Rapid Sea Level Rise in the Southern Hemisphere Subtropical Oceans

The subtropical oceans between 35 degrees and 20 degrees S 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 35 degrees and 20 degrees S 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.