A STUDY ON CLIMATOLOGICAL CHARACTERISTICS OF THE ASIAN SUMMER MONSOON AND ITS VARIABILITY

(1)

A STUDY ON CLIMATOLOGICAL CHARACTERISTICS OF THE ASIAN SUMMER MONSOON AND ITS VARIABILITY

by

P.V.S. RAJU

Thesis submitted

in fulfillment of the requirements for the award of the degree of

DOCTOR OF PHILOSOPHY

CENTRE FOR ATMOSPHERIC SCIENCES

INDIAN IN S TITU TE OF TEC H N O LO G Y, DELHI

HAUZ KHAS, NEW DELHI, INDIA

JANUARY 2004

(2)

I. 1. T. D ELH I.

A « Y

w u,T±t.l-2

n't-\

5 5 1

^■ ^{5 ' ( i ‘}

R f i j -

(3)

Dedicated to My Spiritual Guru H.H. Sri Poornananda Swamij

(4)

C E R T IF IC A T E

This is to certify th at the thesis entitled ” A S tu d y o n C lim a to lo g ic a l C h a r a c te ristic s of th e A sian S u m m e r M o n so o n a n d its V a ria b ility ” being subm itted by Mr. P.V.S. Raju for the award of the degree of D O C T O R O F P H IL O S O P H Y , as a record of the original bonafide research work carried out by him. He has worked under our joint guidance and supervision and to the best of our knowledge has fulfilled the requirements for the submission of this thesis. The results presented in this thesis have not been subm itted in part or full to any other University or Institute for award of any degree/diploma.

Dr. U.C. Mohanty Professor

Centre for Atmospheric Sciences Indian Institute of Technology New Delhi

Scientist C-MMACS Bangalore

Dr. K.J. Ramesh Head

Disaster Management Unit

Planning Department, A.P. Secretariat Hyderabad

(5)

ACKN0WLED6EMFNT.S

I am indebted to many people who have contributed ideas and thoughts during my research.

Without their foresight and willingness to help, this would not have been possible. I will endeavour to thank everyone involved: apologies fo r any glaring omissions. A t the outset, I express my sincere gratitude to Prof. U.C. Mohanty, my thesis supervisor, fo r many insightful conversations during the development of the ideas in this thesis, constant encouragement and support throughout the course period.

I also express my deepest sense of gratitude to Dr. P. Soswami and Dr. K.J. Ramesh, my thesis supervisors, fo r their invaluable guidance and encouragement. Dr. Goswami always inspired me with innovative ideas and kindness fo r providing available computing facilities at C-M M AC S. Dr.

Ramesh took considerable interest in correcting the thesis and offering valuable suggestion to improve the presentation of the work.

I am grateful to Prof. G. Jayaraman, Head, CAS fo r making available necessary facilities in the centre. Many thanks to the faculty at CA S, especially Prof. O.P. Sharma and Prof. S.K. Dash fo r the useful suggestions.

I am grateful to Dr. G. Prathap, Scientist-in-Charge, C-M M ACS, fo r providing the necessary facilities at C-M M ACS. Also I would like to thank Tangavelu, Achintya, Gopal and Raji fo r their help during my stay at C-M M ACS.

I am deeply indebted to G p. Capt. O.P. Madan fo r the valuable discussions and suggestions at various stages of the work. I also extend my sincere thanks to Rajeev Bhatla fo r his cooperation and timely help in academic and non-academic activities.

My special thanks to PLS Rao, Sarkar, Satya, Rama and Potty, fo r their kind help in initial stages of my research at I I T Delhi. I wish to thank all my colleagues past and present at C A S , I I T Delhi, particularly, Azadi, Das, Dimri, Mandal, Manish, Narender, Nelson, Prasad and Sangeet for sharing the companionship, knowledge, and pleasant and invigorating atmosphere to work in.

I gratefully acknowledge the financial support of Council of Scientific and Industrial Research and Department of Science and Technology, India.

I sincerely acknowledge the NCEP/NCAR, U S A fo r providing the reanalysis data sets. I thank Dr. Chi-Fan Shih, NCAR, fo r his kind help in decoding 6R IB data. Also, I acknowledge the India Meteorological Department fo r providing the necessary data.

