numerical simulation of north indian ocean features …
TRANSCRIPT
NUMERICAL SIMULATION OF NORTH INDIAN OCEAN
FEATURES USING ROMS WITH AN EMPHASIS ON THE
BAY OF BENGAL
SANDEEP K K
CENTRE FOR ATMOSPHERIC SCIENCES
INDIAN INSTITUTE OF TECHNOLOGY DELHI
APRIL 2019
© Indian Institute of Technology Delhi (IITD), New Delhi, 2019
NUMERICAL SIMULATION OF NORTH INDIAN OCEAN
FEATURES USING ROMS WITH AN EMPHASIS ON THE
BAY OF BENGAL
by
SANDEEP K K
Centre for Atmospheric Sciences
Submitted
in fulfilment of the requirements of the degree of
DOCTOR OF PHILOSOPHY
to the
INDIAN INSTITUTE OF TECHNOLOGY DELHI
APRIL 2019
Dedicated to my Parents
Certificate
This is to certify that the thesis entitled "Numerical simulation of north Indian
Ocean features using ROMS with an emphasis on the Bay of Bengal" being
submitted by Mr. Sandeep K K to the Indian Institute of Technology Delhi for
the award of the degree of DOCTOR OF PHILOSOPHY is a record of original
bonafide research carried out by him. Mr. Sandeep K K has worked under our
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.
(Prof. Vimlesh Pant) (Prof. A D Rao)
Associate Professor, Professor,
Centre for Atmospheric Sciences Centre for Atmospheric Sciences
Indian Institute of Technology Delhi Indian Institute of Technology Delhi
New Delhi-110016, INDIA New Delhi-110016, INDIA
Acknowledgements
I would like to express my deepest gratitude and sincere thanks to my supervisor
Prof. Vimlesh Pant for his proficient guidance, suggestions, encouragement and
constant support throughout the tenure of PhD. He inspired me with his
innovative thinking, dedication and progressive outlook towards science. His
patience and encouragement helped me to overcome many difficulties and achieve
my best. Without his support I would not have reached this point now.
I am greatly indebted to my co-supervisor Prof. A D Rao for his invaluable
scientific insights and advices during my research work. As a great teacher and
mentor he has taught me to think better and write more carefully and clearly. I
consider myself very fortunate for being able to work with a very considerate
and encouraging professor like him.
My sincere thanks to Prof. Manju Mohan, Head, Centre for Atmospheric Sciences
and all the faculty members of the Centre for their support throughout the course
of the research.
My sincere and heartfelt thanks to Dr. M.S. Girishkumar for his support and
encouragement from the very beginning of my research career. I am grateful to Dr.
P.A. Francis for his timely scientific advices and support.
I would like to thank Indian Institute of Technology Delhi for the fellowship
support, infrastructure, and HPC facility as the computational resource.
I feel very fortunate to have been part of the Ocean Computing Laboratory, CAS
and would like to thank all my fellow colleagues and friends for their support and
kind help to carry out my research work.
I am deeply indebted to my parents, wife and all our family members for their
never ending support and esteem. Above all I am thankful to Almighty God for
his Eternal Blessings and Benevolence.
Sandeep K K
New Delhi
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Abstract
The unique geographical and meteorological characteristics of the Indian Ocean (IO) cause
and enforce variability in its upper oceanic physical processes with a vast range of temporal and
spatial scales. In recent decades, the IO has been identified to have substantial impact on the
regional and global climate variability. Thus, a better understanding of the processes in the IO
through observational and modeling studies has become an important scientific and societal
necessity. In the present thesis, the dynamic and thermodynamic variability of the northern IO,
particularly in the Bay of Bengal (BoB) at different timescales are studied using a high resolution
3D Regional Ocean Modeling System (ROMS). The ROMS model is configured over the tropical
IO extending from 30°E to 120°E and 30°S to 30°N with an eddy permitting grid resolution of
1/8° × 1/8° in horizontal and 40 terrain following sigma levels in the vertical. The daily mean
values of atmospheric variables obtained from the TropFlux and QuikSCAT/ASCAT data are used
for the computation of surface heat and momentum fluxes as surface forcing fields. The K-Profile
Parameterization (KPP) mixing scheme formulation is used for the vertical turbulent mixing.
A brief review of the important pioneer works on IO dynamics and its variability with
special reference to the BoB has been discussed in Chapter 1. Chapter 2 describes the model
formulation and its validation, methodology followed in the study and details of the various data
sets used. In Chapter 3, the impact of continental (riverine) freshwater discharge on the sea surface
salinity (SSS) simulations over the IO is examined. The daily climatological atmospheric forcing
and monthly river discharges are used to force the model. All the rivers draining into the IO from
the Indian subcontinent, Africa, Australia, and Indonesia are incorporated in the model. Two
numerical experiments are carried out with different freshwater forcing to identify the impact of
river discharges on the salinity variability. The first one, i.e., No-River Experiment (NR-Expt),
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treats the evaporation (E) and precipitation (P) as the surface freshwater flux (E-P). The riverine
freshwater discharge (R) is added to the surface freshwater flux in the second experiment, i.e.
