Probabilistic Forecast of North American Decadal Climate for 2011-2020

We present a probabilistic decadal prediction of North American surface temperature and precipitation for 2011-2020. The approach uses atmospheric general circulation models that are constrained by specified sea surface temperature (SST) and sea ice conditions representing plausible scenarios for changes in boundary conditions resulting from anthropogenic greenhouse gas forcing. The ensemble mean 2011-2020 response to our forecast surface boundary forcing includes warming over the entire continent, precipitation increases over Canada, and precipitation decreases over the contiguous United States (relative to 1971-2000 conditions). Temperature increases are predicted to be particularly large over northern Alaska and northern Canada where both SST and sea ice conditions generate warming. Precipitation decreases are predicted in a nearly continuous zonal band across the central to southern U.S. with strongest drying over the far West, the Mississippi Valley, and the Gulf Coast. A widespread increase in precipitation is predicted across high latitudes of North America with particular increases predicted over Alaska and northwestern Canada. We generate probabilistic forecasts for 2011-2020 by comingling the North American forced responses to SST and sea ice change with decadal anomalies resulting from natural oceanic variability alone. Whereas the impact of a particular trajectory of initial natural climate conditions is not treated in our approach, as would be done for instance in initialized coupled model prediction practices, we estimate the statistics of such influences using atmospheric model simulations spanning the 20th Century. Results indicate that warmer decadal surface temperatures during 2011-2020 for area averages of Canada and the contiguous U.S. are very likely to occur, with only about a 10% probability that the effect of natural oceanic decadal fluctuations will overwhelm the warming signal induced by our scenarios of anthropogenic SST/sea ice changes. Likewise, predictions for 2011-2020 suggest a 97% probability for area-averaged wetter conditions over the high latitudes of North America, and a 63% probability for area-averaged reduced precipitation over the contiguous U.S.