Changshui Xia

Three‐dimensional structure of the summertime circulation in the Yellow Sea from a wave‐tide‐circulation coupled model

[1] The three-dimensional structure of the summertime circulation of the Yellow Sea (hereafter YS) is studied by using a prognostic wave-tide-circulation coupled model based on the Princeton Ocean Model (POM) and a surface wave model. The simulated tidal harmonic constants and temperature structure agree with the observations well. The patterns of the simulated salinity generally agree also with the observations. The simulated results show that the horizontal circulation has a three-layer structure: in the surface layer (0–4 m), the prevailing current direction is northeastward; in the upper layer (4–40 m) it is dominated by a basin scale anticlockwise (cyclonic) gyre; in the bottom layer (below 40 m) the water diverges from the center area and there exists a weak southward current along the YS trough. The stream function of the YS shows that the net circulation of the YS is an anticlockwise (cyclonic) one, and the net transport is about 0.1 Sv. Diagnostic analysis of the momentum balance and sensitivity show that the cyclonic circulation in the upper layers is mainly a quasi-geostrophic flow along tidal-induced temperature front, and it is also strengthened by the tide residual currents. The tidal residual current and the compensation for northward surface layer wind transport contribute to the formation of southward flow in the bottom layer. The vertical circulations vary along different sections. A circulation cell is found in the frontal area near the Korean coast, and an upwelling is found along the slope.

Upwelling off the west coast of Hainan Island in summer: Its detection and mechanisms

The summertime upwelling off the west coast of Hainan Island is newly detected by satellite remote sensing sea surface temperature, and confirmed by both historical field observations and numerical modeling. Furthermore, numerical experiments are conducted to gain understanding of the upwelling mechanisms. A tidal mixing front (TMF) is identified as the vital factor triggering the formation of the upwelling. The baroclinic pressure gradient force, which stems from the intense density difference across the TMF, causes a frontal-scale circulation at the TMF. As a result, upwelling appears as a branch of this circulation. The southwest monsoon induces downwelling, which competes with the front-induced upwelling. Climatologically, the upwelling dominates and can reach about 5 m below the sea surface above the slope bottom. In calm weather with no or weak winds, it is expected that the upwelling can reach all the way to the sea surface.

The role of surface waves in the ocean mixed layer

Previously,most ocean circulation models have overlooked the role of the surface waves.As a result,these models have produced insufficient vertical mixing,with an under-prediction of the mixing layer(ML)depth and an over-prediction of the sea surface temperature(SST),particularly during the summer season.As the ocean surface layer determines the lower boundary conditions of the atmosphere,this deficiency has severely limited the performance of the coupled ocean-atmospheric models and hence the climate studies.To overcome this shortcoming,a new parameterization for the wave effects in the ML model that will correct this systematic error of insufficient mixing.The new scheme has enabled the mixing layer to deepen,the surface excessive heating to be corrected,and an excellent agreement with observed global climatologic data.The study indicates that the surface waves are essential for ML formation,and that they are the primer drivers of the upper ocean dynamics;therefore,they are critical for climate studies.

The Statistical Theory of Breaking Entrainment Depth and Surface Whitecap Coverage of Real Sea Waves

Some basic statistics for wave breaking have been derived based on the statistical model of real sea waves. The analytic expressions of breaking entrainment depth and surface whitecap coverage involved with both sea wave characteristics and surface wind velocity have been derived on the basis of the whitecap formation model. The concept of the upper envelope for all the whitecap coverage data versus wind speed has been proposed, and it is assumed to correspond to the whitecap coverage in the case of the infinite wind duration and fetch to determine the model constants. The analytic expressions of breaking entrainment depth and whitecap coverage have been compared with the observations in several ways, and consistently favorable agreement can be found for most observations.

The impact of surface waves on the mixing of the upper ocean

A new three-dimensional numerical model is derived through a wave average on the primitive N-S equations, in which both the“Coriolis-Stokes forcing” and the“Stokes-Vortex force” are considered. Three ideal experiments are run using the new model applied to the Princeton ocean model (POM). Numerical results show that surface waves play an important role on the mixing of the upper ocean. The mixed layer is enhanced when wave effect is considered in conjunction with small Langmuir numbers. Both surface wave breaking and Stokes production can strengthen the turbulent mixing near the surface. However, the influence of wave breaking is limited to a thin layer, but Stokes drift can affect the whole mixed layer. Furthermore, the vertical mixing coefficients clearly rise in the mixed layer, and the upper ocean mixed layer is deepened especially in the Antarctic Circumpolar Current when the model is applied to global simulations. It indicates that the surface gravity waves are indispensable in enhancing the mixing in the upper ocean, and should be accounted for in ocean general circulation models.

