MODELLING OF COASTAL OCEAN
PROCESSES ALONG THE INDIAN COASTS
DEBASIS KUMAR MAHAPATRA Centre for Atmospheric Sciences
Submitted in fulfilment of the requirements of the degree of
DOCTOR OF PHILOSOPHY
INDIAN INSTITUTE OF TECHNOLOGY DELHI HAUZ KHAS, NEW DELHI-110016, INDIA
L I il RY Acc.
cDedicatect to !My Parents and'(Parent-in-Caws.. .
This is to certify that the thesis entitled "Modelling of Coastal Ocean Processes along the Indian Coasts" being submitted by Mr. Debasis Kumar Mahapatra to the Indian Institute of Technology Delhi for the award of the degree of DOCTOR OF PHILOSOPHY is a record of the original bonafide research carried out by him.
Mr. Mahapatra has worked under my guidance and supervision and has fulfilled the requirements for the submission of this thesis. The results contained in this thesis have not been submitted in part or full to any other University or Institute for the award of any degree or diploma.
(Dr. A. D. RAO) Professor Centre for Atmospheric Sciences Indian Institute of Technology Delhi New Delhi-110016, INDIA
I wound hike to express my deepest sense of gratitude to Prof. )f. (D. Woo for his guidance, supervision and painstaking efforts in bringing out this thesis. During the course of this study his keen interest, enormous encouragement, and motivation was a great support without which it would have been impossible to complete this thesis.
It is a matter of great pride to express my profound reverence and warmest regards to Prof S. K, Vu6e for his endearing concern, kind inspiration and motivation throughout the course of this study.
I am thankful to (Prof. S. R (Dash and RProf. V. C. Mohanty for their time to time help and co-operation during the course of this research.
I warmly thank (Dr. A. 7C 9rfitra and 'Dr. E. N. cRjzjagopa! WC9ld RJ4!!F, for their co-operation, concerti and moral support.
I express my heartfelt appreciation for my wife Amita for finding me the fight, whenever it was far away. 'Being the driving force, she always kept my spirits high..Ifer patient love, support and understanding enabled me to complete this work;
Special thanks are due to (Dr. Anumeha (Du6e for offering help in going through this manuscript. I also thank my friends and colleagues in CAS lab, IIlD for mating my stay memorable and in particular time-to-time help offered by Els. 9i4ourani Sinha.
Lastly, I express my sincere thanks to the anonymous reviewers for going through the thesis critically and highly appreciating the work
(De6asis Kumar 9vlahapatra) J\rew Dethi
The present study is broadly divided into three categories: (i) numerical modelling of coastal processes through a regional model, (ii) modelling of basin-scale processes, and (iii) improvement of modelling studies involving both a regional and basin-scale models through nesting processes using the Princeton Ocean Model (POM). The study of coastal processes through a regional model focuses on three important features of the Bay of Bengal (BoB) such as effect of freshwater on the circulation, internal wave signatures and upper ocean response due to the passage of a cyclone. The basin-scale modelling studies involve circulation of the North Indian Ocean (NIO), mini cold pool (MCP) off the southern Indian Peninsula, and the Indian Ocean Dipole (IOD) signature with reference to the Indian Summer Monsoon Rainfall (ISMR).
Finally, the improvement due to nesting of a regional model with a basin-scale model has been established.
The distribution of freshwater from two perennial rivers, present all year round, Krishna and Godavari in the BoB on the east coast of India during the southwest (SW) monsoon has been studied. It is found that assisted by the curvature of the coastline, south of Machilipatnam, the surface circulation redistributes the freshwater from the two rivers as well as the low-saline influx from the huge river system from the north; resulting in anomalous pool(s) of low-salinity waters away from the coast which is endorsed by the available Conductivity Temperature Depth (CTD) data and Ocean Surface Current Analysis Real-Time (OSCAR) currents.
Synthetic Aperture Radar (SAR) images in the northwest (NW) BoB indicate the existence of internal waves. The numerical investigation indicated a predominant activity of internal waves in the north, and the rationale is three- fold. The first one can be attributed to the stable stratification due to freshwater discharge from the head bay major river system, secondly, the significant range of the tides and finally, the bathymetry in the coastal waters off Paradip is about 12% shallower compared to that of Visakhapatnam and further south.
The upper ocean response due to the passage of the 1999 Orissa super cyclone in the BoB is also studied. It is well known that the sea surface temperature (SST) cooling is greater towards the right of the storm track, because of the dominant wind stress forcing there. However, it is found to be true only in the open ocean. The coastal dynamics transformed the scenario as the cyclone translated over the coastal waters owing to the impedance of the coastline to the circulation causing intense upwelling and shifted the region of maximum surface cooling to the left of the cyclone track. The satellite imageries during the period also endorsed the model simulations.
The regional models lack the effect of basin-scale processes that affect the coastal processes. Hence, it is apt to study this using a basin-scale model which is set-up and subjected to years of integration to reach quasi steady- state and validated thoroughly. This model is applied to study the formation of the MCP at the southern tip of India and passage of BoB waters to Arabian Sea and vice versa which in turn affects the active or break phase of the SW and the northeast (NE) monsoon and also the IOD signatures with respect to ISMR.
