An internal solitary wave forecasting model in the northern South China Sea (ISWFM-NSCS)

Abstract

Abstract. Internal solitary waves (ISWs) are a ubiquitous phenomenon in the dynamic ocean system, which play a crucial role in driving transport through turbulent mixing. Over the past few decades, numerical modelling has become a vital approach to investigate the generation mechanism and spatial distribution of ISWs. The northern South China Sea (NSCS) has been treated as a physical oceanographic focus of ISWs in massive numerical studies since the last century. However, there has been no systematic evaluation of a reliable three-dimensional (3D) model about accurately reproducing ISW characteristics in the NSCS. In this study, we implement a 3D ISW forecasting model in the NSCS and quantitatively evaluate the requirements of factors (i.e. model resolution, tidal forcing, and stratification selection) in accurately depicting ISW properties by comparison with observational data at a mooring station in the vicinity of the Dongsha Atoll. Firstly, the 500 m resolution model can basically reproduce the principal ISW characteristics, while the 250 m resolution model would be a better solution to identify wave properties, specifically increasing 40 % accuracy of predicting characteristic half-widths. Nonetheless, a 250 m resolution model spends nearly 5-fold the computational resources of a 500 m resolution model in the same model domain. Compared with the former two, the model with a lower resolution of 1000 m severely underestimates the nonlinearity of ISWs, resulting in an incorrect ISW field in the NSCS. Secondly, the model with 8 (or 13) primary tidal constituents can accurately reproduce the real ISW field in the NSCS, while the one with four main harmonics (M2, S2, K1 and O1) would underestimate averaged wave-induced velocity for about 38 % and averaged mode-1 wave amplitude for about 15 %. Thirdly, the model with the initial condition of field-extracted stratification gives a better performance in predicting some wave properties than the model with climatological stratification, namely 13 % improvement of arrival time and 46 % improvement of characteristic half-width. Finally, background currents, spatially varying stratification and external (wind) forcing are discussed to reproduce a more realistic ISW field in the future numerical simulations.