River Experiment (R-Expt). The model simulated SSS in the R-Expt shows a better agreement
with the observed SSS from Aquarius satellite and buoys, as compared to the NR-Expt. The mixed
layer salt budget analysis highlights the dominant contributions from (E-P) and horizontal
advection terms in controlling the mixed layer salinity (MLS) over most of the IO domain.
Seasonal variability in the salinity and salt budget terms over the ten hydrologically different
regions in the northern and southern tropical IO are analyzed. The northern BoB (N-BoB) is found
to have the highest impact of river discharges on the SSS and MLS simulations during the
southwest monsoon (June-September) season. The salinity is reduced by ~4 psu with salinity
tendency up to -0.05 psu day-1 in the N-BoB, when river discharges are incorporated in the model.
The effect of river discharge on the MLS in the southeastern AS appears in the late winter and pre-
monsoon seasons. The eastern equatorial IO (EEIO) acts as a source of low-saline waters in the
IO.
In Chapter 4, the interannual simulations performed for years 2000-2014, are analyzed after
a validation against available buoy measurements and satellite data over the IO. Model simulated
daily fields are evaluated extensively by using multiple statistical metrics. The model simulated
sea surface temperature (SST) at different moored buoy locations exhibits high correlation
coefficient (R~0.9) with the ranges of standard deviation of simulated SST consistent with the
corresponding buoy observations. Intraseasonal and interannual variability of depth of 20oC
isotherm are simulated reasonably well as observed at the respective buoy locations. The Dipole
Mode Index derived from the simulated SST reproduces the positive/negative Indian Ocean Dipole
(IOD) events that occurred during the simulation period. Interannual variability in temperature,
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currents, and oceanic mixed layer depth is analyzed in response to IOD events. The anomalies in
equatorial currents are found to affect the strength of coastal currents along the Indian coastlines.
Model simulations show that the enhanced (suppressed) coastal upwelling process along the
Sumatra coast that leads to anomalous cooling (warming) off the Sumatra coast during the positive
(negative) IOD events. The composite analysis reveals that the surface meridional salt transports
along the east coast of India and EEIO region are characterized by the reverse salt transports in
response to the positive IOD and negative IOD events. During August-September months of the
positive IOD events, the meridional salt transport is northward in the southwestern boundary of
the BoB, which can be attributed to the northward current anomalies along the east coast of India.
Within the IO, the BoB shows distinct features in the sea surface salinity and surface
currents. The large amount of freshwater added to the BoB through surplus precipitation over
evaporation and river runoff makes it the lowest saline region in the IO. In Chapter 5, the role of
winds in determining the dispersal pattern of the freshwater plumes in the BoB is investigated
using a high-resolution regional ocean model with the realistic coastline, bathymetry, and river
discharge data. Multiple experiments are carried out with the idealized winds having different
directions and speeds, which represent the seasonal wind patterns in the BoB. The plume is
channelized along the coast in the cases of southeasterly and northeasterly winds but is forced
towards the central BoB under the southwesterly winds. Riverine plumes tend to confine in the
northern and northeastern BoB when forced with moderate winds. The spatial coverage of low-
saline waters is observed over most of the domain or in the eastern BoB under low-wind or
moderate wind conditions. The high-wind condition in the bay confines the freshwater mostly in
the northernmost part of the BoB. It is found that an increase in wind speed from 2 to 8 ms-1 results
in the enhanced southward meridional salt transport from 2 to 10 kg m-2 s-1 along 16oN latitude in
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the eastern BoB. Thus, the distinct roles of wind speed and direction on the characteristics of the
freshwater plume in the BoB are evident from the idealized numerical experiments.
The general conclusions from the study and future scope of the work is given in Chapter
6. After a successful validation, ROMS model was used to assess the impact of riverine freshwater
on the SSS simulations in the IO. The northeastern IO shows larger influence of river discharge
on salinity as compared to the western IO. In the southern tropical IO, the effect of river discharge
on mixed layer salinity variability found to be minimum among the different sectors of IO. The
composite analysis is carried out for the IOD events from the interannual simulation for the period
of 2000-2014. It was found from the analysis that the depth of 20oC isotherm shoals from 100-110
m in nIOD to 70-80 m in pIOD during October in the EEIO. The westward salt transport is
enhanced (suppressed) during pIOD (nIOD) events in the eastern equatorial IO. A set of model
sensitivity experiments highlight the crucial role of wind speed and direction in modulating the
freshwater plume pathways and the salt transport in the BoB.
