INDIAN SUMMER MONSOON AND BEHAVIOUR OF NORTHERN AND SOUTHERN ANNULAR MODES

SUMIT VIKRAM SINGH

CONTACT: - sumitvikram2001@gmail.com

TERM –IV, 2013-14

UNDER GUIDANCE OF

Gurmanwant Sandhu

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

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Abstract

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 specified analysis 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.

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1. INTRODUCTION

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 countries

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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 K−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 K−1), and they 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.

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Fig 2.1.a: wind pattern across India in January, march, may, September and November

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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

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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 cycle.)

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

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2.3 INDIAN ANNUAL AVERAGE RAINFALL MAP Under 20:-

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

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2.5 AIR CURRENT ALONG THE INDIAN SUB CONTINENT

January:-

Wind starts retrieving back to ocean

March: -

Low pressure starts developing over land

May: -

Air current brings along monsoon clouds

July:-

Indian summer monsoon begins

September:-

Low pressure above ocean

November:-

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

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2.7 EFFECT OF NAO, NAM AND SAM ON TROPICAL FACTORS RELATED TO MONSOON

TERMINOLOGY: -

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

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2.9 NAM (Northern Annular Mode): -

The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is an index (which varies over time with no particular periodicity) of the dominant pattern of non-seasonal sea-level pressure variations north of 20N latitude, and it is characterized by pressure anomalies of one sign in the Arctic with the opposite anomalies centered about 37–45N

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

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3. EFFECT

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 tropical Indian subcontinent. Some of the studies suggested that NAO is related to Indian monsoon variability. It is noted that April NAO index has negative correlation with Indian summer monsoon of the 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, this 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 Indian Ocean Dipole (IOD) and the other is Tropical Biennial oscillation.  The climate shift during 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 already noted that Indian summer monsoon has relation with extra tropical variability of northern hemisphere . 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.

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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 negative 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 1951 – 2008 is also considered. 

 The SAM and NAO indices are classified (SAMI and NAOI) into four categories. The values of indices above/below +/‐ 0.3 are included for the study, as a result this modes can better represent 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 is also considered

SAM (+) NAO (+)  SAM () NAO (+) SAM (+) NAO () SAM () NAO ()1958 1952 1956 19511970 1954 1966 19531982 1957 1971 19611989 1959 1973 19631990 1962 1978 19751993 1964 1979 19882001 1972 1983 19952002 1977 19842004 1985 20002006 1991 2007

1992 20081994

Fig 4.a: - Table showing positive and negative phases of NAO & SAM in different years.

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5. DISCUSSION: -

 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

6. CONCLUSION

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. 

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The negative  anomaly  over  southwest region is intense andspatially spread during SAMI is positive and NAOI is negative. 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 simultaneous effect of June SAMI and April NAOI. Further the results obtained here will help to investigate the depth of the relationship  of monsoon with extra tropics. 

7. REFERENCES

The following sites, blogs and research papers have been referred in order to complete this report

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 southern oscillation.

Dugam S S Kakade 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

WEBSITES:-

www.weatherquestions.com

www.atmos.colostate.edu

www.alhea.com

http://adsabs.harvard.edu/

http://iopscience.iop.org/

www.nc-climate.ncsu.edu

http://www.imd.gov.in/

http://www.atmos.washington.edu/

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