Rapid Rise in Air Temperature and Variation in Seawater Temperature in Osaka Bay: Analysis of Data Measured with Monitoring Posts

Abstract

The annual average air temperature in Japan had been gradually declining from 2000 to the early 2010s, but since the cool summer of 2014, it has risen rapidly with some fluctuations. We investigated the impact of this rise in air temperature on the water temperature in Osaka Bay from April 2010 to March 2024. In winter (January to March average) , the water temperature has risen in tandem with the air temperature, and the annual minimum water temperature in Akashi Strait has risen by 1.59 ℃ per decade. As a result, the number of days with a daily average water temperature below 10 ℃ has decreased from 84 days in 2011 to 0 days from 2023 onwards. On the other hand, in summer (July to September average) , there was no significant increase in either the air temperature or the surface water temperature. The surface water temperature fluctuated owing to the effects of cold water flowing into the bottom layer of Osaka Bay from the bottom water of Kii Channel and the air temperature. Therefore, even in a hot summer, if cold water flows into the bottom layer, the surface water temperature did not rise. The seasonal variation in lower water temperature was markedly affected by the presence or absence of cold-water inflow.