Cross-shelf variation of internal tides west of the Dongsha Plateau in the northern South China Sea

We examine the cross-shelf variation of internal tides(ITs)west of the Dongsha Plateau in the northern South China Sea based on observations from 4 moorings deployed between August 2017 and September 2018.On the slope,the amplitude of diurnal baroclinic current ellipses are 5 times larger than that of barotropic currents.The baroclinic energy quickly dissipates during cross-shelf propagation,and barotropic currents become dominant on the shelf outside of the Zhujiang River Estuary,with the amplitude of semidiurnal barotropic current ellipses being 10 times larger than that of the baroclinic ones.Dynamic modal decomposition indicates the first baroclinic mode is dominant for both diurnal and semidiurnal ITs.The total horizontal kinetic energy(HKE)of the first three baroclinic modes shows spatiotemporal differences among the 4 moorings.On the slope,the HKE for diurnal ITs is stronger in summer and winter,but weaker in spring and autumn;for semidiurnal ITs there is a similar seasonal variation,but the HKE in winter is even stronger than that in summer.On the shallow shelf,both diurnal and semidiurnal ITs maintain a certain intensity in summer but almost disappear in winter.Further analysis shows that only the upper water column is affected by seasonal variation of stratification on the slope,variation of diurnal ITs is thus controlled by the semi-annual cycle of barotropic energy input from the Luzon Strait,while the incoherent baroclinic currents make a major contribution to the temporal variation of semidiurnal ITs.For the shelf region,the water column is well mixed in winter,and the baroclinic energy largely dissipates when ITs propagate to the shelf zone despite of a strong barotropic energy input from the Luzon Strait.

Numerical modeling of the seasonal circulation in the coastal ocean of the Northern South China Sea

The Finite Volume Community Ocean Model(FVCOM)was adapted to the Northern South China Sea(NSCS)to investigate the seasonality of coastal circulation,as well as along-shelf and cross-shelf transport.In fall and winter,southwestward current dominates the NSCS shelf,while the currenfs direction shifts to northeast in summer.The circulation pattern in spring is more complicated:both southwestward and northeastward currents are detected on the NSCS shelf.The mean shelf circulation pattern in winter does not show the permanent counter-wind South China Sea Warm Current(SCSWC)along the 100-200 m isobaths.Meanwhile,the model results indicate a northeastward current flowing along 50-100 m isobaths in spring.Southwestward along-shelf transport varies from 0.30-1.93 Sv in fall and winter,and it redirects to northeast in summer ranging jfrom 0.44—1.09 Sv.Onshore transport is mainly through the shelf break segment southeast of the Pearl River Estuary.