Meridional Ocean Carbon Transport

Aitor Aldama‐Campino,Filippa Fransner,Sjoerd Groeskamp,Andrew Yool,Malin Ödalen,Jonas Nycander,Kristofer Döös

doi:10.1029/2019gb006336

Abstract

The ocean's ability to take up and store CO2 is a key factor for understanding past and future climate variability. However, qualitative and quantitative understanding of surface‐to‐interior pathways, and how the ocean circulation affects the CO2 uptake, is limited. Consequently, how changes in ocean circulation may influence carbon uptake and storage and therefore the future climate remains ambiguous. Here we quantify the roles played by ocean circulation and various water masses in the meridional redistribution of carbon. We do so by calculating streamfunctions defined in dissolved inorganic carbon (DIC) and latitude coordinates, using output from a coupled biogeochemical‐physical model. By further separating DIC into components originating from the solubility pump and a residual including the biological pump, air‐sea disequilibrium, and anthropogenic CO2, we are able to distinguish the dominant pathways of how carbon enters particular water masses. With this new tool, we show that the largest meridional carbon transport occurs in a pole‐to‐equator transport in the subtropical gyres in the upper ocean. We are able to show that this pole‐to‐equator DIC transport and the Atlantic meridional overturning circulation (AMOC)‐related DIC transport are mainly driven by the solubility pump. By contrast, the DIC transport associated with deep circulation, including that in Antarctic bottom water and Pacific deep water, is mostly driven by the biological pump. As these two pumps, as well as ocean circulation, are widely expected to be impacted by anthropogenic changes, these findings have implications for the future role of the ocean as a climate‐buffering carbon reservoir.

Highlights

The ocean circulation and its redistribution of carbon are important parts of the global carbon cycle and have significant impacts on the atmospheric pCO2 (Ciais et al, 2013; Mikaloff Fletcher et al, 2007; Sabine et al, 2004; Sarmiento & Gruber, 2006)
By further separating dissolved inorganic carbon (DIC) into components originating from the solubility pump and a residual including the biological pump, air-sea disequilibrium, and anthropogenic CO2, we are able to distinguish the dominant pathways of how carbon enters particular water masses
We are able to show that this pole-to-equator DIC transport and the Atlantic meridional overturning circulation (AMOC)-related DIC transport are mainly driven by the solubility pump

Summary

Introduction

The ocean circulation and its redistribution of carbon are important parts of the global carbon cycle and have significant impacts on the atmospheric pCO2 (Ciais et al, 2013; Mikaloff Fletcher et al, 2007; Sabine et al, 2004; Sarmiento & Gruber, 2006). With rising atmospheric pCO2 and related climate change, many studies have focused on air-sea CO2 exchange (e.g., Landschützer et al, 2015; Le Quéré et al, 2007; Takahashi et al, 2002; Yasunaka et al, 2018) and how the physical and biological pumps drive the air-sea exchange (Volk & Hoffert, 2013) How this air-sea CO2 exchange connects the surface to the ocean interior via large scale ocean circulation is, less well constrained (Levy et al, 2013). Estimates of meridional ocean carbon transport have been made through inverse calculations based on observed air-sea CO2 exchange and its variations (Gloor et al, 2003; Gruber et al, 2009; Mikaloff Fletcher et al, 2007) These do not, give any information on the control that the various circulation cells exerts on carbon transport and redistribution within the ocean. They do not explain how the transport is connected globally, nor do they clarify the sources of the transported carbon, that is, whether it originates from the physical or the biological pumps

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