Chapter 1
Introduction and Literature Survey
1. 1 Introduction
Jet Streams are a strong narrow current of air concentrated
along a quasi-horizontal axis, usually in the upper troposphere
characterized by strong vertical and horizontal shears and featuring
one or more velocity maxima. The vertical shear is of the order of 10-20
knots per kilometer and the lateral shear is about 18 knots per
hundred nautical miles. The differences in air temperature at the
surface can produce winds aloft. Where these temperature differences
are most pronounced, the winds aloft are stronger giving rise to Jet
Stream.
A variety of jet streams exist in the atmosphere such as polar
front jet stream, polar night jet stream, subtropical westerly jet stream,
tropical easterly jet stream (Fig. 1.1). The polar front jet stream is found
in the upper troposphere between 300N and 600N near the tropopause.
Towards the poles, warmer mid-latitude air meets cold polar air at the
polar front, and the pronounced temperature differences there result in
the polar jet stream aloft. The polar night jet stream is located in the
upper stratosphere near 600N. Further south, near 30 degrees latitude,
is another region of intense westerly winds known as the subtropical
jet. These jet streams are localized regions of intense westerly flow
where wind speeds exceeds 100 knots. They act to steer weather
systems (i.e. low pressure centers, fronts), and their position is often
referred to as the storm track. The position and strength of the polar
and subtropical jet streams varies throughout the year, and can also be
affected by the change in surface pressure and ocean temperature. All
these jet streams flow from west to east except for the tropical easterly
Chapter 1: Introduction
2
jet which flows from east to west. The tropical easterly jet is located at
about 15°N extending across Indian Peninsula to northeastern Africa.
Usually the subtropical westerly jet stream and the tropical easterly jet
stream in the westerly and easterly wind regimes in the upper
troposphere are the two prominent jet streams that constitute an
important part of the tropical general circulation. These jet streams are
known to have significant bearing on Indian weather and climate.
In addition to these upper tropospheric jet streams, a lower
tropospheric jet stream exists in the monsoon regime known as Somali
jet or Cross Equatorial Low-level Jet Stream (LLJ) and it has a
significant role to play on the southwest monsoon activity over India.
Low-level jet streams, which are found in several parts of the world,
refer to strong low level currents on a monthly or seasonal basis, but
they are much weaker and smaller than the planetary-scale uppertropospheric jet streams. These are generally located in the lowest 1 to
2 km of the troposphere and are strongly influenced by local factors
such as orography, friction and diurnal cycle of heating. Some of the
locations favorable for the generation of low-level jet stream are slopes
of mountains parallel to the anticyclonic flow around the subtropical
anticyclones, north-south oriented coasts near cross equatorial flow
and narrow mountains gaps. Although the low-level jets are weak and
limited in spatial extent, they constitute an important part of the
regional circulation owing to their role in large-scale moisture and
momentum transport. During the Indian southwest monsoon, a strong
cross equatorial flow occurs in the form of a low level jet at a level
between 1 and 1.5 km with a core speed of 15-25 m/s and with strong
horizontal and vertical shears.
Chapter 1: Introduction
3
Fig. 1.1 Jet stream positions in the Atmosphere (Courtesy Britannica
Encyclopedia).
Low-level jets have been observed over every continent (Fig. 1.2),
although the more frequent low level jets are known or suspected to
occur over North America (Bonner, 1968; Douglas, 1993), South
America (Virji, 1981, 1982), Africa (Ardanuy, 1979; Findlater, 1969b;
Jury and Spencer-Smith, 1988; Jury and Tosen, 1989; Kelbe, 1988),
Australia (Keenan et al., 1989; Willson, 1975), Asia (Findlater, 1969b;
Tao and Chen, 1987), and Antarctica (Chiba and Kobayashi, 1986;
Schwerdtfeger, 1975). These regions of frequent LLJ occurrence
typically are located either to the east of a large mountain range or
where large land-sea temperature gradients exist.
Chapter 1: Introduction
4
Fig. 1.2: Regions where low level jet occur (shaded), square shows
regions where low level jets have been observed. (Stensrud, 1996)
1.2 Somali Jet
Somali jet is an essential part of the Indian summer monsoon.
