2015 Vol. 34, No. 5

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Contrasting dynamic characteristics of shear turbulence and Langmuir circulation in the surface mixed layer
LI Guojing, WANG Dongxiao, CHEN Ju, YAO Jinglong, ZENG Lili, SHU Yeqiang, SUI Dandan
2015, 34(5): 1-11. doi: 10.1007/s13131-015-0661-4
Large eddy simulation (LES) is used to investigate contrasting dynamic characteristics of shear turbulence (ST) and Langmuir circulation (LC) in the surface mixed layer (SML). ST is usually induced by wind forcing in SML. LC can be driven by wave-current interaction that includes the roles of wind, wave and vortex forcing. The LES results show that LC suppresses the horizontal velocity and greatly modifies the downwind velocity profile, but increases the vertical velocity. The strong downwelling jets of LC accelerate and increase the downward transport of energy as compared to ST. The vertical eddy viscosity Km of LC is much larger than that of ST. Strong mixing induced by LC has two locations. They are located in the 2δs-3δs(Stokes depth scale) and the lower layer of the SML, respectively. Its value and position change periodically with time. In contrast, maximum Km induced by ST is located in the middle depth of the SML. The turbulent kinetic energy (TKE) generated by LC is larger than that by ST. The differences in vertical distributions of TKE and Km are evident. Therefore, the parameterization of LC cannot be solely based on TKE. For deep SML, the convection of large-scale eddies in LC plays a main role in downward transport of energy and LC can induce stronger velocity shear (S2) near the SML base. In addition, the large-scale eddies and S2 induced by LC is changing all the time, which needs to be fully considered in the parameterization of LC.
Seasonal variability of the Kuroshio Current at the PN Section in the East China Sea based on in-situ observation from 1987 to 2010
WEI Yanzhou, PEI Yuhua, ZHANG Rong-Hua
2015, 34(5): 12-21. doi: 10.1007/s13131-015-0662-3
As the spatio-temporal variability of the Kuroshio is highly influenced by mesoscale eddies, representing its seasonal variability characteristics requires sufficiently long term observations to reduce the uncertainties. Geostrophic velocity data estimated from hydrographic observation from 1987 to 2010 and the shipboard ADCP velocity data from 1993 to 2008 at the PN Section in the central East China Sea are collected to view the seasonal variability objectively. From both types of observation, it is found that the seasonal climatology mean of the Kuroshio Current exhibits significant difference in three areas, which are located at the Kuroshio Current core and its two flanks in a shallow layer less than 300 m, with the weakest northeast current at the core in autumn, the strongest counter current on the right flank in spring, and the strongest northeast current on the left flank in autumn, respectively. The seasonal variance of the Kuroshio Current also exhibits significant difference on the off-shore side of the Kuroshio, with larger variance in spring and summer while smaller variance in autumn and winter. For the current parallel to the PN Section, the ratio of the seasonal variability component to the intraseasonal variability component is relatively smaller than that for the current perpendicular to the PN Section. Further analyses indicate that the seasonal variability at the PN Section is tightly linked to the upstream and downstream current variability.
Effects of interannual salinity variability on the dynamic height in the western equatorial Pacific as diagnosed by Argo
ZHENG Fei, WANG Hui, WAN Liying
2015, 34(5): 22-28. doi: 10.1007/s13131-015-0663-2
In this paper, interannual variations of the ocean dynamic height over the tropical Pacific are diagnosed using three-dimensional temperature and salinity fields from Argo profiles, with a focus on the effects of interannually varying salinity on the El Niño-Southern Oscillation (ENSO) evolutions. The diagnostic analyses clearly demonstrate a significant and large role that the salinity field plays in modulating the sea surface dynamic height (SSDH) in the western tropical Pacific. In particular, the contribution of the interannually varying salinity to the interannual variations in SSDH approximately equals to that of the interannually varying temperature. Over the western equatorial Pacific, the salinity variability was responsible for a 30% to 40% reduction in SSDH anomaly in opposition to the thermal build up in SSDH anomaly, providing an important contribution to modulating the seasonal-to-interannual evolution of the tropical Pacific Ocean and affecting the developing of ENSO events.
