Sea Ice as a Predictor of Atmosphere and Ocean States over the Arctic and Southern Oceans, and Implications for Attribution of Polar Climate Change

Over the Arctic and Southern Oceans, the local atmospheric vertical profile, ocean hydrography, and surface fluxes are well-correlated to local sea ice concentration and thickness. With greenhouse warming, many winter season changes in polar atmospheric structure, upper ocean stratification, and surface fluxes are consistent with sea ice decline: a deeper, warmer, moister, windier, atmospheric boundary layer; a deeper oceanic mixed layer atop a weaker cold halocline; and greater radiative and turbulent fluxes at the atmosphere-ocean interface. Since declining sea ice extent is primarily driven by increased summer season melt, these observations might suggest that winter season polar changes are primarily a response to summer season ice decline.

In this study, we develop a mathematical framework to decompose winter season changes in the coupled polar atmosphere-ocean system into those attributable to receding summer sea ice, and those attributable to changes in the winter season climate state independent of ice decline. We find that while changes in turbulent fluxes, boundary layer depth, and upper ocean hydrography are primarily due to declining sea ice, most other winter season polar changes require contributions from both declining sea ice and adjustments in the polar climate state independent of sea ice changes (including increased atmospheric greenhouse gas loading and ice-independent radiative feedbacks). Furthermore, upper tropospheric changes are only weakly coupled to sea ice changes, and are instead primarily controlled by midlatitude and tropical driving. Our analysis suggests that the transformation of the winter season polar climate state with greenhouse warming can be attributed to a combination of ice-dependent and ice-independent changes; both classes of processes are needed to explain observed and modeled winter season changes over the polar oceans.