This is one of 16 Rapid Response (RAPID) projects funded as the result of a Dear Colleague Letter (NSF 11-006) encouraging diagnostic analyses of climate model simulations prepared for the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5). Research conducted in these projects is expected to lead to more detailed model intercomparisons, better understanding of robust model behaviors, and better understanding and quantification of uncertainty in future climate simulations.
This project addresses the region of zero or negative summertiime temperature trends which occurs over the midwestern US and is commonly referred to as a "warming hole". Work performed under the grant seeks to determine whether the warming hole occurs as the result of the natural variability associated with low-frequency modes, or whether it is caused by some anthropogenic climate forcing (aerosols, for example). The project has three main tasks: 1) evaluating the extent to which the 20th century climate simulations prepared for the AR5 are capable of reproducing the warming hole; 2) assessing whether warming holes, defined as regions of reduced warming over multiple decades, are captured in the internal variability of the models, and, if so, whether they occur in preferred locations, such as the U.S. Midwest; 3) determining whether the model simulations capture the observed local relationships between regional U.S. temperatures and clouds and precipitation and the global teleconnections to sea-surface temperatures.
The broader impact of the project lies in its support of the IPCC AR5, which is intended to provide information on climate change and its consequences to decision makers worldwide. The ability of climate models to represent the warming hole is important for assessing the reliability of future midwest temperature projections from the models.
Climate changes in the 20th Century, even when averaged over many decades, reveal significant regional structures. Prominent among these are regions of reduced warming or of cooling over the United States Midwest , especially in the daytime. These regions have been called "warming holes." In particular, a prominent region centered over the Mississippi Valley shows no significant rising trend in summertime temperatures over the second half of the 20th century. The U.S. warming hole is precisely the sort of climate variability that must be represented – and predicted, if this is possible – accurately, if climate models are to provide actionable projections to guide adaptation and mitigation. In the first phase of this project, we examined the output of climate models used in international climate assessments, considering model runs intended to simulate the past climate - that of the last century. We found that few simulations with these models produced a warming hole. Either there is somethng missing from these models that prevents them from producing a warming hole, or the warming hole is not a predictable response to global warming generated by increasing greenhouse gases. Our results do not allow us to distinguish between these two possibilities. In the second phase, we looked at many simulations from a single model, again simulating past climate, but now including observed information about the temperature of the ocean. Because ocean temperatures are an important part of climate variability, we expected that these model runs would produce a warming hole. They did not, however, indicating that this particular model, at least, cannot represent the processes that produced the warming hole in the real world. While our results do not allow us to determine exactly what these processes are, various pieces of evidence point to problems with how the model handles cloud cover and how it varies with climate.