Turning the North Pacific Over: Earth System Model Experiments Reveal Precession-Driven Deep-Water Formation in the Early Eocene

In the modern ocean, deep-water formation does not occur in the North Pacific. Yet, geological evidence suggests that ocean circulation during past warmhouse climates may have differed fundamentally from today. The Early Eocene, characterized by elevated greenhouse gas concentrations and strong orbital pacing, provides a key test case for exploring alternative modes of deep-ocean ventilation. However, the extent to which individual orbital parameters modulated deep-water formation in the North Pacific remains poorly understood.

Here, we use the intermediate-complexity Earth system model cGENIE to perform a suite of orbital sensitivity experiments under the Early Eocene climate boundary conditions, systematically isolating the effects of eccentricity, obliquity and precession on ocean circulation. By holding background climate boundary conditions constant, our experiments allow direct assessment of the dynamical response of the ocean to orbital forcing alone.

The simulations reveal that precession exerts a substantially stronger control on ocean circulation strength than either eccentricity or obliquity, with the most pronounced response occurring in the North Pacific. Under precession minimum configurations, reduced summer insolation leads to cooler surface waters and enhanced winter buoyancy loss. This promotes deeper winter mixed layer, increases vertical exchange, and enables sustained deep-water formation in the North Pacific. In contrast, precession maximum configurations are associated with warmer surface waters and weaker winter cooling, limiting mixed-layer deepening and favoring the formation of intermediate rather than deep waters.

Our findings highlight precession as a key regulator of deep-water formation in ice-free climates and demonstrate that changes in seasonal insolation can trigger major reorganization of ocean circulation. This provides new mechanistic insight into how orbital forcing may have contributed to variability in ocean ventilation, carbon cycling, and climate stability during past greenhouse worlds.