Effect of turbulent mixing on the formation of intermediate nepheloid layer over the northern continental slope of the Andaman sea

Abstract

An intermediate nepheloid layer (INL) serves as an important conduit for the cross-slope transport of particulate matter, including organic carbon, biological nutrients and other lithogenic minerals. Despite extensive reports on the substantial sediment influx from the Ayeyarwady River into the northern continental slope of the Andaman Sea (AS), the transport route and fate of these river-borne sediments remain poorly understood, due to lack of in situ observations of turbid INL over the slope. In this study, we present direct evidence of an INL over the northern continental slope of the AS during the winter of 2019/2020, accompanied by enhanced mid-water turbulent mixing. Mooring measurements reveal energetic internal tides with high-mode vertical structure in the study region; and beam-like structures of internal tides are observed, which could be responsible for the enhanced mid-water turbulent mixing coinciding with the INL. Moreover, available microstructure profiles reveal energetic turbulent mixing with bottom-intensified turbulent diffusivity over the study area. Numerical experiments suggest that inhomogeneous distribution of turbulent mixing over the continental slope could induce local convergence of the upwelling transport in the upslope direction, resulting in an intrusion from the boundary to the interior and consequently promoting the INL formation. The discovery of the INL and its mixing-driven generation mechanism provide new insights into sediment transport dynamics over the northern continental slope of the AS.