2016 Vol. 35, No. 3

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Turbulent mixing in the upper ocean of the northwestern Weddell Sea, Antarctica
GUO Guijun, SHI Jiuxin, JIAO Yutian
2016, 35(3): 1-9. doi: 10.1007/s13131-016-0816-y
Turbulent mixing in the upper ocean (30-200 m) of the northwestern Weddell Sea is investigated based on profiles of temperature, salinity and microstructure data obtained during February 2014. Vertical thermohaline structures are distinct due to geographic features and sea ice distribution, resulting in that turbulent dissipation rates (ε) and turbulent diffusivity (K) are vertically and spatially non-uniform. On the shelf north of Antarctic Peninsula and Philip Ridge, with a relatively homogeneous vertical structure of temperature and salinity through the entire water column in the upper 200 m, both ε and K show significantly enhanced values in the order of O(10-7)-O(10-6) W/kg and O(10-3)-O(10-2) m2/s respectively, about two or three orders of magnitude higher than those in the open ocean. Mixing intensities tend to be mild due to strong stratification in the Powell Basin and South Orkney Plateau, where ε decreases with depth from O(10-8) to O(10-9) W/kg, while K changes vertically in an inverse direction relative to ε from O(10-6) to O(10-5) m2/s. In the marginal ice zone, K is vertically stable with the order of 10-4 m2/s although both intense dissipation and strong stratification occur at depth of 50-100 m below a cold freshened mixed layer. Though previous studies indentify wind work and tides as the primary energy sources for turbulent mixing in coastal regions, our results indicate weak relationship between K and wind stress or tidal kinetic energy. Instead, intensified mixing occurs with large bottom roughness, demonstrating that only when internal waves generated by wind and tide impinge on steep topography can the energy dissipate to support mixing. In addition, geostrophic current flowing out of the Weddell Sea through the gap west of Philip Passage is another energy source contributing to the local intense mixing.
Low-frequency variability of the shallow meridional overturning circulation in the South China Sea
YANG Zhitong, LUO Yiyong
2016, 35(3): 10-20. doi: 10.1007/s13131-016-0826-9
The low-frequency variability of the shallow meridional overturning circulation (MOC) in the South China Sea (SCS) is investigated using a Simple Ocean Data Assimilation (SODA) product for the period of 1900-2010. A dynamical decomposition method is used in which the MOC is decomposed into the Ekman, external mode, and vertical shear components. Results show that all the three dynamical components contribute to the formation of the seasonal and annual mean shallow MOC in the SCS. The shallow MOC in the SCS consists of two cells:a clockwise cell in the south and an anticlockwise cell in the north; the former is controlled by the Ekman flow and the latter is dominated by the external barotropic flow, with the contribution of the vertical shear being to reduce the magnitude of both cells. In addition, the strength of the MOC in the south is found to have a falling trend over the past century, due mainly to a weakening of the Luzon Strait transport (LST) that reduces the transport of the external component. Further analysis suggests that the weakening of the LST is closely related to a weakening of the westerly wind anomalies over the equatorial Pacific, which leads to a southward shift of the North Equatorial Current (NEC) bifurcation and thus a stronger transport of the Kuroshio east of Luzon.
