Hierarchical Approaches to Climate Dynamics: Mechanisms of Recent Southern Ocean Cooling

March 02, 2026

Dave Bonan

Hosted by Jim Hurrell

Abstract

Earth’s climate is shaped by complex interactions among the atmosphere, ocean, cryosphere, and land, making the causes of historical change difficult to isolate. Hierarchical approaches that progressively build complexity are often needed to explain these changes and resolve model–observation discrepancies. In this talk, I demonstrate how such methods reveal the mechanisms behind a longstanding model–observation discrepancy and improve confidence in 21st-century projections. Since the late 1970s, the Southern Ocean surface has cooled despite ongoing global warming—a signal that remains poorly understood and that most climate models fail to reproduce. Using a conceptual model of the ocean mixed layer, I show that strengthened circumpolar westerlies drive this cooling, but only when atmospheric warming is weak. Observational estimates of the mixed-layer heat budget indicate that climate models capturing the recent cooling do so for the correct physical reasons. However, many underestimate wind trends and overestimate atmospheric warming. The former bias reflects internal variability, whereas the latter may arise from how climate models transmit Southern Ocean cooling to lower latitudes, increasing global warming and overwhelming the wind-driven cooling locally. More broadly, this work highlights the value of integrating observations, theory, and climate models to develop a mechanistic understanding of the processes governing Earth’s climate and its response to external forcing. I conclude by showing how hierarchical approaches provide a foundation for understanding the behavior of Earth’s climate across a wide range of spatial and temporal scales.