Arctic Oscillation and ice severity in the Bohai Sea, East Asia

The Bohai Sea is the southernmost sea in the Northern Hemisphere where seasonal freezing takes place in winter. Climate and ice conditions there are very sensitive to large-scale climatic variations. In the present study, the authors investigated the relationships of ice severity in the Bohai Sea to the planetary Arctic Oscillation (AO) during the period 1954/55-2001/02. It has been found that during low-AO winters the regional mean temperature around the Bohai Sea is evidently lower, and the number of freezing days, freezing degree days and the length of freezing duration all change significantly. Daily temperature distribution shows a significant difference in both mean and variance between high-AO and low-AO phases, and the changes in the temperature means dominate over that of the variance in the context of sea ice severity. The temperature-AO relation is well supported by the corresponding features of the large-scale atmospheric circulation system. AO-related changes in the Siberian High and East Asian Trough cause remarkable changes to the thermal conditions at the surface and to the dynamical conditions in the middle troposphere, consequently affecting the air temperature over the Bohai Sea and ice severity. The AO-sea ice severity relation has been generally stationary over time during the last one hundred years. Analysis of cross-power coherency between the AO and ice indices for the last century shows that the highest and significant covariance stands at periods of about 2.1, 3, and 7.5-14 years. The Bohai Sea ice severity has declined rapidly since the 1970s, which is unprecedented since 1880, and this feature is in agreement with the positively upward trend in the AO and the associated changes in the Siberian sea level pressure, East Asian Trough, and the regional temperature.