Disentangling Warming and Circulation Influences on Winter Precipitation Using Nudging Experiments

As greenhouse gas concentrations increase, surface temperatures continue to rise and hydroclimate patterns, including precipitation, are shifting. Making confident precipitation projections is challenging due to large internal climate variability and the small spatial scales on which precipitation processes occur. Furthermore, climate models have biases in large-scale circulation variability that can lead to precipitation biases. Here, we aim to understand the impact of warming alone on mid-latitude winter precipitation changes. To do this, we produce new simulations with CESM2-CAM6 where meridional and zonal winds from 850 hPa to the top of the atmosphere are nudged towards winds from a reanalysis product (ERA5) globally. In the first experiment we nudge the winds in an atmospheric model forced by historical sea surface temperatures and sea ice extent from 1979-2014. In the second experiment, we nudge to the same historical winds but the atmospheric model is forced by the CESM2 Large Ensemble mean SSTs and sea ice extent from the end of the 21st century (2065-2100). This second experiment allows us to capture the impact of a large greenhouse gas forcing on mid-latitude precipitation changes and directly quantify the thermodynamic contribution to the precipitation change. By also comparing the historical nudged run to an SST forced simulation (without any nudging) we can potentially elucidate biases in the modeled large-scale circulation and their impact on mid-latitude precipitation variability.