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सारााश
ह िद म ासागर (IO) की अहितीय भौगोहिक और मौसम सिबिधी हिशषताएि इसकी ऊपरी म ासागरीय भौहतक
परहियाओि म पररिततनशीिता को वयापक सामहयक और सथाहनक पमान म उतपनन एिि हनधातररत करती । ाि क
दशकोि म, IO को कषतरीय और िहिक जििाय पररिततनशीिता पर पयातपत परभाि डािन क हिए प चाना गया । इस
परकार, अििोकन और मॉडहििग अधययन क माधयम स IO म परहियाओि की ब तर समझ एक म तवपरत िजञाहनक
और सामाहजक आिशयकता बन गई । िततमान थीहसस म उततरी IO क गहतशीि और तापगहतशीि (थमोडायनाहमक)
पररिततनशीिता, हिशष रप स बिगाि की खाडी (BoB) म, अिग-अिग समय पर एक उचच ररजॉलयशन 3-डायमनशन
‘रीजनि ओशन मॉडहििग हससटम’ (ROMS) का उपयोग करक अधययन हकया गया । ROMS मॉडि को
उषणकहिबिधीय IO पर 30 °E स 120°E और 30°S स 30°N तक 1/8° × 1/8° कषहतज और 40 भभाग का अनसरर
करन िाि ऊरधातधर सतरोि क एडडी-अनमहत हिड ररजॉलयशन क साथ कॉननिगर हकया गया । िर ॉपफलकस और
निकसकि / एएससीएिी आिकडोि (डिा) स परापत िायमिडिीय पराचिोि क दहनक औसत मलयोि का उपयोग सत ी ऊषमा
और सििग परिा (फलकस) की गरना कर सत पर फोहसिग क रप म हकया । K- परोफाइि परामीिराइजशन (KPP)
हमनकसिग सकीम सतर का परयोग ऊरधातधर िबतिि हमनकसिग क हिए हकया गया ।
IO की गहतशीिता और इसकी पररिततनशीिता पर म तवपरत अिरी कायो की सिहकषपत समीकषा, BoB क
हिशष सिदभत क साथ, अधयाय 1 म की गई । अधयाय 2 म मॉडि तयार करन और इसकी मानयता, अधययन म परयकत
कायतपररािी, और हिहभनन आिकडोि का िरतन हकया गया । अधयाय 3 म, समदरी सत की ििरता (SSS) पर कॉनिनिि
(ररिररन) ताज पानी (फरशिािर) क हनित न क परभाि की जािच की गई । मॉडि को िोसत करन क हिए दहनक
जििाय िायमिडिीय फोहसिग और माहसक नदी हनित न (ररिर हडसचाजत) आिकडोि का उपयोग हकया गया । भारतीय
उपम ािीप, अफरीका, ऑसटर हिया और इिडोनहशया स IO म ब न िािी सभी नहदयाा मॉडि म शाहमि की गई ।
ििरता पररिततनशीिता पर नदी हनित न क परभाि की प चान करन क हिए अिग-अिग फरशिािर फोहसिग क साथ
दो सिखयातमक परयोग हकए गए । प िा िािा, यानी, नो-ररिर एकसपररमि (NR-Expt), िाषपीकरर (ई) और िषात (पी)
को सत क ताज पानी क परिा (ई-पी) क रप म मानता । नदी क ताज पानी का हनित न (R) दसर परयोग अथातत
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नदी परयोग (R-Expt) म सत क ताज पानी क परिा म जोडा गया । R-Expt म SSS का मॉडि हसमिशन NR-
Expt की तिना म ऐिाररस उपि और पलाि (बॉय) स माप गए SSS क साथ एक ब तर तािमि दशातता । हमहित
परत ििर बजि हिशलषर IO डोमन क अहधकािश भाग पर हमहित परत ििरता (MLS) को हनयिहतरत करन म (ई-
पी) और कषहतज अिसारर स परमख योगदान पर परकाश डािता । उततरी और दहकषरी उषणकहिबिधीय IO म दस
ाइडर ोिॉहजकि रप स हिहभनन कषतरोि म ििरता और ििर बजि क भागोि म मौसमी पररिततनशीिता का हिशलषर
हकया गया । उततरी BoB (N-BoB) म दहकषर-पहिम मानसन (जन-हसतिबर) क मौसम क दौरान SSS और MLS
हसमिशन पर नदी क हनित न का सबस अहधक परभाि पाया जाता । जब नदी क हनित न को मॉडि म शाहमि हकया
जाता तो N-BoB म ििरता ~4 psu और ििरता की परिहतत -0.05 psu हदन -1 तक कम ो जाती । दहकषरपिी
अरब सागर (AS) म MLS पर नदी क हनित न का परभाि सहदतयोि क अित और परी-मॉनसन सीजन म हदखाई दता ।
पिी हिषित IO (EEIO) कम खार पानी क सरोत क रप म कायत करता ।
अधयाय 4 म िषत 2000-2014 क हिए हकए गए इििरएनअि हसमिशन का हिशलषर, IO क ऊपर उपिबध
पलाि (बॉय) माप और उपि डिा क साथ सतयापन क बाद हकया गया । कई सािनखयकीय महिरकस का उपयोग करक
मॉडि हसमििड दहनक पराचिोि का बड पमान पर मलयािकन हकया । मॉडि हसमििड समदर की सत का तापमान
(SST) हिहभनन जग ोि पर सथाहपत पलाि िारा माप गए SST क साथ उचच स सिबिध गरािक (R ~ 0.9) परदहशतत करता
और हसमििड SST क मानक हिचिन की सीमाएि सिबिहधत पलाि परकषरोि क अनरप । 20oC समताप रखा की ग राई
