Satellite-based analysis of surface upwelling in the sea adjacent to Zhoushan islands in China

Abstract

The fish catch in natural upwelling areas, which accounts for only 0.1% of the ocean surface, accounts for more than 40% of the world’s catch. The Zhoushan fishery, which is the largest fishery in China, is mainly formed by a coastal upwelling that features a low temperature. The upwelling in the study area (29.5–31.5° N, 121.5–123.5° E) is a vital factor affecting the formation of the Zhoushan fishery, and the primary productivity and low temperature that are brought by the rising water are important features of the upwelling. This study used global real-time high-resolution multivariate fused satellite (OSTIA) daily sea surface temperature (SST) data developed by the United Kingdom Meteorological Office that were collected from 1981 to 2020 to explore the spatial and temporal variation of the characteristics of the upwelling phenomenon in the study area. The data were processed by a temperature gradient-based upwelling edge detection algorithm to extract information on the central location of the upwelling, the location clusters in the core area, and the intensity index. The quantities of center and core area clusters were counted for each pixel point, and their corresponding probability values were calculated. The results of the spatial and temporal variation of the characteristics of the upwelling show that the upwelling in the study area was generated in April of each year, increased in intensity, and peaked in August, furthermore, the southern part of the upwelling dissipated in September. The region’s upwelling is spatially oblique and elliptical, with its long axis following the northeast and extending as far as the mouth of the Yangtze River. Its central location and core area were relatively stably existing in Ma’an Archipelago and Zhongjieshan Islands, which was consistent with the location of the two marine pastures in Zhoushan. According to our findings, locations with higher probability values in the upwelling center and core area, where upwelling occurs frequently, are usually accompanied by higher productivity and offer the potential to develop fishing grounds. The insights that were drawn from the study observations can, therefore, provide some reference for future artificial upwelling site selection.