Enhanced Arctic Sea-Ice Growth Driven by Atmospheric Warming; A Key Role for Snow

We use a realistic coupled ocean-sea ice model and its adjoint to explore the response of Arctic sea ice to local atmospheric warming. The adjoint enables us to map, among others, the timescales and pathways of sea ice sensitivity to changing near-surface air temperature. These maps confirm the counter-intuitive result, that significant ice growth may occur as a lagged response to Arctic atmospheric warming at the end of the melting season. A series of forward perturbation experiments informed by the sensitivity patterns indicate that the enhanced sea ice growth - which leads to recovery or overshoot of ice volume - is accompanied by a reduction of snow thickness on the sea-ice pack. Detailed analysis of the modeled ocean-ice-snow heat budgets reveals our key finding, namely that the reduced snow thickness is essential for persistence and overshoot of sea ice growth. The underlying mechanism is a strong conductivity feedback, wherein atmosphere-driven snow melt leads to a larger conductive ocean heat loss through the overlying ice layer. Due to the small thermal resistance of sea ice relative to snow, the conductivity feedback associated with atmosphere-driven ice melt is negligible. Our results highlight the need for better observing snow thickness on sea-ice for constraining climate models and initializing sea-ice forecasts.