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Novel Tools for Ocean Biogeochemical Models


Ocean general circulation models of the IPCC class have biases even when simulating

present-day conditions, which may bring into question their predictions of future

conditions. This dissertation is about tools for, and results from assessing biases in the

Community Earth System Model (CESM) ocean component, by itself and when combined

with the Biological Ecosystem Cycling (BEC) model. Newly developed tools and their

applications are listed.

1. An offline matrix tracer transport model for the ocean component of CESM.

2. A fast Newton-Krylov implicit tracer equilibrium solver for both the annually-averaged

and the seasonally-varying circulation.

3. An effective preconditioner for the solver simulating radiocarbon.

Application results:

For a natural radiocarbon simulation, an equilibrium solution was obtained in 23

model-years, a dramatic decrease from the 4000 model-years reported for

time-stepping. The modeled circulation in the deep Pacific Ocean produced

radiocarbon ages twice those of observations.

4. A capability for computing the surface origin of water mass fractions as well as the

age of the various water masses.

Application results:

The North Atlantic was the major supplier of ventilated water to not only the

Atlantic, but also the Pacific and Indian Oceans. A lack of formation of bottom

water in the Southern Ocean was discovered.

5. A capability for restricting the tracer simulation domain to a limited region of the

ocean while retaining the effectiveness of advection and diffusion fields on the

boundary. This reduces computational costs and allows separating local versus

remote impacts of tracer sources on the biogeochemical tracer concentrations. This

capability has the potential to provide a platform for further biogeochemical studies.

Application results:

The Indian Ocean region was isolated. Global versus regional circulation effects

were determined using radiocarbon. Most of the bias within the region was eliminated

by using observational, rather than globally calculated values, on the boundaries.

Oxygen production and consumption from a CMIP5 BEC simulation were used to

drive a regional oxygen model. Boundary values of oxygen from the CMIP5 BEC

simulation were replaced with observations, resulting in less bias within the region.

However, significant bias in the location of the Arabian Sea oxygen minimum

zone remained.

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