Search Results

You are looking at 1 - 10 of 14 items for :

  • Author or Editor: Yao Yao x
  • Journal of Physical Oceanography x
  • Refine by Access: All Content x
Clear All Modify Search
Aifeng Yao and Chin H. Wu

Abstract

Energy dissipation for unsteady deep-water breaking in wave groups on following and opposing currents, including partial wave-blocking conditions, was investigated by detailed laboratory measurements. A range of focusing wave conditions, including current strengths, wave spectrum slopes, and breaking intensities, were examined. Observations show that weak following and opposing currents do not alter the limiting wave steepness. The kinematics of unsteady breaking can be characterized as the one without currents simply by the Doppler shift. In contrast, strong opposing currents can cause partial wave blockings that narrow the spectral frequency bandwidth and increase the mean spectral slope. Dependence of the significant spectral peak steepness on the spectral bandwidth parameter was identified, confirming threshold behavior of breaking inception of nonlinear wave group dynamics. Loss of excessive energy fluxes due to breaking was found to depend strongly on the mean spectral slope. Wave groups of a steeper spectral slope yield fewer energy losses. In addition, the spectral distribution of energy dissipation due to breaking has the following two main characteristics: (a) significant energy dissipation occurred at frequency components that were higher than the spectral peak frequency, and little energy change at the peak frequency was found; (b) below the spectral peak frequency a small energy gain was observed. The energy-gain-to-loss ratio varies with the spectral bandwidth parameter. Higher gain– loss ratios (up to 40%) were observed for breakers on strong opposing currents under the partial wave-blocking condition. Comparison and assessment of proposed and existing parameterizations for breaking-wave energy dissipation were made using the measured data. The new proposed form provides the features for addressing these two main spectral energy distribution characteristics due to breaking with and without currents.

Full access
Neng-Chun Yao, S. Neshyba, and H. Crew

Abstract

Application of rotary cross-bispectra and energy transfer functions to a set of wind and current data measured from the Totem research buoy shows these elements to be non-Gaussian and that nonlinear interactions do occur between the wind stress and current at 14 m depth. Such transfers account for 35% of the total energy estimates and for 100% of the estimates at the inertial frequency. For the latter, the most effective set of nonlinear interacting components include half-day and quarter-day components of wind stress. No energy transfers linearly. The effect of nonlinear interaction in broadening the current response spectrum is noted.

Full access
Neng-Chun Yao, Steve Neshyba, and Henry Crew

Abstract

Bispectrum and cross-bispectrum analyses of the rotary components of stationary random vector processes are more easily interpreted than similar analyses of their scalar components, and have the advantage that the bispectral estimates are invariant to coordinate rotation. Application to some wind-ocean current data shows these to be non-Gaussian and subject to significant nonlinear coupling over a wide range of interacting triplets of rotary components. A set of complex-valued energy transfer functions are developed by which the magnitudes of the linear and quadratic interactions may be compared.

Full access
T. Yao, Howard J. Freeland, and Lawrence A. Mysak

Abstract

Six current-meter mooring were deployed in a line approximately 600 km in length along the continental shelf of British Columbia. Analysis of the low frequency (periods exceeding a day) fluctuations in current for the winter 1981–82 period is discussed. Alongshore currents off Vancouver Island are mutually correlated with time lag less than a day. The region of mutual correlation does not extend north of Vancouver Island, across Queen Charlotte Sound. Coherence is observed between currents south and north of Queen Charlotte Sound only in a frequency band where there is mutual coherence with local wind. A comparison is made between observation and free coastal-trapped wave theory. Off northern Vancouver Island, where the shelf is narrower than off southern Vancouver Island, there is increased vertical shear, a feature of the second coastal-trapped wave mode. A consistency test is applied using the cross spectral matrix of alongshore components of velocity. In the dominant energy-containing frequency bands (periods ≳10 days), the structure of alongshore currents off Vancouver Island is consistent with the two lowest free coastal-trapped wave modes locked in phase.