My parents are biggest strength in my life; their inspiration and unconditional support turned my dream into reality. Many thanks to my brother, sister and brother-in-law fo r their unwavering support through the difficult times.

(P.V.S. R A JU )

(6)

A b stract

A continuing goal in the diagnostic studies of the atmospheric general circulation is to estimate the various quantities that cannot be directly observed. Evolution of all the dynamical terms in the budget equations for kinetic energy, vorticity, heat and mois

ture provides estimates of diabatic heating, source/sinks of energy, vorticity generation, diabatic heating and source/ sinks of moisture. All these are im portant forcings to the cli

mate system. The diagnostic aspects of the circulation features, dynamics and energetics of Asian summer monsoon and its variability are studied with 52 years of National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis d ata sets. The work embodied in this thesis is divided into three segments. In the foremost part, 52 years (1948-1999) climatology of summer monsoon circulations and large scale budget of kinetic energy, vorticity, heat and moisture are studied. Also, the mean circulation features and energetics associated with the composite of surplus and deficient monsoon seasons over India at 95% significant level are investigated. The strong low level cross equatorial flow (Somali jet), Tropical Easterly Jet and Tibetan anticyclone, monsoon heat low and monsoon trough are characteristic features of the monsoon season.

The strong rising motion over the tropics and sinking motion over the extra tropics dur

ing summer monsoon season are observed. A significant contribution is also made by the meridional component of the wind to the adiabatic generation of kinetic energy during summer monsoon season. The monsoon region is characterized as source region of kinetic energy and vorticity. The summer monsoon evinces strong convergence of heat and mois

ture over the monsoon domain. The interesting aspect noticed in this study is th at the large scale budgets of kinetic energy, heat and moisture indicate excess magnitudes over the Arabian Sea and western Indian Ocean during surplus monsoon. On the otherhand the east Bay of Bengal and eastern Indian Ocean indicate show stronger activity dur

ing deficient monsoon. These regions are statistically significant at 95% confidence level.

The anomalous warming over western Indian Ocean is in principle responsible for intense

(7)

Arabian Sea regime, in turn, leads to surplus monsoon over India. In tandem with the warmer eastern Indian Ocean, during drought and normal monsoon season, the Bay of Bengal branch is pretty intense. These aspects are reflected in various budget terms.

In the second part, the association between sub-seasonal oscillation (SSO) activity and interannual variabilities of Indian summer monsoon have been studied using NCEP/NCAR reanalysis data. Spatial structures of the SSO activity show clear contrast between excess and deficit monsoon years. The intensity of Indian summer monsoon is defined in terms of all-India summer monsoon rainfall while the SSO activity is defined in terms of power spectra of wind fields calculated with data from 40 year (1958-1997) NCEP reanalysis.

The results show strong association between Indian summer monsoon and SSO activity, especially over 850 hPa. Significantly, a signal for Indian summer monsoon also exists in the SSO activity in the pre-monsoon (Dec-May) fields. Implications of such an association for prediction of seasonal outlook of monsoon are described.

The third part of the thesis is divided into two section. The first section deals with the mean circulation features, dynamics and thermodynamic characteristic during break and withdrawal phases of the Indian summer Monsoon. The break period that lasted for three and more days are considered in this study. The horizontal flux and adiabatic generation of kinetic energy in the Arabian Sea as well as the Bay of Bengal decrease during the break phase. There is decrease in the horizontal flux of vorticity both in the Arabian Sea and Bay of Bengal from pre-break to break and increases again in revival of monsoon activity. The axis of flux convergence of heat and moisture shifts towards north (south) during break (post break) phase. The post-retreat distribution of flux convergence of heat illustrates the large heating over central Indian Ocean and the flux divergence of heat over east African region. During post-retreat the zone of equatorial Indian Ocean noticed strong flux convergence of heat and moisture as compared to pre-retreat period.

The intense convective activity and moisture flux convergence which is due to the shift of ITCZ towards south during the post-retreat period.

