Understanding of sea-level rise and decadal changes on regional and global scales

Abstract

Present-day sea-level rise is the key indicator of climate change. Since late 1992, the satellite altimeters have clearly shown the global mean sea-level (GMSL) has been rising due to anthropogenic changes in Earth’s climate system, including increased ice-melting and ocean heat uptake. However, despite continuous global warming, the GMSL rise slowed over the 2000s, and the sea level decreased in the eastern tropical Pacific and increased in the western tropical Pacific. After the 2010, the GMSL increased again, and the pattern reversed compared with the earlier decades. Natural climate variability, which is responsible for redistributing ocean heat and land water storage, can obscure the underlying global and regional sea-level rise. Therefore, identifying the climate-driven sea-level change on global and regional scales is essential to enhance our understanding of sea-level changes and mitigate the impact of ongoing sea-level rise. Sea-level changes are caused by several physical processes. Recent advancements in in- situ observation and satellite measurements have enabled comparing the sum of each process with the observed sea-level change within uncertainty. Previous studies have demonstrated the closure of the GMSL budget and the contribution of each process to sea-level rise (Chen et al., 2017; Dieng et al., 2017; WCRP, 2018; Royston et al., 2020; Cha et al., 2021). However, due to each physical process varying in space and time, estimating their contribution to sea-level rise on a regional scale remains a challenging issue. East Asian marginal seas, including the Yellow and East China Seas, South China Sea, and East/Japan Sea, have experienced higher sea-level rise than the GMSL. Thus, sea-level rise is getting more attention because a lot of people live near the East Asian Marginal Seas. In this dissertation, three studies are carried out to improve our understanding of climate- driven sea-level variability and underlying process of sea-level rise on global and regional scales. The first study examines the decade-long fluctuation in the GMSL rise, which can be attributed to climate-related decadal fluctuation in ocean heat storage and hydrology. The GMSL rise rate was slowed down over the 2000s and has increased rapidly after 2011. Over the 2010s, the climate-driven decadal changes led to additional ocean mass gain from land water storage and increased ocean heat uptake, resulting in the GMSL rise rate increasing. This study demonstrates that this decadal fluctuation is linked to Pacific decadal variability and notes that the importance of natural variability in understanding the impact of the ongoing sea- level rise. The second study focuses on a dramatic shift in the tropical Pacific Ocean toward an El Niño-like state, which coincides with a period of a recent resumption of global warming after a hiatus in the 2000s. The observation and model data analysis using ensemble empirical mode decomposition shows that the distinct decadal mode in the tropical Pacific is associated with the suppression and resumption of global warming over this period. This decadal mode is associated with Pacific Decadal Oscillation-related wind variability on a decadal time scale, which can control the sea-level trend, subsurface temperature changes, and the strength of the Equatorial Under Current in the tropical Pacific. Hindcast and model experiments illustrate that decadal climate-related wind forcing can control regional oceanic response in the tropical Pacific has a possible link to global ocean warming. The third study is a process-based assessment of the sea-level rise in the northwestern Pacific marginal seas, including the Yellow and East China Sea, South China Sea, and East/Japan Sea. This study conducts the budget analysis comparing the combination of observation and reanalysis with satellite sea-level change and estimates the contribution of each physical process to sea-level rise. The result shows that the sterodynamic effect and ice- melting are significant processes to sea-level rise and sterodynamic process dominates the spatial pattern and variability. Further estimation using in situ profiles and satellite gravity measurement presents that ocean mass redistribution plays a crucial role in sterodynamic sea- level change along the continental shelves.