Modelling surface ocean carbon

In order to understand the roles of different processes affecting ocean atmosphere exchange of CO2 and DMS it is essential to develop quantitative models for these.

This model development also serves as a unifying focus for the work done in the other parts of the programme. In particular it provides us with a basis for comparing observations in different parts of the ocean and for extrapolating from specific times and locations to estimate annual and area averaged fluxes.

We have developed a model of the processes driving marine carbon chemistry in the ocean mixed layer including air-sea exchange of CO2 and the removal of carbon by marine biota. This has been used to simulate seasonal cycles of surface CO2 concentrations for conditions corresponding to open ocean east of Dunedin and the results have been compared with observations from the ocean atmosphere carbon exchange project.

Results of this work show that estimating and validating the amount of phytoplankton is critical. We have improved a model, originally developed in the UK for conditions in the Atlantic ocean, by incorporating a more realistic treatment of light levels and taking some account of seasonally varying ratios of carbon to chlorophyll in the phytoplankton ecosystem. These model improvements substantially improved the agreement between simulations and observations. 

Simulated partial pressure of carbon dioxide (pCO2) in subantarctic surface water. Temperature increases in summer (Dec–Feb) drive pCO2 up, while phytoplankton photosynthesis drives it down. Increasing temperatures and light levels stimulate a phytoplankton bloom in November/ December which draws down pCO2 but is limited by nutrients. The atmospheric CO2 level used is shown by a dashed horizontal line. When the ocean pCO2 is below (above) this line the ocean acts as a sink (source).
Research subject: Oceans