Kejian Wu

The Global Distribution of Diapycnal Mixing and Mixing Coefficient Tensor in the Upper 2000m Ocean from Argo Observations

Diapycnal Mixing (DM) within the upper 2000m of the global ocean is calculated by a fine-scale parameterization using the multiyear-mean density gridded product that created by employing all the Argo float observations to date through the recently published equation of seawater TEOS-10. The geographic distribution of Argo-derived DM derived in this study is spatial-dependent and varies with latitude and depth. The magnitude and pattern of DM is favorably validated by comparisons with previous studies. Furthermore, the mixing coefficient tensor K is calculated and analyzed. Components of the tensor fitting for the geopotential coordinate models are also presented. It is found that the tensor components in horizontal direction, Kxx and Kyy, have similar magnitude and distribution pattern. In the vertical, Kzz is enhanced over regions with rough topography and strong wind (e.g., Westerly region), suggesting agreement with previous estimates. This work presents a scheme to estimate the DM and mixing coefficient tensor using Argo observations, and offers a useful Argo-based mixing product for the purpose of promoting the study and modeling of ocean circulation and other processes.

The effect of Coriolis-Stokes forcing on upper ocean circulation in a two-way coupled wave-current model

We investigated the Stokes drift-driven ocean currents and Stokes drift-induced wind energy input into the upper ocean using a two-way coupled wave-current modeling system that consists of the Princeton Ocean Model generalized coordinate system (POMgcs), Simulating WAves Nearshore (SWAN) wave model, and the Model Coupling Toolkit (MCT). The Coriolis-Stokes forcing (CSF) computed using the wave parameters from SWAN was incorporated with the momentum equation of POMgcs as the core coupling process. Experimental results in an idealized setting show that under the steady state, the scale of the speed of CSF-driven current was 0.001 m/s and the maximum reached 0.02 m/s. The Stokes drift-induced energy rate input into the model ocean was estimated to be 28.5 GW, taking 14% of the direct wind energy rate input. Considering the Stokes drift effects, the total mechanical energy rate input was increased by approximately 14%, which highlights the importance of CSF in modulating the upper ocean circulation. The actual run conducted in Taiwan Adjacent Sea (TAS) shows that: 1) CSF-based wave-current coupling has an impact on ocean surface currents, which is related to the activities of monsoon winds; 2) wave-current coupling plays a significant role in a place where strong eddies present and tends to intensify the eddy’s vorticity; 3) wave-current coupling affects the volume transport of the Taiwan Strait (TS) throughflow in a nontrivial degree, 3.75% on average.

Numerical Study of the Effects of Wave-Induced Forcing on Dynamics in Ocean Mixed Layer

Numerical experiments using hybrid coordinate ocean model (HYCOM) are designed to quantify the effects of wind wave-induced Coriolis-Stokes forcing (CSF) on mixed layer (ML) dynamics in a global context. CSF calculated by the wave parameters simulated by using the WaveWatch III (WW3) model is introduced as a new driving force for HYCOM. The results show that noticeable influence on ocean circulation in ML can be caused by CSF. Over most of the global oceans the direction of Stokes transport is different from that of the change in current transport caused by CSF. This is not unusual because CSF is normal to Stokes drift. However, the CSF-caused change in current transport and the wave-induced Stokes transport have the same magnitude. The seasonal variabilities of mixed layer temperature (MLT) and mixed layer depth (MLD) caused by CSF are analyzed, and the possible relationship between them is also given.

