XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE Sound Speed Variability over Bay of Bengal from Argo Observations (2011-2020) 1 st Sudip Jana Department of Mathematics Adamas University Kolkata, India sudip.ocn@gmail.com 2 nd Avijit Gangopadhyay School for Marine Science and Technology University of Massachusetts Dartmouth Dartmouth, MA, USA avijit.gangopadhyay@umassd.edu 3 rd Patrick J. Haley, Jr. Department of Mechanical Engineering Massachusetts Institute of Technology Cambridge, MA, USA phaley@mit.edu 4 th Pierre F. J. Lermusiaux Department of Mechanical Engineering Massachusetts Institute of Technology Cambridge, MA, USA pierrel@mit.edu Abstract—In this paper, we study the spatio-temporal variability of the sound speed in the Bay of Bengal (BoB) estimated from the Argo observation data during 2011 - 2020. We perform domain-wide and region specific analysis of the sound speed structure and identify the regions and times of higher variabilities. The domain-wide spatio-temporal variability in the sound speed is maximum in the thermocline layers near 110 m depth. This variability is smaller at around 35-40m depth but increases in the surface layers. The regions of higher temporal and spatial variability vary with depth and time. In the surface layers, the variability is large in the northern part of the Bay but in the subsurface and the layers underneath, it is large along the entire western boundary from the north to south. Due to the combined impact of temperature inversion and the positive salinity gradient, the northern BoB experiences a significant positive vertical gradient in sound speed above the sonic layer depth (SLD) during the postmonsoon and winter periods. This gradient supports strong surface ducting and formation of the shadow zone below the SLD. Keywords—Sound speed variability, Bay of Bengal, Argo data, Sonic layer depth, Surface duct, Uncertainty characterization I. INTRODUCTION Underwater sound propagation has many important implications in acoustic communication, detecting underwater objects, and observing and monitoring the ocean interior and different oceanic processes. The analysis of the sound speed structure and its variability is essential to understand and predict the underwater refraction of sound and hence the formation of surface sound duct and shadow zone. The sound speed in the ocean depends on temperature, salinity and pressure and increases with all these three parameters. Among these three parameters, the temperature and salinity vary widely over space and time depending upon different oceanic and atmospheric conditions. In general the temperature plays the dominant role in controlling the variability in sound speed. However, the salinity impact becomes important in the regions where the salinity contrast is very large. The Bay of Bengal (BoB) is one such region that realizes a remarkable spatial and seasonal contrast in both temperature and salinity due to multiple factors. The excess freshwater influx, intrusion of equatorial currents, seasonally reversing western boundary currents, eddies, basin to sub-basic scale gyres, and equatorial remote forcing substantially modulate the thermohaline structure and its variability in the BoB. The enormous amount of freshwater input from the monsoon rain and the runoff of a number of rivers results in extreme surface freshening in the northern BoB [1]–[6], which leads to the formation of barrier layer and temperature inversions [7]–[11]. The northward flowing Western Boundary Current (WBC) during spring carries salty water towards the north [12] – [14] while the southward flowing East India Coastal Current (EICC) carries low-saline water from the north to the south [4], [15]. The boundary currents and associated eddies lead to significant variability in the thermocline region. The southwest monsoon wind results in upwelling of colder subsurface water to the surface along the west coast during the summer [16]. The BoB remains populated by several eddies of shorter to longer time scales throughout the year. The intrusion of Southwest Monsoon Current (SMC) through the southwest corner during the monsoon period supplies warm and salty water that helps maintain the salinity balance in the BoB [17], [18]. The equatorial remote forcing, such as, the upwelling and downwelling Kelvin waves modulates the currents and the thermohaline structure [19]. All these factors affect the thermohaline structure of different parts of the BoB and eventually affect the sound speed of this region. Understanding of the spatial and seasonal distribution of the sound speed variability is useful for ocean acoustic modeling, sensing, and data assimilation [20]–[23]. Therefore, it is worthwhile to study the sound speed variability in the BoB. A number of previous studies investigates different aspects of the BoB and Arabian Sea sound speed structures. Reference [24] studied the sound speed structure in the BoB and the Arabian Sea using the Levitus annual climatology. Reference [25] estimated SLD from surface parameters using an artificial neural network technique. Reference [26] investigated the seasonal variability of SLD in the central Arabian Sea. Using two climatologies, [27] studied the variability in the sound speed and the SOFAR channel in the Indian Ocean. Reference [28] used World Ocean Atlas (WOA01) climatology and Argo in situ data to study the distribution of the SLD in the Arabian Sea. Reference [29] used hourly mooring profiles and studied Jana, S., A. Gangopadhyay, P.J. Haley, Jr., and P.F.J. Lermusiaux, 2022. Sound Speed Variability over Bay of Bengal from Argo Observations (2011-2020). In: OCEANS 2022 Chennai, February 21-24, 2022, pp. 1-8, in press.