of Dash et al. (2006b), RegCM3 has been successfully integrated to
simulate the salient features of summer monsoon circulation from 1993-1996. Their study
indicates that RegCM3 can be used to study the monsoon processes over the south Asia
region. In order to understand and reduce the model bias for Indian summer monsoon,
detail analysis need to be done using long term RegCM3 integrations.
Extreme temperature events always influence ecosystem and human society. Rise
in global mean temperature and increase in the number of warmer years during the past
two decades have been investigated by a number of researchers (Jones and Briffa, 1992;
Mann et al. 1998; De et al. 2005). Studies suggest that extreme temperature events are
changing over the world. Similar inference over India has been based on observed
TERI University‐Ph.D. Thesis, 2012 19
Introduction
temperature values. However, these observations are not uniform in space and time
duration. Currently, a good quality dataset of daily maximum and minimum temperatures
for the period 1969-2005 is prepared by the India Meteorological Department (IMD) at a
resolution of 1ox1o over the Indian land points. It provides a good scope to study the
changes in different categories of temperature extremes in India. Depending on intensity
and duration, moderate and intense temperature events need to be categorized and
studied.
Kripalani et al. (2007) examined south Asian summer monsoon precipitation
variability in models of IPCC AR4. They found that out of 22 models, 19 could able to
capture maximum rainfall during the summer monsoon season. In order to capture more
regional features of Indian summer monsoon the higher horizontal resolution of the IPCC
models are considered in this study. Only those important models are considered whose
average latitudinal surface resolution varies from 1.1 to 2.0 degree approximately. Based
on this criteria five models are selected that are CCSM3, ECHAM5, GFDL2.1,
MIROC3.2 (hires) and UKMO-HadGEM1. The simulations of these models can be used
to investigate the changes in precipitation, temperature and wind circulation pattern in
future climate with respect to the present climate.
Based on the above discussion, the present thesis has the following specific objectives:
1. To study the characteristics of Eurasian snow depth with respect to Indian summer
monsoon rainfall.
2. To analyze of Indian summer monsoon rainfall and temperature characteristics
using long term integration of RegCM3 and observed data.
3. To re-examine the snow-monsoon relationship using the IITD T80L18 GCM
simulations
4. To compare IPCC AR4 models simulated Indian summer monsoon characteristics
TERI University‐Ph.D. Thesis, 2012 20
Introduction
The thesis has been organized into six chapters. This chapter 1 introduces the topic and
deals briefly with the discussion on the important issues based on earlier literature.
Chapter 2 provides a detailed discussion on the characteristics of Eurasian snow and its
relationship with ISMR. The connections of snow starting dates with monsoon onset
dates at Kerala Coast are examined in detail. Also, the strength of snow depth in different
regions in Eurasia is related with strength of summer monsoon rainfall in five
homogeneous rainfall regions in India.
Chapter 3 presents the experimental work using ICTP regional climate model, RegCM3.
A brief analysis of summer monsoon features simulated by RegCM3 such as seasonal
climatology, intra-seasonal and inter-annual variations and compared those with
observations. Extreme rainfall years are also considered for further verifications.
Chapter 4 proposes the study on spatial and temporal changes in the characteristics of
maximum and minimum temperatures and their extremes in India as a whole as well in
its seven homogeneous temperature regions using IMD observational data. Temperatures
are categorized based on their intensity and duration and analyzed on interannual and
interdecadel time scales.
Chapter 5 is aimed to know the possible changes in ISMR, temperature wind circulation
pattern in warmer climate under forced emission scenarios. Therefore, 20th century
control run (20C3M) and forced scenarios A2, B1 and A1B are considered using the
simulations of IPCC AR4 coupled climate models.
Finally, in Chapter 6, the results of the previous chapters (2 to 5) have been summarized,
and conclusion of the thesis is presented. This chapter also includes the scope to
incorporate new ideas and improvement in present work for future developments.
TERI University‐Ph.D. Thesis, 2012 21
Introduction
TERI University‐Ph.D. Thesis, 2012 22