How important is the Equatorial Undercurrent for biogeochemistry and global climate predictions?
How important is the Equatorial Undercurrent for biogeochemistry and global climate predictions?
Abstract:
The Pacific Equatorial Undercurrent (EUC) is one of the strongest ocean currents on Earth and transports thermocline waters into the upwelling regions of the Eastern Pacific, where they can influence regional biogeochemistry and the global climate. Despite its importance, current earth system models show disparate trends in the response of the EUC to climate change, some models showing a strengthening and others weaken or do not respond.
Here we examine the dynamics of the EUC and its impact on biogeochemistry, in particular, the Oxygen Minimum Zone (OMZ), by contrasting two coupled climate models with a different horizontal resolution, using a coarse (1 deg) model and an eddying (1/10 deg) model. While both models reproduce basic properties of the zonal mean EUC, the coarse model fails to represent key mesoscale processes, leading to a misrepresentation of the EUC along the Equator. The current gradually weakens over the basin, a characteristic commonly found in earth system models of similar resolution. In contrast, the eddying model reproduces a stronger and sustained current across the basin, as observed in nature. We show that which has substantial implications for the OMZ beneath.
The coarse model produces a strong east-west tilt in the upper boundary of the OMZ, while the eddying model produces a flat boundary more akin to observations.
This tilt crucially affects how changes in the equatorial Circulation project onto the shape and variability of the OMZ: A strong OMZ tilt increases the sensitivity to changes in circulation, leading to a strong dependence on circulation changes and overwhelming other relevant processes like changes in biological production.
These findings suggest that uncertainties in the physical processes maintaining the EUC might severely hamper our ability to predict the response of the OMZ and the earth system in general to both multi-annual and long term changes.
Here we examine the dynamics of the EUC and its impact on biogeochemistry, in particular, the Oxygen Minimum Zone (OMZ), by contrasting two coupled climate models with a different horizontal resolution, using a coarse (1 deg) model and an eddying (1/10 deg) model. While both models reproduce basic properties of the zonal mean EUC, the coarse model fails to represent key mesoscale processes, leading to a misrepresentation of the EUC along the Equator. The current gradually weakens over the basin, a characteristic commonly found in earth system models of similar resolution. In contrast, the eddying model reproduces a stronger and sustained current across the basin, as observed in nature. We show that which has substantial implications for the OMZ beneath.
The coarse model produces a strong east-west tilt in the upper boundary of the OMZ, while the eddying model produces a flat boundary more akin to observations.
This tilt crucially affects how changes in the equatorial Circulation project onto the shape and variability of the OMZ: A strong OMZ tilt increases the sensitivity to changes in circulation, leading to a strong dependence on circulation changes and overwhelming other relevant processes like changes in biological production.
These findings suggest that uncertainties in the physical processes maintaining the EUC might severely hamper our ability to predict the response of the OMZ and the earth system in general to both multi-annual and long term changes.