की इििर ासीजनि और इििरएनअि पररिततनशीिता सिबिहधत पलाि सथानोि पर परकषरोि क अनरप यथोहचत रप स
हसमििड । हसमििड SST स परापत डाईपोि मोड इिडकस हसमिशन अिहध क दौरान हई पॉहजहिि / हनगहिि
इिहडयन ओशन डाईपोि (IOD) घिनाओि को दो राता । तापमान, धाराओि, और म ासागरीय हमहित परत की ग राई
की इििरएनअि पररिततनशीिता का हिशलषर IOD घिनाओि क सनदभत म हकया गया । भमधयरखीय धाराओि म
हिसिगहतयाा भारतीय ति रखाओि पर तिीय धाराओि की शनकत को परभाहित करती । मॉडि हसमिशन स पता चिता
हक समातरा ति क साथ बढी हई (घिी हई) तिीय अपिहििग परहिया सकारातमक (नकारातमक) IOD घिनाओि क
दौरान समातरा ति पर हििकषर ठि डी (गमी) करती । समि हिशलषर स पता चिता हक भारत क पिी ति और
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EEIO कषतर क साथ सत मररहडयनि ििर पररि न सकारातमक IOD और नकारातमक IOD घिनाओि क सनदरभ म
हिपरीत ििर पररि न को दशातता । सकारातमक IOD घिनाओि क अगसत-हसतिबर म ीनोि क दौरान, BoB क दहकषर-
पहिमी सीमा म मररहडयनि ििर पररि न उततर की ओर , हजस भारत क पिी ति पर उततरिती धाराओि की
हिसिगहतयोि क साथ समबदध हकया जा सकता ।
IO क भीतर, BoB समदरी सत की ििरता और सत धाराओि म हभनन हिशषताएि हदखाता । िाषपीकरर
स अहधक िषात और नदी हनित न क माधयम स बडी मातरा म ताजा पानी BoB म जड जाता जो इस IO म सबस कम
खारा कषतर बनाता । अधयाय 5 म, BoB म ताज पानी क गबार (पलम) क फिाि क सवरप को हनधातररत करन म िाओि
की भहमका की जााच यथाथतिादी समदर ति, समदरति सत (बथीमीिरी) और नदी क हनित न आिकडोि क साथ एक
उचच-ररजॉलयशन कषतरीय म ासागर मॉडि का उपयोग करक की गई । हिहभनन हदशाओि और गहत िािी आदशत िाओि
क साथ कई परयोग हकए गए जो BoB म मौसमी िा क सवरप का परहतहनहधतव करत । दहकषर-पिी और उततर-पिी
िाओि क मामिोि म गबार ति क साथ सििगन र ता , िहकन दहकषर-पहिमी िाओि क त त गबार मधय-BoB की
ओर अिसर ोता । मधयम िाओि क त त नदी-हनित न गबार उततरी और पिोततर BoB म सीहमत र त । कम- िा
या मधयम िा की नसथहत म कम-खार पानी का सथाहनक फिाि अहधकािश डोमन या पिी BoB म दखा जाता । खाडी
म उचच िा की नसथहत जयादातर BoB क उततरी भाग म ताज पानी को सीहमत रखती । य पाया गया हक िा की
गहत म 2 स 8 ms-1 िनदध ोन स पिी BoB म 16oN अकषािश पर दहकषरिती ििर पररि न 2 स 10 kg m-2 s-1 बढ जाता
। इस परकार, BoB म ताज पानी क गबार की हिशषताओि पर िा की गहत और हदशा की अिग-अिग भहमकाएा
आदशत सिखयातमक परयोगोि स सपषट ोती ।
परसतत कायत क सामानय हनषकषत और भहिषय कायत क अिसर अधयाय 6 म हदए गए । एक सफि सतयापन
क बाद, ROMS मॉडि का उपयोग IO म SSS हसमिशन पर नदी क ताज पानी क परभाि का आकिन करन क हिए
हकया गया । पहिमी IO की तिना म पिोततर IO म ििरता पर नदी क हनित न का अहधक परभाि हदखता । दहकषरी
उषणकहिबिधीय IO म, हमहित परत ििरता पररिततनशीिता पर नदी क हनित न का परभाि IO क हिहभनन कषतरोि क बीच
नयनतम पाया गया। िषत 2000-2014 की अिहध क इििरएनअि हसमिशन स IOD घिनाओि क हिए समि हिशलषर
viii
हकया गया। हिशलषर स य पाया गया हक EEIO म अकटबर क दौरान 20oC समताप रखा की ग राई nIOD म 100-
110 मीिर स घिकर pIOD म 70-80 मीिर र गई। पिी भमधयरखीय IO म पहिमोततर ििर पररि न pIOD (nIOD)
की घिनाओि क दौरान बढा (घिा) । मॉडि सििदनशीिता परयोगोि िारा ताज पानी क गबार क मागत और BoB म
ििर पररि न को सिशोहधत करन म िा की गहत और हदशा की म तवपरत भहमका को उजागर हकया गया ।
ix
Table of Contents Certificate
Acknowledgements
Abstract i-viii
Table of Contents ix-xii
List of Figures xiii-xviii
List of Tables xix
Chapter 1 INTRODUCTION 1-31
1.1 Background of the Study 2
1.2 Major Characteristics of the Indian Ocean 4
1.2.1 Surface winds 4
1.2.2 Sea surface temperature 5
