INDIAN SUMMER MONSOON AND BEHAVIOUR OF
NORTHERN AND SOUTHERN ANNULAR MODES
SUMIT VIKRAM SINGH
CONTACT: - firstname.lastname@example.org
TERM –IV, 2013-14
UNDER GUIDANCE OF
ROLL NO:-120107234, CIVIL 2ND
YEAR, SEC- D
COURSE NAME:- HYDROLOGY
DEPARTMENT OF CIVIL ENGINEERING
SCHOOL OF ENGINEERING AND TECHNOLOGY
SHARDA UNIVERSITY, GREATER NOIDA, U.P, INDIA
The Monsoon of the Indian subcontinent is among several geographically distributed
observations of global monsoon taking place in the Indian subcontinent. In the subcontinent,
it is one of oldest weather observations, an economically important weather pattern and the
most anticipated weather event and unique weather phenomenon.
The main objective of this research paper is to present a deep and specifiedanalysis on
monsoon and its components in India. A brief introduction about monsoon (what is
monsoon?) is followed by types of monsoon in India, their occurrence pattern, rainfall
intensity, wind movement and effect of NAM and SAM and Atlantic oscillations. Subjects
are supported with good clear images in order for better understanding.
Emphasis is given on summer monsoon pattern in India. Effect of NAM (NORTH
ANNULAR MODE), SAM (SOUTH ANNULAR MODE), and NAO (NORTH ATLANTIC
OSCILLATIONS) has also been linked to monsoon patterns in India.
In addition to that, these modes have known to influence the precipitation
pattern over the tropical Indian subcontinent Finally a small conclusion with all the
references have been added at the end of the papers.
Monsoon season is a welcome relief to drought conditions in many areas of the world.
Monsoons can also bring about widespread famine and enough rain to kill hundreds of people
in floods. While the Asia and India monsoons are famous, there are even monsoon season in
the United States. So, what is a monsoon?
Monsoons, or rainy seasons, are a shift in wind direction which causes excessive rainfall in
many parts of the world including Asia, North America, South America, and Africa. The
primary mechanism behind a monsoon is a shift in global wind patterns.
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding
changes in precipitation, but is now used to describe seasonal changes in atmospheric
circulation and precipitation associated with the asymmetric heating of land and sea. Usually,
the term monsoon is used to refer to the rainy phase of a seasonally-changing pattern,
although technically there is also a dry phase
During most of the year, winds blow from land to ocean making the air dry. Winds
originating from land are called continental. During certain months of the year, the winds
begin to blow from the ocean to the land making the air moist. Winds originating over a body
of water are called maritime. This moist ocean air is what causes monsoonal rains over many
2. WORK DONE
2.1 HOW ARE MONSOONS FORMED?
Monsoons are large-scale sea breezes which occur when the temperature on land is
significantly warmer or cooler than the temperature of the ocean(SEE FIG.2.1.a). These
temperature imbalances happen because oceans and land absorb heat in different ways. Over
oceans, the air temperature remains relatively stable for two reasons: water has a relatively
high heat capacity (3.9 to 4.2 J g−1
),and because both conduction and convection will
equilibrate a hot or cold surface with deeper water (up to 50 meters).
In contrast, dirt, sand, and rocks have lower heat capacities (0.19 to 0.35 J g−1
can only transmit heat into the earth by conduction and not by convection. Therefore, bodies
of water stay at a more even temperature, while land temperatures are more variable.
Fig 2.1.a: wind pattern across India in January, march, may, September and November
2.2 TYPES OF MONSOONS IN INDIA:
INDIAN SUMMER MONSOON: -
Summer Monsoons or the south-west monsoon winds travel over the cool air of the oceans
blowing towards the warmer land
During warmer months sunlight heats the surfaces of both land and oceans, but land
temperatures rise more quickly. As the land's surface becomes warmer, the air above it
expands and an area of low pressure develops.
Meanwhile, the ocean remains at a lower temperature than the land, and the air above it
retains a higher pressure. This difference in pressure causes sea breezes to blow from the
ocean to the land(SEE FIG 2.2.b), Bringing moist air inland. This moist air rises to a higher
altitude over land and then it flows back toward the ocean (thus completing the cycle).
However, when the air rises, and while it is still over the land, the air cools. This decreases
the air's ability to hold water, and this causes precipitation over the land. This is why summer
monsoons cause so much rain over land.
Fig 2.2.a: Wind direction during summer monsoon
INDIAN WINTER MONSOON: -
In the colder months, the cycle is reversed. Then the land cools faster than the oceans and the
air over the land has higher pressure than air over the ocean. This causes the air over the land
to flow to the ocean. When humid air rises over the ocean, it cools, and this causes
precipitation over the oceans. (The cool air then flows towards the land to complete the
Winter Monsoons or the north-east monsoon winds travel over the cool Asian land mass
towards the warmer oceans (SEE FIG 2.2.b). Ocean air gains warmth & humidity from the
warm southern waters. As it rises drawing cooler air from the cooling land mass it moves still
gaining moisture only to drop it over Australia and Indonesia etc.
