2013 Vol. 32, No. 7
Display Method:
2013, 32(7): 1-7.
doi: 10.1007/s13131-013-0325-1
Abstract:
In situ observations, satellite data, and the output from an eddy-resolving ocean circulation model were used to study the generations and propagations of an anticyclonic eddy in the northern South China Sea (NSCS) during the winter of 2009-2010. In the NSCS, the anticyclonic eddy firstly appeared to the west of the Luzon Strait,migrated generally along the continental slope and dissipated around the Xisha Archipelago. The evolution of the warmeddy contains three phases: development, maturation, and decay. The eddy mainly stayed near 119.7°E in December and then gradually moved to 118.7°E until January 15, when its intensity, as indicated by the thermocline temperature and salinity anomalies, increased significantly, reflecting the growth of the eddy. The eddy reached its peak on January 15 and persisted until February 23. During this period, the eddy propagated westward to 116.4°E. After, the warm eddy weakened significantly and dissipated finally near the Xisha Archipelago.
In situ observations, satellite data, and the output from an eddy-resolving ocean circulation model were used to study the generations and propagations of an anticyclonic eddy in the northern South China Sea (NSCS) during the winter of 2009-2010. In the NSCS, the anticyclonic eddy firstly appeared to the west of the Luzon Strait,migrated generally along the continental slope and dissipated around the Xisha Archipelago. The evolution of the warmeddy contains three phases: development, maturation, and decay. The eddy mainly stayed near 119.7°E in December and then gradually moved to 118.7°E until January 15, when its intensity, as indicated by the thermocline temperature and salinity anomalies, increased significantly, reflecting the growth of the eddy. The eddy reached its peak on January 15 and persisted until February 23. During this period, the eddy propagated westward to 116.4°E. After, the warm eddy weakened significantly and dissipated finally near the Xisha Archipelago.
2013, 32(7): 8-13.
doi: 10.1007/s13131-013-0326-0
Abstract:
This study uses a large eddy simulation (LES) model to investigate the turbulence processes in the ocean surface boundary layer at Zhangzi Island offshore. Field measurements at Zhangzi Island (39°N, 122°E) during July 2009 are used to drive the LESmodel. The LES results capture a clear diurnal cycle in the oceanic turbulence boundary layer. The process of the heat penetration and heat distribution characteristics are analyzed through the heat flux results from the LES and their differences between two diurnal cycles are discussed as well. Energy balance and other dynamics are investigated which show that the tide-induced shear production is the main source of the turbulence energy that balanced dissipation. Momentum flux near the surface shows better agreement with atmospheric data computed by the eddy correlationmethod than those computed by bulk formula.
This study uses a large eddy simulation (LES) model to investigate the turbulence processes in the ocean surface boundary layer at Zhangzi Island offshore. Field measurements at Zhangzi Island (39°N, 122°E) during July 2009 are used to drive the LESmodel. The LES results capture a clear diurnal cycle in the oceanic turbulence boundary layer. The process of the heat penetration and heat distribution characteristics are analyzed through the heat flux results from the LES and their differences between two diurnal cycles are discussed as well. Energy balance and other dynamics are investigated which show that the tide-induced shear production is the main source of the turbulence energy that balanced dissipation. Momentum flux near the surface shows better agreement with atmospheric data computed by the eddy correlationmethod than those computed by bulk formula.
2013, 32(7): 14-20.
doi: 10.1007/s13131-013-0327-z
Abstract:
The simulation of an ocean general circulation model for the earth simulator (OFES) is transformed to an isopycnal coordinate to investigate the spatial structure and seasonal variability of the Mindanao Undercurrent (MUC). The results show that (1) potential density surfaces, σθ=26.5 and σθ=27.5, can be chosen to encompass theMUC layer. Southern Pacific tropicalwater (SPTW), Antarctic IntermediateWater (AAIW) and high potential density water (HPDW) constitute the MUC. (2) Climatologically, the MUC exists in the formof dual-core. In somemonths, the dual-core structure changes to a single-core structure. (3) Choosing section at 8°N for calculating the transport of the MUC transport is reliable. Potential density constraint provides a good method for calculating the transport of the MUC. (4) The annual mean transport of the MUC is 8.34×106 m3/s and varies considerably with seasons: stronger in late spring and weaker in winter.
