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Satellite data products and high-resolution in situ observations were combined to investigate the evolution and structure of the Kuroshio Extension Front in Spring 2019. The former reveals the variation of the front is influenced by the northward movement of the Kuroshio Extension through transporting warm and saline water to a cold and brackish water region. The latter indicates steep upward slopes of the isopycnals, tilting northward in the frontal zone, as well as several ~300 m thick blobs of North Pacific Intermediate Water between 26.25 and 26.75 kg/m3， where conspicuous thermohaline intrusions occur. Further analysis indicates these thermohaline intrusions prefer to alternate salt fingering and diffusive convection interfaces, and are affected by strong shears.

Abstract. Satellite measurements during April to June in 2019 and direct observations from 28th to 30th May in 2019 about the Kuroshio Extension Front are conducted. The former shows the front experience a process of stable-unstable-stable state caused by the movement of the Kuroshio Extension’s second meander and a pinched-off eddy. The latter indicates the steep upward slopes of the isopycnals tilt northward in the strong frontal zone as well as several over 100 m thick blobs of cold and fresh water in the salinity minimum zone of North Pacific Intermediate Water. Using isopycnal anomaly method and diapycnal spiciness curvature method, characteristic interleaving layers are shown primarily in σθ = 26.3–26.9 kg/m3, which corresponds to large variations of potential spiciness in intermediate layers. Further analysis indicates the development of thermohaline intrusions may be driven by the double diffusive instability and the velocity anomalies. Besides, we find the turbulence mixing attributed to symmetric instability and shear instability is very strong in intermediate layer.

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Using image processing technology to extract important information, such as isoline and weather system of the meteorological facsimile chart, is conducive to integration with other information, and has important practical value in navigation operations, marine weather forecasting, target recognition, and image retrieval. In meteorological facsimile charts, there are many types of medium-value lines, dense lines in some areas, superimposition and presence of multiple information, such as isolines and isoline characters, intersection of specific weather system symbols, etc. For different types of contours, numeric characters, weather system symbols and other object characteristics, the corresponding object extraction and recognition methods are proposed: Remove the latitude and longitude lines and coastline in the meteorological facsimile map by basemap matching; According to the position and shape features of the figure box, extract the meteorological fax figure box, separate and remove the different character tagging information; On the basis of identifying triangles and semicircles in weather symbols of the frontal system, the frontal symbols are extracted based on the circumscribed triangles and template matching. First the contour character on the fax image is expanded into a block connected region. Determine the position of the character information by judging the number of pixels in the connected region, and then use rotation and template matching to identify the numeric character. Using the meteorological facsimile maps of the US Meteorological Center and the Japan Meteorological Center for the main information extraction, experiments show that the method of this paper has a good effect on the complete and accurate symbol extraction of frontal weather systems, and reduces the computational complexity of contour detection, isoline extraction and numerical recognition. The methods can detect some information from weather charts properly and the error rate is very low.