Cold Water Phenomenon in Autumn in the Sea Area of the Xisha Islands

Abstract

Using satellite observation and reanalysis data, we studied the cold water phenomenon in the sea area of the Xisha Islands in autumn from 2003 to 2022. The cold water phenomenon refers to a colder water body relative to its surrounding area, which can partially offset the impact of global warming, thereby contributing to the protection and restoration of marine ecosystems. The cold water with a central position close to the Xisha Islands is distributed in a nearly circular area, and exhibits seasonal and interannual variations. It usually occurs at the end of September, with its intensity rapidly increasing and reaching its peak on October 1, then gradually weakening, and finally disappearing by November 6. The maximum intensity of the cold water phenomenon can reach 1.6°C. This phenomenon notably observed in October 2006, 2009, 2013 and 2022, with intensities reaching 0.9°C, 1.1°C, 1.1°C, and 0.4°C, respectively. The dynamic mechanism of its formation can be described as follows: in autumn of some years, the northeast monsoon occurs in the northern part of the South China Sea (SCS), while the southwest monsoon still prevails in the southern part of the SCS, which causes the generation of strong positive wind stress curl (WSC) near the Xisha sea area. The positive WSC causes cyclonic circulation and further causes upwelling through Ekman pumping, which transports low‐temperature water from the seabed to the sea surface and thus leads to the occurrence of the cold water phenomenon in the Xisha sea area.