Combined effects of climatic factors on extreme sea level changes in the Northwest Pacific Ocean

Extreme sea level events (ESLs) act as a comprehensive consequence of climate change, and a better understanding of the impact processes of climatic factors on ESLs variability is essential for obtaining a coastal sustainable development strategy. In this work, hourly sea-level data from 699 worldwide tide gauges between 1960 and 2013 were used to analyze the ESLs variability, and we selected 72 representative tide gauges in the Northwest Pacific, which stood out in terms of ESLs intensity, duration and occurrence. Under the climate change framework, The Northwest Pacific is a key area that subjected to stronger, longer, and more frequent ESLs after 1990, and reef coast ESLs gradually shifted their sensitivity from intensity to occurrence. TC-summer and winter are the peak seasons of ESLs during the year. The effects of water level discrepancy (WLD) on ESLs are increasing, and the growing impacts even exceeded the impact of sea-level at a few Indonesian tide gauges. Low frequency variations of sea-level are provided as the background of ESLs variabilities on a seasonal time scale and secular change, with more than 80% of tide gauges experiencing accelerated sea-level rise. In terms of magnitude, the high tide level is the major climatic factor to ESLs along the Northwest Pacific coast. The nested Gumbel–Hougaard copula function is built to evaluate the ESLs partial correlation-joint return period, which introduces the combined effect of WLD, sea-level variability, high tide level, and wind. Sea-level variability, WLD, and wind all shorten the ESLs joint return periods at more than 70% tide gauges, particularly, wind exhibits as a significant shortening climatic factor throughout the Northwest Pacific. Meanwhile, the high tide level shows significant and complicated contribution to the ESLs, which fluctuated according to the complex features of the tidal wave system, and this complexity makes it possible to increase the return period. An individual climatic factor influenced weakly on ESLs joint return period when a second climatic factor is introduced considering the interactions between different climatic factors. More tide gauges will likely exhibit complex features of the ESLs joint return period, and some tide gauges that have a shortened ESLs joint return period may even show an increase. Considering the coastal flooding risk framework, coastal flooding risk is growing in the decadal fluctuation, so the enhanced forecasting and prevention strategy against future ESLs will provide an effective blueprint for adaption and mitigation.