Fig 2.2.b: wind direction during winter monsoon
2.3 INDIAN ANNUAL AVERAGE RAINFALL
parts of Rajasthan and Maharashtra
Between 20 and 50: -
Parts of Gujarat, Maharashtra, Andhra Pradesh, UP
Between 60 and 100:-
Parts of Bihar, Tamil Nadu, Karnataka
Between 150 to 250:-
Parts of Assam, Tiripura, Mizoram, Himachal Pradesh
Above 250: -
Arunachal Pradesh, Sikkim, Western Ghats Fig 2.3.a: - Indian annual average rainfall
2.4 DIFFERENCE IN ANNUAL RAINFALL IN SUMMER AND WINTER
Fig 2.4.a: - summer season Fig 2.4.b: - winter season
2.5 AIR CURRENT ALONG THE INDIAN SUB CONTINENT
Wind starts retrieving back to ocean
Low pressure starts developing over land
Air current brings along monsoon clouds
Indian summer monsoon begins
Low pressure above ocean
Indian winter monsoon begins
Fig 2.5.a: - variation in air current per month
2.6 INDIAN MONTHLY RAINFALL DATA
Fig: - Indian monthly rainfall data
2.7 EFFECT OF NAO, NAM AND SAM ON TROPICAL FACTORS RELATED TO
NAO: - North Atlantic Oscillation
NAM: - North Annular Mode
SAM: - South Annular Mode
ENSO: - El Nino Southern Oscillation
NAOI: - NORTHERN ANNULAR OSCILLATION INDEX
(Atmospheric pressure at sea level between Icelandic low and Azores high)
SAMI: - SOUTHERN ANNULAR OSCILLATION IDEX
(Atmospheric pressure at sea level between Icelandic low and Azores high)
2.8 WHAT ARE THESE MODULATIONS?
The Indian summer monsoon is a highly energetic global atmospheric circulation system.
Although the El Nino Southern Oscillation (ENSO) has been statistically effective in
explaining several past droughts in India, in recent decades the ENSO-monsoon relationship
has weakened over the Indian subcontinent. In this context, a tele-connection with other
dominant modes is of interest. The
present study focuses on the mutual
impact of the North Atlantic Oscillation
(NAO) and Southern Annular Mode
(SAM) on the regional variability of the
Indian summer monsoon.
There are two annular modes in Earth's
atmosphere: a Northern annular mode
(NAM) and a Southern annular mode
(SAM). Both annular modes explain
more of the week-to-week, month-to-
month, and year-to-year variance in the
extra tropical atmospheric flow than any
other climate phenomenon
Fig 2.8.a.: -
circulation of air current over continents
2.9 NAM (Northern Annular Mode): -
The Arctic oscillation (AO) or Northern Annular
Mode/Northern HemisphereAnnular Mode (NAM) is
an index (which varies over time with no particular periodicity) of the dominant pattern of
non-seasonal sea-level pressurevariations north of 20N latitude, and it is characterized by
pressure anomalies of one sign in the Arctic with the opposite anomalies centered about 37–
The degree to which Arctic air penetrates into middle latitudes is related to the AO index,
which is defined by surface atmospheric pressure patterns. When the AO index is positive,
surface pressure is low in the polar region. This helps the middle latitude jet stream to blow
strongly and consistently from west to east, thus keeping cold Arctic air locked in the polar
region. When the AO index is negative, there tends to be high pressure in the polar region,
weaker zonal winds, and greater movement of frigid polar air into middle latitudes.
Fig 2.9.a: - Northern Annular Mode
2.10 SAM (Southern Annular Mode): -
The Antarctic oscillation is a low-frequency mode
of atmospheric variability of the southern
hemisphere. It is also known as the Southern
Annular Mode (SAM) or Southern
Hemisphere Annular Mode (SHAM). It is defined
as a belt of westerly winds or low pressure
surrounding Antarctica which moves north or
south as its mode of variability. In its positive
phase, the westerly wind belt contracts
towards Antarctica, while its negative phase
involves this belt moving towards the Equator.
Fig 2.10.a: - Southern Annular Mode
Extra tropical influence on atmospheric parameters are observed through
the anomalous behaviour of North Atlantic Oscillation (NAO), North Annular Mode
(NAM) and Southern Annular Mode (SAM).In this, NAM and SAM are dominant
mode of variability in northern and southern hemisphere respectively.
In addition to that, these modes have known to influence the precipitation pattern over the tro
pical Indian subcontinent. Some of the studies suggested that NAO is related to Indian monso
It is noted that April NAO index has negative correlation with Indian summer monsoon of th
e concurrent year.
Summer precipitation over India has shown teleconnection with various features all over the
globe, and this rainfall has high influence in the socio economic growth of India. El Nino, thi
s Pacific phenomenon is one of the important parameter that is noted to influence the summer
rainfall over India.
But the effect of El Nino to Indian summer monsoon has weakened during recent decades.
