Long-term trends and seasonal variations in environmental conditions in Sesoko Island, Okinawa, Japan

Abstract

Knowledge of environmental factors is crucial in understanding biological and ecological processes. Yet information on the environment around Sesoko Island, Okinawa, Japan, one of the main locations for coral reef research in Japan, remains scarce. Data of air and sea surface temperature (SST), wind velocity, wave height, and frequency of typhoons have been manually recorded at Sesoko Station, Tropical Biosphere Research Station, the University of the Ryukyus from September 1990 to November 2021. Here we describe the seasonal and long-term trends in these environmental variables at Sesoko Island. Some of the key findings were that the air temperature and SST fluctuated by ~9 - 12°C throughout the year. A rise in air temperature and SST between 1990 and 2021 was observed in the winter and autumn season, respectively. The Degree Heating Week (DHW) based on the in-situ data reflected the bleaching obser vations around Sesoko Station. The DHW exceeded the critical bleaching level of 8°C-week in 1998 and the significant bleaching level of 4°C-week in 2001, 2016, and 2017. Weak southerly winds were dominant in summers, while stronger northeasterly winds were dominant in winters. The frequency of winds between 3.4 to 7.9 m/s and northeastern winds have increased through time. Typhoons generally occur between May and October, and the frequency of typhoons has not increased over the past 30 years. Wave heights never exceeded 0.5 m and were highest between July and September. These findings will provide a reliable baseline of the environment at Sesoko Island for further ecological studies.