Causes of seasonal sea level anomalies in the coastal region of the East China Sea
WANG Hui, LIU Kexiu, QI Dongmei, GAO Zhigang, FAN Wenjing, ZHANG Zengjian, WANG Guosong
2016, 35(3): 21-29. doi: 10.1007/s13131-016-0825-x
Based on the analysis of sea level, air temperature, sea surface temperature (SST), air pressure and wind data during 1980-2013, the causes of seasonal sea level anomalies in the coastal region of the East China Sea (ECS) are investigated. The research results show:(1) sea level along the coastal region of the ECS takes on strong seasonal variation. The annual range is 30-45 cm, larger in the north than in the south. From north to south, the phase of sea level changes from 140° to 231°, with a difference of nearly 3 months. (2) Monthly mean sea level (MSL) anomalies often occur from August to next February along the coast region of the ECS. The number of sea level anomalies is at most from January to February and from August to October, showing a growing trend in recent years. (3) Anomalous wind field is an important factor to affect the sea level variation in the coastal region of the ECS. Monthly MSL anomaly is closely related to wind field anomaly and air pressure field anomaly. Wind-driven current is essentially consistent with sea surface height. In August 2012, the sea surface heights at the coastal stations driven by wind field have contributed 50%-80% of MSL anomalies. (4) The annual variations for sea level, SST and air temperature along the coastal region of the ECS are mainly caused by solar radiation with a period of 12 months. But the correlation coefficients of sea level anomalies with SST anomalies and air temperature anomalies are all less than 0.1. (5) Seasonal sea level variations contain the long-term trends and all kinds of periodic changes. Sea level oscillations vary in different seasons in the coastal region of the ECS. In winter and spring, the oscillation of 4-7 a related to El Ni.o is stronger and its amplitude exceeds 2 cm. In summer and autumn, the oscillations of 2-3 a and quasi 9 a are most significant, and their amplitudes also exceed 2 cm. The height of sea level is lifted up when the different oscillations superposed. On the other hand, the height of sea level is fallen down.
Variations of temperature, salinity and current in the southern tidal passage of the Hangzhou Bay, China
HE Zhiguo, HUANGFU Kailong, YUAN Yeping, SONG Dan, LI Li
2016, 35(3): 30-37. doi: 10.1007/s13131-016-0819-8
Field surveys covering a spring-neap tidal period were conducted to investigate the characteristics of tidal dynamics within a curved channel in the southern Hangzhou Bay, China. The channel has a maximum depth of more than 100 m with an average tidal range of 2.5 m, serving as the main tidal passage in the southern part of the Hangzhou Bay. Water salinity, temperature and velocity data were collected from the ship-based transects and mooring measurements. During flood tide, the tidal current intrudes into the Hangzhou Bay through the northern side of the channel with a maximum velocity of about 2 m/s, while retreats through the southern side during ebb tide with a maximum velocity of 1.8 m/s. Due to the pressure, density gradients, the Coriolis force and centrifugal effect, a lateral exchange flow is generated as the tidal current relaxes from flood to ebb. Salinity and temperature data show that the water in the channel is weakly stratified during both spring and neap tides in summer time. However, mixing in the middle region will be enhanced by the lateral circulation. Mooring data indicate that the temperature and salinity are varying at a frequency similar to tidal current but higher than sea level oscillation. Our results support the hypothesis that the high frequency salinity and temperature variations could be generated by combination of the tidal current and the lateral exchanging flow.
Study on the mesoscale eddies around the Ryukyu Islands
HAN Shuzong, XU Changsan, WU Huiming, WANG Gang, PEI Junfeng, FAN Yongbin, WANG Xingchi
2016, 35(3): 38-45. doi: 10.1007/s13131-016-0824-y
Results of the Ocean General Circulation Model for the Earth Simulator (OFES) from January 1977 to December 2006 are used to investigate mesoscale eddies near the Ryukyu Islands. The results show that:(1) Larger eddies are mainly east of Taiwan, above the Ryukyu Trench and south of the Shikoku Island. These three sea areas are all in the vicinity of the Ryukyu Current. (2) Eddies in the area of the Ryukyu Current are mainly anticyclonic, and conducive to that current. The transport of water east of the Ryukyu Islands is mainly toward the northeast. (3) The Ryukyu Current is significantly affected by the eddies. The lower the latitude, the greater these effects. However, the Kuroshio is relatively stable, and the effect of mesoscale eddies is not significant. (4) A warm eddy south of the Shikoku Island break away from the Kuroshio and move southwest, and is clearly affected by the Ryukyu Current and Kuroshio. Relationships between the mesoscale eddies, Kuroshio meanders, and Ryukyu Current are discussed.