Variation of Indo-Pacific upper ocean heat content during 2001-2012 revealed by Argo
WU Xiaofen, LIU Zenghong, LIAO Guanghong, WU Lingjuan
2015, 34(5): 29-38. doi: 10.1007/s13131-015-0664-1
Understanding of the temporal variation of oceanic heat content (OHC) is of fundamental importance to the prediction of climate change and associated global meteorological phenomena. However, OHC characteristics in the Pacific and Indian oceans are not well understood. Based on in situ ocean temperature and salinity profiles mainly from the Argo program, we estimated the upper layer (0-750 m) OHC in the Indo-Pacific Ocean (40°S-40°N, 30°E-80°W). Spatial and temporal variability of OHC and its likely physical mechanisms are also analyzed. Climatic distributions of upper-layer OHC in the Indian and Pacific oceans have a similar saddle pattern in the subtropics, and the highest OHC value was in the northern Arabian Sea. However, OHC variabilities in the two oceans were different. OHC in the Pacific has an east-west see-saw pattern, which does not appear in the Indian Ocean. In the Indian Ocean, the largest change was around 10°S. The most interesting phenomenon is that, there was a long-term shift of OHC in the Indo-Pacific Ocean during 2001-2012. Such variation coincided with modulation of subsurface temperature/salinity. During 2001-2007, there was subsurface cooling (freshening) nearly the entire upper 400 m layer in the western Pacific and warming (salting) in the eastern Pacific. During 2008-2012, the thermocline deepened in the western Pacific but shoaled in the east. In the Indian Ocean, there was only cooling (upper 150 m only) and freshening (almost the entire upper 400 m) during 2001-2007. The thermocline deepened during 2008-2012 in the Indian Ocean. Such change appeared from the equator to off the equator and even to the subtropics (about 20°N/S) in the two oceans. This long-term change of subsurface temperature/salinity may have been caused by change of the wind field over the two oceans during 2001-2012, in turn modifying OHC.
Impact of observational MJO forcing on ENSO predictability in the Zebiak-Cane model: Part I. Effect on the maximum prediction error
PENG Yuehua, SONG Junqiang, XIANG Jie, SUN Chengzhi
2015, 34(5): 39-45. doi: 10.1007/s13131-015-0665-0
With the observational wind data and the Zebiak-Cane model, the impact of Madden-Julian Oscillation (MJO) as external forcing on El Niño-Southern Oscillation (ENSO) predictability is studied. The observational data are analyzed with Continuous Wavelet Transform (CWT) and then used to extract MJO signals, which are added into the model to get a new model. After the Conditional Nonlinear Optimal Perturbation (CNOP) method has been used, the initial errors which can evolve into maximum prediction error, model errors and their join errors are gained and then the Niño 3 indices and spatial structures of three kinds of errors are investigated. The results mainly show that the observational MJO has little impact on the maximum prediction error of ENSO events and the initial error affects much greater than model error caused by MJO forcing. These demonstrate that the initial error might be the main error source that produces uncertainty in ENSO prediction, which could provide a theoretical foundation for the adaptive data assimilation of the ENSO forecast and contribute to the ENSO target observation.
An evaporation duct prediction model coupled with the MM5
JIAO Lin, ZHANG Yonggang
2015, 34(5): 46-50. doi: 10.1007/s13131-015-0666-z
Evaporation duct is an abnormal refractive phenomenon in the marine atmosphere boundary layer. It has been generally accepted that the evaporation duct prominently affects the performance of the electronic equipment over the sea because of its wide distribution and frequent occurrence. It has become a research focus of the navies all over the world. At present, the diagnostic models of the evaporation duct are all based on the Monin-Obukhov similarity theory, with only differences in the flux and character scale calculations in the surface layer. These models are applicable to the stationary and uniform open sea areas without considering the alongshore effect. This paper introduces the nonlinear factor αv and the gust wind item wg into the Babin model, and thus extends the evaporation duct diagnostic model to the offshore area under extremely low wind speed. In addition, an evaporation duct prediction model is designed and coupled with the fifth generation mesoscale model (MM5). The tower observational data and radar data at the Pingtan island of Fujian Province on May 25-26, 2002 were used to validate the forecast results. The outputs of the prediction model agree with the observations from 0 to 48 h. The relative error of the predicted evaporation duct height is 19.3% and the prediction results are consistent with the radar detection.
