Investigating the Impact of Changing Ocean Ventilation, due to Climate Change, on the Biological Carbon Pump

There is compelling evidence that with increased warming the ocean is experiencing a number of changes to its circulation. For example, reduction in ocean ventilation due to 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 investigate the effect of ocean ventilation on the biological carbon pump, two idealised box models for oceanic 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 being represented by a relaxation timescale that varies with increased warming from emissions. The second model is more complex, including a thermocline with a dynamically controlled thickness and meridional overturning circulation (that also varies with increased warming), which determines the extent of ocean ventilation. These models have previously been used to carry out climate projections by investigating the ocean’s carbon and thermal response to annual anthropogenic emissions, but there has been no investigation on how changing circulation impacts the biological carbon pump. A simple nutrient-phytoplankton-zooplankton-detritus (NPZD) biological model is introduced to model the impact of macronutrient concentrations on phytoplankton and zooplankton growth. Using the extended models, we undertake simulations with both constant and changing circulation, in order to investigate the effect of slower circulation and increased ocean stratification on the biological carbon pump, primary production and nutrient distribution.