Xianwen Bao

M2 tidal dynamics in Bohai and Yellow Seas: a hybrid data assimilative modeling study

A high-resolution hybrid data assimilative (DA) modeling system is adapted to study the M2 barotropic tidal characteristics and dynamics in the Bohai and Yellow Seas. In situ data include tidal harmonics extracted from both coastal sea level and bottom pressure observations. The hybrid DA system consists of both forward and inverse models. The former is three-dimensional, finite-difference, nonlinear Regional Ocean Modeling System (ROMS). The latter is a three-dimensional, linearized, frequency-domain, finite-element model TRUXTON. The DA system assimilates in situ observations via the inversion of the barotropic tidal open boundary conditions (OBCs). Model skill is evaluated by comparing misfits between the observed and modeled tidal harmonics. The assimilation scheme is found effective and efficient in correcting the tidal OBCs, which in turn improves ROMS tidal solutions. Up to 50% reduction of model/data misfits is achieved after data assimilation. M2 co-tidal maps constructed from the posterior (data assimilative) ROMS solutions agree well with observational analysis of (Fang et al. 2004). Detailed analyses on tidal mixing, residual current, energy flux, dissipation, and momentum term balance dynamics are performed for M2 constituent, revealing complex M2 tidal characteristics in the study region and the important role of coastal geometry and topography in affecting regional tidal dynamics.

Comparison of summer thermohaline field and circulation structure of the Bohai Sea between 1958 and 2000

The analysis of observed salinity data over 35 years (1961–1996) at four stations around the Bohai Sea, i.e. Huludao, Qinhuangdao, Tanggu and Beihuangcheng, reveals that the salinity of the 4 observation stations has increased 1.1, 1.6, 1.9 and 0.4, respectively. The data also show that over the past 35 years, there have been at least 5 large salinity variation processes. The salinity data from two cruises of the Bohai Sea in August 1958 and 2000, show that the salinity pattern of the Bohai Sea has changed markedly. Low salinity in the sea surface layer around the old Yellow River mouth in August 1958 had been replaced by high salinity in August 2000 and the maximum variation of salinity is over 10.0. In addition, the values and distribution of salinity were almost the same from surface to bottom there in August 2000, but there existed significantly different salinity levels between the surface layer and the deep layer in August 1958. When a comparison is made between the salinity levels of the above-mentioned two years, it is found that the salinity in August 2000 is on average 2.0 higher than that of August 1958 in the main part of the Bohai Sea. The change of temperature and salinity field in the Bohai Sea leads to the change of the circulation. The numerical simulation shows that in comparison with the circulation structure of the Bohai Sea in August 1958, the circulation in August 2000 changes markedly. The significant changes of circulation appeared in Bohai Bay, Laizhou Bay and in the middle of the Bohai Sea. The clockwise current loop outside of the Bohai Bay and counterclockwise current loop outside of the Laizhou Bay in August 1958 disappeared in August 2000, and the counterclockwise current loop of the Bohai Bay migrated obviously outward. The flow direction in the Laizhou Bay turned 180° around. Corresponding to the variation of the Bohai Sea circulation, the amount of water exchange between the Bohai Sea and the Yellow Sea has also changed. The water exchange rate through the Bohai Strait decreases, on average, 0.7 ×104 m3/s in August 2000 in contrast to that of August 1958.

The assessment of extactable tidal energy and the effect of tidal energy turbine deployment on the hydrodynamics in Zhoushan

In this study, we construct one 2-dimensional tidal simulation, using an unstructured Finite Volume Coastal Ocean Model (FVCOM). In the 2-D model, we simulated the tidal turbines through adding additional bottom drag in the element where the tidal turbines reside. The additional bottom drag was calculated from the relationship of the bottom friction dissipation and the rated rotor efficiency of the tidal energy turbine. This study analyzed the effect of the tidal energy turbine to the hydrodynamic environment, and calculated the amount of the extractable tidal energy resource at the Guishan Hangmen Channel, considering the rotor wake effect.

Climate projections of spatial variations in coastal storm surges along the Gulf of Mexico and U.S. east coast

Using statistically downscaled atmospheric forcing, we performed a numerical investigation to evaluate future climate’s impact on storm surges along the Gulf of Mexico and U.S. east coast. The focus is on the impact of climatic changes in wind pattern and surface pressure while neglecting sea level rise and other factors. We adapted the regional ocean model system (ROMS) to the study region with a mesh grid size of 7–10 km in horizontal and 18 vertical layers. The model was validated by a hindcast of the coastal sea levels in the winter of 2008. Model’s robustness was confirmed by the good agreement between model-simulated and observed sea levels at 37 tidal gages. Two 10-year forecasts, one for the IPCC Pre-Industry (PI) and the other for the A1FI scenario, were conducted. The differences in model-simulated surge heights under the two climate scenarios were analyzed. We identified three types of responses in extreme surge heights to future climate: a clear decrease in Middle Atlantic Bight, an increase in the western Gulf of Mexico, and non-significant response for the remaining area. Such spatial pattern is also consistent with previous projections of sea surface winds and ocean wave heights.

