Variability Of Ocean Dynamics And Associated Mechanisms Over The Tropical Indian Ocean

The Indian Ocean is the warmest and third-largest ocean in the world.The dynamics and behavior are different from other oceans as it is highly exposed to the tropical climate.The ocean also plays an important role in the summer monsoon,and its study will have a significant impact on the socio-economic dimensions of the Indian subcontinent.The areas of study for this thesis are in the tropical Indian Ocean.The evolution of sea surface temperature(SST)anomalies in the tropical Indian Ocean mainly involves coupled ocean-atmosphere processes that are either generated by a large-scale atmospheric force linked to El Ni?o/La Nina-Southern Oscillation(ENSO)in the tropical eastern Pacific or by an internal independent ocean mechanism such as the Indian Ocean Dipole(IOD)can affect the interannual variability of SST.In fact,these SST anomalies,driven by various ocean dynamics(such as horizontal and vertical advection,surface-based energy flows,and horizontal and vertical wind turbulence),are mainly responsible for causing extreme weather conditions in the Indian Ocean during the monsoon and influence weather and climate over adjoining land area.The dynamics of such variations in monsoon rainfall and extreme weather conditions are not well understood,but it is well recognized that the atmosphere interacts with the upper ocean rather than the surface alone,suggesting that the forecast of the monsoon in the Indian Ocean can be improved by using the upper ocean parameters.Due to sparse observations of surface parameters over the tropical Indian Ocean(TIO),a detailed and systematic study of physical processes controlling the variability of these parameters could not be earned out for long.However,the development of recent Argo programs and remote sensing techniques(space-based observations of ocean and atmosphere)have boosted such studies.The expansion of Argo arrays in the Indian Ocean has considerably enhanced the surface and subsurface parameters’ database.This study attempts an innovative way based on simulation and modeling techniques to address such gaps by documenting variability in upper ocean parameters and associated mechanisms over the TIO.The study is needed to anticipate extreme weather occurrences in the tropical Indian Ocean in applications such as air-sea interaction investigations,monsoon studies,underwater acoustic studies,and statistical dynamic models.The first and foremost focus of this study is the analysis of the surface and subsurface parameters variability and associated mechanisms in the northwestern Indian Ocean.The monsoon cycle has a significant impact on the circulation of SST and sea surface salinity(SSS)in the north Indian Ocean,both spatially and temporally.Argo data was from 2004 to 2019,with temperature and salinity values are taken closest to the sea surface(z≤ 5 m),which is comparable to satellite SST and SSS observations.From the seasonal evolution of SST and SSS,this study ingeniously investigates the co-existence of two water masses in the Arabian Sea region:1)the Arabian Sea High Saline Water(ASHSW)that is in excess of salinity(>36.9)and occupying the north and central basins of the Arabian Sea and 2)the Bay of Bengal Water(BBW)that is inferior in salinity(<35)and occupying the south basin of the Arabian Sea.The second part of the study focuses on the interaction between the ocean and the atmosphere.Two years were chosen with contrasting SST anomalies,2016 and 2017,marked by strong El Nino and weak La Nina episodes,respectively,in the tropical eastern Pacific Ocean.The powerful El Nino and weak La Nina triggered extreme negative and weak positive Indian Ocean Dipole(IOD)events in the Indian Ocean during 2016 and 2017,respectively.Much work has been done in the past to investigate global climate change and its impact on the evolution of IOD.The dynamic behind it,however,is still not well understood.Using various satellite datasets,the present study explores the mechanisms behind these events and their impacts on SST variability in the TIO in a novel approach.For this,the monthly mean SST anomalies data were computed from NOAA Optimum Interpolation Sea Surface Temperature(OISST)on the basis of 30-year mean daily climatology(1981-2010).The relationship established between wind and SST suggests that SST is controlled by different dynamics in the West and East pole of the tropical Indian Ocean.Another prominent feature of this study is the observation of weak upwelling along the Omani-Arabian coast,which warmed the SST by 1℃ in the summer of 2017(as compared to 2016).This warming led to increased precipitation in the Bay of Bengal(BoB)region during the summer of 2017.These findings indicate that the tropical Indian Ocean is a key region for the generation of SST anomalies and variability of rainfall in 2016 and 2017.The study further investigates,in a disparate departure from the past research,the seasonal variability of SST and isothermal layer depth(ILD)in association with the monsoon cycle in the Arabian Sea and Sea of Oman regions for these two contrasting years,2016 and 2017.The ILD climatology is computed from temperature profiles provided by the Argo floats using a threshold technique(T(z)≥SST-1℃)to study the region with stronger and weaker monsoon wind forcing.The study suggests that summer monsoon is more pronounced in the south-central basin of the Arabian Sea,where ILD is deepening(>100 m in September 2016)mainly due to stronger wind forcing in this region.The Sea of Oman region,on the hand,contrary,has a smaller ILD amplitude(<10 m in June 2016)and higher SST meaning,indicating that it is weakly influenced by the summer monsoon.The seasonal relationship established between ILD variability and the monsoon cycle for 2016 and 2017 shows that ILD could be a useful indicator for predicting the summer monsoon in the Arabian Sea regions.Finally,a set of simulations carried out with a limited area has been used to understand the impact of mesoscale ocean vortices on the sound field.This study employs the Okubo-Weiss(OW)approach to extract and identify vortices using real-time data from the AIPOcean 1.0 numerical product.In a novel way,a theoretical model of sound field calculation under the influence of vortex is then developed in COMSOL software based on the finite element method.The influence of mesoscale ocean vortex on low-frequency sound propagation is investigated and addressed using this model.The COMSOL-based results are consistent with previous research,indicating that this model could be useful for analyzing underwater acoustic filed influenced by mesoscale vortices.