The simulation of an ocean general circulation model for the earth simulator (OFES) is transformed to an isopycnal coordinate to investigate the spatial structure and seasonal variability of the Mindanao Undercurrent (MUC). The results show that (1) potential density surfaces, σθ=26.5 and σθ=27.5, can be chosen to encompass theMUC layer. Southern Pacific tropicalwater (SPTW), Antarctic IntermediateWater (AAIW) and high potential density water (HPDW) constitute the MUC. (2) Climatologically, the MUC exists in the formof dual-core. In somemonths, the dual-core structure changes to a single-core structure. (3) Choosing section at 8°N for calculating the transport of the MUC transport is reliable. Potential density constraint provides a good method for calculating the transport of the MUC. (4) The annual mean transport of the MUC is 8.34×106 m3/s and varies considerably with seasons: stronger in late spring and weaker in winter.
2013, 32(7): 21-28.
doi: 10.1007/s13131-013-0328-y
Abstract:
Surface currentsmeasured by high frequency (HF) radar arrays are assimilated into a regional oceanmodel over Qingdao coastal waters based on Kalman filter method. A series of numerical experiments are performed to evaluate the performance of the data assimilation schemes. In order to optimize the analysis procedure in the traditional ensemble Kalman filter (ENKF), a different analysis scheme called quasiensemble Kaman filter (QENKF) is proposed. The comparisons between the ENKF and the QENKF suggest that both them can improve the simulated error and the spatial structure. The estimations of the background error covariance (BEC) are also assessed by comparing three different methods: Monte Carlo method; Canadian quick covariance (CQC) method and data uncertainty engine (DUE) method. A significant reduction of the root-mean-square (RMS) errors between model results and the observations shows that the CQC method is able to better reproduce the error statistics for this coastal ocean model and the corresponding external forcing. In addition, the sensibility of the data assimilation system to the ensemble size is also analyzed by means of different scales of the ensemble size used in the experiments. It is found that given the balance of the computational cost and the forecasting accuracy, the ensemble size of 50 will be an appropriate choice in the Qingdao coastal waters.
Surface currentsmeasured by high frequency (HF) radar arrays are assimilated into a regional oceanmodel over Qingdao coastal waters based on Kalman filter method. A series of numerical experiments are performed to evaluate the performance of the data assimilation schemes. In order to optimize the analysis procedure in the traditional ensemble Kalman filter (ENKF), a different analysis scheme called quasiensemble Kaman filter (QENKF) is proposed. The comparisons between the ENKF and the QENKF suggest that both them can improve the simulated error and the spatial structure. The estimations of the background error covariance (BEC) are also assessed by comparing three different methods: Monte Carlo method; Canadian quick covariance (CQC) method and data uncertainty engine (DUE) method. A significant reduction of the root-mean-square (RMS) errors between model results and the observations shows that the CQC method is able to better reproduce the error statistics for this coastal ocean model and the corresponding external forcing. In addition, the sensibility of the data assimilation system to the ensemble size is also analyzed by means of different scales of the ensemble size used in the experiments. It is found that given the balance of the computational cost and the forecasting accuracy, the ensemble size of 50 will be an appropriate choice in the Qingdao coastal waters.
2013, 32(7): 29-37.
doi: 10.1007/s13131-013-0329-x
Abstract:
Using in situmeasurement data fromMay-June, 1998, and data fromthe Asian seas international acoustics experiment (ASIAEX) from2001 in the South China Sea (SCS), the spectral density function and the dissipation spectrum function are estimated. In the infra-gravity wave (IGW) band, the power spectra of velocity (u, v, w) are universal functionswith respect to characteristic frequencies, which correspond to the peak frequencies of the dissipation spectrum (PFDS). This suggests that high-frequency internal waves in the IGW band have similar dynamical characteristics. In addition, the evolution of these characteristic frequencies is explored and its highest value is 8.8 cph (cycles per hour, 1 cph=2.778×10-3 Hz).
