Multidecadal Sea Level Rise in the Southeast Indian Ocean: The Role of Ocean Salinity Change

Abstract

Abstract Regional sea level rise in the southeast Indian Ocean (SEIO) exerts growing threats to the surrounding Australian and Indonesian coasts, but the mechanisms of sea level rise have not been firmly established. By analyzing observational datasets and model results, this study investigates multidecadal steric sea level (SSL) rise of the SEIO since the mid-twentieth century, underscoring a significant role of ocean salinity change. The average SSL rising rate from 1960 through 2018 was 7.4 ± 2.4 mm decade−1, and contributions of the halosteric and thermosteric components were ∼42% and ∼58%, respectively. The notable salinity effect arises primarily from a persistent subsurface freshening trend at 400–1000 m. Further insights are gained through the decomposition of temperature and salinity changes into the heaving (vertical displacements of isopycnal surfaces) and spicing (density-compensated temperature and salinity change) modes. The subsurface freshening trend since 1960 is mainly attributed to the spicing mode, reflecting property modifications of the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the southern Indian Ocean. Also noteworthy is a dramatic acceleration of SSL rise (20.3 ± 7.0 mm decade−1) since ∼1990, which was predominantly induced by the thermosteric component (16.3 ± 5.5 mm decade−1) associated with the heaving mode. Enhanced Ekman downwelling by surface winds and radiation forcing linked to global greenhouse gas warming mutually caused the depression of isopycnal surfaces, leading to the accelerated SSL rise through thermosteric effect. This study highlights the complexity of regional sea level rise in a rapidly changing climate, in which the role of ocean salinity is vital and time-varying.