Sensitivity of the Midlatitude Circulation to the Tug-of-War between the Upper-Level Tropical Warming and Surface Arctic Warming in the Coming Century

The future response of the midlatitude atmospheric circulation to increased anthropogenic forcing is uncertain in large part due to competing influences of the projected warming at the surface in the Arctic, and at upper levels in the tropics. We will discuss the influence of each of these two climate change signals on the response of the mid-latitude atmospheric circulation at the end of the 21^st century focusing on the North Atlantic and European sector. Using CMIP5 and CESM large ensemble simulations, we find that the tug-of-war between the two effects influences by how much wintertime cold extremes diminish at the end of the 21^st century. Models with a dominant tropical warming exhibit a less pronounced decline in cold extremes over Europe than models with a dominant Arctic warming. This is due to the maintenance of cooler conditions in the subpolar North Atlantic when tropical warming dominates. To further isolate the role of tropical vs polar changes, we conduct perturbation experiments with a high-top atmospheric global climate model (WACCM). In these experiments we nudge the atmospheric fields in each region separately to the projected values at the end of the century. A third experiment will impose nudging in both regions at the same time allowing us to assess their combined influence as well as possible linearity in the response.