Using in situmeasurement data fromMay-June, 1998, and data fromthe Asian seas international acoustics experiment (ASIAEX) from2001 in the South China Sea (SCS), the spectral density function and the dissipation spectrum function are estimated. In the infra-gravity wave (IGW) band, the power spectra of velocity (u, v, w) are universal functionswith respect to characteristic frequencies, which correspond to the peak frequencies of the dissipation spectrum (PFDS). This suggests that high-frequency internal waves in the IGW band have similar dynamical characteristics. In addition, the evolution of these characteristic frequencies is explored and its highest value is 8.8 cph (cycles per hour, 1 cph=2.778×10-3 Hz).
2013, 32(7): 38-46.
doi: 10.1007/s13131-013-0330-4
Abstract:
A vertical two-dimensional numerical model has been applied to solving the Reynolds Averaged Navier- Stokes (RANS) equations in the simulation of current and wave propagation through vegetated and nonvegetated waters. The k-ε model is used for turbulence closure of RANS equations. The effect of vegetation is simulated by adding the drag force of vegetation in the flow momentum equations and turbulence model. To solve the modified N-S equations, the finite difference method is used with the staggered grid system to solver equations. The Youngs’ fractional volume of fluid (VOF) is applied tracking the free surface with second-order accuracy. The model has been tested by simulating dam break wave, pure current with vegetation, solitary wave runup on vegetated and non-vegetated channel, regular and random waves over a vegetated field. Themodel reasonably well reproduces these experimental observations, themodeling approach presented herein should be useful in simulating nearshore processes in coastal domains with vegetation effects.
A vertical two-dimensional numerical model has been applied to solving the Reynolds Averaged Navier- Stokes (RANS) equations in the simulation of current and wave propagation through vegetated and nonvegetated waters. The k-ε model is used for turbulence closure of RANS equations. The effect of vegetation is simulated by adding the drag force of vegetation in the flow momentum equations and turbulence model. To solve the modified N-S equations, the finite difference method is used with the staggered grid system to solver equations. The Youngs’ fractional volume of fluid (VOF) is applied tracking the free surface with second-order accuracy. The model has been tested by simulating dam break wave, pure current with vegetation, solitary wave runup on vegetated and non-vegetated channel, regular and random waves over a vegetated field. Themodel reasonably well reproduces these experimental observations, themodeling approach presented herein should be useful in simulating nearshore processes in coastal domains with vegetation effects.
2013, 32(7): 47-56.
doi: 10.1007/s13131-013-0331-3
Abstract:
The existing high-resolution hydrographic data in the western tropical Pacific Ocean are used to explore the spatial distribution and primary characteristics of thermohaline intrusions in the thermocline. Statistics show that the vertical scales of intrusions are 20-40 min the upper thermocline (22.0-26.0σ0) and 40-80 m in the lower thermocline (26.0-27.2σ0). In the upper thermocline, themost intensive intrusions exist at the equatorial front (EF) where north/south Pacific water masses converge, and the westward spreading of the north Pacific tropical water (NPTW) in the Philippines Sea also produces patches of intrusions surrounding its high-salinity tongue. In the lower thermocline, intrusions are also strong at the tropical front (TF) which is the boundary between the north Pacific subtropical/tropical waters. At the bottom of the thermocline (at about 27.0σ0), intrusionsmainly exist near the western boundary, which are produced by intermediate water convergence through the advection of subthermocline western boundary flows. Most strikingly a “C”-shape distribution of intrusions at around 26.4σ0 is revealed, covering the vicinity of the EF, the TF, and theMindanao Current (MC), i.e., the western boundary pathway of the north Pacific subtropical cell (STC). Synoptic section analysis reveals that intrusions aremore prominent on the warm/salty flank of the fronts, implying more cross-front tongues of cold/fresh water. Among the intrusions, those at the EF are of best lateral coherence which implies a unique drivingmechanism involving near-inertial velocity perturbations near the equator.
