Seafloor temperature variability in the Zhoushan Archipelago: Patterns and mechanisms

Abstract

Sea water temperature is one of the most critical environmental parameters for characterizing and regulating marine biogeochemical processes. Due to limited observational data, past researches have primarily focused on water temperature in the upper ocean, with relatively little attention given to the spatiotemporal evolution and modulation mechanisms of seafloor temperature (SFT). Utilizing continuous, high-frequency in situ SFT data collected by submarine cable observation systems at four stations (Dongji 1, Dongji 2, Dajishan, and Zhairuoshan) in the Zhoushan Archipelago area (ZA) from October 2020 to December 2021, we examined the patterns and regulatory mechanisms of SFT variability within this region. Results show that at the Dongji 1, Dongji 2, and Dajishan stations, the diurnal standard deviation (STD) of SFT exhibits a remarkable seasonal pattern, being higher in winter and summer, and lower in spring and autumn, at the same time, SFT displays significant semi-diurnal and diurnal periodic signals during winter and summer, which nearly disappear in spring and autumn; at the Zhairuoshan station, periodic variations in SFT are minimal throughout the year. Further analyses of multi-source sea temperature data and results from Lagrangian particle tracking point out that the diurnal STD of SFT at Dongji 1, Dongji 2, and Dajishan stations is mainly dominated by the distinct sea temperature background and water exchange between the ZA and adjacent water: due to strong tidal mixing and shallow topography, the ZA, Hangzhou Bay and along with the southern regions of the Yangtze River Estuary exhibits weak stratification throughout the year. In winter, under the control of tidal currents and the southward Zhejiang-Fujian Coastal Current (ZFCC), daily water exchange in ZA primarily with the colder waters originating from Hangzhou Bay and the southern Yangtze River Estuary. While during summer, the ZA is occupied by cold upwelling, and dominated by tidal currents and the northward ZFCC, thus ZA mainly exchange with warmer waters from Hangzhou Bay and the southeastern side of the archipelago within a day. Therefore, this seasonal variation results in larger diurnal STDs of SFT in winter and summer. In contrast, during the monsoon transition period of spring and autumn, the horizontal and vertical distribution of sea temperature across the ZA and its adjacent water is generally uniform, leading to smaller diurnal STD of SFT. The relatively enclosed hydrological environment of Zhairuoshan station leads to limited daily water exchange, resulting in consistently weak diurnal STD of SFT year-round.