The rapidly warming Arctic has already led to a widespread retreat of sea ice, which has been especially pronounced in the Pacific Arctic in recent years. This regional ice loss is expected to continue for the foreseeable future and result in the time-mean winter sea ice margin to become situated well north of the Bering Strait. Climate models project that this transition to ice-free waters during winter will be colocated with the sharpest decline in atmospheric sea level pressure (SLP) of any region or season. Furthermore, surface winds are likely to strengthen and exacerbate coastal erosion with the transition from sea ice to open water, due to a combination of a smoother surface, enhanced turbulent mixing in the atmospheric boundary layer, and stronger geostrophic winds from a poleward-shifting Aleutian Low. These changes are robust in past CMIP global model simulations and the latest Community Earth System Model, Version 2 (CESM2), but the precise causes of this joint ice-ocean-atmosphere response are still unresolved.

In this study we use CESM2’s Large Ensemble of 50 independent realizations of future climate from 2015-2100 with the SSP3-7.0 greenhouse forcing to investigate the wintertime climatic response in the Pacific Arctic. Our main diagnostic tool is the TempestExtremes framework, which automates the detection and tracking of cyclones from reanalysis and climate model output.

Projections show a northward migration and poleward stretching of the climatological Aleutian Low by the end of the century, leading to a very large 7 hPa maximum SLP decline north of the Bering Strait where the ice pack transitions to open water. In this region the surface air temperature increases by up to 20 K, along with a large enhancement in the latent heat flux of over 40 W/m², both of which help fuel more frequent and extreme cyclones. The strongest cyclones (< 950 hPa) double in frequency and engender a significant increase in heat and moisture transport conducive to additional sea ice loss. We use the maximum Eady growth rate to characterize the more favorable environment for cyclones and to decompose the relative contributions from weakened atmospheric stability and stronger vertical wind shear in strengthening future cyclones.