(8)

In the second section, the dynamic and thermodynamic characteristics of the Asian summer monsoon during onset of the Indian summer monsoon (over Kerala Coast) and its variability are investigated with reanalysis datasets. Daily averaged (0000 and 1200 UTC) reanalysis datasets of NCEP during pre-onset, onset and post onset period (1948-1999) have been used to study the mean circulation characteristics and the large scale energetics of the Asian summer monsoon at the time of onset over India. The energetics delineate remarkable transition in their magnitude with the onset phases. The flux divergence of kinetic energy over Somali Coast in the low levels intensify with the evolution of monsoon over India. The flux convergence zone is developed over central Arabian Sea in the onset period, it strengthened and extended over Indian Peninsula during post-onset period. The upper level features delineate flux divergence over the Bay of Bengal and flux convergence over east Africa, western Indian Ocean and south Arabia during onset period and intensify further with the advance of summer monsoon over India. Prior to the onset of summer monsoon, the flux convergence of heat, moisture convergence and large convective activity are observed in the east Bay of Bengal, and further intensify in the onset and post-onset periods. Once the monsoon sets over India, the flux convergence of heat, diabatic heating and moisture convergence developed in the onset period over south Arabian Sea and further strengthen in the post-onset period. These changes in the energetics could be used as potential predictors of the onset of summer monsoon. An attem pt has been made to emphasis on the prediction of onset dates of summer monsoon over India based on energetics and basic meteorological parameters with multivariate statistical technique.

The regression technique has been developed with the data of May and June for 42 year (1948-1989) and tested with independent data sets of NCEP analysis for 1990-1999. The results indicate the predicted onset dates are in well agreement with the onset dates reported by India Meteorological Department (IMD).

(9)

C O N T E N T S

A b stract

i

List o f T ables

&

F igu res

vi

C h ap ter-1 In tro d u ctio n

1.1 Introduction 1

1.2 A Brief Review of Monsoon Studies 4

1.3 Experimental Studies 7

1.4 Interannual Variability 8

1.5 Intraseasonal Variability 10

1.6 Brief Outline of the Thesis 11

C hapter-2 M ean C lim atological C h aracteristics o f th e A sia n Sum m er M o n so o n

2.1 Introduction 14

2.2 Methodology 16

2.3 Data and Analysis System 19

2.4 Results and Discussion 21

2.4.1 Reliability of NCEP reanalysis 21

2.4.2 Mean characteristics of Asian summer monsoon 24

2.4.3 Contrasting features of Asian summer monsoon 38

2.5 Conclusion 51

C hapter-3 S u b -S eason al O scillation s and In teran n u al V ariab ilities

3.1 Introduction 53

3.2 The Hypothesis and a Conceptual Model 55

3.2.1 The hypothesis 55

3.2.2 A conceptual model 56

3.3 Data and Methodology 57

3.3.1 Index of SSO activity 57

IV

(10)

3.4.1 Yearly SSO activity 59

3.4.2 Monsoon SSO activity 62

3.4.3 Pre-monsoon SSO activity 71

3.4.4 Statistical significance 75

3.4.5 Forecast potential 77

3.5 Conclusion 79

C h ap ter-4 B rea k and W ith d raw al o f th e In d ia n S um m er M o n so o n

4.1 Introduction 81

4.3 Break Features over India 85

4.3.1 Circulation characteristics 85

4.3.2 Kinetic energy budget 93

4.3.3 Heat and moisture budget 98

4.4 Withdrawal Features 105

4.5 Conclusions 110

C h a p ter-5 O n set an d A d van cem en t o f th e In d ia n S u m m er M o n so o n

5.1 Introduction 113

5.3.1 Circulation characteristics 116

5.3.2 Kinetic energy budget 120

5.3.3 Heat and moisture budget ' 128

5.3.4 Evolution and advancement of summer monsoon 135

5.3.5 Onset prediction: Hindcast skill 145

5.4 Conclusions 150

C h ap ter-6 Sum m ary

6.1 Summary 152

5.2 Future Studies 157

R eferen ces

¹⁵⁹

V