Showing papers by "Quanan Zheng published in 2022"

An overview on water masses in the China seas

Abstract: This paper overviews progress in the research of water mass classification in the China seas, consisting of the Bohai Sea, the Yellow Sea, the East China Sea, the Taiwan Strait and the South China Sea. So far, oceanographers have achieved systematic research results in terms of the concept and definition, theory and method, characteristics and variation of water masses in the China seas. Here 3-D visualized diagrams are developed to illustrate spatial distributions of classified water masses in the China seas. Meanwhile, core values or ranges of temperature and salinity for individual water mass are integrated. Seasonal variations and formation mechanisms of water masses are briefly analyzed.

Numerical simulations of internal solitary wave evolution beneath an ice keel

Abstract: The deformation and evolution of internal solitary waves (ISWs) beneath an ice keel can enable potential diapycnal mixing and facilitate upper ocean heat transport, despite a poor understanding of the underlying physics and energetics of ISWs in Polar environments. This study aims to understand the dynamic processes and mixing properties during the evolution of ISWs beneath ice keels in the Arctic Ocean using high-resolution, non-hydrostatic simulations. Ice keels can destabilize ISWs through overturning events. Consequently, the initial ISW disintegrates and transfers its energy into secondary smaller-scale waves. During the ISW-ice interaction, ISW-induced turbulent mixing can reach O(10−3) W/kg with a magnitude of resultant heat flux of O(10)W/m. Sensitivity experiments demonstrated that the ISW-ice interaction weakened as the ice keel depth decreased, and consequently, the resultant turbulent mixing and upward heat transfer also decreased. The ice keel depth was critical to the evolution and disintegration of an ISW beneath the ice keel, while the approximate ice keel shape had little effect. Our results provide an important but previously overlooked energy source for upper ocean heat transport in the Arctic Ocean.

Variations in Flow Patterns in the Northern Taiwan Strait Observed by Satellite-Tracked Drifters

Abstract: This study investigates the variations in flow patterns in the northern Taiwan Strait in summer using high-frequency (HF) radar measurements, satellite-tracked drifter trajectories and numerical models. There is an obvious interaction between intra-diurnal tides and ocean currents in northwestern Taiwan. When the tide changes between high tide and low tide, the change in direction of the nearshore flow occurs before the change in the offshore flow. Drifter trajectories show that there are three different drifting paths in the Taiwan Strait in summer. One path is along the west coast of Taiwan from the southwest coast to the northeast coast. Another path is the same as the first one but leads northward to the East China Sea instead of eastward to the northeast coast of Taiwan. The other path exists along the west coast of Taiwan, some distance out, after being deflected by the bottom ridge. The regional ocean modeling system model was used in this study to clarify the influencing factors that lead to these three paths. The results of multiple simulations and HF radar data indicate that the bifurcation of the first two drift paths in northwestern Taiwan is caused by ebb and flood tide transitions. The different routes of the latter two paths are due to the significant speed difference between the nearshore current and the offshore current approximately 45 km from the coast.

Statistical Analysis of Mesoscale Eddies Entering the Continental Shelf of the Northern South China Sea

Abstract: An Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO) mesoscale eddy trajectory atlas product is used to analyze the path type and temporal variability of the eddies that entered the continental shelf area of the northern South China Sea (SCS) from 1993 to 2016. A total of 184 mesoscale eddies entered the continental shelf area of the northern SCS during a 24-year period. We classify the mesoscale eddies into four types according to the motion trajectories: along-the-isobath type, intrusion-of-continental-shelf type, local wandering type, and shelf-internal-generation type. The occurrence numbers of these four types were 87, 38, 23, and 36, respectively. The mean amplitude and radius of the along-the-isobath type are the largest, about 18 cm and 153 km, respectively; furthermore, their average lifetime is also the longest, about 93 days. The mean amplitude, radius, and lifetime are the smallest for the shelf-internal-generation type, about 16 cm, 146 km, and 74 days, respectively. The direction and velocity of the background flow field affects the intrusion path of the mesoscale eddies onto the continental shelf of the northern SCS. The seasonal distribution of the mesoscale eddies quantity is also related to the direction and velocity of the corresponding background flow field.

Coherence of Eddy Kinetic Energy Variation during Eddy Life Span to Low-Frequency Ageostrophic Energy

Abstract: The evolution of mesoscale eddies is crucial for understanding the ocean energy cascade. In this study, using global reanalysis sea surface velocity data and a mesoscale eddy trajectory product tracked by satellite altimeters, we aimed to reveal the coherence of eddy kinetic energy (EKE) variation to low-frequency ageostrophic energy during the eddy life span. The variation in EKE throughout the eddy life span was highly coherent to that of the seven-day low-passed ageostrophic kinetic energy, with a correlation coefficient of −0.94. The low-frequency ageostrophic motions supplied 38% of the EKE variation in the growing stage of mesoscale eddies and absorbed 42% in the decaying stage. The evolution rate of the EKE during the eddy life span was consistent with the barotropic conversion rate of the low-frequency ageostrophic motions, further confirming the dominant role of low-frequency ageostrophic motions in eddy growth and decay.