1.2.3 Sea surface salinity
1.2.4 Surface fluxes
1.2.5 Upper-ocean circulation
1.2.6 Unique features of the Arabian Sea and the Bay of Bengal
7
8
14
16
1.3 Literature Review 22
1.4 Motivation of the Study 29
1.5 Outline of the Thesis 30
Chapter 2 MODEL, DATA AND METHODOLOGY 32-63
2.1 Introduction 33
2.2 Model Formulation 34
2.2.1 Governing equations 34
2.2.2 Vertical boundary conditions 36
2.2.3 Horizontal boundary conditions
2.2.4 Horizontal discretization
2.2.5 Vertical discretization
2.2.6 Viscosity and diffusivity
2.2.7 Point sources
37
38
39
40
41
x
2.3 Model Configuration 42
2.3.1 Domain and bathymetry 42
2.3.2 Model input
2.3.3 Pre- and post-processing tools
44
47
2.4 Reference Data 47
2.4.1 RAMA
2.4.2 OSCAR
2.4.3 TMI-AMSRE SST
2.4.4 North Indian Ocean Climatological Atlas (NIOA)
47
48
48
48
2.5
Model Validation
2.5.1 Annual mean and seasonal variability of SST
2.5.2 Annual mean and seasonal SSS distribution over the IO
2.5.3 Validation of model simulated daily parameters with
RAMA buoy data
2.5.4 Annual mean and seasonal variability of surface currents
49
49
52
55
57
2.6 Conclusions 62
Chapter 3 IMPACT OF RIVER DISCHARGE ON THE
THERMOHALINE CHARACTERISTICS OF THE
INDIAN OCEAN
64-90
3.1 Introduction 65
3.2 Model and Methodology
3.2.1 Model configuration
3.2.2 Incorporation of continental (river) discharge and numerical
experiments
3.2.3 Mixed layer salinity budget analysis
67
67
69
71
3.3 Results and Discussion
3.3.1 Mixed layer salinity budget
3.3.2 Effect of continental freshwater discharge
73
73
80
3.4 Conclusions 88
xi
Chapter 4 INTERANNUAL VARIABILITY OF THE INDIAN OCEAN
FEATURES
91-125
4.1 Introduction 92
4.2 Model Configuration 94
4.3 Data and Methodology 94
4.4 Results and Discussion
4.4.1 Interannual variations in the IO thermodynamic features
4.4.1.1 Sea surface temperature
4.4.1.2 Subsurface temperature
4.4.1.3 Depth of 20oC isotherm
4.4.1.4 Zonal and meridional currents
4.4.2 IOD events during the simulation period
4.4.2.1 Variability of surface currents in response to IOD
Events
4.4.2.2 Coastal oceanic processes
4.4.2.3 Surface salt transport
96
96
96
103
109
110
113
114
116
120
4.5 Conclusions 123
Chapter 5 EFFECT OF WIND ON THE FRESHWATER PLUME IN
THE BAY OF BENGAL: A SENSITIVITY STUDY
126-155
5.1 Introduction 127
5.2 Numerical Modelling and Methodology
5.2.1 Model configuration and forcing
5.2.2 Incorporation of river discharge
5.2.3 Numerical experiments and methodology
5.2.3.1 Equivalent depth of freshwater
5.2.3.2 Surface salt transport estimates
129
129
131
132
134
134
5.3 Results and Discussion
5.3.1 Model validation
5.3.2 Role of wind direction in plume dispersion
5.3.3 Role of wind speed in plume dispersion
135
135
137
140
xii
5.3.4 Dynamical mechanism in the plume dispersion under
different wind conditions
5.3.5 Vertical extent of freshwater under different wind
conditions
5.3.6 Surface salt transport variability and its response to winds
143
148
150
5.4 Conclusions 154
Chapter 6 CONCLUSIONS AND FUTURE SCOPE OF THE WORK 156-165
6.1 Conclusions 157
6.2 Future Work 164
References 166-184
List of Acronyms 185-188
List of Websites 189
Biographical Sketch 190-193
xiii
List of Figures
Figure No. Title Page
No.
