2016 Vol. 35, No. 7

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The shallow meridional overturning circulation in the South China Sea and the related internal water movement
ZHANG Ningning, LAN Jian, MA Jie, CUI Fengjuan
2016, 35(7): 1-7. doi: 10.1007/s13131-016-0900-3
The structure of the annual-mean shallow meridional overturning circulation (SMOC) in the South China Sea (SCS) and the related water movement are investigated, using simple ocean data assimilation (SODA) outputs. The distinct clockwise SMOC is present above 400 m in the SCS on the climatologically annual-mean scale, which consists of downwelling in the northern SCS, a southward subsurface branch supplying upwelling at around 10°N and a northward surface flow, with a strength of about 1×106 m3/s. The formation mechanisms of its branches are studied separately. The zonal component of the annual-mean wind stress is predominantly westward and causes northward Ekman transport above 50 m. The annual-mean Ekman transport across 18°N is about 1.2×106 m3/s. An annual-mean subduction rate is calculated by estimating the net volume flux entering the thermocline from the mixed layer in a Lagrangian framework. An annual subduction rate of about 0.66×106 m3/s is obtained between 17° and 20°N, of which 87% is due to vertical pumping and 13% is due to lateral induction. The subduction rate implies that the subdution contributes significantly to the downwelling branch. The pathways of traced parcels released at the base of the February mixed layer show that after subduction water moves southward to as far as 11°N within the western boundary current before returning northward. The velocity field at the base of mixed layer and a meridional velocity section in winter also confirm that the southward flow in the subsurface layer is mainly by strong western boundary currents. Significant upwelling mainly occurs off the Vietnam coast in the southern SCS. An upper bound for the annual-mean net upwelling rate between 10° and 15°N is 0.7×106 m3/s, of which a large portion is contributed by summer upwelling, with both the alongshore component of the southwest wind and its offshore increase causing great upwelling.
The mean properties and variations of the Southern Hemisphere subpolar gyres estimated by Simple Ocean Data Assimilation (SODA) products
DUAN Yongliang, LIU Hongwei, YU Weidong, HOU Yijun
2016, 35(7): 8-13. doi: 10.1007/s13131-016-0901-2
Based on the Simple Ocean Data Assimilation (SODA) products, we study the mean properties and variations of the Southern Hemisphere subpolar gyres (SHSGs) in this paper. The results show that the gyre strengths in the SODA estimates are (55.9±9.8)×106 m3/s for the Weddell Gyre (WG), (37.0±6.4)×106 m3/s for the Ross Gyre (RG), and (27.5±8.2)×106 m3/s for the Australian-Antarctic Gyre (AG), respectively. There exists distinct connectivity between the adjacent gyres and then forms an oceanic super gyre structure in the southern subpolar oceans. And the interior exchanges are about (8.0±3.2)×106 m3/s at around 70°E and (4.3±3.1)×106 m3/s at around 140°E. The most pronounced variation for all three SHSGs occurs on the seasonal time scale, with generally stronger (weaker) SHSGs during austral winter (summer). And the seasonal changes of the gyre structures show that the eastern boundary of the WG and AG extends considerably further east during winter and the interior exchange in the super gyre structure increases accordingly. The WG and RG also show significant semi-annual changes. The correlation analyses confirm that the variations of the gyre strengths are strongly correlated with the changes in the local wind forcing on the semi-annual and seasonal time scales.
