Despite the backdrop of continuous global warming, sea ice extent has been found not to consistently decrease across the globe, and instead exhibit heterogeneous variability at middle to high latitudes. However, the existing studies are focused primarily on high latitude frozen seas, while studies on the long-term variability of sea ice cover at middle latitudes are generally lacking. Afforded by continuous satellite imagery, evolution of sea ice cover over nearly three decades from 1988 to 2015 in the Bohai Sea as a peculiar mid-latitude frozen sea area is reported for the first time. An anomalous trend of slight overall increase of 1.38 ± 1.00% yr–1 (R = 1.38, i.e. at a statistical significance of 80%) in Bohai Sea ice extent was observed over the 28 year period. The detrended annual average ice area (AAIA) was further found to correlate with a slight decreasing mean ice-period average temperature (IAT, r = –0.58, p < 0.01) of 11 meteorological stations around the Bohai Sea as well as a mild increasing cumulative freezing degree days (CFDD, r = 0.65, p < 0.01). Correlation with decreasing Arctic Oscillation (AO) index (r = –0.60, p < 0.01) and North Atlantic Oscillation (NAO) index (r = –0.69, p < 0.01) over the study period suggested AO and NAO as the primary large-scale climate factors for Bohai Sea ice. In addition, the seasonal cycle of ice cover showed a single peak with longer freezing phase than melting phase, due to the different temperature change rate during the freezing and melting phases. The results can provide important references for monitoring the recent climate change in the region and beyond.

Sea-ice hazard causes serious harm to aquaculture, marine navigation, offshore oil production and other activities in the Bohai Sea, China. To study the spatial distribution characteristics of sea-ice-hazard risk in Bohai is therefore desirable. The thickness and area of sea ice in the Bohai Sea during the winters (December–March) of 1987–2011 were estimated using data from the NOAA (US National Oceanic and Atmospheric Administration) satellite. The sea-ice thickness was converted into a sea-ice-hazard index after defining this index, and the different sea-ice-hazard risk grades were classified. The occurrence probability of sea-ice hazard was also calculated using fuzzy risk theory, and the spatial distribution characteristics of sea-ice-hazard risk in the Bohai Sea were studied. The results show that the sea-ice-hazard risk for offshore aquaculture decreased as the offshore distance increased. All the oilfields in Liaodong Bay are influenced by sea-ice hazard, two of the fields in Bohai Bay are slightly affected and the remaining fields are not influenced. The risk for marine navigation is related to the location of the port and the distance from the port. The risk in the port area is the highest; it is reduced by more than 30% at distances 10 km away from the port.

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ABSTRACT. To investigate the spatio-temporal characteristics of sea-ice resource, we used sea-ice volume to measure the amount of sea-ice resource in the Bohai Sea, China. The sea-ice area was extracted from Advanced Very High Resolution Radiometer (AVHRR) remote-sensing images using the zonal threshold method. The sea-ice thickness was estimated using a sea-ice model based on shortwave radiation theory and field measurements. The spatio-temporal characteristics of sea-ice volume were then analysed using GIS technology. The results indicate that the Bohai Sea experienced two sea-ice volume peaks in winter 2009/10. The largest sea-ice volume was in Liaodong Bay ($80.26% of the entire sea-ice volume of the Bohai Sea). Bohai Bay had the second largest ice volume, and Laizhou Bay the smallest. The relationship between sea-ice volume and distance from shore is essentially exponential. The proportion of total sea-ice volume that is 0-10 km from shore is $42.43%, whereas the proportion that is 100-110 km from shore is only 0.002%.