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

海洋中二氧化碳源和汇的变化具有高度的不确定性，因此可能对未来的大气二氧化碳水平产生重大影响。热带太平洋的二氧化碳排放是这种强大的温室气体向大气的最大自然来源。该项目将结合现有的和新的地球系统模式（ESM）模拟，研究热带太平洋上空海气二氧化碳交换的当前和未来变化。研究方法将集中在21世纪气候和海洋生物地球化学模拟中控制赤道释气变化的强大机制。这里提出的研究可以提供关于热带海洋继续减缓二氧化碳排放的能力的答案，从而有助于减缓大气中二氧化碳的增加。此外，提出的采样偏差分析是了解热带海洋中海洋碳观测系统未来扩展需求的第一步。最后，本项目将支持1名早期职业研究员和1名博士后学者，接受气候动力学和海洋生物地球化学的跨学科培训。先进的诊断和新的模拟相结合将用于识别和理解强大机制的动力学和物理学，即那些可以与简单的动力学和物理过程相联系，因此独立于参数化过程的机制。将进行新的模拟，目的是隔离二氧化碳增加、海洋环流和温度升高对赤道释气的影响。将使用多模型集成的现有输出来探索结果的模型依赖性。该项目还将探索在随后的几个世纪中，随着大气中二氧化碳的稳定和碳循环接近新的平衡，这些机制是如何运作的。将考虑利用1984-2012年期间观测到的变化评估模拟机制的策略。重点将放在减少由于赤道太平洋上二氧化碳分压时空变异性采样有限而产生的不确定性上。还将使用新的仅针对海洋的模拟来研究太平洋气候持续多年代际变化的影响，这些模拟旨在将二氧化碳增加的影响与环流变化隔离开来。这项工作的结果是多方面的，包括1)研究模型预测中的鲁棒机制；2) 2100年以后赤道脱气的长期响应评价；3)对热带太平洋现有观测系统进行系统评价；4)太平洋气候多年代际变化对热带碳循环观测变化的影响。

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.