Study on the amplitude inversion of internal waves at Wenchang area of the South China Sea
ZHANG Xudong, WANG Jing, SUN Lina, MENG Junmin
2016, 35(7): 14-19. doi: 10.1007/s13131-016-0902-1
The field experiment is conducted from April 16, 2005 to July 20, 2005 at Wenchang area east of Hainan Island (19°35'N, 112°E) of China. Internal wave packets are observed frequently with thermistor chains during the experiment. Meanwhile, internal waves are also detected from a synthetic aperture radar (SAR) image on June 19, 2005 and several other moderate-resolution imaging spectroradiometer (MODIS) images near a mooring position. The distance between the positive and negative peaks induced by the internal wave can be obtained from satellite images. Combined with remote sensing images and in situ data, a new method to inverse the amplitude of the internal wave is proposed based on a corrected nonlinear Schrödinger (NLS) equation. Two relationships are given between the peak-to-peak distance and the characteristic wavelength of the internal wave for different nonlinear and dispersion coefficients. Based on the satellite images, the amplitude inversion of the internal waves are carried out with the NLS equation as well as the KdV equation. The calculated amplitudes of the NLS equation are close to the observation amplitude which promise the NLS equation a reliable method.
On analyzing space-time distribution of evaporation duct height over the global ocean
YANG Kunde, ZHANG Qi, SHI Yang, HE Zhengyao, LEI Bo, HAN Yina
2016, 35(7): 20-29. doi: 10.1007/s13131-016-0903-0
The statistical features of the evaporation duct over the global ocean were comprehensively investigated with reanalysis data sets from the National Centers for Environmental Prediction. These data sets have time and spatial resolutions of 1 h and 0.313°×0.312°, respectively. The efficiency of the analysis was evaluated by processing weather buoy data from the Pacific Ocean and measuring propagation loss in the Yellow Sea of China. The distribution features of evaporation duct height (EDH) and the related meteorological factors for different seas were analyzed. The global EDH is generally high and demonstrates a latitudinal distribution for oceans at low latitudes. The average EDH is approximately 11 m over oceans beside the equator with a latitude of less than 20°. The reasons for the formation of the global EDH features were also analyzed for different sea areas.
Position variability of the Kuroshio Extension sea surface temperature front
WANG Yanxin, YANG Xiaoyi, HU Jianyu
2016, 35(7): 30-35. doi: 10.1007/s13131-016-0909-7
High spatial resolution sea surface temperature (SST) data from 1993 to 2013 are used to detect the position of the Kuroshio Extension sea surface temperature front (KEF) from 141°E to 158°E, and the seasonal, monthly and interannual-to-decadal variations of the KEF position are investigated. The latitudinal position of the KEF varies with longitudes:the westernmost part of the KEF from 141°E to 144°E is relatively stable, whereas the easternmost part from 153°E to 158°E exhibits the largest amplitude of its north-south displacement. In the light of the magnitudes of the standard deviations at longitudes, then the KEF is divided into three sections:western part of the KEF (KEFw, 141°-144°E), central part of the KEF (KEFc, 144°-153°E) and eastern part of the KEF (KEFe, 153°-158°E). Further analysis reveals that the KEFw position is dominated by the decadal variability, while the KEFc and KEFe positions change significantly both on interannual and decadal time scales. In addition, the KEFw position is well correlated with the KEF path length. The possible mode leading to the decadal oscillation of the KEFw is further discussed. The KEFw position exhibits significant connections with the Pacific decadal oscillation (PDO) index and the north Pacific gyre oscillation (NPGO) index with a time lag of 40 and 33 months, respectively.
Arctic autumn sea ice decline and Asian winter temperature anomaly
LIU Na, LIN Lina, WANG Yingjie, KONG Bin, ZHANG Zhanhai, CHEN Hongxia
2016, 35(7): 36-41. doi: 10.1007/s13131-016-0911-0
Associations between the autumn Arctic sea ice concentration (SIC) and Asian winter temperature are discussed using the singular value decomposition analysis. Results show that in recent 33 years reduced autumn Arctic sea ice is accompanied by Asian winter temperature decrease except in the Tibetan plateau and the Arctic Ocean and the North Pacific Ocean coast. The autumn SIC reduction excites two geopotential height centers in Eurasia and the north Arctic Ocean, which are persistent from autumn to winter. The negative center is in Barents Sea/Kara Sea. The positive center is located in Mongolia. The anomalous winds are associated with geopotential height centers, providing favorable clod air for the Asian winter temperature decreasing in recent 33 years. This relationship indicates a potential long-term outlook for the Asian winter temperature decrease as the decline of the autumn sea ice in the Arctic Ocean is expected to continue as climate warms.