Verification of an operational ocean circulation-surface wave coupled forecasting system for the China’s seas

An operational ocean circulation-surface wave coupled forecasting system for the seas off China and adjacent areas (OCFS-C) is developed based on parallelized circulation and wave models. It has been in operation since November 1, 2007. In this paper we comprehensively present the simulation and verification of the system, whose distinguishing feature is that the wave-induced mixing is coupled in the circulation model. In particular, with nested technique the resolution in the China’s seas has been updated to (1/24)° from the global model with (1/2)° resolution. Besides, daily remote sensing sea surface temperature (SST) data have been assimilated into the model to generate a hot restart field for OCFS-C. Moreover, inter-comparisons between forecasting and independent observational data are performed to evaluate the effectiveness of OCFS-C in upper-ocean quantities predictions, including SST, mixed layer depth (MLD) and subsurface temperature. Except in conventional statistical metrics, non-dimensional skill scores (SS) is also used to evaluate forecast skill. Observations from buoys and Argo profiles are used for lead time and real time validations, which give a large SS value (more than 0.90). Besides, prediction skill for the seasonal variation of SST is confirmed. Comparisons of subsurface temperatures with Argo profiles data indicate that OCFS-C has low skill in predicting subsurface temperatures between 100 m and 150 m. Nevertheless, inter-comparisons of MLD reveal that the MLD from model is shallower than that from Argo profiles by about 12 m, i.e., OCFS-C is successful and steady in MLD predictions. Validation of 1-d, 2-d and 3-d forecasting SST shows that our operational ocean circulation-surface wave coupled forecasting model has reasonable accuracy in the upper ocean.

The numerical investigation of seasonal variation of the cold water mass in the Beibu Gulf and its mechanisms

A wave-tide-circulation coupled model based on the Princeton Ocean Model is established to explore the seasonal variation of the cold water mass in the Beibu Gulf and its mechanisms. The results show that the cold water mass starts forming in March, reaches the maximum strength during June and July, and fades away since October. Strong mixing in winter transports the cold water from sea surface to bottom. The cold water mass remains in the bottom layer as the thermocline strengthens during spring, except for the shallow water where the themocline is broken by strong tidal mixing, which gradually separate the cold water mass from its surrounding warm water. Further analysis on the ocean current and stream function confirms that the cold water mass in the Beibu Gulf is locally developed, with an anticlockwise circulation caused by a strong temperature gradient. Sensitivity experiments reveal that the cold water mass is controlled by the sea surface heat flux, while the terrain and tidal mixing also play important roles.

Case study on the three-dimensional structure of meso-scale eddy in the South China Sea based on a high-resolution model

Meso-scale eddies are important features in the South China Sea (SCS). The eddies with diameters of 50–200 km can greatly impact the transport of heat, momentum, and tracers. A high-resolution wave-tide-circulation coupled model was developed to simulate the meso-scale eddy in the SCS in this study. The aim of this study is to examine the model ability to simulate the meso-scale eddy in the SCS without data assimilations The simulated Sea Surface Height (SSH) anomalies agree with the observed the AVISO SSH anomalies well. The simulated subsurface temperature profiles agree with the CTD observation data from the ROSE (Responses of Marine Hazards to climate change in the Western Pacific) project. The simulated upper-ocean currents also agree with the main circulation based on observations. A warm eddy is identified in winter in the northern SCS. The position and domain of the simulated eddy are confirmed by the observed sea surface height data from the AVISO. The result shows that the model has the ability to simulate the meso-scale eddy in the SCS without data assimilation. The three-dimensional structure of the meso-scale eddy in the SCS is analyzed using the model result. It is found that the eddy center is tilted vertically, which agrees with the observation. It is also found that the velocity center of the eddy does not coincide with the temperature center of the eddy. The result shows that the model has the ability to simulate the meso-scale eddy in the SCS without data assimilations. Further study on the forming mechanism and the three-dimensional structure of the meso-scale eddies will be carried out using the model result and cruise observation data in the near future.

A numerical investigation into the long-term behaviors of Fukushima-derived ~（137）Cs in the ocean

The Fukushima nuclear accident in 2011 released large amounts of radionuclides, including 137Cs, into the Pacific Ocean. A quasi-global ocean radioactive transport model with horizontal grid spacing of 0.5°×0.5° and 21 vertical layers was thereafter established to study the long-term transport of the Fukushima-derived 137Cs in the ocean. The simulation shows that the plume of 137Cs would be rapidly transported eastward alongside the Kuroshio Current and its extensions. Contaminated waters with concentrations lower than 2 Bq/m3 would reach the west coast of North America 4 or 5 years after the accident. The 137Cs tends to be carried, despite its very low concentration, into the Indian and South Pacific Oceans by 2016 via various branches of ocean currents. Meanwhile, the 137Cs concentrations in the western part of the North Pacific Ocean decrease rapidly with time. Up to now the highly contaminated waters have remained in the upper 400 m, showing no evidence of significant penetration to deeper layers.

Sensitive study of the long and short surface wave-induced vertical mixing in a wave-circulation coupled model

The previous studies by the MASNUM research team have shown the effectiveness of the wave-induced mixing (Bv) in improving the simulation of upper-ocean thermal structure. The mechanisms of Bv are further investigated by incorporating different Bv products into the MASNUM wave-circulation coupled model. First, experiments were designed to explore the effects of Bv, which contain the contributions at different wave lengths (l). The results of three experiments, the non-Bv case, the short-wave case (l <300 m), and the long-wave case (l >300 m) are compared, and it is found that the long waves are the most important component for Bv to generate mixing in the upper ocean. As the swell plays dominant role in mixing, the parameterization of Bv into wind may be not a proper way. Second, Bv effects at different time-scales, including daily and monthly, were examined. The results show that the monthly averaged Bv has larger impact than the daily averaged Bv, especially in summer.