Figure 1.1 The IO domain used for the present study along with the bathymetry
derived from the modified ETOPO2v2.
4
Figure 1.2 Climatology of the seasonal wind stress in Nm-2 (contour-fill) and wind
speed in ms-1 (vectors) over the IO derived from TropFlux data.
5
Figure 1.3 Seasonal SST climatology over the IO. 6
Figure 1.4 Seasonal SSS climatology over the IO. 8
Figure 1.5 Climatology of seasonal LHF over the IO derived from TropFlux data. 10
Figure 1.6 Seasonal SHF climatology over the IO derived from TropFlux data. 11
Figure 1.7 Seasonal net SWR climatology over the IO derived from TropFlux data. 12
Figure 1.8 Seasonal LWR climatology over the IO derived from TropFlux data. 13
Figure 1.9 Seasonal NHF climatology over the IO derived from TropFlux data. 13
Figure 1.10a A schematic showing the major summer (southwest) monsoon current
pattern and circulation in the IO.
15
Figure 1.10b A schematic showing the major winter (northeast) monsoon current
pattern and circulation in the IO.
16
Figure 1.11 The SSS annual mean distribution in the northern IO showing the
contrasting surface salinity characteristics of the AS and the BoB. The
locations of the last gauging station of major rivers draining into the two
basins according to Dai et al., 2009 are also shown.
17
Figure 1.12 A schematic showing the comparison between the feedback cycles in the
AS and BoB.
19
Figure 2.1 Illustration of the model state variables arranged over an Arakawa C-grid
in the horizontal direction of the ROMS grids.
39
Figure 2.2 Illustration of the model state variables arranged in the vertical direction
of the ROMS grids.
40
Figure 2.3 Illustration of the indexing of the point source in ROMS. In case (a), river
enters into ,i j cell from the left u-face. In case (b), river enters from the
right u-face into the same cell.
42
Figure 2.4 The model domain used for the present study along with the bathymetry
(m) derived from the modified ETOPO2v2 data.
43
xiv
Figure 2.5 Sections of the model sigma-coordinate vertical layers along 65°E in the
domain.
43
Figure 2.6 Comparison of annual mean SST (oC) over the IO from the TMI (top-right
panel) and the same from the ROMS model (top-left panel).The
difference (Model-TMI) in SST is shown in the middle panel. SD of
monthly mean SST from TMI (bottom-right panel) and ROMS model
(bottom-left).
50
Figure 2.7 Comparison of the seasonal SST (oC) over the IO as simulated by ROMS
model (left panel) and from the TMI data (middle panel) and the
difference between them (right panel).
51
Figure 2.8 Comparison of annually averaged SSS (psu) over the IO as observed from
the Aquarius (top-right panel) and from the ROMS model (top-left panel).
The SSS difference (Model-Aquarius) is shown in the middle panel. SD
of monthly mean SSS from both Aquarius (bottom-right panel) and
ROMS model (bottom-left).
54
Figure 2.9 Comparison of the seasonal SSS (psu) over the IO as simulated by ROMS
model (left panel) and from the Aquarius data (middle panel) and the
difference between them (right panel). The locations of the RAMA buoys
used for validation over the IO are shown with stars in the middle panel of
the top row.
55
Figure 2.10 Comparison of the daily climatological SSS (psu) and SST (oC) estimated
from the daily RAMA buoy data in the IO and the same extracted from
the ROMS model.
