Novel Insights Into the Zonal Flow and Transport in the Luzon Strait Based on Long-Term Mooring Observations

The zonal flow and volume transport in the Luzon Strait play a crucial role in modulating the circulation, heat and material balance, and biogeochemical processes in the South China Sea (SCS), but the quantitative values remain unclear due to lack of long-term direct observations. Here, this knowledge gap is bridged by analyzing 4-year velocity measurements from a mooring array along 119.9 degrees E. Based on the novel data, the mean value of the upper-layer Luzon Strait transport (i.e., LST_up) is estimated at -4.54 +/- 1.69 Sv. Seasonally, the westward LST_up attains its peak and trough in January and June with values of -6.80 +/- 1.46 Sv and -2.59 +/- 0.76 Sv, respectively. At the interannual time scale, the LST_up was strongest in 2016-2017 but weakest in 2017-2018. Further analysis suggested that local winds and the combination of local winds and upstream Kuroshio transport are likely the dominant modulation mechanisms for its seasonal and interannual variations, respectively. In the middle layer, a quasi-steady cyclonic flow structure is identified and the volume transport is therefore trivial. We further found that seasonal variation of the middle-layer transport is dominated by the eastward flow of this cyclonic structure. Corresponding to the gravest-mode response in the Luzon Strait, seasonal-to-interannual variations of this middle-layer eastward flow are nearly in-phase with the upper-layer westward flow. Overall, the above observational results provide a benchmark for the flow and transport in the Luzon Strait which can help understand the dynamics of the SCS circulation and validate and improve regional numerical simulations. Plain Language Summary The Luzon Strait is the main channel connecting the South China Sea (SCS) with the Pacific Ocean and the volume transport through the strait is crucial in regulating physical and biogeochemical processes in the SCS. However, given the sparsity of long-term direct observations, the knowledge of zonal flow and