Ocean Tracer and Biogeochemical Modeling at MIT.

Here we briefly describe some of our ocean tracer and biogeochemistry studies.

Aims:

Our overarching aim is to better understand the interactions between the climate system and the global carbon cycle. What is the role of ocean circulation and its variability in controlling and modulating global biogeochemical cycles? What are the feedbacks between climate and biogeochemical processes?

You can find pre-prints, reprints and abstracts of of our papers on these topics here.


Transient Tracers.

Ocean distributions of anthropogenic transient tracers, such as chlorofluorocarbons (CFCs) and radiocarbon (14C) reveal pathways, rates and mechanisms of ocean circulation and mixing. CFC-11 is entirely man-made, and its concentration in the atmosphere increased rapidly from the 1930s to the 1990s. It is soluble and has been invading the oceans, marking waters which have recently been near the surface. You can see an animation of the invasion of CFC-11 into the deep North Atlantic in one particular configuration of the MIT ocean circulation model.

We have examined model simulations of the penetration of bomb-radiocarbon (the 14C perturbation from atmospheric nuclear tests in the 1960's) into the ocean. The comparison shows an improved simulation in models which (partially) resolve mesoscale eddies - the oceanic equivalent of weather systems which are typically tens of kilometres in lateral scale.

The figure shows the modeled 1972 distribution of bomb-radiocarbon on the sigma-theta=27.0 density surface from a coarse resolution ocean model (top, 1 degree resolution) and an ``eddy permitting'' model (bottom, 1/3 degree resolution). The finer resolution model shows significant ventilation of the western subtropical thermocline due to mesoscale eddy stirring. From Follows and Marshall (1996); read the abstract here.

Comparing model simulations of ocean tracers with observed data can inform us about the veracity of our ocean circulation models. We have taken part part in the Ocean Carbon Model Intercomparison Project, OCMIP in which a number of numerical ocean circulation models are used to simulate a suite of ocean tracers. Comparisons show significant differences between models and data, and between the different models.

Interannual Variability of Air-Sea Gas Fluxes.

How do changes in ocean circulation and mixing on interannual timescales affect the air-sea exchange of trace gases, such as carbon dioxide and oxygen? We use numerical models of ocean circulation and biogeochemistry to examine the possible effect of interannual changes in meteorological forcing and upper ocean circulation on the fluxes of carbon dioxide and oxygen between the ocean and atmosphere. With graduate student Galen Mckinley, using global ocean circulation estimates from the ECCO consortium we find a significant interannual variability of the air-sea exchange of carbon dioxide (of similar magnitude to those estimated from observed atmospheric data) related to changes in upper ocean mixing.

The figure shows the time-mean air-sea flux of carbon dioxide from a global, interannually varying simulation made by Galen McKinley. The ocean model absorbs carbon from the atmosphere in regions of sea surface heat loss and biological uptake of carbon (e.g. North Atlantic). The ocean model is losing carbon dioxide from the ocean in regions where carbon rich deep waters upwell to the surface (e.g. Tropical Pacific). Interannual variability in the global flux of carbon dioxide across the model's sea surface show general agreement with inferences from observed data showing a strong signature of changes in the Tropical Pacific associated with El Nino phenomenon.

Regional and Interannual Variability of Ocean Biological Productivity

We have examined the relationships between regionanal and interannual variations in meteorological forcing and the variability of the spring bloom of biological productivity in the North Atlantic ocean. We have examined chlorophyll distributions estimated from remotely sensed ocean color by the SeaWiFs mission. We have also used a numerical model of the time dependent ocean circulation with a highly simplified representation of the ecosystem of the North Atlantic. This project has been a collaboration with Dr. Stephanie Dutkiewicz, of MIT, and Dr. Watson Gregg of NASA/Goddard Space Flight Center.

The figure shows the mean surface ocean chlorophyll concentration in the Atlantic basin (image from the SeaWiFs website). Yellow and light blue indicate high chlorophyll concentrations. The high concentrations in the subpolar gyre reflect the strong late spring and summer bloom in that region. The subtropics are depleted in nutrients year round, reflected in low chlorophyll concentrations (dark blue).

An animation, showing the seasonal and interannual variability of the North Atlantic spring bloom, as observed from space by SeaWiFS, can be seen here (1.9Mb animated gif, images produced by Stephanie Dutkiewicz). Monthly images are sequenced, for the two years 1998 and 1999.

We have studied the intensity of the bloom in the subtropical and subpolar North Atlantic, and its relationship to meteorological forcing, using analysis of the SeaWiFS data and numerical models. These studies are described in a series of papers (click link to download PDF's): Follows and Dutkiewicz (2001)., Dutkiewicz et al. (2001). and Williams et al. (2001).

Ocean Carbon Cycle Model Intercomparison Project (OCMIP)

We have participated in OCMIP, an international comparison of simulations of the ocean uptake of chlorofluorocarbons and the distribution and air-sea exchange of nutrients, carbon and oxygen in the ocean. We have used a coarse resolution configuration of the MIT ocean circulation model overlain with parameterizations of biogeochemical processes as defined by the OCMIP HOWTO documents.

Selected results from the MIT OCMIP simulations may be viewed, and downloaded here


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