01/07/2014

# Climate Models: New Method to Enable Ocean Age Tracking and Testing with Radiocarbon

## Summary

Natural radiocarbon is deposited onto the Earth and ocean surface, where it decays as it gets slowly mixed into the deep ocean. Using radiocarbon in climate models to track and test the age of deep ocean circulation can be very useful. However, because the ocean circulation is slow, the model takes a long time for the radiocarbon to reach a steady state and the method is computationally too expensive to routinely use with moderate- to high-resolution ocean models. To overcome this difficulty, U.S. Department of Energy-funded scientists have developed a mathematical method that greatly accelerates the time needed for the model to equilibrate, allowing the ocean to reach equilibrium in a few decades. Essentially, the method “preconditions” the ocean with a very coarse resolution result and uses this as a starting point for the more precise solution, so that equilibrium in the high-resolution model is achieved more quickly. The method was implemented in the Parallel Ocean Program (POP) of the Community Earth System Model (CESM) and was used to simulate the prebomb equilibrium radiocarbon distribution. The model resolution (1×1 degree in the horizontal and 60 levels in the vertical) is the highest of all natural radiocarbon simulations performed to date. Once it reached equilibrium, the modeled radiocarbon distribution was compared to observations, and the team was able to identify clear biases (errors) in the model circulation of the deep Pacific Ocean. The biases had prebomb radiocarbon ages that are twice the observed values and were significantly larger than those of coarser resolution models. The new method will make it possible to take advantage of natural radiocarbon observations when calibrating processes in models of increasing resolution.

## References

Bardin, A., F. Primeau, and K. Lindsay. 2014. “An Offline Implicit Solver for Simulating Prebomb Radiocarbon,” *Ocean Modelling* **73**, 45-58. DOI: 10.1016/j.ocemod.2013.09.008.