Toward a mechanistic model of the ocean biological carbon pump

Photosynthesis by phytoplankton at the surface of the ocean absorb CO2 from the atmosphere to produce organic matter. At the end of their life cycle these marine organisms aggregate into large, rapidly sinking particles. This sinking organic matter is in turn fed on by bacteria and zooplankton, respiring CO2 that can remain dissolved in the deep ocean for thousands of years. This set of processes, collectively known as the "biological carbon pump" (BCP), is a major pathway by which carbon is transported from the atmosphere to the deep sea. Understanding the complex processes that control the efficiency of the BCP and hence the relative partitioning of carbon between the ocean and atmosphere and, ultimately, climate, is one of the leading problems in oceanography and climate science, and the subject of the NERC-funded COMICS (Controls over Ocean Mesopelagic Interior Carbon Storage) project supporting this studentship. Unfortunately, important as it is, current models of the BCP embedded within global climate models do not have a mechanistic representation of the BCP. They are thus largely incapable of responding to environmental changes and cannot be used to investigate how the BCP will evolve in the future or how it may have operated in the past. The primary objective of this project is to obtain a mechanistic understanding of the BCP and it's response to environmental conditions. To achieve this goal, a global model that represents the processes through which marine particles stick together or break apart will be developed. Starting with the growth of phytoplankton at the surface the model will explicitly consider the main processes affecting the sinking of organic particles through the ocean. This model of particles and biogeochemistry will interact with ocean circulation as simulated by models such as UKESM.

Specifically, the goal is to take a stochastic approach to modeling the dynamics of marine particles in which the interaction between individual particles, biology and ocean circulation are explicitly simulated. Such a Lagrangian approach has not previously been attempted in 3-d because of its computational expense. We plan to address this by exploiting novel computational hardware such as programmable Graphics Processing Units GPUs to achieve computational speedup of the model. The student will not only lead the development of the model-acquiring training and skills in marine biogeochemistry, oceanography and high performance computing and numerical modeling-but also perform experiments to mechanistically explore the BCP's response to climate change, especially changes in circulation, atmospheric CO2 and ocean chemistry. The student will actively collaborate with COMICS team members at the National Oceanography Centre, Southampton, and other observational and modeling groups in the US, Germany and France.