Development of an airmass-based regional climate change scenario

Summary In middle latitudes, regional climates are largely determined by the frequency and character of different airmasses advected across the region. Airmass characteristics and frequencies are expected to be different in a warmer world. General circulation models are, for example, unanimous in projecting large temperature changes for high latitudes, the source region for polar airmasses. Conventional approaches to the construction of regional climate change scenarios are not able to capture such differences between airmasses. Here we present a new approach that assigns each day in the observed and model-produced records to one of three classes based on the upper-level flow, the steering current for airmasses. This approach permits an evaluation of a model's ability to reproduce the observed regional climate in terms of airmasses which is more insightful than a comparison of monthly means. The model used here, the CCM0 version of the NCAR model, was found to reproduce many of the observed December airflow features (the month chosen to demonstrate the approach) for the Lake Superior basin. The approach also permits a more insightful analysis of the projected changes under 2*CO₂ conditions. The CCM0 projects a significant warming and moistening only for the northerly airflows. The northerly flows are also projected to become more frequent. To illustrate the significance of these results, daily scenarios of climate change were constructed from these projections and used in a lake evaporation model. It is found that the changes in the northerly flows projected by this model translate into a 19% reduction in the evaporative power of the air over Lake Superior (wind speeds held at present level).With 3 Figures