Asymmetry of upper ocean salinity response to the Indian Ocean dipole events as seen from ECCO simulation
ZHANG Ying, DU Yan, ZHANG Yuhong, GAO Shan
2016, 35(7): 42-49. doi: 10.1007/s13131-016-0904-z
The interannual variability of salinity and associated ocean dynamics in the equatorial Indian Ocean is analyzed using observations and numerical simulations by the Estimating the Circulation and Climate of the Ocean (ECCO) model. The results show that salinity anomalies in the upper ocean are asymmetrically associated with the Indian Ocean dipole (IOD) events, with stronger response during their positive phases. Further investigations reveal that zonal currents along the equator, the Wyrtki jets, dominate the salinity transport. During the positive IOD events, the Wyrtki jets have stronger westward anomalies. The positive skewness of the IOD explains that the amplitude of the anomalous Wyrtki jets is stronger in the positive IOD events than that in the negative events.
WindSat satellite comparisons with nearshore buoy wind data near the U.S. west and east coasts
ZHANG Lei, SHI Hanqing, YU Hong, YI Xin
2016, 35(7): 50-58. doi: 10.1007/s13131-016-0905-y
Nearshore wind speeds retrieved by WindSat are validated by a comparison with the moored buoy observations near the U.S. west and east coasts. A 30 min and 25 km collection window is used for the WindSat wind data and buoy measurements from January 2004 to December 2014. Comparisons show that the overall root-mean-square error is better than 1.44 m/s near the U.S. coasts, and the result for the east coast is better than that for the west coast. The retrieval accuracy of the descending portions is slightly better than that of the ascending portions. Most buoy-to-buoy variations are not significantly correlated with the coastal topography, the longitude and the distance from the shore or satellite-buoy separation distance. In addition, comparisons between a polarimetric microwave radiometer and a microwave scatterometer are accomplished with the nearshore buoy observations from 2007 to 2008. The WindSat-derived winds tend to be lower than the buoy observations near the U.S. coasts. In contrast, the QuikSCAT-derived winds tend to be higher than the buoy observations. Overall, the retrieval accuracy of WindSat is slightly better than that of QuikSCAT, and these satellite-derived winds are sufficiently accurate for scientific studies.
Characteristics of cyclone climatology and variability in the Southern Ocean
WEI Lixin, QIN Ting
2016, 35(7): 59-67. doi: 10.1007/s13131-016-0913-y
A new climatology of cyclones in the Southern Ocean is generated by applying an automated cyclone detection and tracking algorithm (developed by Hodges at the Reading University) for an improved and relatively highresolution European Centre for Medium-Range Weather Forecasts atmospheric reanalysis during 1979-2013. A validation shows that identified cyclone tracks are in good agreement with a available analyzed cyclone product. The climatological characteristics of the Southern Ocean cyclones are then analyzed, including track, number, density, intensity, deepening rate and explosive events. An analysis shows that the number of cyclones in the Southern Ocean has increased for 1979-2013, but only statistically significant in summer. Coincident with the circumpolar trough, a single high-density band of cyclones is observed in 55°-67°S, and cyclone density has generally increased in north of this band for 1979-2013, except summer. The intensity of up to 70% cyclones in the Southern Ocean is less than 980 hPa, and only a few cyclones with pressure less than 920 hPa are detected for 1979-2013. Further analysis shows that a high frequency of explosive cyclones is located in the band of 45°-55°S, and the Atlantic Ocean sector has much higher frequent occurrence of the explosive cyclones than that in the Pacific Ocean sector. Additionally, the relationship between cyclone activities in the Southern Ocean and the Southern Annular Mode is discussed.
