Mechanisms of projected sea-level trends and variability in the Southeast Asia region based on MPI-ESM-ER

The low-lying and densely populated Southeast Asia (SEA) region is threatened by sea-level change. To better understand the mechanism of sea-level change in this region, the sea-level trends and variability in the SEA region are investigated over the historical period 1950–2014 and during 2015–2099 under Shared Socio-economic Pathway5-8.5 scenario forcing, based on the output of the high-resolution (~ 0.1° for ocean) global climate model MPI-ESM-ER. The results confirm that the SEA sterodynamic sea level and its components (i.e., thermosteric, halosteric, and manometric sea level) are rising with accelerations, which are superimposed on El Niño-Southern Oscillation (ENSO) driven variabilities. To understand changes of thermosteric and halosteric sea level, regional-mean ocean heat and freshwater contents are analysed based on physical processes of ocean transports and air-sea fluxes. We show that ENSO variability impacts the thermosteric sea level mainly through lateral ocean heat transports, while it impacts the halosteric sea level mainly through surface freshwater flux. In the projection, a decreased volume transport of the relatively cold water (with respect to the SEA average) from the Pacific warms the SEA region. At the same time, a freshening of the transported saline water (with respect to the SEA average) results in an increased ocean freshwater transport into the SEA region. Locally, the pathway of volume transport from the Pacific to the SEA region is shifting northward, which results in a weakened Indonesian throughflow and an enhanced South China Sea throughflow, both leading to changes of regional sea-level pattern.