High-latitude Continental Warming due to Sensitivity of Arctic Air Formation to Temperature and CO2

The temperature of high latitude continents in winter has increased much more rapidly than the global-mean temperature over the past decades, and the maintenance of warm, above-freezing conditions in continental interiors in winter has been a particular puzzle of the equable climates of the deep past. To investigate the sensitivity of continental interior temperatures in winter to the temperature of the adjacent high-latitude ocean, we perform idealized simulations of Arctic air formation with a single-column version of the WRF model. We prescribe the initial sounding of the atmosphere representing an air column starting over the ocean, and then allow it to evolve for two weeks in the absence of any solar heating and with a very low heat capacity surface underneath, representing the movement of the air column over high-latitude sea ice or a continental interior. This setup allows for rapid surface cooling and formation of surface-based inversions, in which clouds modulate the rate of surface cooling. Low-level liquid or mixed-phase clouds slow the surface cooling considerably, until cloud ice formation and mixed-phase precipitation processes eliminate cloud liquid and thin the cloud deck, leading to enhanced surface cooling and strong inversions (e.g., Curry, 1983). Across a range of microphysical parameterizations, we find that as the initial surface temperature is increased, representing a warmer ocean, formation of cloud ice is delayed, and the warming effects of clouds persist longer. If CO₂ increases, and lower-tropospheric lapse rates of the initial sounding are also allowed to steepen with warming, then the average surface temperature over the two weeks can increase by over two degrees for each degree increase of the initial surface air temperature. The time it takes for the surface air temperature to drop below freezing increases steeply and nonlinearly as the initial surface air temperature of the sounding rises above 10^oC, and can exceed 10 days for initial surface air temperatures of 15^oC. Our results suggest that the “lapse rate feedback” in simulations of anthropogenic climate change may be related to the influence of low clouds on the stratification of the lower troposphere, and also indicate that optically thick low clouds could help to maintain above-freezing winter continental interiors in equable climates.