A parameter inversion for sea bridge based on high-resolution polarimetric synthetic aperture radar
LIU Genwang, ZHANG Jie, ZHANG Xi, MENG Junmin, WANG Guoyu
2016, 35(7): 68-75. doi: 10.1007/s13131-016-0912-z
Each reflection return of a bridge over water is displayed as wide stripe in a high-resolution synthetic aperture radar (SAR) image, which lead to difficulties in a parameter inversion. Therefore, a method of bridge parameter inversion is proposed for high-resolution full polarimetric SAR (PolSAR). First, the single, double and triplebounce returns from each component of the bridge are distinguished by the polarization scattering features. Then the reasons which lead to the backscatter echoes of the bridge over water being displayed as stripes are analyzed, using a principle of microwave reflection, as well as an extraction method for each reflection return, and a parameter retrieval method is obtained. Finally, the parameters of the bridge, including the height (top and bottom surfaces of the sea bridge), width, thickness, span, and height of the bridge tower, are retrieved using full polarimetric AIRSAR data. When a comparison of the measured data is completed, the results indicate that the proposed method can invert the parameters with a high accuracy, and that the inversion error of the bridge height (bottom surface) is only 1.3%. Moreover, the results also show that for the high-resolution SAR, the C and L-band images have the same ability in regards to parameter retrieval.
The sound velocity and bulk properties of sediments in the Bohai Sea and the Yellow Sea of China
ZHENG Jiewen, LIU Baohua, KAN Guangming, LI Guanbao, PEI Yanliang, LIU Xiaolei
2016, 35(7): 76-86. doi: 10.1007/s13131-016-0906-x
In order to investigate the correlation between a sound velocity and sediment bulk properties and explore the influence of frequency dependence of the sound velocity on the prediction of the sediment properties by the sound velocity, a compressional wave velocity is measured at frequencies of 25-250 kHz on marine sediment samples collected from the Bohai Sea and the Yellow Sea in laboratory, together with the geotechnical parameters of sediments. The results indicate that the sound velocity ranges from 1.232 to 1.721 km/s for the collected sediment samples with a significant dispersion within the series measuring frequency. Poorly sorted sediments are highly dispersive nearly with a positive linear relationship. The porosity shows a better negative logarithmic correlation with the sound velocity compared with other geotechnical parameters. Generally, the sound velocity increases with the increasing of the average particle size, sand content, wet and dry bulk densities, and decreasing of the clay content, and water content. An important point should be demonstrated that the higher correlation can be obtained when the measuring frequency is low within the frequency ranges from 25 to 250 kHz since the inhomogeneity of sediment properties has a more remarkably influence on the laboratory sound velocity measurement at the high frequency.
Wave-dominated, mesotidal headland-bay beach morphodynamic classsfications of the Shuidong Bay in South China
YU Jitao, DING Yuanting, CHENG Huangxin, CAI Lailiang, CHEN Zishen
2016, 35(7): 87-95. doi: 10.1007/s13131-016-0907-9
Beach morphodynamic classifications have achieved extensive acceptance in foreign coastal geomorphological studies. Three beaches located in different zones of a headland-bay coast are classified according to a dimensionless fall parameter, a relative tide range parameter and a dimensionless embayment scaling parameter. Synchronous data, including wave, tide, sediment and beach morphology, are respectively collected from the tangential beach, the transitional beach and the shadow beach of the Shuidong Bay during each spring tide for 16 successive months. The research results indicate that (1) the beach in the tangential zone falls between two major categories which are low tide terrace beaches with rips and barred beaches; the beach in the transitional zone exhibits two main types which are low tide bar/rip beaches and barred dissipative beaches; and the beach in the shadow zone mainly mirrors dissipative states with presence or absence of bars; and (2) the sequential changes and differences of beach states in different coastal zones reflect spatial and temporal variabilities of the headlandbay coast, totally meeting the actual measured beach morphology changes, showing that studies on wavedominated, meso-macrotidal beaches need to consider the influences of the tides. Meanwhile, the research mainly provides a framework about beach state studies, due to different beach states with different erosion patterns, which requires the need to strengthen the researches in this respect, in order to further enrich theoretical basis for a beach topography evolution, beach morphodynamic processes and beach erosion prevention in China.
