Polar amplification in idealized climates: the role of ice, moisture, and seasons
- Nicole Feldl
- Timothy Merlis (McGill)
Polar amplification of climate change is simulated across models with various representations of local feedbacks and poleward energy transports. Yet uncertainty in attribution remains due to the interactive nature of the physical mechanisms and the different perspectives afforded by different diagnostic methods. Here, the role of sea-ice processes, moist energy transport, and the seasonal cycle of insolation are systematically investigated in two models, an energy balance model (EBM) and an idealized general circulation model (GCM). Filling this gap in the modeling hierarchy reveals that, while annual-mean polar amplification is insensitive to the inclusion of a seasonal cycle with a simple ice-albedo feedback, the seasonal warming pattern and change in polar energy flux convergence are profoundly affected by the combined influence of seasons and thermodynamic-ice processes, in a manner consistent with state-of-the-art climate model projections. In particular, a minimum in summer polar warming occurs where temperatures are in their melting regime, and a maximum occurs where thick, cold ice preconditions a large radiatively forced temperature response. Despite this large winter warming, the annual-mean polar amplification is reduced compared to the case of a simple ice-albedo feedback. When the effect of latent heat transport on the EBM’s diffusive representation of atmospheric energy transport is disabled, polar amplification is further reduced by a factor of 1.7 across the range of ice representations, suggestive of a superposition of warming contributions by the ice-albedo feedback and moist processes. Together, the results imply that the change in surface heat capacity associated with ice loss leads to surface warming that would promote a destabilizing wintertime lapse rate feedback and that the increase in moist energy transport would lead, in more complex models, to additional warming via a water vapor feedback. Strong consistency of the climate responses in the EBM and GCM further support the broader relevance of our findings.