Full access
Yao Xu, Hailun He, Jinbao Song, Yijun Hou, and Funing Li

Abstract

Buoy-based observations of surface waves during three typhoons in the South China Sea were used to obtain the wave characteristics. With the local wind speeds kept below 35 m s−1, the surface waves over an area with a radius 5 times that of the area in which the maximum sustained wind was found were mainly dominated by wind-wave components, and the wave energy distribution was consistent with fetch-limited waves. Swells dominated the surface waves at the front of and outside the central typhoon region. Next, the dynamics of the typhoon waves were studied numerically using a state-of-the-art third-generation wave model. Wind forcing errors made a negligible contribution to the surface wave results obtained using hindcasting. Near-realistic wind fields were constructed by correcting the idealized wind vortex using in situ observational data. If the different sets of source terms were further considered for the forcing stage of the typhoon, which was defined as the half inertial period before and after the typhoon arrival time, the best model performance had mean relative biases and root-mean-square errors of −0.7% and 0.76 m, respectively, for the significant wave height, and −3.4% and 1.115 s, respectively, for the peak wave period. Different sets of source terms for wind inputs and whitecapping breaking dissipation were also used and the results compared. Finally, twin numerical experiments were performed to investigate the importance of nonlinear wave–wave interactions on the spectrum formed. There was evidence that nonlinear wave–wave interactions efficiently transfer wave energy from high frequencies to low frequencies and prevent double-peak structures occurring in the frequency-based spectrum.

Full access
Gengxin Chen, Weiqing Han, Yuanlong Li, Jinglong Yao, and Dongxiao Wang

Abstract

By analyzing in situ observations and conducting a series of ocean general circulation model experiments, this study investigates the physical processes controlling intraseasonal variability (ISV) of the Equatorial Undercurrent (EUC) of the Indian Ocean. ISV of the EUC leads to time-varying water exchanges between the western and eastern equatorial Indian Ocean. For the 2001–14 period, standard deviations of the EUC transport variability are 1.92 and 1.77 Sv (1 Sv ≡ 106 m3 s−1) in the eastern and western basins, respectively. The ISV of the EUC is predominantly caused by the wind forcing effect of atmospheric intraseasonal oscillations (ISOs) but through dramatically different ocean dynamical processes in the eastern and western basins. The stronger ISV in the eastern basin is dominated by the reflected Rossby waves associated with intraseasonal equatorial zonal wind forcing. It takes 20–30 days to set up an intraseasonal EUC anomaly through the Kelvin and Rossby waves associated with the first and second baroclinic modes. In the western basin, the peak intraseasonal EUC anomaly is generated by the zonal pressure gradient force, which is set up by radiating equatorial Kelvin and Rossby waves induced by the equatorial wind stress. Directly forced and reflected Rossby waves from the eastern basin propagate westward, contributing to intraseasonal zonal current near the surface but having weak impact on the peak ISV of the EUC.

Full access
Qiang Wang, Lili Zeng, Jian Li, Ju Chen, Yunkai He, Jinglong Yao, Dongxiao Wang, and Weidong Zhou

Abstract

Cross-shelf flow induced by mesoscale eddies has been investigated in the northern South China Sea (NSCS) using velocity observations from Long Ranger ADCP moorings. Mesoscale eddies influenced the three mooring stations during almost all the observation period. Four quadrants have been defined with the mooring location as the origin, and it is found that warm (cold) mesoscale eddies induce onshore (offshore) movement in the eastern two quadrants and offshore (onshore) movement in the western two quadrants. When an eddy propagates past a mooring station, net cross-shelf flow at the mooring station can be induced by asymmetry in the horizontal and vertical structure of the eddy and by its evolution. As an eddy propagates westward, its shape changes continually and the vertical modes also transform from high to lower modes, which contributes to the net cross-shelf flow. Based on the quasigeostrophic potential vorticity equation, it is confirmed that the net cross-shelf flow is mainly induced by the eddy evolution and suppressed by nonlinear effect. Because of dispersion characteristics of the mesoscale eddy, barotropic mode will restructure at the baroclinic mode area after separating from the baroclinic mode, which will be enhanced by topography slope.