The Reanalysis of Currents and Throughflow Volume Transport in the Taiwan Strait

Currents and throughflow transport in the Taiwan Strait (TS) are systematically analyzed using a new regional ocean reanalysis dataset (China Ocean ReAnalysis, CORA in brief), recently released by National Marine Data and Information Service (NMDIS) in 2009, covering Chinas coastal waters and adjacent seas. The total throughflow volume transport through TS presents a seasonal variability with the annual-mean of 1.49Sv. It reaches maximum in summer with mean value of 2.33Sv, while in winter the mean transport falls to 0.78Sv on average. The estimated throughflow transport is in good agreement with the observational results. The comparisons of the results with historical observations and numerical simulations also show that CORA performs well on reproducing the currents and temperature field in TS region. In addition, CORA not only successfully reproduced the known oceanic phenomena in TS such as temperature front, up-wind flow in fall, winter northward flow blocking in Penghu Channel, and the winter anticyclonic eddy located at north of Changyun Rise (CYR), but it also found a new cyclonic eddy in north TS in winter.

Wave effect on the ocean circulations through mass transport and wave-induced pumping

The wave Coriolis-Stokes-Force-modified ocean momentum equations are reviewed in this paper and the wave Stokes transport is pointed out to be part of the ocean circulations. Using the European Centre for Medium-Range Weather Forecasts 40-year reanalysis data (ERA-40 data) and the Simple Ocean Data Assimilation (SODA) version 2.2.4 data, the magnitude of this transport is compared with that of wind-driven Sverdrup transport and a 5-to-10-precent contribution by the wave Stokes transport is found. Both transports are stronger in boreal winter than in summers. The wave effect can be either contribution or cancellation in different seasons. Examination with Kuroshio transport verifies similar seasonal variations. The clarification of the efficient wave boundary condition helps to understand the role of waves in mass transport. It acts as surface wind stress and can be functional down to the bottom of the ageostrophic layer. The pumping velocities resulting from wave-induced stress are zonally distributed and are significant in relatively high latitudes. Further work will focus on the model performance of the wave-stress-changed-boundary and the role of swells in the eastern part of the oceans.

Influence of the physical environment on the migration and distribution of Nibea albiflora in the Yellow Sea

This study investigates the migration and distribution of the warm-temperate fish Nibea albiflora. Their spawning migration and wintering migratory routes within in the Yellow Sea are described in detail. Considering the main physical features and environment of the Yellow Sea, it appears to be have one wintering ground and three migratory routes from the wintering ground to the spawning grounds. The fish begin to migrate from the wintering ground to the spawning grounds in the northwest region of the Yellow Sea in late March. The Yellow Sea has three spawning grounds. The first is located near the Yalu River on the Liaodong Peninsula and the second one is located in Rushan Bay of Shandong Peninsula. The third spawning ground is located in Haizhou Bay in the southern region of the Yellow Sea. This study found that the temperature of the Yellow Sea influences the migration of N. albiflora, and that the migratory routes coincide with the thermal fronts in the sea. Nutrients for juvenile fish are taken from the coastal upwelling area. Chlorophyll is a good environmental indicator of phytoplankton biomass and thereby provides the status of biological resources. Different types of sediment in near-shore zones are also of practical significance for the growth of fish. The study of the effects of marine environments on the migration of various fishes is not only significant to the fishing industry, but can also provide a scientific basis for the understanding of the ecological implications of the relevant physical processes.

The signal of La Niña in wave transport

Abstract La Nina phenomena have significant impact on extreme weather and marine disasters in China. Large-scale water transport is one of the primary factors affecting the development of La Nina events; however, the contribution of swell transport from the Southern Ocean to the sea surface temperature anomaly (SSTA) in the eastern equatorial Pacific (EEP) has been ignored in previous studies. Through various statistical techniques, this study used seven sets of satellite altimeter and reanalysis data to reveal the relationship between swell transport and La Nina. Based on the characteristics of significant SSTA changes in the Nino 3 region (5°S–5°N, 90°–150°W) following an El Nino event, two cases of La Nina and non-La-Nina were defined. Wave transport in the first case presents an anomaly to the north of the 30°S section, i.e., cold water carried by swell transport to the EEP increases prior to the onset of a La Nina event. However, the characteristics of swell transport in the non-La-Nina scenario are contrary to the first case. Wave transport anomalies in the two considered cases produce sufficient and necessary conditions for the development of La Nina and non-La-Nina events. In addition, the impact of swell transport from the westerlies on La Nina events is confirmed further based on heat flux calculations. Wave transport has considerable effect on SSTA variability in the EEP; therefore, it should not be neglected in future studies.

