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.
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.
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.
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