Projected Changes in Atmospheric Circulation and Precipitation Variability over the North Pacific and North America in a Coupled Model Large Ensemble
While much attention has been given to understanding how anthropogenic radiative forcing influences the mean state of the climate system, far less scrutiny has been paid to how it may modulate naturally occurring modes of variability. In this study, we investigate forced changes to unforced modes of wintertime atmospheric circulation variability and associated impacts on precipitation over the North Pacific and North America based on the 40-member CESM1 Large Ensemble during 1920-2100. Each simulation is subject to the same radiative forcing protocol but starts from a slightly different initial condition, leading to different sequences of internal variability. Evolving forced changes in the amplitude and spatial character of the leading internal modes of 500 hPa geopotential height variability are determined by applying Empirical Orthogonal Function analysis across the ensemble dimension at each time step. The results show that the leading modes of internal variability intensify and expand their region of influence in response to anthropogenic forcing, with concomitant impacts on precipitation. Linkages between the Pacific and Atlantic, and between the tropics and extratropics, are also enhanced in the future. These projected changes are driven partly by teleconnections from amplified ENSO activity and partly by dynamical processes intrinsic to the extra-tropical atmosphere. The marked influence of anthropogenic forcing on the characteristics of internal extratropical atmospheric circulation variability presents fundamental societal challenges to future water resource planning, flood control, and drought mitigation.