Wind-wave hindcast in the Yellow Sea and the Bohai Sea from the year 1988 to 2002
HE Hailun, XU Yao
2016, 35(3): 46-53. doi: 10.1007/s13131-015-0786-5
We performed long-term wind-wave hindcast in the Yellow Sea and the Bohai Sea from the year 1988 to 2002, and then analyzed the regional wave climate. Comparisons between model results and satellite data are generally consistent on monthly mean significant wave height. Then we discuss the temporal and spatial characteristics of the climatological monthly mean significant wave heights and mean wave periods. The climatologically spatial patterns are observed as increasing from northwest to southeast and from offshore to deep-water area for both significant wave height and mean wave period, and the patterns are highly related to the wind forcing and local topography. Seasonal variations of wave parameters are also significant. Furthermore, we compute the extreme values of wind and significant wave height using statistical methods. Results reveal the spatial patterns of N-year return significant wave height in the Yellow Sea and the Bohai Sea, and we discuss the relationship between extreme values of significant wave height and wind forcing.
Assessment and adjustment of sea surface salinity products from Aquarius in the southeast Indian Ocean based on in situ measurement and MyOcean modeled data
XIA Shenzhen, KE Changqing, ZHOU Xiaobing, ZHANG Jie
2016, 35(3): 54-62. doi: 10.1007/s13131-016-0818-9
The in situ sea surface salinity (SSS) measurements from a scientific cruise to the western zone of the southeast Indian Ocean covering 30°-60°S, 80°-120°E are used to assess the SSS retrieved from Aquarius (Aquarius SSS). Wind speed and sea surface temperature (SST) affect the SSS estimates based on passive microwave radiation within the mid-to low-latitude southeast Indian Ocean. The relationships among the in situ, Aquarius SSS and wind-SST corrections are used to adjust the Aquarius SSS. The adjusted Aquarius SSS are compared with the SSS data from MyOcean model. Results show that:(1) Before adjustment:compared with MyOcean SSS, the Aquarius SSS in most of the sea areas is higher; but lower in the low-temperature sea areas located at the south of 55°S and west of 98°E. The Aquarius SSS is generally higher by 0.42 on average for the southeast Indian Ocean. (2) After adjustment:the adjustment greatly counteracts the impact of high wind speeds and improves the overall accuracy of the retrieved salinity (the mean absolute error of the Zonal mean is improved by 0.06, and the mean error is -0.05 compared with MyOcean SSS). Near the latitude 42°S, the adjusted SSS is well consistent with the MyOcean and the difference is approximately 0.004.
Role of ocean upper layer warm water in the rapid intensification of tropical cyclones:A case study of typhoon Rammasun (1409)
SUN Jia, ZUO Juncheng, LING Zheng, YAN Yunwei
2016, 35(3): 63-68. doi: 10.1007/s13131-015-0761-1
Rammasun intensified rapidly from tropical storm to super typhoon in the northern South China Sea (NSCS) before its landfall on Hainan Island. Analysis of observed data shows that the anomalous ocean upper layer warm water (WW) is important to the rapid intensification of Rammasun. During the period of Rammasun, sea surface temperature (SST) in the NSCS was much warmer than the climatological SST. The anomalous WW supplied more energy to Rammasun, resulting in its rapid intensification. Numerical simulations further confirm that the NSCS WW plays an important role in the rapid intensification of Rammasun. As the WW is removed, the intensification of Rammasun is only 25 hPa, which is 58.1% of that in the original SST-forced run.