Full access
Hui Zhou, Hengchang Liu, Shuwen Tan, Wenlong Yang, Yao Li, Xueqi Liu, Qiang Ren, and William K. Dewar

Abstract

The structure and variations of the North Equatorial Countercurrent (NECC) in the far western Pacific Ocean during 2014–16 are investigated using repeated in situ hydrographic data, altimeter data, Argo data, and reanalysis data. The NECC shifted ~1° southward and intensified significantly with its transport exceeding 40 Sv (1 Sv ≡ 106 m3 s−1), nearly double its climatology value, during the developing phase of the 2015/16 El Niño event. Observations show that the 2015/16 El Niño exerted a comparable impact on the NECC with that of the extreme 1997/98 El Niño in the far western Pacific Ocean. Baroclinic instability provided the primary energy source for the eddy kinetic energy (EKE) in the 2015/16 El Niño, which differs from the traditional understanding of the energy source of EKE as barotropic instability in low-latitude ocean. The enhanced vertical shear and the reduced density jump between the NECC layer and the North Equatorial Subsurface Current (NESC) layer renders the NECC–NESC system baroclinically unstable in the western Pacific Ocean during El Niño developing phase. The eddy–mean flow interactions here are diverse associated with various states of El Niño–Southern Oscillation (ENSO).

Open access
Qiang Wang, Lili Zeng, Yeqiang Shu, Jian Li, Ju Chen, Yunkai He, Jinglong Yao, Dongxiao Wang, and Weidong Zhou

Abstract

Topographic Rossby waves (TRWs) are reported to make a significant contribution to the deep-ocean current variability. On the northern South China Sea (NSCS) continental slope, TRWs with peak spectral energy at ~14.5 days are observed over about a year at deep moorings aligned east–west around the Dongsha Islands. The TRWs with a group velocity of O(10) cm s−1 contribute more than 40% of total bottom velocity fluctuations at the two mooring stations. The energy propagation and source are further identified using a ray-tracing model. The TRW energy mainly propagates westward along the NSCS continental slope with a slight downslope component. The possible energy source is upper-ocean 10–20-day fluctuations on the east side of the Dongsha Islands, which are transferred through the first baroclinic mode (i.e., the second EOF mode). These 10–20-day fluctuations in the upper ocean are associated with mesoscale eddies. However, to the west of the Dongsha Islands, the 10–20-day fluctuations in the upper ocean are too weak to effectively generate TRWs locally. This work provides an interesting insight toward understanding the NSCS deep current variability and the linkage between the upper- and deep-ocean currents.

Full access
Yuhong Zhang, Yan Du, W. N. D. S Jayarathna, Qiwei Sun, Ying Zhang, Fengchao Yao, and Ming Feng

Abstract

A prolonged high-salinity event in the northern Arabian Sea, to the east of the Gulf of Oman, during 2014–17 was identified based on Argo datasets. The prolonged event was manifested as enhanced spreading of the surface Arabian Sea high-salinity water and the intermediate Persian Gulf water. We used satellite altimetric data and geostrophic current data to understand the oceanic processes and the salt budget associated with the high-salinity event. The results indicated that the strengthened high-salinity advection from the Gulf of Oman was one of the main causes of the salinity increase in the northern Arabian Sea. The changes of the seasonally dependent eddies near the mouth of the Gulf of Oman dominated the strengthened high-salinity advection during the event as compared with the previous 4-yr period: the westward shifted cyclonic eddy during early winter stretched to the remote western Gulf of Oman, which carried the higher-salinity water to the northern Arabian Sea along the south coast of the Gulf. An anomalous eddy dipole during early summer intensified the eastward Ras Al Hadd Jet and its high-salinity advection into the northern Arabian Sea. In addition, the weakened low-salinity advection by coastal currents along the Omani coast caused by the weakened southwest monsoon contributed to the maintenance of the high-salinity event. This prolonged high-salinity event reflects the upper-ocean responses to the monsoon change and may affect the regional hydrography and biogeochemistry extensively.

Free access