Recent decline of sea ice area in the Bohai Sea over the period 2001 to 2023

The Bohai Sea, China's northernmost sea, experiences sea ice during winter that impacts shipping, ports, mariculture, and infrastructure, while also serving as vital habitat for the spotted seal. Monitoring sea ice variability is crucial for both marine economic and ecological sustainability. This study utilizes MODIS satellite images with minimal or no cloud cover and employs a sample-based object-oriented feature extraction method, along with visual interpretation corrections, to construct a daily (though discontinuous) time series of sea ice area in the Bohai Sea from 2001 to 2023. The average sea ice area decreased by 0.15 × 10⁴ km² (16.36 %) from 2001–2010 to 2011–2023. The longer initial freezing period (∼5 weeks) and shorter melting period (∼3 weeks) show an asymmetric unimodal pattern in daily average sea ice area variation. The severe freezing phase lasts nearly 6 weeks, typically reaching its peak in late January or early February. Spatially, sea ice freezing frequency decreases from north to south and nearshore to offshore, with frequencies above 40 % concentrated along the northern and eastern coasts of Liaodong Bay. The overall trend of sea ice freezing frequency per pixel shows a decline in 56.98 % of the entire sea area, with an average rate of −0.16 %. The trend of freezing frequency is declining in Liaodong Bay (−0.25 %) and Bohai Bay (−0.10 %) but slightly increasing in Laizhou Bay (0.02 %). Local climate factors, including air temperature, northward wind, and surface sensible heat flux, play a crucial role in sea ice variation in the Bohai Sea, while the AO (r = −0.71, p < 0.01) and NAO (r = −0.61, p < 0.01) indices serve as important large-scale climatic factors.