This jet constitutes the strongest cross-equatorial flow in the lower
troposphere and forms part of a major low-level air current in the
monsoon system. The movement of the southwest monsoon towards
India is supported to a great extent by the crucial Somali jet which
originates
near
well
know
“Mascarene
high”
in
the
southern
hemisphere. The Somali jet commences its journey over “Mascarene
high” near Madagascar in the southern hemisphere. This low level jet
(LLJ) stream flows intermittently from the vicinity of Mauritius, over the
northern tip of Madagascar, to reach Kenya coast as southerlies.
Sometimes, this southeasterly jet from the Mauritius area is joined by
or even replaced temporarily by low level jet streams moving northward
through the Mozambique Channel after bursts of cooler air come
around the tip of southern Africa. During April this LLJ flows across
the northern tip of Madagascar, then enters into East Africa during
May and, subsequently, crosses the northern parts of the Arabian Sea
Chapter 1: Introduction
5
before reaching India in June (Fig. 1.3(a)). In the month of July two
maxima are observed; one near the Somali coast and the second over
the northern tip of Madagascar and also branching can be seen around
100N (Fig. 1.3(b)).
Fig. 1.3: (a) Mean Monthly position and (b) mean velocity (1Km) during
July, of the Somali Jet over the Indian Ocean (Findlater, 1971).
1.3 Importance of Somali Jet
Bunker (1965) and Findlater (1966) brought to the attention of
the world the existence of the low level jet stream flowing close to the
coast of Kenya and Somalia during the northern summer season.
Observations propose that during the Asian Summer Monsoon
strongest cross equatorial flow from the southern to the northern
hemisphere is in the region of the low level jet. Additionally one
important fact related to the LLJ is the coincidence of a zone of coastal
upwelling with its path around 90N. After the low level jet moves
towards the Indian coastline around 90N, it separates into two
Chapter 1: Introduction
6
branches (Fig. 1.4). One branch flows towards the west coast of the
Indian Peninsula while the second branch recurves towards the
southern half of the Indian coastline and Sri-Lanka.
It must be acknowledged that Findlater’s analysis highlighted the
existence and importance of the East African Low-Level Jet (LLJ).
Findlater (1966) showed that maximum intensity occurs between June
to September and jet core found between 1 and 1.5 km above mean sea
level, which (Joseph and Raman, 1966) is regularly observed over
peninsular India. In his subsequent work, Findlater (1969b) extended
the analysis to areas south and north of the equator. Findlater found
that the LLJ has its origin in the south Indian Ocean north of the
Mascarene High and flows as an easterly current which crosses the
equator along the east coast of Africa as southerly current and turns
into a westerly current over the Arabian Sea and passes through India.
According to their computation in the lower troposphere this jet
contributes
nearly
half
the
inter-hemispheric
transport
of
air.
Keshavamurty (1968) also documented the presence of southwesterly
winds over the Arabian Sea, peninsular Indian and the Bay of Bengal.
Later Findlater (1969a) explored the mean flow patterns for
January and July which reveal the jet’s significant contribution to mass
transport of air between the southern and the northern hemispheres.
Also a multicore structure was noticed over the low-lying areas by
Findlater (1972). This phenomenon was discovered by Findlater (1971)
using monthly mean winds.
But it is still not clear why the jet
separates into two branches. It is also not clear why the major flow of
air from the southern to northern hemisphere takes place along a
narrow preferred zone off the East African coast and these still
intrigued meteorologists. Because of the extensive work by Findlater
this low level jet is also known as Findlater’s jet.
Chapter 1: Introduction
7
Fig. 1.4: August wind field over the Indian Ocean (Findlater (1971)).
Thick line shows the axis of Somali Jet and broken line shows the
Isotach.
Washington and Daggupaty (1975) using National Center for
Atmospheric Research (NCAR) General Circulation Model (GCM) has
simulated some of the features of the LLJ but the core speed of the
simulated jet was rather low (~12m/s). Krishnamurti et al. (1976)
showed that many feature of the LLJ could be simulated by a one-level
primitive equation model incorporating the East African Mountains, βeffect and lateral forcing from the east around 750E longitude. In
another study Anderson (1976) considered the LLJ essentially as a
boundary current bounded on the west by the East African mountain
chain. Hart (1977) felt that vertical diffusion was more important than
Anderson (1976) horizontal diffusion.
