Tides-generated nonlinear internal waves transport sediment in the deep-sea bottom boundary layer

Abstract

As a globally distributed high-energy oceanic motion, the ability of nonlinear internal waves (NLIWs) to penetrate the thermocline into the deep-sea bottom boundary layer (BBL) and lead to sediment transport remains controversial due to the scarcity of direct in situ observations. To address this issue, we deployed a tripod-mooring system at 2011 m water depth on the northern slope of the South China Sea, an area renowned for its vigorous NLIWs and significant terrigenous sediment input to the deep sea. Our observation confirms that NLIWs begin shoaling and breaking, thereby triggering sediment resuspension, when reaching the deep slope rather than being confined to the shelf and upper slope. Two distinct types of NLIWs originating from the Luzon Strait exhibit different effects on the deep-sea BBL. Type-A NLIWs, whose induced flows partially align with the background flows, amplify BBL flows and contribute 50.28% of the total sediment flux despite accounting for only 1.63% of the observation time. In contrast, type-B NLIWs, whose induced flows oppose the background flows, weaken BBL flows and inhibit 29.22% of the total sediment flux. Notable linear correlations between NLIWs’ amplitude and the BBL temperature, flow velocity, and shear velocity has been established for the first time, which will help to advance the parameterization of NLIW-BBL interactions and provide crucial insights into the role of NLIWs in deep-sea sediment transport.