Regional and Seasonal Characteristics of the Recent Expansion of the Tropics in the Northern Hemisphere

It has been widely reported that the subsiding regions of Earth’s Hadley circulation have been shifting poleward in recent decades, pushing subtropical dry zones closer to populated midlatitude regions. While the characteristics of this tropical expansion are approximately zonally symmetric in the Southern Hemisphere, the recent tropical expansion in the Northern Hemisphere has unique seasonal and regional characteristics. Examining subtropical sea level pressure from eight observational data sets, we find that the recent tropical expansion is focused in the North Pacific and North Atlantic basins, with the largest expansion occurring during autumn months.

Previous studies have attributed the recent expansion of the tropics in the Northern Hemisphere to a number of factors, ranging from natural variability (El Niño-Southern Oscillation or Pacific Decadal Oscillation) to anthropogenic forcing (increasing greenhouse gases and/or anthropogenic aerosols). Using output from CMIP5 models, we identify the seasonal and regional characteristics of the tropical expansion expected from each of these forcing factors. We then compare and contrast these fingerprints of natural variability, greenhouse gas forcing, and aerosol forcing with the observed trends, providing a new perspective on the potential causes of recent tropical expansion that cannot be captured using a traditional annual-mean, zonal-mean perspective.