Desai et al. (1976) with the help of surface and upper air
observations taken during the Indo-Soviet Monsoon Experiment
(ISMEX) and observations over the East African coast and the Indian
Peninsula confirmed the earlier belief that active to vigorous monsoon
Chapter 1: Introduction
conditions
over
the
8
West
Indian
coast
were
preceded
by
the
strengthening of the LLJ over the Arabian Sea.
In one of the models of Hart (1977) represented the idea of
potential vorticity conservation and the advection of the potential
vorticity across the equator resulting in the formation of a low level jet
in the presence of western boundary mountains. Thereafter a series of
experiments were performed with a simple barotropic primitive
equation (P.E.) model by Bannon (1979a, 1979b) where author was able
to simulate features of Somali jet which resemble the real jet. These
studies also have described some of the dynamics involved.
Stommel and Fieux (1978) studied the structure of the Surface
winds during the onset of monsoon in the Arabian sea with the help of
historical meteorological ship reports available between the equator and
20°N, and between Africa and India for the period between 1900 and
1973. They tried to find a link between onset of monsoon near the
Somali coast and the rainfall over the Indian peninsula.
Krishnamurti and Wong (1979a) concluded that the meridional
motion of air across the equator from the Southern to the northern
hemisphere toward lower pressure results in an acceleration and an
enhancement of the horizontal advective terms in the balance forces. In
continuation to this study Krishnamurti et al. (1983) with the help of
three dimensional planetary boundary layer model were able to
simulate more realistically its curvature and the position of its
maximum
strength
Ramanathan
(1982)
over
the
showed
Arabian
that
the
Sea.
overall
Krishnamurti
and
development
and
strengthening of the low-level zonal flow during onset is highly sensitive
to the large scale field of differential heating. Cadet and Desbois (1980)
studied the fluctuation in the intensity of the Somali low-level jet
flowing along the East African coast during the Indian summer
monsoon. On the basis of preliminary analysis Cadet and Desbois
Chapter 1: Introduction
9
(1979) suggested that the quasi-biweekly oscillation of the jet may be
associated with an interaction between mid-latitude perturbations of
Southern Hemisphere and the southeast trade wind of the equatorial
southern Indian Ocean.
Based on trajectory calculations, Kuettner and Unninayar (1981)
suggest that the low level monsoon jet over the Arabian Sea is an
inertial current steered by the cross-equatorial pressure gradient.
Grossman and Friehe (1986) hypothesized that the vertical
structure of the LLJ can be explained by the mixture of two dynamical
effects: the thermal wind or vertical shear of the geostrophic wind, and
transfer of horizontal momentum from the atmosphere to the ocean by
turbulent momentum exchange within the atmospheric boundary layer.
Their study reveals that geostrophic wind shear is dominant above the
low-level maximum while turbulent momentum exchange is dominant
below.
Rodwell and Hoskins (1995) studied the LLJ using a time
dependent primitive equation model having specified zonal flow,
mountains and diabatic heating. They found that, surface friction and
diabatic heating provide mechanisms for material modification of
potential vorticity (PV) of the flow and both were found to be important
for the maintenance of the LLJ.
It is necessary to mention that the possible reason behind
consideration of low level jet in studies of global climate is its
relationship with convective activity. The low level jet stream can also
affect climate in various ways that are not related to convective activity.
As shown by several studies (Kuo and Seitter, 1985; Lemaitre and
Brovelli, 1990; Mastrantonio et al., 1976; Raymond, 1978) the low level
jet not only helps in transporting moisture but also responsible for
instabilities that can be produced by a LLJ wind profile and influence
convective development (Stensrud, 1996).
Chapter 1: Introduction
10
Fieux and Stommel (1977) showed that near Somalia the onset
duration is nearly one week. Later Halpern and Woiceshyn (1999) made
detail study of the onset of the Somali Jet in order to describe Arabian
Sea winds and pointed out that the onset time of the Somali jet is not
the same throughout the Arabian Sea. According to them, minimum
duration was about three inertial periods, which is the approximate
time for development of Ekman currents and the zonal component of
wind direction must be eastward.