Comparison and combination of EAKF and SIR-PF in the Bayesian filter framework
SHEN Zheqi, ZHANG Xiangming, TANG Youmin
2016, 35(3): 69-78. doi: 10.1007/s13131-015-0757-x
Bayesian estimation theory provides a general approach for the state estimate of linear or nonlinear and Gaussian or non-Gaussian systems. In this study, we first explore two Bayesian-based methods:ensemble adjustment Kalman filter (EAKF) and sequential importance resampling particle filter (SIR-PF), using a well-known nonlinear and non-Gaussian model (Lorenz '63 model). The EAKF, which is a deterministic scheme of the ensemble Kalman filter (EnKF), performs better than the classical (stochastic) EnKF in a general framework. Comparison between the SIR-PF and the EAKF reveals that the former outperforms the latter if ensemble size is so large that can avoid the filter degeneracy, and vice versa. The impact of the probability density functions and effective ensemble sizes on assimilation performances are also explored. On the basis of comparisons between the SIR-PF and the EAKF, a mixture filter, called ensemble adjustment Kalman particle filter (EAKPF), is proposed to combine their both merits. Similar to the ensemble Kalman particle filter, which combines the stochastic EnKF and SIR-PF analysis schemes with a tuning parameter, the new mixture filter essentially provides a continuous interpolation between the EAKF and SIR-PF. The same Lorenz '63 model is used as a testbed, showing that the EAKPF is able to overcome filter degeneracy while maintaining the non-Gaussian nature, and performs better than the EAKF given limited ensemble size.
Calibration of HY-2A satellite significant wave heights with in situ observation
PENG Hailong, LIN Mingsen
2016, 35(3): 79-83. doi: 10.1007/s13131-015-0758-9
Significant wave height (SWH) can be computed from the returning waveform of radar altimeter, this parameter is only raw estimates if it does not calibrate. But accurate calibration is important for all applications, especially for climate studies. HY-2a altimeter has been operational since April 2012 and its products are available to the scientific community. In this work, SWH data from HY-2A altimeters are calibrated against in situ buoy data from the National Data Buoy Center (NDBC), Distinguished from previous calibration studies which generally regarded buoy data as "truth", the work of calibration for HY-2A altimeter wave data against in situ buoys was applied a more sophisticated statistical technique-the total least squares (TLS) method which can take into account errors in both variables. We present calibration results for HY-2A radar altimeter measurement of wave height against NDBC buoys. In addition, cross-calibration for HY-2A and Jason -2 wave data are talked over and the result is given.
Research on polarization of oil spill and detection
CAI Yang, ZOU Yarong, LIANG Chao, ZOU Bin
2016, 35(3): 84-89. doi: 10.10.1007/s13131-015-0817-x
The SAR (Synthetic Aperture Radar) has the capabilities for all-weather day and night use. In the case of determining the effects of oil spill dumping, the oil spills areas are shown as dark spots in the SAR images. Therefore, using SAR data to detect oil spills is becoming progressively popular in operational monitoring, which is useful for oceanic environmental protection and hazard reduction. Research has been conducted on the polarization decomposition and scattering characteristics of oil spills from a scattering matrix using all-polarization of the SAR data, calculation of the polarization parameters, and utilization of the CPD (Co-polarized Phase Difference) of the oil and the sea, in order to extract the oil spill information. This method proves to be effective by combining polarization parameters with the characteristics of oil spill. The results show that when using Bragg, the oil spill backscattering machine with Enopy and a mean scatter α parameter. The oil spill can be successfully identified. However, the parameter mechanism of the oil spill remains unclear. The use of CPD can easily extract oil spill information from the ocean, and the polarization research provides a base for oil spill remote sensing detection.
Assessment of intraseasonal variabilities in China Ocean Reanalysis (CORA)
ZHANG Min, ZHOU Lei, FU Hongli, JIANG Lianghong, ZHANG Xiangming
2016, 35(3): 90-101. doi: 10.1007/s13131-016-0820-2
A regional reanalysis product-China Ocean Reanalysis (CORA)-has been developed for the China's seas and the adjacent areas. In this study, the intraseasonal variabilities (ISVs) in CORA are assessed by comparing with observations and two other reanalysis products (ECCO2 and SODA). CORA shows a better performance in capturing the intraseasonal sea surface temperatures (SSTs) and the intraseasonal sea surface heights (SSHs) than ECCO2 and SODA do, probably due to its high resolution, stronger response to the intraseasonal forcing in the atmosphere (especially the Madden-Julian Oscillation), and more available regional data for assimilation. But at the subsurface, the ISVs in CORA are likely to be weaker than reality, which is probably attributed to rare observational data for assimilation and weak diapycnal eddy diffusivity in the CORA model. According to the comparison results, CORA is a good choice for the study related to variabilities at the surface, but cares have to be taken for the study focusing on the subsurface processes.
