Mechanisms of polar-amplified warming: understanding hemispheric and seasonal asymmetries

Abstract

The Arctic has warmed four times faster than the global average in recent decades. This polar-amplified warming has wide-ranging impacts on local ecosystems and global climate, yet there is still debate over the drivers of polar amplification, how they interact with each other, and their relative importance. Moreover, future warming uncertainty is larger for the poles than for any other region on Earth. I motivate key research questions in Chapter 1 before examining contributions to polar warming and its uncertainty in the latest generation of climate models in Chapter 2. I find that a larger albedo feedback than previously shown contributes comparably to temperature feedbacks towards greater warming in the Arctic than in the tropics or Antarctic. The albedo feedback and its positive covariance with other polar feedbacks are also the dominant source of model uncertainty in polar warming. I also show that increased atmospheric moisture transport to the poles is the third largest contributor to Arctic amplification and the largest contributor to projected Antarctic warming. In Chapter 3, I complement these comprehensive climate models with idealized models and theory to identify a central mechanism for seasonality in Arctic warming. As frozen sea ice melts and transitions to open ocean, the increasing thermal inertia of the surface layer slows the rate of seasonal cooling, which can alone produce the observed pattern of peak Arctic warming in early winter. In Chapter 4, I investigate the seasonality of projected changes in poleward heat and moisture transport to the Arctic, which have mainly been studied in the annual-mean or in winter. This seasonality can be explained by down-gradient diffusion of anomalies in dry and moist static energy, yielding a winter peak in reduced dry heat transport and a summer peak in increased moisture transport. I hypothesize that even for compensating annual-mean changes in moist and dry heat transports, their opposite seasonality will produce non-compensating impacts on Arctic warming. Despite its winter peak, Arctic warming is impacted by forcing in all seasons, highlighting a need for further research to investigate how seasonality in poleward heat transport and other feedbacks mediates their effect on Arctic warming. I present preliminary results and future work addressing this question in Chapter 5, in addition to summarizing our conclusions.