The tele-connection of Somali jet over the Arabian Sea with ElNiño / La-Niña has been explained by Arpe et al. (1998). A dynamic
interaction of Somali jet with the Western Ghat (Sahyadri Mountain)
also has been suggested by Wu et al. (1999).
Halpern and Woiceshyn (2001) studied the interannual variation
of Somali Jet linked with the El-Niño and La-Niña episodes during the
period 1988-1999. They found that the average date of Somali Jet onset
was two days later in El-Niño events in comparison with La-Niña
conditions.
Swapna and RameshKumar (2002) examined the role of low level
flow on the monsoon activity using monsoon period 850hPa winds for
two contrasting years 1987 (drought) and 1988 (good). They found that
wind speed was more over the western Arabian Sea during good
monsoon year as compared to bad monsoon year.
Sam and Murty (2002) described the variation in the monsoon
LLJ from an active phase of the monsoon to a break phase using
temperature and wind data for 1965 and 1966. They found that during
the active phase of monsoon the low level jet has a core speed of 55
knots and during the break phase of monsoon the core speed was less
than 10 knots. Moreover, they mentioned that this jet stream is
expected to have a good effect on the distribution of rainfall with
Chapter 1: Introduction
11
respect to its axis and is therefore important in forecasting of monsoon
rainfall.
Recently
the
process
of
Somali
jet
formation
has
been
hypothesized by Taniguchi and Koike (2003) which involves a)
atmospheric heating over the western part of Tibetan Plateau, b)
formation of the low in the lower troposphere (the monsoon trough) as a
consequence of the change in the upper troposphere, c) formation of
pressure gradient between the monsoon trough and the anticyclone in
the southeastern part of Africa and finally d) generation of Somali jet.
Moreover through examination of the seasonal march of the wind field
at 850hPa, they recognized that Somali Jet is one of the key factors of
forming the Asian monsoon at its initial stage
Joseph and Sijikumar (2004) suggested that adequate care
should be taken by monsoon modelers to see that LLJ and associated
deep convection are properly simulated in the models.
Boos and Emanuel (2009) examined the onset of Somali jet and
the associated monsoon in a convective quasi-equilibrium framework.
They observed that the Jet onset is accompanied by a large (O(100
W/m2)) increase in surface enthalpy flux over the Arabian Sea that is
nearly collocated with, and linearly related to, the concurrent increase
in deep tropospheric ascent. Camberlin et al. (2010); Joseph and
Sijikumar (2004); Roja Raman et al. (2011) discussed different
characteristics of the Somali jet associated with the active-break cycle
of the Indian summer monsoon (ISM) rainfall.
1.3.1
Monson Onset
India is a country where nearly 60% of the population derives
their livelihood from agriculture (Sharma et al., 2003). Beginnings of
various agricultural activities are linked with the onset of the monsoon
because of which information about the onset of the monsoon is eagerly
Chapter 1: Introduction
12
awaited by farmers. Therefore prediction of the date of onset of
monsoon is an important aspect for the Indian farmers. The normal
date of onset of monsoon over Kerala is 1 June with a standard
deviation of 7days (Fig.1.5). Any failure or even late arrival of monsoon
rains results in wide spread starvation and economic disaster.
Every year India Meteorological Department (IMD) declares the
monsoon onset over Kerala operationally. Although, there is no precise
definition
of
the
onset
of
the
monsoon,
conventionally
Indian
meteorologists identify the date of onset over the Kerala coast based on
a sharp increase and characteristic persistency of the rainfall
(Ananthakrishnan et al., 1968). Besides this fact there exist a number
of definitions of onset of monsoon over Kerala (Ananthakrishnan et al.,
1967; Ananthakrishnan and Soman, 1988; Fasullo and Webster, 2003;
Gadgil and Joseph, 2003; Goswami and Xavier, 2005; Lin and Wang,
2002; Xavier et al., 2007), the most commonly used definition is based
on rainfall over a number of stations exceeding a threshold that is
sustained for minimum period of time (Ananthakrishnan and Soman,
1988). Also appreciable changes occur in a) winds, b) temperature and
c) rainfall on account of monsoon.