57
Figure 2.11 Annual mean surface currents (ms-1) over the IO from the ROMS model
(top panel), OSCAR data (middle panel), and the corresponding
difference between them (bottom panel).
60
Figure 2.12 Comparison of the seasonally averaged surface currents (ms-1) in the
northern IO from the ROMS model (left panels) and the corresponding
OSCAR currents (right panel). The thick black arrows represent the
prominent currents in the region.
61
Figure 2.13 Comparison of the surface currents (ms-1) in the STIO from the model
(upper panel) and from the OSCAR data (lower panel). The thick black
arrows represent the prominent currents in the region.
62
Figure 3.1 Model domain along with the bathymetry (meters) derived from modified
ETOPOv2. Locations of the last gauging stations of all the rivers draining
into the IO (Dai et al., 2009), which incorporated in the model are shown as
red dots. The boxes marked in the figure represent regions used for the
salt budget analysis.
69
Figure 3.2 Monthly climatology of river discharge (m3s-1) from major rivers (having
peak discharge > 500 m3s-1) included in the model. The bottom panel
shows total river discharge in different sectors of the IO as marked in
71
xv
Figure 3.1.
Figure 3.3 The annual mean mixed layer salt budget terms (psu day-1) in the top and
middle rows. The annual mean salinity (psu) and currents (ms-1) within
the mixed layer are shown in the bottom row.
76
Figure 3.4 Model simulated (R-Expt) seasonal variability of the salinity (psu) shown
in shades overlaid with the currents (ms-1) in vectors within the mixed
layer.
77
Figure 3.5 The mixed layer salt budget terms (psu day-1) from the R-Expt experiment
for the summer monsoon season (June-September) over the IO.
79
Figure 3.6 Same as in Figure 3.5 except for the winter monsoon season (December-
February).
80
Figure 3.7 Comparison of model simulated SSS (psu) in the two experiments. First
column-without river (NR-Expt) simulations, second column- with river
(R-Expt) simulations, third column- difference (R-Expt minus NR-Expt).
81
Figure 3.8 Time series of the area averaged SSS (psu) within the ten regions (marked
boxes in Figure 3.1) before (dashed lines, NR-Expt.) and after (solid lines,
R-Expt.) adding the riverine freshwater discharges into the model.
83
Figure 3.9 Balance of mixed layer salt budget terms (psu day-1) over the ten selected
regions (boxes marked in Figure 3.1) in the IO before (dashed lines, NR-
Expt.) and after (solid lines, R-Expt.) adding the riverine freshwater
discharges into the model.
85
Figure 4.1 Model domain along with bathymetry derived from ETOPO2v2. The
locations of the RAMA buoys used for validating the ROMS model are
also shown as white boxes.
94
Figure 4.2 Comparison of the SST data from RAMA buoys of IO at different
locations and ROMS for 2013.
97
Figure 4.3 Comparison of the SST (°C) data from RAMA buoys of IO at different
locations and ROMS for 2014.
99
Figure 4.4 Scatter plot diagram for SST (°C) from the RAMA buoys located at
different locations in different years and corresponding data from ROMS.
101
Figure 4.5 Comparison of model simulated SST (°C) for the entire simulation years
(2000 to 2014) with the two buoys (located at (1.5oS, 90oE) and (5oS,
95oE)).
101
Figure 4.6 Comparison of multi-year (2000-2014) seasonal average and SD of SST
(oC) from model and TMI for the DJF, MAM, JJAS and ON seasons. The
seasonal average surface currents (ms-1) from the model and OSCAR data
are overlaid over the SST average panels. The boxes marked in the figure
(j) show the different geographic regions in IO used for present analysis.
103
xvi
Figure 4.7 Comparison of subsurface temperature profile, D20 (dashed lines) and
MLD (continuous lines) from the model simulation and Argo data area
averaged over the three different regions (AS, BoB and CEIO) in the IO.
105
Figure 4.8 Comparison of vertical temperature profile (°C) and D20 (m) from
RAMA buoys located at different locations and ROMS. The D20 are
shown as black continuous line.
107
Figure 4.9 Scatter plot for the vertical temperature profile (°C) from RAMA buoy
and ROMS.
108
Figure 4.10 Statistical comparison of vertical temperature profiles (°C) from different
buoys located over the IO. (a) SD of vertical temperature at each RAMA
buoy locations (continuous lines) and the corresponding ROMS (dashed
lines), (b) RMSD between model and buoy SST (°C) at each locations.
108
Figure 4.11 Comparison of D20 (m) from RAMA buoy located at (1.5oS, 90oE) and
ROMS for different years.
110
Figure 4.12 Meridional velocity component (ms-1) from RAMA buoys and ROMS
model at different locations in the IO.