A modeling study of the characteristics and mechanism of the westward coastal current during summer in the northwestern South China Sea

The characteristics and dynamical mechanism of summer-time coastal current over the North South China Sea shelf have been investigated based on a high resolution unstructuredgrid finite volume community ocean model (FVCOM). Modeldata comparison demonstrates that the model describes and explains well the coastal dynamics over the North South China Sea shelf. The coastal current on the North South China Sea shelf is greatly influenced by monsoon and the freshwater discharge of the Pearl River. Strong southwesterly wind drives the coastal current northeastward. However, under weak southwest monsoon, the coastal current west of the Pearl River estuary (PRE) advects toward the southwest, and splits into two parts when reaching east of the Qiongzhou Strait, with one branch entering the Gulf of Tonkin through the Qiongzhou Strait, transporting low salinity water into the Gulf of Tonkin, and the other part flows cyclonic and interacts with the northeastward current around southeast of Hainan Island, forming a cyclonic eddy east of the Qiongzhou Strait. A variety of model experiments focused on freshwater discharge, wind forcing, tidal rectification, and stratification are performed to study the physical mechanism of the southwestward coastal current which is usually against the summer wind. Process-oriented experiment results indicate that the southwest monsoon and freshwater discharge are important factors influencing the formation of southwestward coastal current during summer.

A numerical study on the circulation and tide in a zigzag bay

The Shacheng Bay (SCB) is one of the most complex coastal bays in southeast China and due to the fact of complicated geometry and dynamic coastal processes, it is considered as a challenging area for the numerical simulation of its hydrodynamic characteristics. The most advanced finite volume ocean model, finite-volume coastal ocean model (FVCOM), has adopted to simulate this hydrodynamic system, where tidal currents, tidal residual current and dye diffusion processes were studied and analyzed quantitatively. The validation of this numerical model matches well with various observation data, including elevation and current data. The misfit of a tidal elevation has a relative standard error of 3.66% and 4.67% for M2 and S2 tide components. The current validation shows a good match with an average error of 10 cm/s and 8° in the speed major axis and its direction respectively between the simulation and the measurement. This proves the robustness and reliability of this model. It is also found that the cape effect is significant and important in this system. The dye diffusion simulations show a 53 d flushing period for the whole inner bay waterbody. The results are of its first kind for understanding the hydrodynamic system in the SCB and they can provide helpful and trustful scientific information for others.

An effective method for improving the accuracy of Argo objective analysis

Based on the optimal interpolation objective analysis of the Argo data, improvements are made to the empirical formula of a background error covariance matrix widely used in data assimilation and objective analysis systems. Specifically, an estimation of correlation scales that can improve effectively the accuracy of Argo objective analysis has been developed. This method can automatically adapt to the gradient change of a variable and is referred to as “gradient-dependent correlation scale method”. Its effect on the Argo objective analysis is verified theoretically with Gaussian pulse and spectrum analysis. The results of one-dimensional simulation experiment show that the gradient-dependent correlation scales can improve the adaptability of the objective analysis system, making it possible for the analysis scheme to fully absorb the shortwave information of observation in areas with larger oceanographic gradients. The new scheme is applied to the Argo data objective analysis system in the Pacific Ocean. The results are obviously improved.

Statistical downscaling of IPCC sea surface wind and wind energy predictions for U.S. east coastal ocean, Gulf of Mexico and Caribbean Sea

A multivariate statistical downscaling method is developed to produce regional, high-resolution, coastal surface wind fields based on the IPCC global model predictions for the U.S. east coastal ocean, the Gulf of Mexico (GOM), and the Caribbean Sea. The statistical relationship is built upon linear regressions between the empirical orthogonal function (EOF) spaces of a cross- calibrated, multi-platform, multi-instrument ocean surface wind velocity dataset (predictand) and the global NCEP wind reanalysis (predictor) over a 10 year period from 2000 to 2009. The statistical relationship is validated before applications and its effectiveness is confirmed by the good agreement between downscaled wind fields based on the NCEP reanalysis and in-situ surface wind measured at 16 National Data Buoy Center (NDBC) buoys in the U.S. east coastal ocean and the GOM during 1992–1999. The predictand-predictor relationship is applied to IPCC GFDL model output (2.0°×2.5°) of downscaled coastal wind at 0.25°×0.25° resolution. The temporal and spatial variability of future predicted wind speeds and wind energy potential over the study region are further quantified. It is shown that wind speed and power would significantly be reduced in the high CO2 climate scenario offshore of the mid-Atlantic and northeast U.S., with the speed falling to one quarter of its original value.

Barotropic current fluctuations coupled with sea level drawdown in Yellow and Bohai Seas

Sub-tidal barotropic current variations coupled with residual sea level fluctuation in the Bohai and Yellow Seas during wintertime are addressed in this study. The temporal evolution and spatial distribution of current fluctuation are investigated using moored acoustic Doppler current profiler data in a three-dimensional numerical model. It is found that a southward current followed by a northward current occurred in the northern Yellow Sea during the fluctuation, concurrent with a significant outflow followed by inflow through the Bohai Strait. The process is consistent from surface to bottom and is coupled with remarkable residual sea level fluctuation. This quasi three-day fluctuation with amplitude 0.2–0.3 m/s leads to 1 m/1.2 m drawdown in the northern Yellow and Bohai Seas, respectively, strongly influencing water exchange between those seas. Because this a prominent feature in the seas, it is necessary to evaluate its effect on fluctuation during winter in future studies, in particular, the northward current during the recovery phase of sea level in the Bohai and Yellow Seas regarding seasonal variation.

Effects of Wave–Current Interactions on Suspended-Sediment Dynamics during Strong Wave Events in Jiaozhou Bay, Qingdao, China

AbstractWave–current interactions are crucial to suspended-sediment dynamics, but the roles of the associated physical mechanisms, the depth-dependent wave radiation stress, Stokes drift velocity, ...