CERTAIN ASPECTS OF THE NUMERICAL SIMULATION OF CIRCULATION IN THE BAY OF BENGAL AND THE ARABIAN SEA

Transcription

1 CERTAIN ASPECTS OF THE NUMERICAL SIMULATION OF CIRCULATION IN THE BAY OF BENGAL AND THE ARABIAN SEA by INDU JAIN A THESIS SUBMITTED IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Centre for Atmospheric Sciences INDIAN INSTITUTE OF TECHNOLOGY, DELHI N EW DELHI , INDIA JUNE 1994

2 DEDICATED TO MY PARENTS

3 C E R T I F I C A T E This is to certify that the thesis entitled 'CERTAIN ASPECTS OF THE NUMERICAL SIMULATION OF CIRCULATION IN THE BAY OF BENGAL AND THE ARABIAN SEA" being submitted by INDU JAIN for the sward of the degree of DOCTOR OF PHILOSOPHY, is a record of the original bonafied research work carried out by her. She has worked under our joint guidance and supervision and has fulfilled the requirements for the submission of this thesis. The results presented in this thesis have not been submitted in part or full to any other University or Institute for the award of any degree/diploma. (S.K. DUBE) Principal Scientific Officer, Centre for atmospheric sciences Indian Institute of Techonology New Delhi (U.C. MOHANTY) Joint Director, National Centre for Medium Range Weather forecasting Lodi Road New Delhi I. I. T. DELHI LIBRARY _

4 ACKNOWLEDGEMENT I sincerely express my deepest gratitude to my supervisors, Dr. S.K. Dube and Dr. U.C. Mohanty, for their painstaking guidance, constant encouragement and generous cooperation during the period of the research work. I am extremely grateful to Dr. P.C. Sinha, Head, Centre for Atmospheric Science', for his immense help throughout the course of this work. I wish to express my gratitude to Prof. M.K. Jain, Emeritus professor, Department of Mathematics, Indian Institute of Technology New Delhi, for his invaluable suggestions during this study. I highly appreciate the help rendered by Dr. A.D. Rao and Dr. K.J. Ramesh at various stages of this work. I am also thankful to all my friends, and colleagues for their help and cooperation. In particular, I appreciate the help from P. Chittibabu, B. Bhaskaran and Shailendre during the initial stage of my work. This work would not have been possible without the moral support and encouragement received from my mother and father during the course work. I acknowledge the immense contribution of my brother, sisters, sister-in-law and loving nephew whose love and affection gave me continuous encouragement. And finally, I thank my husband JR.K. Kansal for his patience and understanding while I was busy with this thesis. June, 1994 INDU JAIN

5 ABSTRACT The climatic changes in the atmosphere have their counter parts in the ocean and, consequently, the changes in the conditions in the sea, reflect climatic conditions. The surface wind stress at the ocean-atmosphere interface is the main driving force for the tropical ocean dynamics. North Indian Ocean is of special significance to oceanographers because of its unique features of currents that switches direction on an annual cycle. The general circulation pattern of the North Indian Ocean largely depends on the seasonal reversal of monsoon winds over the Indian subcontinent. The main objective of the thesis is to develop numerical models for studying the wind-induced circulation in the Bay of Bengal and the Arabian Sea. It consists of seven chapters. In Chapter 1, a brief review of the literature relevant to the studies made in the thesis has been presented. It also provides a description of different modelling/observational studies on the wind-induced circulation for the Bay of Bengal and the Arabian Sea. In Chapter 2, an oceanic climatological circulation model for the Bay of Bengal has been developed, which explains seasonal variability of currents. This chapter describes the theoretical formulation of a vertically integrated non-linear model, with realistic basin geometry which has been used for the simulation of the wind-induced circulation in the Bay of Bengal. The model is forced by the monthly mean wind stress derived from the

6 30-years ( ) of Comprehensive Oceanographic and Atmospheric Data sets (COADS)- The computed wind-induced circulation of the Bay of Bengal is found to be in good agreement with the climatological pattern. The effect of riverine runoff from one of the greatest river systems of the world, that is, Ganga-Brahmaputra-Meghana in the head Bay has been discussed in Chapter 3. Oceanic climatological circulation model for the Bay of Bengal described in this chapter, includes the riverine discharge in the head Bay from the northern river system. The model described in Chapter 2 was suitably modified by applying an open boundary condition at the location, where Ganga-Brahmaputra-Meghana rivers join the Bay of Bengal. * Sensitivity experiments have been performed with varying water inputs and a comparative study of the results with and without the inclusion of river discharge, have been made. The simulation provide definitive evidence that, the river discharge during monsoon season into the Bay of Bengal suppresses the upwelling along the northern parts of the east coast of India. In Chapter 4, a wind-driven barotropic circulation model for the eastern sector of the Arabian Sea is described. Using vertically integrated numerical model, sensitivity experiments have been performed with six open boundary conditions and a comparative study of the results have been made. The results

