Interannual variation of a bottom cold water mass in the Seto Inland Sea, Japan

A bottom cold water mass (BCWM) is a widespread physical oceanographic phenomenon in coastal seas, and its temperature variability has an important effect on the marine ecological environment. In this study, the interannual variation of the BCWM in Iyo-Nada (INCWM), a semi-enclosed bay in the Seto Inland Sea, Japan, from 1994 to 2015 and its response to air–sea heat flux change were investigated using monthly observational data and a hydrodynamic model. Surrounded by the isotherm of 18 ∘C, the observed multi-year average water temperature inside the INCWM was 17.58 ∘C with a standard deviation of 0.27 ∘C, while the mean area of INCWM was 5.73 × 105 m2 with a standard deviation of 4.35 × 105 m2. The interannual variation of average water temperature of INCWM showed a negative correlation with its area that indicates a low temperature corresponds to a big area. In addition, the interannual variation of the average temperature inside INCWM showed positive correlations with the local water temperature from April to July and with remote water temperature below 10 m in an adjacent strait in July. Differing from previously studied BCWMs, which had interannual variations depending closely on the water temperature before the warming season, the interannual variation of INCWM is more sensitive to the air–sea heat flux during the warming season than that in the previous winter. Further, by comparing several BCWMs, we found that the BCWM size is a key factor in understanding the heat transfer process responsible for the interannual variation of BCWMs in coastal seas. These findings will help us to understand the response of bottom cold water mass in coastal seas to sea surface forcing change.