Development of a sea-sediment coupled model incorporating ocean bottom heat flux

Abstract: Previous in-situ observations have suggested that bottom water temperature variations in shelf seas can drive significant ocean bottom heat flux (BHF) by heat conduction. The BHF-driven bottom water temperature variations, however, have been overlooked in ocean general circulation models. In this study, we established a sea-sediment fully coupled model through incorporating the BHF. The coupled model included a sediment temperature module/model, and the BHF was calculated based on the sediment heat content variations. Meanwhile, we applied temporally varying BHF in the calculation of the bottom water temperature, which further determined the sediment temperature. The two-way coupled BHF process presents a more complete and physically reasonable heat budget in the ocean model and a synchronously varying sediment temperature profile. The coupled model was validated using a one-dimensional test case, and then it was applied in a domain covering the Bohai and Yellow Seas. The results suggest that when a strong thermocline exists, the BHF can change the bottom water temperature by more than 1°C because the effects of the BHF are limited to within a shallow bottom layer. However, when the water column is well mixed, the BHF changes the temperature of the entire water column, and the heat transported across the bottom boundary is ventilated to the atmosphere. Thus, the BHF has less effect on water temperature and may directly affect air-sea heat flux. The sea-sediment interactions dampen the amplitude of the bottom water temperature variations, which we propose calling the seabed dampening ocean heat content variation mechanism (SDH).

Improved upper-ocean thermodynamical structure modeling with combined effects of surface waves and M2 internal tides on vertical mixing: a case study for the Indian Ocean

Abstract: Abstract. Surface waves and internal tides have a great contribution to vertical mixing processes in the upper ocean. In this study, three mixing schemes, including non-breaking surface-wave-generated turbulent mixing, mixing induced by the wave transport flux residue and internal-tide-generated turbulent mixing, are introduced to study the effects surface waves and internal tides on vertical mixing. The three schemes are jointly incorporated into the Marine Science and Numerical Modeling (MASNUM) ocean circulation model as a part of the vertical diffusive terms, which are calculated by the surface wave parameters simulated from the MASNUM wave model and the surface amplitudes of the mode-1 M2 internal tides extracted from satellite altimetry data using a two-dimensional plane wave fit method. The effects of the mixing schemes on Indian Ocean modeling are tested by five climatological experiments. The surface waves and internal tides enhance the vertical mixing processes in the sea surface and ocean interior, respectively. The combination of the mixing schemes is able to strengthen the vertical water exchange and draw more water from the sea surface to the ocean interior. The simulated results show significant improvement in the thermal structure, mixed layer depth and surface currents if the three schemes are all adopted.

Validation of the CFOSAT Scatterometer Data With Buoy Observations and Tests of Operational Application to Extreme Weather Forecasts in Taiwan Strait

Abstract: The China‐France Oceanography Satellite (CFOSAT) launched in 2018 is equipped with the Chinese Scatterometer (CSCAT), which is designed to measure high‐precision sea surface wind fields in the global ocean. This study aims to validate the CFOSAT wind fields with moored buoy‐measured ground truth data from August 2019 to July 2021. The test area was chosen as the Taiwan Strait, which is an important navigation channel and rich fishery grounds in the East Asia. It is also a high wind speed area particularly during typhoons and cold air passages. Thus, the accurate wind forecasts are crucially needed to avoid capsizing and casualties. The validation results give the correlation coefficient (R2) of 0.92, the root mean square error (RMSE) of 1.75 m s−1, and the absolute deviation (AD) of 1.10 m s−1 for the wind speeds, and R2 of 0.96, the RMSE of 21.87°, and the AD of 1.17° for the wind directions. During cold air passages, the AD of wind speeds decreases to 1.03 m s−1, which is smaller than 1.17 m s−1 during typhoons. Using the CFOSAT wind as historical observation data for refining of forecast work was tested by model output statistics revised method. The test results show that the refined forecast results of wind fields are improved up to 4%–17%. Thus, the validation and test results obtained in this study indicate that updating satellite winds would have great contributions to refined forecasting of local sea surface winds.

Modulation effects of mesoscale eddies on sea surface wave fields in the South China Sea derived from a wave spectrometer onboard the China‐France Ocean Satellite

Abstract: This study aims to examine the modulation effects of mesoscale eddies on sea surface wave fields in the South China Sea using data measured by a wave spectrometer named Surface Wave Investigation and Monitoring onboard the China-France Ocean Satellite from July 2019 to August 2021. The statistical results show that the significant wave heights (SWHs) decrease (increase) from inside to outside of warm (cold) eddies, while the wavelengths have minimum values at the eddy edge in all cases. In cases of strong warm eddies, the maximum variations in SWH, wavelength, and wave direction caused by mesoscale eddy modulation reach up to 18%, 24%, and 28.4°, respectively. The eddy modulation on wave parameters is stronger at low sea states. The correlation analysis reveals that the wave parameter variations across eddies result from the eddy current shear and the direction difference between the eddy current and the waves. The dynamic analysis indicates that the deformation term of the eddy current is a dominant term affecting the SWH at the eddy edge, and the eddy current divergence and its product with wave direction determine the wavelength variation. The eddy vorticity deflects the wave direction by approximately 2°–3°, and the wave-eddy angle and eddy-modulated wind variation also contribute to the wave direction variation. The background current-wave interaction is negligible in the eddy cases studied.

ocean circulation modeling with combined effects of surface waves and M2 internal tides on vertical mixing: a case study for the Indian Ocean”

Abstract: . The surface waves and internal tides have great contribution to the vertical mixing processes in the upper ocean. In this study, three mixing schemes, including the non-breaking surface-wave-generated turbulent mixing, the mixing induced by the wave transport flux residue, and the internal-tide-generated turbulent mixing, are introduced to study the effects the surface waves and the internal tides on the vertical mixing. The three schemes are jointly incorporated into the Marine Science 20 and Numerical Modeling (MASNUM) ocean circulation model as a part of the vertical diffusive terms, which are calculated by the surface wave parameters simulated from the MASNUM wave model and the surface amplitudes of the mode-1 M 2 internal tides extracted from the satellite altimetry data using a two-dimensional plane wave fit method. The effects of the mixing schemes on the Indian Ocean modeling are tested by five climatological experiments. The surface waves and internal tides lead to enhance the vertical mixing processes in the sea surface and ocean