Rapid Sea-Level Rise in the Southern-Hemisphere Subtropical Oceans
The subtropical oceans between 35o and 20oS in the Southern Hemisphere (SH) have exhibited prevailingly rapid sea-level rise (SLR) rates since the mid-twentieth century, amplifying damages of coastal hazards and exerting increasing threats to South America, Africa, and Australia. Yet, mechanisms of the observed SLR have not been firmly established, and its representation in climate models has not been examined. By analyzing observational sea level estimates, ocean reanalysis products, and ocean model hindcasts, we show that the steric SLR of the SH subtropical oceans between 35o and 20oS is faster than the global mean rate by 18.2% ± 9.9% during 1958–2014. However, present climate models—the fundamental bases for future climate projections—generally fail to reproduce this feature. Further analysis suggests that the rapid SLR in the SH subtropical oceans is primarily attributable to the persistent upward trend of the southern annular mode (SAM). Physically, this trend in SAM leads to the strengthening of the SH subtropical highs, with the strongest signatures observed in the southern Indian Ocean. These changes in atmospheric circulation promote regional SLR in the SH subtropics by driving upper-ocean convergence. Climate models show systematic biases in the simulated structure and trend magnitude of SAM and significantly underestimate the enhancement of subtropical highs. These biases lead to the inability of models to correctly simulate the observed subtropical SLR. This work highlights the paramount necessity of reducing model biases to provide reliable regional sea-level projections.
The present study aims to investigate the characteristics and mechanisms of the regional SLR in the SH subtropical oceans between 35o and 20oS and evaluate its representations in climate models. By analyzing multiple observation- and model-based datasets and performing model experiments, we demonstrate that the rapid regional SLR of the SH subtropics was primarily caused by the persistent upward trend of the
SAM since the mid-twentieth century. It is also found that the state-of-the-art climate models significantly underestimate the observed subtropical SLR owing to systematic biases in the simulated structure and trend magnitude of SAM.
Observation-based and OGCM datasets reveal that the sea level rise of the Southern Hemisphere subtropics (35o–20oS) is faster than the global mean rate by 18.2% ± 9.9% for the 1958–2014 period. This rapid sea-level rise is primarily attributable to the persistent upward trend of the southern annular mode (SAM), while the influence of tropical climate variability is minimal in the South Indian Ocean (SIO). The upward trend of SAM gave rise to enhancements of the subtropical highs in the Southern Hemisphere, which is favorable for regional SLR by driving upper-ocean convergence. Systematic biases are detected in the simulated structure and trend magnitude of the SAM in climate models, with a robust underestimation of the SAM’s signatures in the Southern Hemisphere subtropics. These systematic biases of climate models are the key to the underestimated SLR. As a result, climate models are not able to correctly simulate the observed SLR between 35o and 20oS. Our results suggest that the observed Southern Hemisphere subtropical sea level rise is largely driven by the upward SAM trend associated with ozone depletion. Given this, there might be mitigation of sea-level rise over the coming decades, owing to the recovery of Antarctic ozone depletion. Nevertheless, forced by the increasing emissions of greenhouse gases, the future sea-level rise is expected to be continuously rapid in the Southern Hemisphere subtropics, with increasing severity and seriousness of the threat to surrounding countries. The probabilistic prediction of regional sea-level rise is essential for formulating adaption strategies, which rely largely upon climate models. To improve the model representation of the sea level rise over the Southern Hemisphere oceans, some steps seem to be requisite, such as using more realistic ozone forcing, reducing biases in the Southern Ocean climatology, better taking into account small-scale eddies. Overall, these results have important implications for understanding and predicting regional sea-level rise in the ongoing anthropogenic climate change, and this work contributes to helping the hundreds of millions of people residing in coastal areas of South America, Africa, and Australia adapt to future global climate change, with implications for other areas of the world.