Mesoscale ocean fronts and eddies are important mesoscale marine environment characteristics. The ocean front is the interface of water masses with different properties. In the area where an ocean front exists

corresponding hydrological factors (e.g.

temperature

chlorophyll concentration

and salinity) present a high horizontal gradient. Seawater convergence and vertical motion are enhanced in fields where fronts appear; this enhancement leads to the enrichment of nutrients and provides a rich diet to plankton

fish

and so on. Therefore

the sea area where fronts appear can serve as a good fishing ground (e.g.

Zhoushan and Minnan fishing grounds in China). Mesoscale eddy plays an important role in ocean circulation and is an important undertaker of energy transport and ocean material transfer in oceans. Furthermore

eddies can influence the distribution of hydrological factors

such as temperature and salinity

and is thus one of the important factors of marine hydrological variation. Eddies associated with local lifting flow

such as the upwelling associated with cold eddies

carry nutrients to euphotic zones from the bottom of the ocean

greatly improve the primary productivity of the ocean

and influence the distribution of the fishing ground. Thus

they affect the development of the marine economy. Remote sensing data demonstrate excellent continuity and synchronization and can effectively reflect the spatial distribution characteristics of marine hydrological elements and the sea surface height. Therefore

remote sensing data

such as sea surface temperature

sea surface height

and sea level anomaly

have been widely used in the extraction of mesoscale ocean fronts and eddies. Research on the extraction of mesoscale ocean fronts and eddies based on remote sensing data is thus significant to marine ecosystem research

fishery stock assessment

and fishing condition forecasting. We aim to provide a reference and ideas for the extraction of mesoscale ocean fronts and eddies by summarizing and analyzing the methods of extracting mesoscale ocean fronts and eddies. This study summarizes and analyzes the methods of front extraction

such as gradient method

canny algorithm

wavelet analysis method

and algorithms based on the law of universal gravity

as well as the methods of eddy extraction

such as OW

WA

SSH-based and HD methods. Insights and new ideas are then provided. To show the difference among various front extraction methods intuitively

fronts are extracted from same area by using gradient

Sobel

and canny algorithms. The extraction results are presented in a figure. When gradient and Sobel algorithms are used to extract fronts

the thresholds used to distinguish the background and front pixels are obtained by the iterative method. Afterward

the Zhang-Suen method is applied to implement binary image thinning. The center line of the front is then obtained. When the canny algorithm is utilized to extract fronts

0.25 is selected as the low threshold and 0.9 as the high threshold. With sea surface temperature (SST) data on the northern South China Sea (SCS) in February 2014

gradient

Sobel

and canny algorithms are used to extract the temperature front in northern SCS. A figure is subsequently drawn for the temperature front distribution of this area.Results show that among the various front extraction methods

the gradient method is simple but influenced greatly by noise. The canny algorithm presents a great advantage in front positioning accuracy

continuity

and computational efficiency. Wavelet analysis can be used in multi-scale analysis

but the computation is complex. The algorithm based on the law of universal gravity considers the influence of the value and position of the center and neighborhood pixels

so it has better anti-noise ability and accuracy than the gradient method. Among the various eddy extraction methods

the OW method can identify the eddy core region well

but the extraction result is greatly influenced by the selection of the W value threshold. The WA method presents excellent accuracy but requires extensive calculation to obtain the streamline. The SSH-based method is simple but can only extract the eddy boundary and not the core area of the eddy. Early eddy extraction methods ignore the condition that multi-core eddy structures may appear in oceans

and these methods are unable to identify multi-core eddy structures. The HD method combines the advantages of the OW and SSH-based methods. It can extract the boundary and core area of an eddy simultaneously and can identify multi-core eddy structures. According to the summary and analysis of various front and eddy extraction methods

threshold selection for front and eddy extraction is difficult but is important to the quality of the extraction result. Many researchers have examined the threshold selection method. In addition

edge detection methods

such as gradient method and the canny algorithm

that are used widely in front extraction are designed for extracting sharp edges (e.g.

solid edge). However

sea water is fluid and characterized by a weak edge. In other words

the edge of water masses with different properties is not obvious and more difficult to identify than a solid edge. Therefore

traditional edge detection methods are unsuitable for mesoscale front extraction due to the weak edge characteristic of marine environment element images. Given that the ocean front is the interface of different water masses

the region growing algorithm

which is used widely in image segmentation

can be utilized to segment the marine environmental element image of the study area into several independent parts that represent different water masses. Then

the boundaries we wish to extract (i.e.

the front) are searched.