Mechanisms of Internal Tropical Atlantic Multidecadal Variability: Seasonality, Model Diversity, and Climate Impacts

The North Atlantic exhibits pronounced variability on decadal to multidecadal timescales, commonly referred to as Atlantic Multidecadal Variability (AMV). AMV has been linked to climate variability in many regions across the globe. Modelling studies indicate that the global teleconnections of AMV are sensitive to how the tropical branch is represented, though understanding the processes governing its development has received little attention. Moreover, coupled climate models show substantial diversity in simulating the tropical arm of AMV, yet the mechanisms responsible for this inter-model spread remain poorly understood. Here we show that tropical AMV exhibits a seasonal cycle in observations, with growth during boreal summer and decay during boreal winter. Coupled models differ markedly in their ability to capture this observed seasonality. Models that reproduce the observed seasonal evolution of tropical AMV provide insight into its underlying dynamics, revealing that variations in latent heat flux, shortwave radiation, and mixed-layer depth driven by changes in the trade winds are central to the growth and decay of tropical AMV. In contrast, models that fail to represent trade wind weakening and associated ocean-atmosphere interaction processes exhibit substantial deficiencies in simulating tropical AMV. Consequently, models that correctly capture the observed seasonality of tropical AMV also reproduce its associated climate impacts, including variability in Sahel rainfall and the Indian summer monsoon, whereas models that do not capture this seasonality fail to simulate these impacts properly. Given the sensitivity of global climate to tropical AMV, these results highlight the importance of accurately representing the processes linking the extratropical North Atlantic and tropical ocean-atmosphere interactions in coupled climate models.