Using hourly sea level data from four tide gauges, the changes of the extreme sea level in the Bohai Sea were analyzed in this work. Three components (i.e., mean sea level, tide and surge) as well as the tide–surge interaction were studied to find which component was important in the changes of extreme sea levels. Significant increasing trends exist in the mean sea level at four tide gauges from 1980 to 2016, and the increase rate ranges from 0.2 to 0.5 cm/year. The mean high tide levels show positive trends at four tide gauges, and the increasing rate (0.1 to 0.3 cm/year) is not small compared with the long-term trends of the mean sea levels. However, the mean tidal ranges show negative trends at Longkou, Qinhuangdao and Tanggu, with the rate from about −0.7 to −0.2 cm/year. At Qinhuangdao and Tanggu, the annual surge intensity shows explicit long-term decreasing trend. At all four tide gauges, the storm surge intensity shows distinct inter-annual variability and decadal variability. All four tide gauges show significant tide–surge interaction, the characteristics of the tide–surge interaction differ due to their locations, and no clear long-term change was found. Convincing evidence implies that the extreme sea levels increase during the past decades from 1980 to 2016 at all tide gauges, with the increasing rate differing at different percentile levels. The extreme sea level changes in the Bohai Sea are highly affected by the changes of mean sea level and high tide level, especially the latter. The surge variation contributes to the changes of extreme sea level at locations where the tide–surge interaction is relatively weak.

The risk of marine disasters based on flooding is one of the most significant natural disasters in coastal zones. It can be said that flooding in coastal zones has typical sea–land characteristics. Yet, relatively little research has been done in this area. Thus, by using the characteristics of marine disaster risk based on flooding in Qingdao and combining marine science and land science methods, this paper constructs a targeted indicator system for the flooding risk from marine disasters from the perspectives of the disaster natural chain and flooding process. According to the results, the Integrated Risk Index of marine disasters based on flooding in Qingdao is 0.3694, which represents a medium risk level for natural disasters in China’s major coastal areas. The first- and third-level indicators with large contribution rates are almost all natural indicators. This indicates that the natural disaster process and disaster chain greatly affect the flooding disaster risk in Qingdao. However, although natural factors play large roles in the risk of disaster, preventive methods implemented by humans can still have a positive effect on disaster reduction. Therefore, human society should still proceed with understanding disasters from natural processes, change their passive response to active adaptation, and actively strengthen preventive measures to alleviate the adverse impacts of increasingly serious natural disasters.

Using bulk formulas, two-year platform (fastened to the seabed) hourly observations from 2016 to 2017 in the East China Sea (121.6° E, 32.4° N) are used to investigate the role of the tide-induced surface elevation in changing the fixed observational height and modifying the momentum and air-sea turbulent heat fluxes. The semidiurnal tide-dominated elevation anomalies ranging from −3.6 to 3.9 m change the fixed platform observational height. This change causes hourly differences in the wind stress and latent and sensible heat fluxes between estimates with and without considering surface elevation, with values ranging from −1.5 × 10⁻³ Nm⁻², −10.2 Wm⁻², and −3.6 Wm⁻² to 2.2 × 10⁻³ Nm⁻², 8.4 Wm⁻², and 4.6 Wm⁻², respectively. More significant differences occur during spring tides. The differences show weak dependence on the temperature, indicating weak seasonal variations. The mean (maximum) difference percentage relative to the mean magnitude is approximately 3.5% (7%), 1.5% (3%), and 1.5% (3%) for the wind stress and latent and sensible heat fluxes, respectively. The boundary layer stability (BLS) can convert from near-neutral conditions to stable and unstable states in response to tide-induced changes in the observational height, with a probability of occurrence of 2%. Wind anomalies play dominant roles in determining the hourly anomalies of the latent heat flux, regardless of the state of the BLS. Extreme cases, including the cold air outbreak in 2016, tropical cyclones Meranti in 2016, and Ampil in 2018, are also examined. This study will facilitate future observation-reanalysis comparisons in the studied coastal region where ocean–atmosphere-land interactive processes are significant.