7 suggest that Modified Orlanski (MOE) radiation boundary condition is probably the best choice for limited area barotropic ocean circulation models. Computed results show that the vertically integrated model can well be used for studying the response of the eastern Arabian Sea to monsoon wind stress forcing. It is well known that during the summer monsoon strong winds prevail over the Bay of Bengal and their effects are observed on the surface as well as in the sub surface layers of the ocean. To study the vertical structure of the wind-driven circulation in the Bay of Bengal, a multi-level homogeneous model has been developed for the simulation of wind-driven circulation in the Bay of Bengal and is described in Chapter 5. Conservation equations are applied for mass, momentum and turbulence energy. The model uses an energy based scheme to parameterize the Reynolds stress together with a transformed set of coordinates to facilitate the numerical solution of the free surface problem. The model domain was forced by the mean monthly wind stress forcing derived from the 30 years ( ) data sets (COADS). The model was able to reproduce many of the observed features of wind-driven circulation in the Bay of Bengal. The computed results show that the circulation pattern is essentially three-dimensional in nature. The model can be used to identify the locations of upweiling and downwelling in the Bay of Bengal. iii

8 In Chapter 6, a Reduced Gravity model for the Bay of Bengal has been discussed. A wind-induced, barotropic circulation models have been described in earlier chapters. Although the models are able to produce the surface general circulation features of the North Indian Ocean, but for more detailed and realistic simulation of the currents the inclusion of baroclinicity is desirable. A Reduced Gravity model includes the baroclinicity in terms of density difference of the two layers. The basic dynamical formulation of the model is the same as that used by Luther and O'Brien and Luther et al The results presented from the two representative months namely July (SW monsoon) and February (NE monsoon) show that the inclusion of baroclinicity is necessary for the computation of wind-induced circulation of the Bay of Bengal. Chapter 7 is the last chapter of the thesis. Chapter details the conclusions drawn from the studies included in the thesis. In order to have a better understanding of the dynamics of the circulation in the Bay of Bengal and the Arabian Sea, suggestions are also given for future work. iv

9 C O N T E N T S PAGE NO. ABSTRACT LIST OF FIGURES (i) (v) CHAPTER 1 : INTRODUCTION 1.1 Background Main Features of the Seasonally Reversing Winds and Currents in the North Indian Ocean Upwelling and Downwelling in the North Indian Ocean Review of the Indian Ocean Modelling Problem of Open Boundary Conditions (OBC) Objective of the Thesis 27 CHAPTER 2 : MEAN MONTHLY WIND-DRIVEN CLIMATOLOGICAL CIRCULATION MODEL OF THE BAY OF BENGAL 2.1 Introduction Basic Equations Boundary and Initial- Conditions Coordinate Transformation Numerical Procedure Implementation of OBCs at the Southern Boundary Stability of the Scheme 64

10 2.8 Numerical Experiments Results and Discussions Conclusions 81 CHAPTER 3 : IMPLICATIONS OF VARIATIONS IN THE GANGA-BRAHMAPUTRA-MEGHANA RUNOFF ON THE WIND-INDUCED CIRCULATION OF THE BAY OF BENGAL 3.1 Introduction Model Equations Boundary and Initial Conditions Numerical Experiments Results and Discussions Conclusions 102 CHAPTER 4 : WIND-DRIVEN BAR0TR0PIC CIRCULATION MODEL OF THE EASTERN SECTOR OF THE ARABIAN SEA 4.1 Introduction, Formulation Boundary and Initial Conditions Finite-Difference Formulation Numerical Experimentation Results and Discussions Conclusions 127 CHAPTER 5 : A THREE DIMENSIONAL MODEL OF THE BAY OF BENGAL 5.1 Introduction Formulation 131

11 Coordinate Transformation Numerical Solution Numerical Experiments Results and Discussions Conclusions : A REDUCED GRAVITY MODEL OF THE BAY OF BENGAL Introduction Basic Equations Formulation Reduced Gravity Equations Numerical Procedure Open Boundary Conditions (OBCs) The Wind Forcing Results and Discussions Conclusions : CONCLUSIONS AND FUTURE WORK PLAN Bay of Bengal Arabian Sea Future work