The results showed that the occurrence of MCP during the summer season is
mainly due to the upwelling caused by the divergence of the near-surface circulation off southern tip of India and advection of the cold upwelled water from the western Arabian Sea (southwest coast of India). In contrast, during the winter monsoon, the model studies suggest that circulations driven by positive Ekman dynamics and outgoing heat flux are mainly responsible for the formation of MCP off southern tip of India during December-February.
The IOD signature with respect to ISMR has also been investigated to emphasize the model capability to capture large-scale signals. The IOD plays an important role as a modulator of the Indian monsoon rainfall and influences the correlation between the ISMR and El-Nino Southern Oscillations (ENSO).
The positive and negative mode of IOD with respect to ISMR through SSTA is demonstrated by considering two years namely, the weak monsoon cases of 2002 and 2004 and the normal monsoon cases of 2006 and 2007. The sea surface salinity anomaly (SSSA) and the vertical profile of the IOD signature is also investigated.
Whether it is a basin-scale model or a regional model, no part of the BoB or the Arabian Sea can be studied in isolation as they lack the effect of basin- scale processes on the coastal processes. In order to include both, it is more appropriate to nest a regional model with a basin-scale model or use unstructured grids. Therefore, a regional model for the west coast of India is nested with the Indian Ocean Model through elevation and surface velocities.
The overall result suggests a remarkable improvement in the model simulation of coastal processes.
Table of Contents
List of Figures vii-x
List of Tables xi
List of Acronyms xii-xiii
Chapter 1 GENERAL INTRODUCTION 1-30
1.1.1 Indian Ocean, Arabian Sea and Bay of Bengal 1
1.2 Regional Scale Processes 4
1.2.1 Complexity induced by the presence of 4 freshwater in BoB
1.2.2 Internal waves in BoB 7
1.2.3 Extreme weather events-cyclones 10
1.3 Basin-Scale Processes 12
1.3.1 Basin-scale model integration 12 1.3.2 Mini cold pool (MCP) off Indian peninsula 13
1.3.3 Indian ocean dipole 17
1.4 Nesting Procedure 21
1.5 Thesis Objectives and Layout 24
Chapter 2 MODEL FORMULATION 31-60
2.1 Introduction 31
2.2 Model Set-up 37
2.2.1 The governing equations 37
2.2.2 Turbulence closure scheme 40
2.2.3 Boundary conditions 42
2.2.4 The Smagorinsky diffusivity scheme 45 2.2.5 Vertical coordinate representation 45 2.2.6 The vertically integrated equations 48
2.3 The Numerical Scheme 50
2.3.1 External-internal mode interaction 51 2.3.2 Structure of the internal mode calculation 54
2.3.3 Stability constraints 56
2.3.4 Grid arrangement 56 2.4 Sensitivity Studies of the Model with Different 57
Open Boundary Conditions
2.5 Model Configuration for NIO, East Coast of India 60 (BoB) and West Coast of India (Arabian Sea)
Chapter 3 STUDY OF COASTAL PROCESSES WITH A 61-105 REGIONAL MODEL
3.1 Introduction 61
3.2 Complexity Induced due to the Presence of River 61 Discharge to BoB
3.2.1 Introduction 61
3.2.2 Model and experimental set-up 62
3.2.3 Results and discussion 70
3.2.4 Conclusions 77
3.3 Modelling Study of Low-frequency Internal Waves in BoB 78
3.3.1 Introduction 78
3.3.2 Data analysis 79
3.3.3 Model and experimental set-up 87
3.3.4 Results and discussion 90
3.3.5 Conclusions 98
3.4 Modelling of Upper Ocean Response to the Passage 100 of a Cyclone in BoB
3.4.1 Introduction 100
3.4.2 Model and experimental set-up 100
3.4.3 Results and discussion 102
3.4.4 Conclusions 104
Chapter 4 MODELLING OF BASIN-SCALE PROCESSES 106-173
4.1 Introduction 106
4.2 Modelling of Basin-Scale Processes for NIO 110
4.2.1 Model grid for NIO 111
4.2.2 Bathymetry and initial conditions 112
4.2.3 Methodology and results 117
4.2.4 Conclusions 133
4.3 Study of MCP off Southern Indian Peninsula 133
4.3.1 Results and discussion 138
4.3.2. Conclusions 159
4.4 Study of IOD Signature with Reference to ISMR 161 4.4.1 Model and experimental set-up 161
4.4.2 Results and discussion 162
4.4.3 Conclusions 172
Chapter 5 STUDY OF COASTAL PROCESSES OFF THE 174-208 WEST COAST OF INDIA WITH A NESTED MODEL
5.1 Introduction 174
5.2 Nested Region of the Model 176
5.2.1 Analysis area and bathymetry 176
5.2.2 Data and observations 178
5.3 Nesting Procedure 186
5.4 Results and Discussions 189
5.4.1 Circulation off west coast of India during the 191 NE and SW monsoon
5.5 Conclusions 206
Chapter 6 CONCLUSIONS AND FUTURE SCOPE OF WORK 209-214
6.1 Conclusions 209
6.2 Future Scope of Work 212
RESEARCH PUBLICATIONS 235