Very Strong Correlation between the Northern Hemisphere Jet Response and Arctic-Minus-Subtropical Warming across CMIP5 Models

The response of the atmospheric circulation to global warming is still not well known. A specific area of interest is the westerlies of the Northern Hemisphere, most notably the strengthening or weakening of the westerlies, which could change mid-latitude weather patterns. Previous studies have termed the phrase tug-of-war theory, which is the interaction between the warming of the tropics and the warming of the Arctic, with opposite jet response to each. This study focused on the difference between specific areas of the tropics and Arctic using 30-year RCP8.5 minus 30-year historical climate model run outputs for temperature change and zonal wind change. Using 41 climate models from CMIP5 we found a very strong correlation between the strengthening (weakening) of the westerlies and warming of the tropics (Arctic). Specifically, for every degree of difference in warming between the Arctic troposphere (60-90°N, 925-300 hPa) and subtropical upper troposphere (20-40°N, 600-200 hPa) the mid-latitude zonal wind response (30-70°N, 1000-200 hPa) decreased by -0.38 ms^-1 (r = -0.91). This indicates the tug-of-war theory very successfully predicts the circulation response. Similar relationships using the Arctic lower stratosphere and deep-tropical upper troposphere were not as strong, contrary to other studies. For future work we will use reanalysis and/or microwave satellite data with ensembles of historical model runs to determine which models are likely to be more accurate.