The movement of the southwest monsoon towards India is greatly
supported by the Somali jet that transits through Kenya, Somalia and
Sahel and exits the African coast at 90N at low level. Also the criteria
adopted by IMD for operational declaration of monsoon onset contain
set in of westerlies as one of the parameters which in turn shows the
crucial importance of Somali jet in the prediction of monsoon onset.
But it is necessary to mention here that there is no medium range
method available for the prediction of date of onset of monsoon over
Kerala.
Chapter 1: Introduction
13
Fig.1.5: Normal dates for onset of monsoon over India (Obtained from
IMD, http://www.imd.gov.in/section/nhac/dynamic/newnormalonset.
jpg)
1.3.2
West Coast Rainfall
West coast of India is one of the regions which receives active
monsoon spell during the monsoon season. The reason lies behind the
orientation of the Western Ghat (Fig. 1.6). The mountain ranges are
parallel to the coast about 50 km inland between 8.5 and 210N latitude
and have a mean altitude of 800m, though individual peaks are much
higher. The variable rainfall distribution is noticeable during the
monsoon season (June - September) over the Indian Peninsula. Mainly
the orographic influence is dominant for this variation in distribution of
rainfall as the winds are almost at right angles against the Western
Ghats during the monsoon season.
Chapter 1: Introduction
14
Fig. 1.6: Topographic map of India (Elevation in meters) (Courtesy
USGS topography data)
The onset of Somali jet is believed to be the precursor of the
onset of rainfall along the west coast of India (Desai et al., 1976;
Krishnamurti et al.,1981; Yadav and Kelkar, 1989). The Western Ghats
parallel to the coast also play an important role in concentrating
rainfall over this region (Ogura and Yoshizaki, 1988; Prakash et al.,
2013; Suprit and Shankar, 2008; Wu et al., 1999). The well known LLJ
during the June-September months produces intense rainfall over the
west coast of India due to the convergence of wind flow and uplift by
the Western Ghats (Joseph and Raman, 1966).
A relationship between the low level cross equatorial winds which
gets locked by the topography of eastern Africa and the rainfall over
parts of western India has been found by Findlater (1969b), although
this question needs careful examination (Raghavan et al., 1975). Desai
et al. (1976) also confirmed the earlier belief that active to vigorous
monsoon conditions over the West Indian coast were preceded by the
strengthening
of
the
LLJ
over
the
Arabian
Sea.
Furthermore
Krishnamurti and Bhalme’s (1976) spectral analysis of the intensity of
the jet shows the presence of quasi biweekly oscillation of the intensity
of the jet and it may be related to likely oscillations observed in rainfall
Chapter 1: Introduction
15
over India during active and break monsoon. Occasionally, intense
rainfall over the west coast, associated with the formation/movement of
troughs and vortices, occurs during the months of July and August
(Francis and Gadgil, 2006).
The Somali jet carries a large amount of moisture from the
southern Indian Ocean and the Arabian Sea (Simon and Desai, 1986),
pours them as heavy rainfall over the windward side of the ghats when
the LLJ is forced to ascend over the Western Ghats mountain chain
(Grossman and Durran, 1984). On the basis of monthly mean intensity
of Somali jet Halpern and Woiceshyn (2001) mentioned that when the
intensity of Somali jet was above (below) normal, there was an excess
(deficit) of rainfall along the west coast of India. Due to very small
sample size the statistical confidence in their results was low and
results may be considered to be a demonstration of concept. But still
there is no study for the prediction of the occurrence of active monsoon
spell over the west coast of India.
1.3.3
Cross-equatorial transport
The important parameters which play a vital role during the
Indian summer monsoon season are the moisture and the momentum.
It has been observed that 70% of the water vapor that reaches the west
coast of India is from southern hemisphere and 30% comes from
evaporation occurring over the Arabian Sea (Cadet and Reverdin, 1981).
Cadet and Reverdin (1981) showed that in bringing the monsoonal
rains over India, the cross-equatorial flow of moisture and wind as well
as the evaporation over the north Indian Ocean and adjoining seas are
known to be equally important. Also it has been observed that cross
equatorial winds are stronger during good monsoon as compared to
bad monsoon (Krishnamurti et al., 1976). Thus the large amount of
cross equatorial air flux over the Indian Ocean points towards the
Chapter 1: Introduction
16
importance of moisture transport occurring across the equatorial
region.
