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

Satellite observations indicate a significant overall expansion of the Antarctic sea ice cover during recent decades. This is at odds with basic physical intuition about how sea ice should respond to rising global temperatures, and it is also at odds with current coupled global climate models, which typically simulate a receding Antarctic sea ice cover in response to increasing greenhouse forcing during this time period. A range of previous hypotheses have been proposed to explain this engima, including internal climate variability and the transient response to ozone depletion, but no consensus has emerged. Here we investigate the hypothesis that this discrepancy between climate models and observations occurs due to simulation biases in the sea ice drift velocity. As a control we use the NCAR CESM Large Ensemble, which has 40 realizations that all use identical historical and future forcing but differ in their initial conditions, and 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 ice motion, and we simulate 3 realizations of recent climate change. In contrast with the control runs, the Antarctic sea ice expands in all 3 of these realizations, with the simulated spatial structure of the expansion bearing resemblance to observations. This suggests that the reason CESM fails to simulate the observed Antarctic sea ice expansion is due to simulation biases in the sea ice drift velocity. In order to understand what is driving the biases in the simulated sea ice velocity, we carry out 3 additional simulations in which the sea ice surface momentum flux is calculated using historical surface winds from reanalysis rather than the simulated surface wind field. These realizations all fail to capture the observed sea ice expansion, implying that the simulated wind field is not the main reason for the bias. This leaves biases in the sea ice rheology, sea ice drag coefficients, and ocean surface currents as the main candidates for the cause of the Antarctic sea ice extent trend bias in the climate model. These results have implications regarding the physical processes driving the observed Antarctic sea ice expansion as well as how the Antarctic sea ice cover will change during future decades.