Observed features of temperature, salinity and current in central Chukchi Sea during the summer of 2012
HE Yan, LIU Na, CHEN Hongxia, TENG Fei, LIN Lina, WANG Huiwu
2015, 34(5): 51-59. doi: 10.1007/s13131-015-0624-7
During the summer of 2012, the fifth CHINARE Arctic Expedition was carried out, and a submersible mooring system was deployed in M5 station located at (69°30.155'N,169°00.654'W) and recovered 50d later. A set of temperature, salinity and current profile records was acquired. The characteristics of these observations are analyzed in this paper. Some main results are achieved as below. (1) Temperature generally decreases while salinity generally increases with increasing depth. The average values of all records are 2.98℃ and 32.21 psu. (2) Salinity and temperature are well negatively correlated, and the correlation coefficient between them is -0.84. However, they did not always vary synchronously. Their co-variation featured different characters during different significant periods. (3) The average velocity for the whole water column is 141 mm/s with directional angle of 347.1°. The statistical distribution curve of velocity record number gets narrower with increasing depth. More than 85% of the recorded velocities are northward, and the mean magnitudes of dominated northward velocities are 100-150 mm/s. (4) Rotary spectrum analysis shows that motions with low frequency take a majority of energy in all layers. The most significant energy peaks for all layers are around 0.012 cph (about 3.5 d period), while the tidal motion in mooring area is nonsignificant. (5) Velocities in all layers feature similar and synchronous temporal variations, except for the slight decrease in magnitude and leftward twist from top to bottom. The directions of velocity correspond well to those of surface wind. The average northward volume transport per square meter is 0.1-0.2 m3/s under southerly wind, but about -0.2 m3/s during northerly wind burst.
Validation of Chinese HY-2 satellite radar altimeter significant wave height
YE Xiaomin, LIN Mingsen, XU Ying
2015, 34(5): 60-67. doi: 10.1007/s13131-015-0667-y
Chinese Haiyang-2(HY-2) satellite is the first Chinese marine dynamic environment satellite. The dual-frequency (Ku and C band) radar altimeter onboard HY-2 has been working effective to provide operational significant wave height (SWH) for more than three years (October 1, 2011 to present).We validated along-track Ku-band SWH data of HY-2 satellite against National Data Buoy Center (NDBC) in-situ measurements over a time period of three years from October 1, 2011 to September 30, 2014, the root mean square error (RMSE) and mean bias of HY-2 SWH is 0.38 m and (-0.13±0.35) m, respectively. We also did cross validation against Jason-2 altimeter SWH data,the RMSE and the mean bias is 0.36m and (-0.22±0.28) m, respectively. In order to compare the statistical results between HY-2 and Jason-2 satellite SWH data, we validated the Jason-2 satellite radar altimeter along-track Ku-band SWH data against NDBC measurements using the same method. The results demonstrate the validation method in this study is scientific and the RMSE and mean bias of Jason-2 SWH data is 0.26 m and (0.00±0.26) m, respectively. We also validated both HY-2 and Jason-2 SWH data every month, the mean bias of Jason-2 SWH data almost equaled to zero all the time, while the mean bias of HY-2 SWH data was no less than -0.31m before April 2013 and dropped to zero after that time. These results indicate that the statistical results for HY-2 altimeter SWH are reliable and HY-2 altimeter along-track SWH data were steady and of high quality in the last three years. The results also indicate that HY-2 SWH data have greatly been improved and have the same accuracy with Jason-2 SWH data after April, 2013. SWH data provided by HY-2 satellite radar altimeter are useful and acceptable for ocean operational applications.
