Exploring the Impact of Changing Ocean Circulation on Carbon Storage due to the Biological Carbon Pump: An Idealised Modelling Approach

Compelling evidence indicates that ocean circulation is undergoing significant changes due to global warming. These changes include reduced ocean ventilation caused by increased stratification and the weakening of the Atlantic Meridional Overturning Circulation (AMOC). Consequently, this will alter carbon, oxygen, heat and nutrient distribution, and will therefore affect primary production and, by extension, the biological carbon pump. Due to the ocean’s huge capacity for carbon storage, it is imperative that we understand the consequences of these changes.

To examine how ocean ventilation influences the biological carbon pump and overall oceanic carbon storage, two idealised box models of ocean carbon and heat uptake are extended to include biological processes and nutrient cycling. The first model is a one-dimensional box model, with ocean ventilation parameterised by a relaxation timescale that responds to emission-driven warming. The second model is more complex, including a thermocline with a dynamically controlled thickness and meridional overturning circulation, both of which vary with increasing temperatures, determining the extent of ocean ventilation.

These models, previously employed to analyse the ocean’s carbon and thermal response to anthropogenic emissions, are now adapted to explore the effects of changing circulation on the biological carbon pump. A simple nutrient-phytoplankton-zooplankton-detritus (NPZD) biological model is introduced to simulate the role of macronutrient concentrations on phytoplankton and zooplankton growth. Simulations are conducted under scenarios of both constant and changing circulation to investigate the impacts of slower circulation and increased stratification on the biological carbon pump and its contribution to oceanic carbon storage.