The existing high-resolution hydrographic data in the western tropical Pacific Ocean are used to explore the spatial distribution and primary characteristics of thermohaline intrusions in the thermocline. Statistics show that the vertical scales of intrusions are 20-40 min the upper thermocline (22.0-26.0σ0) and 40-80 m in the lower thermocline (26.0-27.2σ0). In the upper thermocline, themost intensive intrusions exist at the equatorial front (EF) where north/south Pacific water masses converge, and the westward spreading of the north Pacific tropical water (NPTW) in the Philippines Sea also produces patches of intrusions surrounding its high-salinity tongue. In the lower thermocline, intrusions are also strong at the tropical front (TF) which is the boundary between the north Pacific subtropical/tropical waters. At the bottom of the thermocline (at about 27.0σ0), intrusionsmainly exist near the western boundary, which are produced by intermediate water convergence through the advection of subthermocline western boundary flows. Most strikingly a “C”-shape distribution of intrusions at around 26.4σ0 is revealed, covering the vicinity of the EF, the TF, and theMindanao Current (MC), i.e., the western boundary pathway of the north Pacific subtropical cell (STC). Synoptic section analysis reveals that intrusions aremore prominent on the warm/salty flank of the fronts, implying more cross-front tongues of cold/fresh water. Among the intrusions, those at the EF are of best lateral coherence which implies a unique drivingmechanism involving near-inertial velocity perturbations near the equator.
2013, 32(7): 57-65.
doi: 10.1007/s13131-013-0332-2
Abstract:
The impact of assimilating Argo data into an initial field on the short-termforecasting accuracy of temperature and salinity is quantitatively estimated by using a forecasting system of the western North Pacific, on the base of the Princeton ocean model with a generalized coordinate system (POMgcs). This system uses a sequential multigrid three-dimensional variational (3DVAR) analysis scheme to assimilate observation data. Two numerical experimentswere conductedwith andwithout Argo temperature and salinity profile data besides conventional temperature and salinity profile data and sea surface height anomaly (SSHa) and sea surface temperature (SST) in the process of assimilating data into the initial fields. The forecast errors are estimated by using independent temperature and salinity profiles during the forecasting period, including the verticaldistributions of the horizontally averaged rootmean square errors (H-RMSEs) and the horizontal distributions of the vertically averaged mean errors (MEs) and the temporal variation of spatially averaged root mean square errors (S-RMSEs). Comparison between the two experiments shows that the assimilation of Argo data significantly improves the forecast accuracy, with 24% reduction of H-RMSE maximum for the temperature, and the salinity forecasts are improved more obviously, averagely dropping of 50% for H-RMSEs in depth shallower than 300 m. Such improvement is caused by relatively uniform sampling of both temperature and salinity fromthe Argo drifters in time and space.
The impact of assimilating Argo data into an initial field on the short-termforecasting accuracy of temperature and salinity is quantitatively estimated by using a forecasting system of the western North Pacific, on the base of the Princeton ocean model with a generalized coordinate system (POMgcs). This system uses a sequential multigrid three-dimensional variational (3DVAR) analysis scheme to assimilate observation data. Two numerical experimentswere conductedwith andwithout Argo temperature and salinity profile data besides conventional temperature and salinity profile data and sea surface height anomaly (SSHa) and sea surface temperature (SST) in the process of assimilating data into the initial fields. The forecast errors are estimated by using independent temperature and salinity profiles during the forecasting period, including the verticaldistributions of the horizontally averaged rootmean square errors (H-RMSEs) and the horizontal distributions of the vertically averaged mean errors (MEs) and the temporal variation of spatially averaged root mean square errors (S-RMSEs). Comparison between the two experiments shows that the assimilation of Argo data significantly improves the forecast accuracy, with 24% reduction of H-RMSE maximum for the temperature, and the salinity forecasts are improved more obviously, averagely dropping of 50% for H-RMSEs in depth shallower than 300 m. Such improvement is caused by relatively uniform sampling of both temperature and salinity fromthe Argo drifters in time and space.