Zooplankton diel vertical migration and influence of upwelling on the biomass in the Chukchi Sea during summer
WANG Huiwu, CHEN Hongxia, XUE Liang, LIU Na, LIU Yanliang
2015, 34(5): 68-74. doi: 10.1007/s13131-015-0668-x
The diel vertical migration (DVM) of zooplankton and the influence of upwelling on zooplankton biomass were examined using water column data of current velocity and mean volume backscattering strength (MVBS) collected by moored acoustic Doppler current profilers (ADCPs) deployed in the southeastern Chukchi Sea during the 5th Chinese National Arctic Research Expedition (CHINARE) in summer 2012, combined with the satellite observational data such as sea surface temperature (SST), wind, and chlorophyll a (Chl a). Hourly acoustic data were continuously collected for 49-d in the mooring site. Spectral analysis indicated that there were different migrating patterns of zooplankton, even though precisely classifying the zooplankton taxa was not available. The prevailing 24-h cycle corresponded to the normal DVM with zooplankton swimming upwards at sunrise and returning to deep waters at sunset. There was a clear DVM in the upper 17 m of the water column during the period with distinct day-night cycles, and no active DVM throughout the water column when the sun above the horizon (polar day), suggesting that light intensity was the trigger for DVM. Also there was a second migrating pattern with 12-h cycle. The upwelling event occurring in the northwest of Alaskan coastal area had important influence on zooplankton biomass at the mooring site. During the upwelling, the SST close to the mooring site dropped significantly from maximal 6.35℃ to minimal 1.31℃ within five days. Simultaneously, there was a rapid increase in the MVBS and Chl a level, suggesting the aggregation of zooplankton related to upwelling.
Sound velocity and related properties of seafloor sediments in the Bering Sea and Chukchi Sea
MENG Xiangmei, LI Guanbao, HAN Guozhong, KAN Guangming
2015, 34(5): 75-80. doi: 10.1007/s13131-015-0669-9
The Bering Sea shelf and Chukchi Sea shelf are believed to hold enormous oil and gas reserves which have attracted a lot of geophysical surveys. For the interpretation of acoustic geophysical survey results, sediment sound velocity is one of the main parameters. On seven sediment cores collected from the Bering Sea and Chukchi Sea during the 5th Chinese National Arctic Research Expedition, sound velocity measurements were made at 35, 50, 100, 135, 150, 174, 200, and 250 kHz using eight separate pairs of ultrasonic transducers. The measured sound velocities range from 1 425.1 m/s to 1 606.4 m/s and are dispersive with the degrees of dispersion from 2.2% to 4.0% over a frequency range of 35-250 kHz. After the sound velocity measurements, the measurements of selected geotechnical properties and the Scanning Electron Microscopic observation of microstructure were also made on the sediment cores. The results show that the seafloor sediments are composed of silty sand, sandy silt, coarse silt, clayey silt, sand-silt-clay and silty clay. Aggregate and diatom debris is found in the seafloor sediments. Through comparative analysis of microphotographs and geotechnical properties, it is assumed that the large pore spaces between aggregates and the intraparticulate porosity of diatom debris increase the porosity of the seafloor sediments, and affect other geotechnical properties. The correlation analysis of sound velocity and geotechnical properties shows that the correlation of sound velocity with porosity and wet bulk density is extreme significant, while the correlation of sound velocity with clay content, mean grain size and organic content is not significant. The regression equations between porosity, wet bulk density and sound velocity based on best-fit polynomial are given.