Pattanaik et al. (2005) showed that presence of strong crossequatorial flow from the beginning of May is conducive for the early
onset of monsoon while subdued cross equatorial flow till the first week
of June is responsible for the late onset, However,
they have
considered only two years of data (2003 and 2004) to conclude. Kakade
and Dugam (2008b) studied the impact of cross equatorial flow on intra
seasonal variability of Indian summer monsoon rainfall and found that
850hPa zonal wind gradient anomaly plays an important role during
the last 20 days of May. In earlier studies many authors have tried to
find out the amount of moisture over the Arabian Sea, but due to lack
of observations these studies are not conclusive. In addition to this fact
there is difference in the amount of cross-equatorial flow computed by
Rao (1964), Findlater (1969a,b), Saha (1970), Saha and Bavadekar
(1973). This shows the need of reexamination of this problem. Also in
this regard Hart et al. (1978) suggested that for accurate computation
of water vapor flux one should use good low altitude wind and humidity
observations, whereas Cadet and Reverdin (1981) says that “much
more needs to be done to estimate more accurately the transport of
water vapour over the Indian Ocean, to understand its variation and
their relationship with rainfall over India”.
1.3.4
Structure of the Somali Jet
Several studies have been made to find out the structure of
Somali jet (Ardanuy, 1979; Bannon, 1979a, 1979b, 1982; Chakraborty
et al., 2009; Desai et al., 1976; Farquharson, 1939; Findlater, 1966,
1967, 1969a, b, 1971, 1972, 1974; Halpern et al., 1998; Halpern and
Woiceshyn, 1999, 2001; Hart et al., 1978; Jambunathan and
Ramamurthy, 1974; Joseph and Raman, 1966; Krishnamurti et al.,
1976; Krishnamurti and Wong, 1979a; Krishnamurti et al., 1983; Pant,
Chapter 1: Introduction
17
1982; Wu et al., 1999). Most of them provide brief information in the
form of case studies or over a specific location but not over the entire
Arabian Sea over which Somali jet flows during the summer monsoon
season and acts as a lifeline.
It is well known that during the Indian summer monsoon season
upwelling occurs near the Somali coast. The inversion over the Arabian
sea was first detected by Colon (1964) later Narayanan and Rao (1981)
also
detected
inversion
over
the
Arabian
sea
using
satellite
observations. Sen and Das (1986) have also studied some aspects of
the low level inversion over the Arabian Sea using MONEX-79
(Monsoon
Experiment,
1979)
data.
Pant
(1982)
studied
some
characteristic features using Indo-Soviet Monsoon Experiment 1973
(ISMEX-73) and Monsoon Experiment 1977 (MONEX-77) data. He
found two jet maxima one near 1.0 km in the north and other at South
at 2 km outside the inversion. Additionally, Findlater (1971) has shown
that the Somali jet splits into two branches out of which one moves
towards the west coast of India while the other moves towards the
southern tip of India. But still it is a topic open for investigation due to
differing opinion on whether jet splits or not.
1.3.5
Simulation of the effect of Mascarene high on
the strength of Somali Jet
The Mascarene high is a high pressure area south of the equator.
The name Mascarene high came from the Mascarene Islands east of
Madagascar. The center of this anticyclone is located near 300S and
500E. Krishnamurti and Bhalme (1976)studied the various elements of
monsoon system, namely Monsoon trough, Mascarene high, Low-level
cross equatorial jet, Tibetan high, Tropical easterly Jet, Monsoon
cloudiness, Monsoon rainfall dry and moist static stability. In their
paper they have also mentioned that “synoptic meteorologists and
operational weather forecasters in these regions have long recognized
Chapter 1: Introduction
18
that Mascarene high has some importance during the monsoon
months, although no definitive analysis of its importance has been
demonstrated”.
Simple and handy way for understanding the different aspects of
Indian monsoon is the use of the climate models. Numerical simulation
of the Indian monsoon has advantage over conventional analysis of
data. It enables us to perform control experiments by which we could
learn about the aggregate of physical processes that make up the
monsoon.
