The Poleward Shift of Storm Tracks Under Climate Change: A Lagrangian Perspective

Extratropical cyclones dominate the distribution of precipitation and wind in the midlatitudes, and therefore their frequency, intensity, and paths have a significant effect on weather and climate. Comprehensive climate models forced by enhanced greenhouse gas emissions suggest that under a climate change scenario, the latitudinal band of storm tracks would shift poleward. While the poleward shift is a robust response across most models, there is currently no consensus on what is the dominant dynamical mechanism. Here we suggest a new mechanism for the poleward shift, which is based on a Lagrangian view of the storm tracks, and involves changes in the propagation characteristics of the storms. We show, by employing a storm-tracking algorithm on an ensemble of CMIP5 models, that the poleward shift in the genesis latitude of the storms (often associated with the expansion of the Hadley Cell) can not account solely to the observed shift. Rather, it is also the latitudinal displacement of the storms that increases. A mechanism for enhanced poleward motion of cyclones in a warmer climate is proposed, which is supported by idealized global warming experiments, and relates the shift to changes in upper level jet and atmospheric water vapour content. Our results imply that under the RCP8.5 climate change scenario, the averaged latitude of peak cyclone intensity shifts poleward by about $1.{\mathrm{2^o }}^{}$($\mathrm{1.0^o}$) in the Atlantic (Pacific) storm track in the Northern Hemisphere (NH), and by about $1.{\mathrm{6^o}}^{ }$in the Southern Hemisphere (SH) storm track. These changes in cyclone tracks can have significant impact on local climate and weather in the midlatitudes, especially in regions close to the eastern ocean boundaries.