Sea ice thickness analyses for the Bohai Sea using MODIS thermal infrared imagery
ZENG Tao, SHI Lijian, MARKO Makynen, CHENG Bin, ZOU Juhong, ZHANG Zhiping
2016, 35(7): 96-104. doi: 10.1007/s13131-016-0908-8
Level ice thickness distribution pattern in the Bohai Sea in the winter of 2009-2010 was investigated in this paper using MODIS night-time thermal infrared imagery. The cloud cover in the imagery was masked out manually. Level ice thickness was calculated using MODIS ice surface temperature and an ice surface heat balance equation. Weather forcing data was from the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. The retrieved ice thickness agreed reasonable well with in situ observations from two off-shore oil platforms. The overall bias and the root mean square error of the MODIS ice thickness are -1.4 cm and 3.9 cm, respectively. The MODIS results under cold conditions (air temperature < -10℃) also agree with the estimated ice growth from Lebedev and Zubov models. The MODIS ice thickness is sensitive to the changes of the sea ice and air temperature, in particular when the sea ice is relatively thin. It is less sensitive to the wind speed. Our method is feasible for the Bohai Sea operational ice thickness analyses during cold freezing seasons.
Sea ice thickness estimation in the Bohai Sea using geostationary ocean color imager data
LIU Wensong, SHENG Hui, ZHANG Xi
2016, 35(7): 105-112. doi: 10.1007/s13131-015-0760-2
A method to estimate the thickness of the sea ice of the Bohai Sea is proposed using geostationary ocean color imager (GOCI) data and then applied to the dynamic monitoring of the sea ice thickness in the Bohai Sea during the winter of 2014 to 2015. First of all, a model is given between the GOCI shortwave broadband albedo and the reflectance of each band with high temporal resolution GOCI data. Then, the relationship model between the sea ice thickness and the GOCI shortwave broadband albedo is established and applied to the thickness extraction of the sea ice in the Bohai Sea. Finally, the sea ice thickness extraction method is tested by the results based on the MODIS data, thermodynamic empirical models (Lebedev and Zubov), and the in situ ice thickness data. The test results not only indicated that the sea ice thickness retrieval method based on the GOCI data was a good correlation (r2>0.86) with the sea ice thickness retrieved by the MODIS and thermodynamic empirical models, but also that the RMS is only 6.82 cm different from the thickness of the sea ice based on the GOCI and in situ data.
Oblique wave scattering by a semi-infinite elastic plate with finite draft floating on a step topography
GUO Yunxia, LIU Yong, MENG Xun
2016, 35(7): 113-121. doi: 10.1007/s13131-015-0760-2
On the basis of a potential theory and Euler-Bernoulli beam theory, an analytical solution for oblique wave scattering by a semi-infinite elastic plate with finite draft floating on a step topography is developed using matched eigenfunction expansions. Different from previous studies, the effects of a wave incident angle, a plate draft, three different plate edge conditions (free, simply supported and built-in) and a sea-bottom topography are all taken into account. Moreover, the plate edge conditions are directly incorporated into linear algebraic equations for determining unknown expansion coefficients in velocity potentials, which leads to a simple and efficient solving procedure. Numerical results show that the convergence of the present solution is good, and an energy conservation relation is well satisfied. Also, the present predictions are in good agreement with known results for special cases. The effects of the wave incident angle, the plate draft, the plate edge conditions and the sea-bottom topography on various hydrodynamic quantities are analyzed. Some useful results are presented for engineering designs.