The Jet Stream Response to Global Warming is Further Deciphered
Given the close relationship between how atmospheric circulation and the global hydrological cycle and related extreme weather events respond to global warming, researchers studied the underlying mechanisms of the jet stream shift. The diversity of mechanisms described by previous studies underscores the complexity of the jet stream shift as a mid-latitude circulation response to global warming and motivates this study that used a large ensemble of simulations from two atmospheric general circulation models.
The poleward shift of the jet stream in response to global warming is one of the most robust and challenging aspects of climate change impacts. Bridging the gap between previous theoretical advances and realistic simulations of the circulation response to warming, this study furthers the understanding of the poleward jet shift and its linkages to mid-latitude extreme events. The diagnostic framework built from this study has a great potential to be further developed for understanding the linkage between a local climate forcing and its global impacts.
The research team, including a Department of Energy scientist at Pacific Northwest National Laboratory, conducted a series of large-ensemble simulations using two state-of-the-art atmospheric general circulation models. In the simulations, sea-surface temperature warming was abruptly switched on from January 1st to focus on the wintertime circulation adjustment to the warming. They performed a hybrid, finite amplitude wave activity budget analysis to quantify the irreversible mixing effect due to wave breaking as the zonal wind adjusts toward a poleward-shifted state. This analysis tool was then applied to the large-ensemble simulations. The modeling results confirm earlier results of more idealized experiments, particularly revealing the dominance of the mid-latitude decrease of effective diffusivity in reducing the dissipation of wave activity, which increases the survival of mid-latitude waves. The surviving waves, upon reaching the upper troposphere through wave propagation, are subject to the influence of the increase of reflection phase speed on the poleward side of the mean jet. Hence more waves are reflected equatorward across the jet, which gives rise to a poleward transport of momentum and a resulting eddy-momentum flux convergence for the poleward shift. This study pinpointed the dynamics of the jet shift and the intimate linkages to the subtropical and mid-latitude extremes associated with Rossby wave breakings.