Five Stages of the Alaskan Arctic Cold Season with Ecosystem Implications

We divide the Alaskan Arctic cold season into five stages based on transitions in climatological and thermophysical conditions in the atmosphere, snowpack, and soil active layer. Each of these stages has distinct characteristics which drive ecosystem processes. During the two autumnal stages (Early Snow and Early Cold) soils remain warm, unfrozen water is present, and the highest rates of cold-season soil respiration occur. The next two stages (Deep Cold and Late Cold) are characterized by a frozen active layer with decreasing temperature. Thaw is critical in determining the length of the growing season and the resumption of biological processes. Deep Cold and Late Cold result from a radiation deficit, show little interannual variation, and will be resistant to change under almost any reasonable climate change scenario. These are also the stages with the least amount of biological activity and have the least impact on the ecosystem. However, Early Snow, Early Cold and Thaw stages vary significantly from year to year, have more ecosystem implications, and are also the most likely to undergo significant change in timing and character as the arctic climate changes. This 5-fold subdivision is useful for framing discussions of biophysical interactions during the arctic winter and for focusing attention on critical cold-season periods.