A quantitative approach to monitoring new sand cay migration in Nansha Islands
ZHU Haitian, JIANG Xingwei, MENG Xuelian, FENG Qian, CUI Songxue, LIANG Chao
2016, 35(3): 102-107. doi: 10.1007/s13131-016-0827-8
Sand cay is a special kind of islet formed by coral detritus and bioclast, which is common in Nansha Islands of China. Some sand cays play an important role in ocean strategy and economy, but surprisingly we know little about them, especially those recently formed sand cays. In this research, we monitor migration of a new sand cay in Nanxun Jiao (Gaven Reef) using a series of QuickBird and WorldView -2 satellite images between June 2006 and August 2013. We conduct a regression between migration distance and wind observational data to examine the migration patterns of the new sand cay. The migration distance is calculated based on the sand cay locations extracted based on Normalized Difference Water Index (NDWI). The wind observational data downloaded from NOAA are reformed into four wind direction vectors. Based on the results of regression, we concluded that the migration of the new sand cay on Nanxun Jiao was significantly associated with the east, west and north wind. East wind was the main influence factor of the migration; its impact strength was almost twice as the west and north wind. The south wind has little effect on the migration of the sand cay, which is partly blocked by the artificial structure in the south.
Impacts of climatic and marine environmental variations on the spatial distribution of Ommastrephes bartramii in the Northwest Pacific Ocean
YU Wei, CHEN Xinjun, YI Qian, GAO Guoping, CHEN Yong
2016, 35(3): 108-116. doi: 10.1007/s13131-016-0821-1
Ommastrephes bartramii is an ecologically dependent species and has great commercial values among the Asia-Pacific countries. This squid widely inhabits the North Pacific, one of the most dynamic marine environments in the world, subjecting to multi-scale climatic events such as the Pacific Decadal Oscillation (PDO). Commercial fishery data from the Chinese squid-jigging fleets during 1995-2011 are used to evaluate the influences of climatic and oceanic environmental variations on the spatial distribution of O. bartramii. Significant interannual and seasonal variability are observed in the longitudinal and latitudinal gravity centers (LONG and LATG) of fishing ground of O. bartramii. The LATG mainly occurred in the waters with the suitable ranges of environmental variables estimated by the generalized additive model. The apparent north-south spatial shift in the annual LATG appeares to be associated with the PDO phenomenon and is closely related to the sea surface temperature (SST) and sea surface height (SSH) on the fishing ground, whereas the mixed layer depth (MLD) might contribute limited impacts to the distribution pattern of O. bartramii. The warm PDO regimes tend to yield cold SST and low SSH, resulting in a southward shift of LATG, while the cold PDO phases provid warm SST and elevated SSH, resulting in a northward shift of LATG. A regression model is developed to help understand and predict the fishing ground distributions of O. bartramii and improve the fishery management.
Study on wave energy resource assessing method based on altimeter data-A case study in Northwest Pacific
WAN Yong, ZHANG Jie, MENG Junmin, WANG Jing, DAI Yongshou
2016, 35(3): 117-129. doi: 10.1007/s13131-016-0804-2
Wave energy resource is a very important ocean renewable energy. A reliable assessment of wave energy resources must be performed before they can be exploited. Compared with wave model, altimeter can provide more accurate in situ observations for ocean wave which can be as a novel method for wave energy assessment. The advantage of altimeter data is to provide accurate significant wave height observations for wave. In order to develop characteristic and advantage of altimeter data and apply altimeter data to wave energy assessment, in this study, we established an assessing method for wave energy in local sea area which is dedicated to altimeter data. This method includes three parts including data selection and processing, establishment of evaluation indexes system and criterion of regional division. Then a case study of Northwest Pacific was performed to discuss specific application for this method. The results show that assessing method in this paper can assess reserves and temporal and spatial distribution effectively and provide scientific references for the siting of wave power plants and the design of wave energy convertors.