Impacts of Shifting Subtropical Highs on North American Climate

Numerous observational studies have found that the Hadley cells have expanded poleward in both the Northern and Southern Hemispheres, and model results suggest that such expansion is likely to continue throughout the 21st century as a result of global warming.  This has led to concerns about future impacts of Hadley cell expansion, including a poleward shift of the subtropical dry zone.  However, climatic changes associated with Hadley cell expansion are zonally asymmetric—especially in the Northern Hemisphere—suggesting that a more regional focus may be necessary.  In this study, we consider the influence of the Northern Hemisphere subtropical highs, and contrast this with the influence of Hadley cell expansion. 

Specifically, we consider the North Pacific and North Atlantic subtropical highs and define, for each high, three indices representing longitude, latitude, and strength.  We find that 21st century trends in variables as diverse as precipitation, sea-level pressure, winds, and ocean upwelling in eastern boundary currents are all driven more by the trends of these subtropical high indices than by the expansion of the Hadley cell.  We conclude that 21st century trends in subtropical high positions and strengths are crucial to understanding the future of Northern Hemisphere climate.  Further work will be needed to determine the dynamical drivers of these subtropical high trends.