112
Figure 4.13 Zonal velocity component (ms-1) from RAMA buoy at (1.5oN, 90oE) and
ROMS model for different years.
113
Figure 4.14 Comparison of DMI (IOD index) during the simulation period (2000 to
2014) dashed lines are drawn at +/-0.48oC for qualification as IOD.
114
Figure 4.15 Comparison of model simulated SST (°C) and current anomalies (ms-1)
estimated from the pIOD, nIOD and normal year composites during 2000
-2014.
116
Figure 4.16 Model simulated temperature profile (°C) composite over the southeastern
box of IOD estimated for the (a) normal year, (b) nIOD, and (c) pIOD
years during 2000-2014. The thick black line denotes the D20 in each
figure.
118
Figure 4.17 Composites of model simulated zonal (a, b and c) and meridional (d, e
and f) velocity components (ms-1) estimated from the pIOD, nIOD, and
normal years during 2000-2014.
119
Figure 4.18 Composites of model simulated MLD (shaded, in meters) and D20
(contours, in meters) estimated from the pIOD, nIOD, and normal years
during 2000-2014 period.
120
Figure 4.19 Comparison of the composites of surface meridional salt transport (kg m-2
s-1) estimated from model simulations for the pIOD, nIOD, and normal
years during 2000-2014.
121
Figure 4.20 Comparison of the composites of surface zonal salt transport (kg m-2s-1)
estimated from model simulations for the pIOD, nIOD, and normal years
122
xvii
during 2000-2014.
Figure 5.1 The model domain along with bathymetry (m) derived from the modified
Etopo2v2. Locations of river mouths (nearest location where the data is
available before discharge) of major rivers also marked with red circles on
the map.
130
Figure 5.2 Monthly climatology of freshwater discharges (m3s-1) from the seven
major rivers in the BoB.
132
Figure 5.3 The idealized winds (ms-1) used to force the model. 133
Figure 5.4 The maximum volume of monthly climatological freshwater discharge
(m3s-1) of the seven major rivers in the BoB.
133
Figure 5.5 Comparison of the model simulated seasonal SSS (psu) with NIOA
climatology, Aquarius and the difference (ROMS-NIOA).
136
Figure 5.6 Comparison of the model simulated seasonal surface current vectors with
OSCAR currents data. Magnitude of current (ms-1) represented by colour
shading.
137
Figure 5.7 Temporal evolution of simulated SSS (psu) under different wind
directions in the BoB. The isohaline contour at 31 psu is shown with a
white thick line.
138
Figure 5.8 Vertical section of salinity (psu) under different idealized wind directions.
The isohaline contour at 31 psu is shown with a thick line.buoy.
140
Figure 5.9 Temporal evolution of simulated SSS (psu), showing features of low
saline plume, under varied wind speeds (low, moderate, high and very
high wind). The isohaline contour at 31 psu is shown with a white thick
line.
142
Figure 5.10 Comparison of vertical section of model simulated salinity (psu) along
21oN in the BoB under different wind speeds after 15 days, 30 days, 45
days, and 60 days of model integration.
143
Figure 5.11 Comparison of salinity profiles along the zonal cross section at 20°N in
the BoB and the associated meridional momentum balancing terms in the
plume from the sensitivity experiments with different wind directions.
145
Figure 5.12 Comparison of salinity profiles along the meridional cross section at 90°E
in the BoB and the associated zonal momentum balancing terms in the
plume from the sensitivity experiments with different wind directions.
146
Figure 5.13 The comparison of vertical salinity structure along the zonal cross section
at 20°N in the BoB and the associated meridional momentum balancing
terms in the plume from the sensitivity experiments with different wind
speeds.
147
xviii
Figure 5.14 The comparison of vertical salinity structure along the meridional cross
section at 90°E in the BoB and the associated zonal momentum balancing
terms in the plume from the sensitivity experiments with different wind
speeds.
148
Figure 5.15 The equivalent depth of freshwater (m) in the BoB for different idealized
wind directions.
149
Figure 5.16 The equivalent depth of freshwater (m) in the BoB for different idealized
wind speeds.
150
Figure 5.17 Zonal and meridional surface salt transport (kg m-2s-1) variability in the
BoB estimated from model simulations (a-h). Time-Longitude section of
surface salt transport along 16oN latitude (shown as black line in figure
(h)) in BoB (i-j).
152
Figure 5.18 Inter-comparison of meridional and zonal surface salt transport (kg m-2s-1)
under different wind directions (a-h) and wind speeds (i-p).
154
xix
List of Tables
Table No. Title Page
No.
Table 2.1 List of variables used in the governing equations of the model. 37
Table 2.2 Details of the parameters used in the ROMS model configuration. 46