A new statistical model of wave heights based on the concept of wave breaking critical zone
YANG Jiaxuan, LI Xunqiang, ZHU Shouxian, ZHANG Wenjing, WANG Lei
2015, 34(5): 81-85. doi: 10.1007/s13131-015-0670-3
When waves propagate from deep water to shallow water, wave heights and steepness increase and then waves roll back and break. This phenomenon is called surf. Currently, the present statistical calculation model of surf was derived mainly from the wave energy conservation equation and the linear wave dispersion relation, but it cannot reflect accurately the process which is a rapid increasing in wave height near the broken point. So, the concept of a surf breaking critical zone is presented. And the nearshore is divided as deep water zone, shallow water zone, surf breaking critical zone and after breaking zone. Besides, the calculation formula for the height of the surf breaking critical zone has founded based on flume experiments, thereby a new statistical calculation model on the surf has been established. Using the new model, the calculation error of wave height maximum is reduced from 17.62% to 6.43%.
The assessment of extactable tidal energy and the effect of tidal energy turbine deployment on the hydrodynamics in Zhoushan
HOU Fang, BAO Xianwen, LI Benxia, LIU Qianqian
2015, 34(5): 86-91. doi: 10.1007/s13131-015-0671-2
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.
Numerical study of resonance induced by wave action on multiple rectangular boxes with narrow gaps
NING Dezhi, SU Xiaojie, ZHAO Ming, TENG Bin
2015, 34(5): 92-102. doi: 10.1007/s13131-015-0672-1
By introducing a source term into the Laplace equation, a two-dimensional fully nonlinear time-domain numerical wave flume (NWF) is developed to investigate the resonance induced by the interaction between waves and multiple objects with narrow gaps. In the numerical model, the fully nonlinear kinematic and dynamic boundary conditions are satisfied on the instantaneous free surface and the constant artificial damping is employed in the gaps to approximate the viscous dissipation due to vortex motion and flow separation. The computational domain is discretized using a higher-order boundary element method (HOBEM). The proposed model is firstly validated against the published experimental data and numerical results of the wave height in the narrow gap between two boxes, the wave heights in the two gaps of three boxes, and wave loads on the boxes. Then, the extensive numerical experiments are performed to study the influences of the number of the boxes and the gap spacing on the resonant frequency, reflected and transmitted wave heights and wave loads on the boxes.
A mathematical model of calculating local tsunami wave source constraints and propagation
LI Daming, LI Yangyang
2015, 34(5): 103-109. doi: 10.1007/s13131-015-0673-0
This paper presents a local tsunami simulation, including the initial displacement field model of tsunami source and tsunami wave propagation model. We deduced the tsunami wave equation; applied the matching of interior and exterior solutions method and water mass method to determine the initial displacement field in different bottom topography. Tsunami wave propagation model was based on the Boussinesq equation. Difference format was based on the ADI method which discretized in alternating direction in the form of implicit scheme. The open boundary of ADI had been revised considering the influence of wave propagation in the equation of motion. The local tsunami mathematical model was used in the simulation of 2011 Japan tsunami, and the results and the observation data match well.
Research Notes
Regional characteristics of sea ice thickness in Canadian shelf and Arctic Archipelago measured by Ground Penetrating Radar
LI Tao, ZHAO Jinping, JIAO Yutian, HOU Jiaqiang, MU Longjiang
2015, 34(5): 110-116. doi: 10.1007/s13131-015-0612-0
Ground Penetrating Radar (GPR) measurements of sea ice thickness including undeformed ice and ridged ice were carried out in the central north Canadian Archipelago in spring 2010. Results have shown a significant spatial heterogeneity of sea ice thickness across the shelf. The undeformed multi-year fast ice of (2.05±0.09) m thick was investigated southern inshore zone of Borden island located at middle of the observational section, which was the observed maximum thickness in the field work. The less thick sea ice was sampled across a flaw lead with the thicknesses of (1.05±0.11) m for the pack ice and (1.24±0.13) m for the fast ice. At the northernmost spot of the section, the undeformed multi-year pack ice was (1.54±0.22) m thick with a ridged ice of 2.5 to 3 m, comparing to the multi-year fast ice with the thickness of (1.67±0.16) m at the southernmost station in the Prince Gustaf Adolf Sea.