Coupling of Surface Ocean Heat and Carbon Perturbations over the Subtropical Cells under Twenty-First Century Climate Change

K B Rodgers,M Ishii,R D Slater,O Aumont,K Toyama,S Schlunegger,T L Frölicher

doi:10.1175/jcli-d-19-1022.1

Abstract

AbstractIt is well established that the ocean plays an important role in absorbing anthropogenic carbon Cant from the atmosphere. Under global warming, Earth system model simulations and theoretical arguments indicate that the capacity of the ocean to absorb Cant will be reduced, with this constituting a positive carbon–climate feedback. Here we apply a suite of sensitivity simulations with a comprehensive Earth system model to demonstrate that the surface waters of the shallow overturning structures (spanning 45°S–45°N) sustain nearly half of the global ocean carbon–climate feedback. The main results reveal a feedback that is initially triggered by warming but that amplifies over time as Cant invasion enhances the sensitivity of surface pCO2 to further warming, particularly in the warmer season. Importantly, this “heat–carbon feedback” mechanism is distinct from (and significantly weaker than) what one would expect from temperature-controlled solubility perturbations to pCO2 alone. It finds independent confirmation in an additional perturbation experiment with the same Earth system model. There mechanism denial is applied by disallowing the secular trend in the physical state of the ocean under climate change, while simultaneously allowing the effects of heating to impact sea surface pCO2 and thereby CO2 uptake. Reemergence of Cant along the equator within the shallow overturning circulation plays an important role in the heat–carbon feedback, with the decadal renewal time scale for thermocline waters modulating the feedback response. The results here for 45°S–45°N stand in contrast to what is found in the high latitudes, where a clear signature of a broader range of driving mechanisms is present.

Highlights

An important priority in climate research is to identify and understand processes that modulate or limit the rate at which the ocean can absorb anthropogenic carbon from the atmosphere under future climate change
The main objective of this study is to demonstrate that a heat–carbon feedback over the low latitudes (458S–458N) is triggered through heating of surface waters, with amplification through time occurring through the cumulative invasion flux of anthropogenic carbon boosting the sensitivity of sea surface pCO2 to warming, with this amplification being larger in the warmer season
The global ocean CO2 uptake for the coupled simulation (COU) case is reduced by 1.3 PgC yr21 during the 2090s in comparison with the biogeochemically coupled (BGC) case, indicating the well-known positive global carbon–climate feedback, with the contrast between COU and BGC being larger than the contrast between radiatively coupled (RAD) and preindustrial conditions (PIN) of 0.19 PgC yr21

Summary

Introduction

An important priority in climate research is to identify and understand processes that modulate or limit the rate at which the ocean can absorb anthropogenic carbon from the atmosphere under future climate change. For the case where anthropogenic perturbations to the ocean state may result in more CO2 being left in the atmosphere and a resulting radiative heating of the Earth system, we refer to this as a positive climate feedback. There is long-standing interest in how anthropogenic heat entering the ocean can trigger climate feedbacks through reduced ocean CO2 uptake

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