Tropical Atlantic multidecadal variability is dominated by external forcing.

Area of Science:

• Climate Science
• Atmospheric Science
• Oceanography

Background:

• Tropical Atlantic climate exhibits multidecadal variability in sea surface temperatures (SSTs), Sahel rainfall, and hurricane activity.
• The physical origins of these interconnected climate systems remain debated due to model and observational uncertainties.

Purpose of the Study:

• To investigate the role of the cross-equatorial gradient in tropical Atlantic SSTs as a driver of Atlantic hurricane formation and Sahel rainfall.
• To evaluate the performance of CMIP6 Earth system models in simulating these climate relationships.

Main Methods:

• Analysis of historical climate data and a large ensemble of CMIP6 Earth system models.
• Identification and correction of model biases in simulating Northern vs. Southern Hemisphere warming trends and associated atmospheric changes.
• Correlation analysis between corrected SST gradients and climate impacts.

Main Results:

• The cross-equatorial tropical Atlantic SST gradient, influenced by radiative forcings (anthropogenic emissions, volcanic aerosols), is a key determinant of hurricane activity and Sahel rainfall.
• CMIP6 models overestimate Northern Hemisphere warming trends, obscuring the SST-climate relationship.
• Correcting for model biases reveals robust correlations between SST gradients and climate impacts, particularly since 1950.

Conclusions:

• The tropical Atlantic SST gradient is a more critical driver of tropical climate impacts than broader North Atlantic SSTs.
• Radiative forcing, including anthropogenic emissions and volcanism, can predict Atlantic hurricane activity and Sahel rainfall variations.
• Future prediction accuracy is constrained by the signal-to-noise paradox and uncertainties in future climate forcings.