Interannual Variability and Scenarios Projection of Sea Ice in Bohai Sea Part I: Variation Characteristics and Interannual Hindcast

The Bohai Sea is one of the southernmost areas for sea ice formation in the northern hemisphere. Sea ice disasters in this body of water severely affect marine activities and the safety of coastal residents. In this study, we analyze the variation characteristics of the sea ice in the Bohai Sea and establish an annual regression model based on predictable mode analysis method. The results show the following: 1) From 1970 to 2018, the average ice grade is (2.6±0.8), with a maximum of 4.5 and a minimum of 1.0. Liaodong Bay (LDB) has the heaviest ice conditions in the Bohai Sea, followed by Bohai Bay (BHB) and Laizhou Bay (LZB). In-terannual variation is obvious in all three bays, but the linear decreasing trend is significant only in BHB. 2) Three modes are obtained from empirical orthogonal function analysis, namely, single polarity mode with the same sign of anomaly in all of the three bays and strong interannual variability (82.0%), the north-south dipole mode with BHB and LZB showing an opposite sign of anomalies to that in LDB and strong decadal variations (14.5%), and a linear trend mode (3.5%). Critical factors are analyzed and regression equations are established for all the principal components, and then an annual hindcast model is established by synthesizing the results of the three modes. This model provides an annual spatial prediction of the sea ice in the Bohai Sea for the first time, and meets the demand of operational sea ice forecasting.