Other parameters that have altered its relation to Indian summer monsoon, one of which is In
dian Ocean Dipole (IOD) and the other is Tropical Biennial oscillation. The climate shift dur
ing 1970’s has also contributed to increase in the break spells of Indian monsoon
thereby adversely affected the circulation and moisture properties .All this distorted
relationship of monsoon
with various parameters has lead to investigate new relationship of monsoon .It hasbeen alrea
dy notedthat Indian summer monsoon has relation with extra tropical variability of northern h
emisphere . North Atlantic Oscillation has shown significant inverse relation with
Madden Julian Oscillation (MJO)
during break spells of summer rainfall over India and the researchers added
that it could be used as a predictor of active/break spells.
4.DATA AND METHODOLOGY: -
The primary data used in the study is the updated Southern Annular Mode Index (SAMI) and
North Atlantic Oscillation Index (NAOI) for a period from 1951 –
2008 (Nan and Li, 2003). The SAMI developed by Nan and Li (2003) has shown better nega
tive correlation in the zonal‐mean sea level pressure anomalies between 40 S and 70 S
For the rainfall analysis, gridded rainfall data (10 x 10) obtained from Indian Meteorological
Department (Rajeevan et al., 2006), and updated for a period of 58 years extending from 195
1 – 2008 is also considered.
The SAM and NAO indices are classified (SAMI and NAOI) into four categories. The value
s of indices above/below +/‐ 0.3 are included for the study, as a result this modes can better re
present their characteristics. The average precipitation during July‐August is observed for this
relationship. The difference between positive and negative SAMI during the NAOI negative
phase is observed primarily.
Similarly the different between positive and negative SAMI during the NAOI positive phase i
s also considered
SAM (+) NAO (+) SAM (-) NAO (+) SAM (+) NAO (-) SAM (-) NAO (-)
1958 1952 1956 1951
1970 1954 1966 1953
1982 1957 1971 1961
1989 1959 1973 1963
1990 1962 1978 1975
1993 1964 1979 1988
2001 1972 1983 1995
2002 1977 1984
2004 1985 2000
2006 1991 2007
Fig 4.a: - Table showing positive and negative phases of NAO & SAM in different years.
During the positive phase of
both SAMI and NAOI, rainfall seems to decrease in the southwest coastal stations and also in
the north and eastern part of central India. Negative
departure of rainfall anomaly is intense over the extreme southwest region.
But in the north east region rainfall anomaly increases during the positive
phase of indices. The variation due to high phase of both indices are also observed in SST
and moisture transport.
During the positive SAMI and negative NAOI the intensity as well as spatial spread of
the negative anomaly in the
southwest region is more compared to the high phases of both SAMI and
NAOI. Negative anomaly is also intense over all regions of India during
positive SAMI and negative NAOI. In northeast, the difference between them
is well evident, where positive anomaly is observed along 850 E in positive
phase of SAMI and NAOI.
Using other criteria i.e., the difference of high and low phase of Southern
Annular Mode Index by keeping North Atlantic Oscillation Index as negative
is used to understand the monsoon variability. It is noticed that negative
anomaly of rainfall occurs in the southwest, north and eastern part of central
India during positive phase of SAMI and negative phase of NAO
But for the positive SAMI and negative phase of
NAOI shows negative anomaly along 850 E rainfall while positive anomaly is
observed in the extreme northeast. The observed rainfall anomaly over the
southwest and northeast due to the coupling effect shows a condition similar
to break spells. During break spells, decreased rainfall over western regions
of India and increased rainfall over eastern India is observed
Monsoon over India shows strong indication of linkage to June Southern
Annular Mode Index. The relationship between SAMI and Indian monsoon is
different when the counterpart oscillation North Atlantic Oscillation Index of
April has considered. The impact of Southern modulation on Indian monsoon is analysed
separately during positive and negative phase of NAOI. It is noticed that the positive
phase of SAMI and negative NAOI results high
variation in monsoon parameters than during the positive phase of SAMI and
NAOI. The precipitation shows strong negative anomaly in southwest, north and eastern
part of central India, while northeast region shows positive anomaly.
The negative anomaly over
southwest region is intense andspatially spread during SAMI is positive and NAOI is negativ
e. During this phase decrease in moisture transport is also intense in the southern India and
nearby regions, the SST also shows weak gradient to develop the monsoon system.
Moreover SST shows slight variation during the two
thresholds in Arabian Sea and Bay of Bengal region and also along the areas
of IOD. The study indicates a probable predictability factor of JulyAugust rainfall using the si
multaneous effect of June SAMI and April NAOI. Further the results obtained here will help t
o investigate the depth of the relationship of monsoon with extra tropics.
The following sites, blogs and research papers have been referred in order to complete this
Alexander BP and Elena N V 1992 The NAO & ENSO Teleconnection; TOGA
Chen W Y 1982 /fluctuations in northern hemisphere 700 Mb height field associated with the
Dugam S SKakade S b 1999 Interactive connection between ENSO & NAO and its relation
with indian summer monsoon
Kriplani R H and Kulkarni A 1997 Rainfall variability over South-East Asia- Connection
with indian monsoon
Verma R K 1970 Recent monsoon variability in the global climate perspective ; Mausam