Towards detecting and attributing long-term changes in CO2 outgassing in the equatorial Pacific

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Changes in carbon dioxide (CO2) sources and sinks in the ocean are highly uncertain, and could therefore have a significant influence on future atmospheric CO2 levels. Outgassing of CO2 in the tropical Pacific is the largest natural source of this powerful greenhouse gas to the atmosphere. This project will use a combination of existing and new Earth System Model (ESM) simulations to study ongoing and future changes in sea-air CO2 exchange over the tropical Pacific. The research approach will focus on robust mechanisms governing changes in equatorial outgassing in 21th Century simulations of climate and ocean biogeochemistry. The research proposed here could provide answers regarding the ability of the tropical oceans to continue to slow their outgassing of CO2, thus contributing to moderating the increase of atmospheric CO2. In addition, the proposed analysis of sampling biases is a first step towards understanding the requirements of future expansions of the ocean carbon observing system in the tropical oceans. Last, this project will support one early career researcher and one post-doctoral scholar, who will receive cross-disciplinary training in climate dynamics and ocean biogeochemistry. A combination of advanced diagnostics and new simulations will be used to identify and understand the dynamics and physics of the robust mechanisms, i.e. those that can be linked to simple dynamical and physical processes and therefore are independent of parameterized processes. New simulations will be performed with the objective of isolating the effects of increasing CO2, ocean circulation, and warmer temperatures on equatorial outgassing. Model dependence of the results will be explored using existing output from a multi-model ensemble. The project will also explore how these mechanisms operate in subsequent centuries as atmospheric CO2 stabilizes and the carbon cycle approaches a new equilibrium. Strategies for evaluating the simulated mechanisms using observed changes during the 1984-2012 period will be considered. Emphasis will be placed on reducing uncertainty due to limited sampling of spatial and temporal variability of pCO2 over the equatorial Pacific. The effect of ongoing multi-decadal changes in Pacific climate will also be studied using new ocean-only simulations designed to isolate the effect of increasing CO2 from changes in circulation. The outcomes of this work are manifold, including 1) study of robust mechanisms in model projections; 2) evaluation of the long-term response of equatorial outgassing beyond year 2100; 3) a systematic evaluation of the existing observing system in the tropical Pacific; and 4) evaluation of the effect of multi-decadal changes in Pacific climate on the observed changes in the tropical carbon cycle.