2013, 32(7): 66-77.
doi: 10.1007/s13131-013-0333-1
Abstract:
Based on the optimal interpolation objective analysis of the Argo data, improvements are made to the empirical formula of a background error covariancematrixwidely 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. Thismethod can automatically adapt to the gradient change of a variable and is referred to as “gradient-dependent correlation scalemethod”. Its effect on the Argo objective analysis is verified theoretically with Gaussian pulse and spectrumanalysis. 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 systemin the Pacific Ocean. The results are obviously improved.
Based on the optimal interpolation objective analysis of the Argo data, improvements are made to the empirical formula of a background error covariancematrixwidely 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. Thismethod can automatically adapt to the gradient change of a variable and is referred to as “gradient-dependent correlation scalemethod”. Its effect on the Argo objective analysis is verified theoretically with Gaussian pulse and spectrumanalysis. 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 systemin the Pacific Ocean. The results are obviously improved.
2013, 32(7): 78-87.
doi: 10.1007/s13131-013-0334-0
Abstract:
Using data from the European remote sensing scatterometer (ERS-2) from July 1997 to August 1998, global distributions of the air-sea CO2 transfer velocity and flux are retrieved. A new model of the air-sea CO2 transfer velocity with surface wind speed and wave steepness is proposed. The wave steepness (δ) is retrieved using a neural network (NN) model fromERS-2 scatterometer data, while the wind speed is directly derived by the ERS-2 scatterometer. The new model agrees well with the formulations based on the wind speed and the variation in the wind speed dependent relationships presented inmany previous studies can be explained by this proposed relationwith variationinwave steepness effect. Seasonally globalmaps of gas transfer velocity and flux are shown on the basis of the newmodel and the seasonal variations of the transfer velocity and flux during the 1 a period. The globalmean gas transfer velocity is 30 cm/h after area-weighting and Schmidt number correction and its accuracy remains calculation with in situ data. The highest transfer velocity occurs around 60°N and 60°S, while the lowest on the equator. The total air to sea CO2 flux (calculated by carbon) in that year is 1.77 Pg. The strongest source of CO2 is in the equatorial east Pacific Ocean, while the strongest sink is in the 68°N. Full exploration of the uncertainty of this estimate awaits further data. An effectual method is provided to calculate the effect of waves on the determination of air-sea CO2 transfer velocity and fluxes with ERS-2 scatterometer data.
Using data from the European remote sensing scatterometer (ERS-2) from July 1997 to August 1998, global distributions of the air-sea CO2 transfer velocity and flux are retrieved. A new model of the air-sea CO2 transfer velocity with surface wind speed and wave steepness is proposed. The wave steepness (δ) is retrieved using a neural network (NN) model fromERS-2 scatterometer data, while the wind speed is directly derived by the ERS-2 scatterometer. The new model agrees well with the formulations based on the wind speed and the variation in the wind speed dependent relationships presented inmany previous studies can be explained by this proposed relationwith variationinwave steepness effect. Seasonally globalmaps of gas transfer velocity and flux are shown on the basis of the newmodel and the seasonal variations of the transfer velocity and flux during the 1 a period. The globalmean gas transfer velocity is 30 cm/h after area-weighting and Schmidt number correction and its accuracy remains calculation with in situ data. The highest transfer velocity occurs around 60°N and 60°S, while the lowest on the equator. The total air to sea CO2 flux (calculated by carbon) in that year is 1.77 Pg. The strongest source of CO2 is in the equatorial east Pacific Ocean, while the strongest sink is in the 68°N. Full exploration of the uncertainty of this estimate awaits further data. An effectual method is provided to calculate the effect of waves on the determination of air-sea CO2 transfer velocity and fluxes with ERS-2 scatterometer data.
