The Hadley circulation in a changing climate.

Area of Science:

• Atmospheric Science
• Climate Dynamics
• Earth System Science

Background:

• The Hadley circulation (HC) is a vital global atmospheric system influencing Earth's energy and moisture transport.
• Anthropogenic climate change theoretically drives HC poleward expansion, yet its strength changes are debated.
• Existing climate models and reanalyses show consensus on HC widening but lack agreement on strength trends.

Purpose of the Study:

• To analyze the widening and strength changes of the Hadley circulation.
• To investigate the factors contributing to uncertainties in HC projections.
• To examine the distinct regional evolution of the Hadley circulation.

Main Methods:

• Analysis of reanalyses and climate simulations over the past four decades.
• Decomposition of the global HC into three regional components.
• Assessment of various forcing factors including aerosols and ozone.

Main Results:

• The Hadley circulation has significantly widened over the last four decades and is projected to continue widening.
• There is no consensus on past or future changes in HC strength, with conflicting mechanisms proposed.
• Regional HCs exhibit different evolutionary patterns, with projected Southern Hemisphere expansion and complex Northern Hemisphere changes.

Conclusions:

• The Hadley circulation's widening is a robust finding, but its strength remains an area of significant uncertainty.
• Natural variability, model deficiencies, and multiple forcing factors contribute to HC projection uncertainties.
• Future HC changes display regional heterogeneity, particularly in the Northern Hemisphere.