UC San Diego

The surface temperature in the Arctic has warmed at twice the rate of the global mean temperature during recent decades. This Arctic amplification of global warming has been a striking feature of climate change, and many studies have investigated what processes contribute to this phenomenon. Many of these processes are often described in the context of climate feedbacks using analyses focused on top-of-the-atmosphere radiative changes. In this context, regional surface warming can then be partitioned into contributions from each feedback process. However, this partitioning can be complicated by interactions between feedbacks themselves and atmospheric heat transport. In the second chapter, we instead apply a feedback-locking approach and evaluate the resulting changes in surface temperature. These results are strikingly different from previous feedback analyses, highlighting the important role of interactions within the climate system. This chapter and many other previous studies focus only on the role of atmospheric and surface processes in Arctic amplification. However, substantial questions remain regarding the role of ocean heat transport. In the third chapter, we investigate changes in oceanic heat fluxes under global warming. We find a mechanism associated with the presence of sea ice that drives enhanced horizontal ocean heat transport into the Arctic region and can contribute substantially to Arctic amplification if this heat is allowed to reach the surface. Currently, only a small amount of the heat stored at depth in the Arctic Ocean can reach the surface, but recent observational studies have argued that sea ice retreat could result in enhanced vertical mixing. In the fourth chapter, we investigate the impacts of a positive feedback whereby increased vertical mixing due to sea ice retreat causes the previously isolated subsurface Arctic Ocean heat to melt more sea ice. We find that an abrupt “tipping point” can occur under global warming, with an associated hysteresis window, for a limited range of parameters. Throughout the thesis, we use idealized models to show how ocean and climate processes can impact Arctic warming, providing insights into possible physical mechanisms that could be at play now or in the future.