Impact of an open Central American Seaway on the Pacific oxygen minimum zone from simulations with global climate models

The tectonic transition from the open to closed CAS during the mid-Miocene to mid-Pliocene (~16-3 Ma BP) is often thought of as a key factor for the development of the tropical Pacific oxygen minimum zone. In this study we investigate the impact of an open Central American Seaway (CAS) on the equatorial current system and oxygen minimum zone in the tropical Pacific. We compare simulations with two independent global climate models (Kiel Climate Model and HadCM3) where the sill depth of the open CAS was set to the same different levels, ranging from shallow to deep.

Both models show a substantial increase in oxygen concentrations in the subsurface eastern tropical Pacific waters in response to the open CAS. Detailed multi-model analysis reveals that the CAS opening results in two main oxygen anomalies in the eastern Pacific, one located below the surface (more northward) and another in deeper (more equatorward) water masses.

Estimates of the water mass transport from west to east driven by the equatorial undercurrent (EUC), the north equatorial counter current (NECC) and the northern subsurface counter current (NSCC) agree well between both models for preindustrial (closed CAS) as well as for the open CAS experiments. Both models show that the open CAS is associated with an enhanced eastward subsurface NSCC in the northeastern tropical Pacific that transports oxygen-rich waters from the western tropical Pacific toward the eastern equatorial Pacific. This mechanism can explain the simulated deeper (and more equatorward) oxygen enrichment in the eastern equatorial Pacific. Potential factors responsible for the more northern anomaly are also analysed.