ESRL/PSD Seminar Series

Abstract

Arctic sea ice loss has recently drawn a lot of media attention because of its possible link to extreme weather events in the mid-latitudes. Some of the proposed mechanisms, however, are being challenged and it is still poorly understood how Arctic sea ice loss affects/will affect the atmospheric circulation and surface climate. To address these issues, we conducted two 161-year simulations using NCAR's "high-top" atmospheric model, Whole Atmospheric Community Climate Model (WACCM): one with a repeating seasonal cycle of Arctic sea ice for the late twentieth century (1980-99), taken from the fully-coupled WACCM historical run; the other with Arctic sea ice for the late twenty-first century (2080-99), obtained from the fully-coupled WACCM RCP8.5 run.

In agreement with previous studies, the tropospheric circulation response to Arctic sea ice loss is characterized by a negative phase of the Northern annular mode (NAM), while the stratospheric response transits from a weakening of the polar vortex in winter to a strengthening of the polar vortex in spring. Further analysis indicates that different geographical locations of ice loss impact the stratospheric circulation in different ways. In particular, the polar vortex weakens in response to ice loss in the central Arctic, but strengthens in response to ice loss in the marginal ice zones. Thus, the stratospheric circulation response to the combined pattern of sea ice loss can be understood as a consequence of largely canceling effects from these two regions. Moreover, the comparison of the NAM response between WACCM and CAM (NCAR's "low-top" model) suggests that the stratosphere may play a role in the tropospheric circulation response to Arctic sea ice loss.

The winter surface climate response to Arctic sea ice loss shows strong (maximum value of 10 K) warming over the high-latitude continents. In the mid-latitudes, North America warms but central Eurasia cools slightly. A heat budget analysis reveals that the warming over North America is mainly caused by warm air advection by the mean sub-monthly transient eddies acting on the anomalous meridional temperature gradient, while the cooling over central Eurasia is due to cold air advection by the anomalous monthly mean circulation (e.g., the polar high pressure center of the NAM) acting on the mean temperature gradient. This geographic dependence highlights the complexity of the surface climate response to the Arctic sea ice loss. Our results also suggest that in North America, the risk of winter extreme cold events will decrease, not increase, contradicting the notion that there will be more winter extreme cold events due to continued Arctic sea ice loss.