The numerical models have been used extensively to study LLJ
development and evolution and have reproduced the basic features of
many observed LLJs (Stensrud, 1996). Typically these studies have
been conducted using either a case study approach (Brill et al., 1985;
Doyle and Warner, 1993; Lapenta and Seaman, 1990) or simplified
analytic initial conditions to examine LLJ sensitivities to various model
parameters (Fast and McCorcle, 1990; Krishnamurti et al., 1976;
McNider and Pielke, 1981; Paegle and Rasch, 1973; Savijarvi, 1991).
The advantage of using numerical models is the ability to separate the
effects of various physical processes on the jet evolution. These studies
point towards the usefulness of numerical models for simulating and
understanding the monsoon.
1.4 Main objectives and outline of the thesis
Somali jet plays an important role during Indian monsoon. In
this thesis an attempt has been made to provide a broad view of Somali
jet. Since most of the earlier studies are based on only a few years data
or over/along a particular region. Therefore an attempt has been made
to provide detailed view over the entire Arabian Sea. The thesis also
sets the goal of formulating a criterion for the prediction of the date of
onset of monsoon over Kerala and active monsoon spell over the west
Chapter 1: Introduction
19
coast of India. Additionally the thesis also aims at finding out the
amount of cross equatorial flux of moisture as well as momentum
during the monsoon months. An experiment has been conducted in
order to find out the effect of Mascarene high on the strength of Somali
Jet.
Therefore, keeping these objectives in mind the first chapter is
designed to provide brief information about the topic, which includes:
brief information about various jet streams present in the atmosphere,
Somali Jet and its importance. Also an attempt is made to provide brief
details about the previous studies involving theoretical as well as
modeling studies related with Somali Jet, onset of monsoon, west coast
rainfall, and cross-equatorial transport.
Chapter 2 gives details about the reanalysis data sets i.e. NCEP
FNL and ERA-Interim as well as provides brief details about the NOAA
interpolated Outgoing Longwave Radiation (OLR) data used in this
study. Also it includes information about the IMD high resolution daily
gridded rainfall dataset. Brief information has been provided about the
India’s first satellite Kalpana-1 launched for Meteorological purpose
whose data have been used in this study.
In spite of its importance, there are not many studies related to
the prediction of the date of onset of monsoon over Kerala. The existing
method of IMD does not give any medium range forecast. The IMD has
stipulated some criteria based on which they declare onset date. They
do not predict the date of onset over Kerala. Therefore in chapter 3 an
attempt has been made to devise criteria for the prediction of the date
of onset of monsoon over Kerala with some lead time in medium range
scale.
The west coast of India is one of the places which receive heavy
rainfall during the southwest monsoon season. Mumbai, the financial
capital of India is also situated on the west coast of India. However
Chapter 1: Introduction
20
there is hardly any attempt to predict the active monsoon spell over the
west coast of India. Thus, chapter 4 deals with the investigation
regarding the prediction of active monsoon spell over the west coast of
India. In this chapter an effort has been made to formulate criteria for
the prediction of heavy rainfall using wind and OLR data.
The availability of high resolution data and also due to divergent
opinions about the cross-equatorial transport during the monsoon
season an attempt is made in chapter 5 to revisit the problem of
computation of cross-equatorial transport of moisture as well as
momentum during the pre-monsoon and monsoon season. Also
investigation is conducted to find a region which can provide precursor
for the prediction of monsoon performance in the following months.
Chapter 6 deals with the structure of Somali Jet. This chapter
presents detailed structure of Somali Jet highlighting the spatial extent
of maximum wind and the level at which jet maxima occur. This also
deals with the behavior of jet maxima during inversion episode. At the
end of this chapter a study is conducted regarding the splitting of
Somali jet as there were diverse opinions about the splitting.
The Mascarene high near to which Somali jet commences its
journey has profound impact on the Indian summer monsoon. In this
regard examination is carried out in chapter 7, in which an attempt has
been made to simulate the effect of Mascarene high on the strength of
Somali jet using WRF model.
Although the main findings have been presented at the end of
each chapter, important results of the thesis are summarized in
chapter 8 with future scope of the study.
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