Long Term Variability of Extreme Sea Surface Temperature in the Gulf of Tonkin, China

Till date, no study on trends in extreme sea surface temperature (SST) for different return periods has been done over the Gulf of Tonkin (GoT). Based on a 84-year (1940-2023) ERA5 Reanalysis data sets, this study for the first time, examined the spatio-temporal pattern in extreme SST for different return periods. Findings showed that more significant moderate to fast warming trends (0.1-0.16 °C per decade (dec-1)) only existed for the 2 year return period. Temporal trends in the 99th percentile SST are insignificant for all return periods. By using the linear regression method, the variability in extreme SST was obtained. Results showed that moderate warming trends dominated a large portion of GoT. Stronger trends, up to 0.018°C yr-1 are noticed near Guangdong, Haikou and southern Sanya in south China. Extreme SST exhibited a slow warming trend of 0.008 °C per year (yr-1) all through the study years. The SST is most stable in most waters in the southern GoT and few waters surrounding Dongfang. Temporal warming rates of SST revealed that 1940-1958, 1976-1994 and 2003-2012 were years of more coastal upwelling and could affect aquatic lives. The strongest warming rate of 0.07°C dec-1 occurred during 1994-2003. The GoT appeared warmer during spring. Spatial decadal variability of SST revealed that moderate warming trends occurred in few regions in the southern GoT and in larger portion of the central and northern GoT. The rise in SST between 1980 and 2020 in the GoT is not limited to increased anthropogenic activities.