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Received：17 May 2019，
Revised：2019-8-29，
Accepted：05 September 2019，
Published：16 February 2020
移动端阅览
目的
2
西北太平洋是我国远洋渔船从事大规模商业性捕捞的重要作业海域，也是中尺度涡的活跃区域，因此中尺度涡的提取方法研究对渔场分析有十分重要的意义。传统的研究方法多依赖栅格数据进行涡旋提取，并用标准圆形来拟合涡旋，难以提取涡旋的多核结构，本研究对此进行了改进。
方法
2
利用海表面高度异常数据，充分考虑海洋涡旋形状多样性以及矢量数据结构的优点，结合过去研究中的涡旋约束条件，基于无阈值的闭合等值线法，确定涡旋边界和中心，实现对西北太平洋黑潮延伸区的不规则矢量涡旋的提取。
结果
2
由于原始的海表面高度异常栅格数据图最接近海洋中涡旋实际存在的形状和位置，因此分别以1997年6月1日、2000年6月1日、2003年6月1日、2006年6月1日、2009年6月1日、2015年6月1日的涡旋提取结果为例与同一时间同一研究区域内原始海表面高度异常数据栅格图以及传统研究方法中标准圆形的涡旋提取结果进行比较。选取1994年6月1日的提取结果为例，同等比例放大同一位置涡旋，计算重叠率，重叠率的最大值为89%，最小值为22%，平均重叠率为65%。结果表明，本文的提取结果更接近海洋中涡旋的实际形状和位置，且精度更高。
结论
2
本文基于海洋卫星高度计的涡旋提取方法，充分考虑了海洋涡旋形状不规则的特点，综合了矢量数据结构的优点，更符合海洋中涡旋的实际形状和位置，比传统方法得到的结果具有更高的准确性和易用性，适用于涡旋与渔场关系的研究。
Objective
2
The Northwest Pacific Ocean is an important operational area for the large-scale commercial fishing of offshore fishing vessels of China and an active area of mesoscale eddies. Mesoscale eddies are important marine phenomena in the upper oceans. They play indispensable roles in ocean circulation
heat and mass transport
entrainment of marine nutrients
and marine life. However
traditional eddy extraction methods have shortcomings. First
they have difficulties in extracting the multicore of the eddy. Second
the ocean eddy is affected not only by other eddies in the process of movement but also by the strong flow
such as the Kuroshio extension in the Pacific Ocean. The eddy could be dragged and detached. Therefore
the shape of the ocean eddy is generally not a standard circle on the horizontal direction. However
most traditional research methods rely on the standard circle to fit the shape of the ocean eddy.The extraction result of traditional research methods is different from the irregularity and complexity of the shape and structure of the actual ocean eddy. In the end
raster data are always applied to display the ocean eddy in the traditional research. Vector data are more accurate than raster data. Thus
the present study improves the extraction method of eddy.
Method
2
Given the eddy constraints in previous studies
the threshold-free closed contour method (TFCCM) fully considered the ocean eddy shape diversity and the advantages of the vector data structure. This method was used to determine the eddy boundary and core on the basis of the sea surface height anomaly (SSHA) data. The TFCCM did not need to provide a new value to judge the existence of the eddy and extract its core and boundary. This method used the extreme point in the local range of the eddy as the core and the outermost contour containing its core as the boundary. In case of missing data points in the study area
the invalid values of the SSHA data were interpolated by the inverse distance weighting method. After generating the SSHA data contour
the topological relationship between the contours was determined. The eddy boundary was the outermost contour and should satisfy the limiting condition of the amplitude and spatial scale. Considering the deformation rate in the latitude and longitude direction of the earth ellipsoid
the spatial scale of the eddy was calculated as the spherical distance on the earth ellipsoid. On the basis of the SSHA difference between the core and boundary of the eddy
the extracted eddy was judged whether it was cyclonic or anticyclonic. In addition
when extracting the eddy boundary
several unreasonable burrs may appear in the extraction result
and the eddy boundary had to be smoothed.
Result
2
The original SSHA raster data map was close to the shape and location where the eddy actually exists in the ocean. The extraction of eddies in this paper were compared with the original SSHA data raster map and the standard circular eddy extraction results of the traditional research method in the same study area. Results of the eddy extraction by TFCCM were compared with raster eddies on June 1
1997
2000
2003
2006
2009
and 2015. Comparison results showed that the eddy boundary in the raster data graph mode was generally blurred. After continuing to enlarge
the image in the raster format exhibited a mosaic phenomenon. Thus
accurately determining the shape and position of the eddy was difficult. TFCCM results were close to the actual shape and position of the eddy in the ocean. They can maintain the smoothness when scaling to analysis. For quantitative analysis
the extraction result on June 1
1994
was used as an example to amplify the eddy at the same position with the same ratio. The overlap ratio was calculated by the results in further analysis. The maximum overlap rate was 89%. The minimum value was 22%. The average overlap rate was 65%. These advantages may bring convenience in the further vector data analysis
such as topology analysis and buffer overlay
in studying the relationship between eddy and fishery.
Conclusion
2
The TFCCM was applied to achieve the extraction of the irregular vector eddy based on the SSHA data in the Northwest Pacific Ocean. Unlike previous studies
only the SSHA data were used in the eddy extraction without any other data resources. The deformation of the ellipsoidal of the Earth was considered. The TFCCM did not need to rely on the threshold of the SSHA data. Thus
the TFCCM could reduce the variable and noise source to simplify calculation. The format of the eddy extraction results belonged to the irregular vector data structure. These results displayed a clear multicore structure. Without considering the interaction between eddies
the eddy extraction results were compared with the original SSHA raster data maps and then overlaid on the traditional standard circular eddy extraction. Finally
comparison results were analyzed and discussed. The overlap rate was calculated
showing that the extraction results by the TFCCM did not deviate much from the actual shape and position of the eddy in the ocean. The extraction method achieved improved fitting degree and accuracy. It is suitable for the practical application. In the future work
the irregular vector results of eddies can be performed in fishery field analysis to improve the accuracy in judging the fishery area
which is affected by the eddy. On the basis of the analysis and discussion
the TFCCM can obtain marine environmental characteristic information that is close to the actual situation and achieve high reliability when combined with fishery data for subsequent research. This finding provides reference for the subsequent study of the relationships between eddies and fishery distribution.
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