Observed Antarctic sea ice expansion reproduced in a climate model after correcting biases in sea ice drift velocity

Shantong Sun,Ian Eisenman

doi:10.1038/s41467-021-21412-z

Abstract

The Antarctic sea ice area expanded significantly during 1979–2015. This is at odds with state-of-the-art climate models, which typically simulate a receding Antarctic sea ice cover in response to increasing greenhouse forcing. Here, we investigate the hypothesis that this discrepancy between models and observations occurs due to simulation biases in the sea ice drift velocity. As a control we use the Community Earth System Model (CESM) Large Ensemble, which has 40 realizations of past and future climate change that all undergo Antarctic sea ice retreat during recent decades. We modify CESM to replace the simulated sea ice velocity field with a satellite-derived estimate of the observed sea ice motion, and we simulate 3 realizations of recent climate change. We find that the Antarctic sea ice expands in all 3 of these realizations, with the simulated spatial structure of the expansion bearing resemblance to observations. The results suggest that the reason CESM has failed to capture the observed Antarctic sea ice expansion is due to simulation biases in the sea ice drift velocity, implying that an improved representation of sea ice motion is crucial for more accurate sea ice projections.

Highlights

The Antarctic sea ice area expanded significantly during 1979–2015
This implies that sea ice motion exerts a relatively strong control on the spatial structure of the sea ice area changes. These results suggest that the reason Community Earth System Model (CESM) fails to simulate the observed Antarctic sea ice expansion is due to simulation biases in the sea ice drift velocity
The ERAWind runs have a slower sea ice retreat than the LENS runs (Supplementary Fig. 6), but the ice does not expand like in the observed ice motion (ObsVi) runs, implying that surface wind biases may be partially responsible for the relevant biases in the simulated sea ice drift velocity

Summary

Introduction

The Antarctic sea ice area expanded significantly during 1979–2015. This is at odds with state-of-the-art climate models, which typically simulate a receding Antarctic sea ice cover in response to increasing greenhouse forcing. When the sum of the model-simulated internal variability and the model-simulated response to historical greenhouse forcing is considered, the observations fall deep within the tail of the model results[2] Overall, these studies suggest that internal variability can give rise to Antarctic sea ice expansion in some cases, a highly unusual realization of internal climate variability would be required to have occurred in the observations for this to explain the observed changes in the Antarctic sea ice. Alternatively, anthropogenic ozone depletion has been suggested to strengthen the Southern Hemisphere westerly surface winds, leading to an anomalous equatorward Ekman transport that initially causes cooling and sea ice expansion, followed by a slower warming due to upwelling of the warmer deep water[6,7,8]. Trends in the Southern Ocean winds have been found to be weaker in climate models than in observations[17,18], which has been suggested to influence the sea ice[19]

Methods

Results

Conclusion