Extreme wave climate variability in South China Sea

Abstract Extreme wave climate variability in the South China Sea (SCS) was investigated using significant wave height (SWH) data simulated by the third generation wave model WAVEWATCH-III (WW-III) for the period 1976–2014. The wind forcing data was using the objective reanalysis wind dataset from the weather research and forecasting (WRF) model. The simulated SWH was well validated by observation data from satellite altimeter and in-situ buoys. A generalized extreme value (GEV) model was applied to analyze the extreme wave climate variability. Monthly significant wave height maxima is used to characterize the seasonal variability. The spatial distributions of positional and scale parameters are achieved. The positional parameter values reach the maximum in winter, while the scale parameter values are greater in summer and autumn due to the impact of typhoon. The regression analysis of harmonic functions was applied to give the annual cycle. Interannual climatology of extreme wave climate from Empirical Orthogonal Function (EOF) decomposition method and spectrum analysis method revealed a dominant 12 months period and a 22 months period, respectively. Results show that El Nino may significantly affect the extreme wave climate variability in the SCS.

Preliminary results of assessing the mixing of wave transport flux residualin the upper ocean with ROMS

The effects of the mixing of wave transport flux residual (Bvl) on the upper ocean is studied through carrying out the control run (CR) and a series of sensitive runs (SR) with ROMS model. In this study, the important role of Bvl is revealed by comparing the ocean temperature, statistical analysis of errors and evaluating the mixed layer depth. It is shown that the overestimated SST is improved effectively when the wave-induced mixing is incorporated to the vertical mixing scheme. As can be seen from the vertical structure of temperature 28°C isotherm changes from 20 min CR to 35 m in SR3, which is more close to the observation. Statistic analysis shows that the root-mean-square errors of the temperature in 10 m are reduced and the correlation between model results and observation data are increased after considering the effect of Bvl. The numerical results of the ocean temperature show improvement in summer and in tropical zones in winter, especially in the strong current regions in summer. In August the mixed layer depth (MLD) which is defined as the depth that the temperature has changed 0.5°C from the reference depth of 10 m is further analyzed. The simulation results have a close relationship with undetermined coefficient of Bvl, sensitivity studies show that a coefficient about 0.1 is reasonable value in the model.

Modelling the upwelling offthe east Hainan Island coast in summer 2010

A synoptic-scale upwelling event that developed offthe east coast of the Hainan Island (EHIU) in the summer of 2010 is defined well via processing the Moderate Resolution Imaging Spectroradiometer (MODIS) sea surface temperature (SST) data. The Regional Ocean Modeling System (ROMS) with high spatial resolution has been used to investigate this upwelling event. By comparing the ROMS results against tide station data, Argo float profiles and MODIS SST, it is confirmed that the ROMS reproduces the EHIU well. The cooler-water core (CWC) distinguished by waters < 27.5°C in the EHIU, which occurred in the east Qiongzhou Strait mouth area and was bounded by a high temperature gradient, was the focus of this paper. Vertical structure of the CWC suggests that interaction between the westward flow and the bathymetry slope played a significant role in the formation of CWC. Numerical experiments indicated that the westward flow in the Qiongzhou Strait was the result of tidal rectification over variable topography (Shi et al., 2002), thus tides played a critical role on the development of the CWC. The negative wind stress curl that dominated the east Qiongzhou Strait mouth area suppressed the intensity of the CWC by 0.2–0.4°C. Further, nonlinear interaction between tidal currents and wind stress enhanced vertical mixing greatly, which would benefit the development of the CWC.