Human-induced weakening of the Northern Hemisphere tropical circulation

By accounting for most of the poleward atmospheric heat and moisture transport in the tropics, the Hadley circulation largely affects the latitudinal patterns of precipitation and temperature at low latitudes. To increase our preparednesses for human-induced climate change, it is thus critical to accurately assess the response of the Hadley circulation to anthropogenic emissions(1-3). However, at present, there is a large uncertainty in recent Northern Hemisphere Hadley circulation strength changes(4). Not only do climate models simulate a weakening of the circulation(5), whereas atmospheric reanalyses mostly show an intensification of the circulation(4-8), but atmospheric reanalyses were found to have artificial biases in the strength of the circulation(5), resulting in unknown impacts of human emissions on recent Hadley circulation changes. Here we constrain the recent changes in the Hadley circulation using sea-level pressure measurements and show that, in agreement with the latest suite of climate models, the circulation has considerably weakened over recent decades. We further show that the weakening of the circulation is attributable to anthropogenic emissions, which increases our confidence in human-induced tropical climate change projections. Given the large climate impacts of the circulation at low latitudes, the recent human-induced weakening of the flow suggests wider consequences for the regional tropical-subtropical climate.

Automated summary

The Hadley circulation plays a crucial role in determining precipitation and temperature patterns at low latitudes. However, there is currently a large uncertainty in the recent changes of the Hadley circulation in the Northern Hemisphere. This study demonstrates that the circulation has considerably weakened over recent decades and attributes the weakening to anthropogenic emissions, which enhances our confidence in projections of tropical climate change caused by human activities.