Ocean carbon uptake and storage influenced by wind bias in global climate models

In global climate model pre-industrial control simulations the Southern Hemisphere westerly winds show a systematic bias in position and strength relative to estimates of their actual position and strength. These wind-stress biases impact the simulated transport of the Antarctic Circumpolar Current(1) and the nature of Southern Ocean water-mass formation(1) and may affect the rate of meridional overturning of the global ocean(2). The effect they have on oceanic carbon uptake and storage is unknown, however. Here we demonstrate, using a coupled carbon-climate model(3), that the wind-stress biases reduce equilibrium ocean carbon storage, redistribute carbon within the ocean and increase oceanic carbon uptake in climate change simulations. The wind-stress biases act directly by influencing Ekman pumping dynamics in the Southern Ocean and also seem to have an indirect effect on the overturning circulation and carbon distribution through the Agulhas leakage and Indo-Atlantic salt flux. Our results indicate that carbon-climate model simulations with the typical pre-industrial wind-stress bias will over-estimate ocean carbon sequestration, and thereby under-estimate atmospheric carbon dioxide concentrations in the twenty-first century, relative to unbiased simulations. The new generation of coupled carbon-climate models may be subject to these wind biases, which could alter their carbon-climate response, although it is worth noting that the uncertainty arising from wind biases that we demonstrate here is one of several uncertainties that affect modelled ocean carbon uptake(4).