The Pacific Arctic as an Emerging “Hot Spot” in the Changing Polar Climate

The Arctic region has been warming rapidly in recent years, roughly two to four times faster than the global average. The warming to date has already caused widespread retreat of sea ice, which has been especially pronounced in the Pacific Arctic (northern Bering Sea to the Chukchi Sea and adjacent seas). Climate models have projected that this region is especially sensitive to future climate change as the sea ice margin migrates northward, accompanied by strengthening surface winds conducive to enhanced coastal erosion. The most pronounced regional climatic shifts in coming decades are expected to occur during winter, when there is likely to be a transition from ice-covered to ice-free waters. This fundamental change will probably trigger an extreme climate transformation consisting of very large increases in temperature, precipitation, and turbulent heat fluxes. Coinciding with these near-surface changes is a projected sharp transition to a more cyclonic circulation regime with a much lower winter mean sea level pressure and more prevalent strong cyclones.

In this study we use the CESM2 Large Ensemble (CESM2-LE) of 50 independent realizations of future climate from 2015-2100 with SSP3-7.0 greenhouse forcing to investigate the wintertime climatic response in the Pacific Arctic. We also utilize TempestExtremes, which automates detection and tracking of cyclones from the climate model output. CESM2-LE simulates a northward migration and poleward stretching of the climatological Aleutian Low through the end of this century, leading to a very large SLP decline of up to 7 hPa north of the Bering Strait where the ice pack transitions to open water. The strongest cyclones (< 950 hPa) double in frequency and foster additional heat and moisture transport that promotes further sea ice loss.

We diagnose the dynamical causes of these storminess changes using two metrics: (1) the maximum Eady growth rate (EGR) as a measure of baroclinic instability, and (2) mid-tropospheric vorticity advection. The EGR allows a breakdown of the relative contributions from changes in atmospheric stability and vertical wind shear in strengthening future cyclones. Vorticity advection reveals regions favorable for ascending air and therefore cyclonic conditions. Our analysis suggests that the robust nature and large magnitude of enhanced future cyclonicity during winter in the Pacific Arctic stems from a mutually supportive combination of bottom-up forcing due to weakening surface stability and top-down forcing due to increasing positive vorticity advection.