Robinson, R. (2021). Constraining the Northern Hemisphere Mid-Latitude Jet Response to Climate Change Using the Difference Between Arctic and Subtropical Warming. Unc Charlotte Electronic Theses And Dissertations.
Greenhouse gases’ continuously increasing emissions have a variety of implications for the future of our climate patterns. This includes potential changes in the northern hemisphere jet strength and location, which are influenced by the warming in the subtropics and the Arctic. Many climate models disagree on the northern jet response to climate change. Some predict a strengthened jet with a poleward shift in response to increased subtropical warming, and others predict a weakened jet with an equatorward shift in response to increased Arctic warming. This results in the "tug-of-war" hypothesis, which states that whichever part of the atmosphere, the subtropics or the Arctic, "wins" the tug-of-war by warming more will control the future jet response. This study applies the newer Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations to replicate previous analysis that used the correlation of the difference between upper-tropospheric subtropical warming and Arctic warming with zonal wind speed to predict models’ future jet response. This study also furthers previous research by comparing reanalysis and microwave sounding unit (MSU) observations over the satellite era to the historical CMIP6 output to determine how well individual models capture the recent historical trends in Arctic minus subtropical warming and the change in jet speed. By understanding how each model performs over the satellite era, the validity of their warming and jet response is determined, illuminating the models that are more trustworthy and allowing predictions of the future jet response to be refined.The findings of this study show that CMIP6 follows the same pattern as previous research, further showing that the Arctic minus subtropical warming difference and the zonal wind change are strongly correlated, and the warming difference is useful as a predictor of future jet response. Repeating this analysis over the satellite era from 1980-2014 shows the correlation exists over recent modeled history as well. The CMIP6 historical simulations are capturing the recent trends in the warming difference between the Arctic and the subtropics better than earlier CMIP generations. However, they still fail to capture the magnitude of this difference by continuing to underestimate the warming in the Arctic. Comparing the historical simulations to reanalysis output and satellite MSU data allows us to discard 8 out of 45 models based on the methods of this study. With so few of the models discarded, this further emphasizes that the models appear to be performing better than they were before. However, many of the same models that show increased warming in the Arctic over the satellite era still tend to produce more warming in the tropics and subtropics in the future, suggesting that a future where the tropics win the tug-of-war may be consistent with the current trends that the Arctic is winning the tug-of-war.