Distribution of surface layer temperature inversion in the North Indian Ocean and associated acoustic propagation characteristics

Abstract

Surface Layer Temperature Inversion (SLTI), a warm layer between the surface and subsurface colder waters, is a significant oceanographic feature that has a profound impact on the oceanic acoustic propagation. Strong inversions are observed in northern BoB and in the South Eastern Arabian Sea (SEAS) with a temperature difference of ~ 5 °C and ~ 3 °C, respectively. Here, we identify and characterise the prominent SLTI regions in the North Indian Ocean. Representative temperature profiles that have been measured from different regions of the North Indian Ocean are selected and classified into six types with respect to the dominant causative factors of SLTI. Then, the acoustic propagation in the surface layer is examined in the presence and absence of SLTI in terms of sonic layer depth, cut-off frequency and limiting ray angle. For the representative profiles analysed in this study, except at stations, where the dominant causative factor of SLTI is river water influx, the sonic layer depth (SLD) increases. Acoustic implications of SLTI are also examined utilizing the ray model (cTraceo). In cases where the dominant cause of SLTI is freshwater influx, the sonic layer shoals and the acoustic waves insonify comparatively farther and deeper regions during non-inversion. This study addresses the variability of acoustic propagation all along the North Indian Ocean driven by SLTI.