Sonic layer depth (SLD) plays a critical role in upper-ocean acoustics and is strongly modulated by stratification processes. Using a decade-long dataset of Argo float observations (2011–2020), this study examines structural characteristics and spatiotemporal variability of the SLD across the Bay of Bengal and quantifies the relative contributions of thermal and haline stratification. SLD exhibits pronounced variability, exceeding 65 m during winter (January–February) in the northern and central basin in association with subsurface temperature inversions within the barrier layer. During the pre-monsoon (April–May), intensified surface heating enhances thermal stratification, leading to a shallow SLD (<10 m). A distinct seasonally reversing spatial gradient is identified, with north-westward deepening in spring and a reversal in summer. Freshwater-driven barrier layer formation in the northern basin results in a decoupling of SLD from mixed layer depth (MLD), while maintaining correspondence with isothermal layer depth (ILD), except under strong thermal-inversion conditions. In the southern basin, weaker haline stratification leads to greater coherence among SLD, MLD, and ILD. Case-specific analyses further reveal that salinity-driven stratification associated with surface freshening can substantially modulate SLD, in some instances overriding thermal controls. These results underscore the coupled influence of temperature and salinity in governing SLD variability and its implications for acoustic propagation.