Allied forces of climate change in the West: The confluence of the Pacific North American pattern and anthropogenic forcing

Regional changes in climate arise in response to changes in both radiative forcing and atmospheric circulation. Observations over the last half-century show that changes in the Pacific North American (PNA) pattern have augmented warming during late winter and spring across Western North America. This alignment of “natural” and anthropogenic forcing has hastened the decline in water resources across portions of the Western US, contributing to nearly a third of the reduction of precipitation falling as snow in the lower elevation mountains in the Cascades and Northern Rockies during late winter. Observations suggest that degree to which dynamical and radiative perturbations align over the next half-century will modify the pace of regional climate change. While that the PNA is well resolved in global climate models, they generally fail to capture observed changes in the PNA response over the late 20th century. However, a majority of models favor a shift in atmospheric circulation toward the positive phase of the PNA by the mid-21st century. The alignment of radiative and dynamical forcing result in amplified regional warming across western North America reasoned to further tax hydrologic resources. These results provide further motivation in resolving the mechanisms, and ultimately, the predictability of large-scale atmospheric circulation as a response to anthropogenic forcing.