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

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-20th 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 depths. 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 rapid-changing climate, in which the role of ocean salinity is vital and time-varying.

The codependence of contributors to regional sea-level rise

<p>Science-based policy for coastal protection requires accurate estimates of the uncertainty in regional sea-level rise. These estimates are strongly influenced by the codependence of individual contributors: thermosteric expansion, ocean dynamics, and mass loss from glaciers and ice sheets. In this study, we use model output and parameterisations to quantify the projected total sea-level rise from a set of 15 Earth System Models from the Coupled Model Intercomparison Project (CMIP) 5. We use these model-based estimates of total sea-level rise to quantify the codependence of individual contributors, determined by the full climate response. We find that assumptions on codependence made in recent reports of the Intergovernmental Panel on Climate Change (IPCC) lead to an overestimation in the uncertainty in regional sea-level rise by 20 to 60%. We further conclude that global mean surface temperature rise is a poor indicator for the inter-model difference in regional sea-level rise as it does not account for inter-model differences in atmospheric and oceanic heat distribution and precipitation patterns. The codependencies derived in this study are suitable for application to new projections, allowing for accurate and consistent estimates of the uncertainty in global and regional sea-level rise.</p>