Global Earth system model simulations of ocean carbon export flux are commonly interpreted only at a fixed depth horizon of 100 m, despite the fact that the maximum annual mixed layer depth (MLD_max) is a more appropriate depth horizon to evaluate export‐driven carbon sequestration. We compare particulate organic carbon (POC) flux and export efficiency (e‐ratio) evaluated at both the MLD_max and 100‐m depth horizons, simulated for the 21st century (2005–2100) under the RCP8.5 climate change scenario with the Biogeochemical Elemental Cycle model embedded in the Community Earth System Model (CESM1‐BEC). These two depth horizon choices produce differing baseline global rates and spatial patterns of POC flux and e‐ratio, with the greatest discrepancies found in regions with deep winter mixing. Over the 21st century, enhanced stratification reduces the depth of MLD_max, with the most pronounced reductions in regions that currently experience the deepest winter mixing. Simulated global mean decreases in POC flux and in e‐ratio over the 21st century are similar for both depth horizons (8%–9% for POC flux and 4%–6% for e‐ratio), yet the spatial patterns of change are quite different. The model simulates less pronounced decreases and even increases in POC flux and e‐ratio in deep winter mixing regions when evaluated at MLD_max, since enhanced stratification over the 21st century shoals the depth of this horizon. The differing spatial patterns of change across these two depth horizons demonstrate the importance of including multiple export depth horizons in observational and modeling efforts to monitor and predict potential future changes to export.

中文翻译：

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21世纪海洋碳出口通量预测对出口深度水平选择的敏感性

尽管每年最大混合层深度（MLD _max）是评估出口驱动的碳固存的更合适的深度这一事实，但通常仅在100 m的固定深度范围内才解释海洋碳出口通量的全球地球系统模型模拟。。我们比较了两种MLD_最大值下评估的颗粒有机碳（POC）通量和出口效率（e-比率）在RCP8.5气候变化情景下，使用嵌入在社区地球系统模型（CESM1-BEC）中的生物地球化学元素周期模型，模拟了21世纪（2005-2100年）的100 m深度层。这两种深度层位选择会产生不同的基线全球速率以及POC通量和比率的空间格局，在冬季深交地区会发现最大的差异。在21世纪，强化分层降低了MLD _max的深度，而目前经历最深冬季混合的地区降幅最大。在两种深度范围内，模拟的21世纪POC通量和e比率的全球平均下降率相似（两个POC通量为8％–9％，e比率为4％–6％），但变化的空间模式是很不一样。当以MLD _max进行评估时，该模型模拟了冬季较深混合区POC通量和e比率的降低幅度不明显，甚至增加，因为21世纪增强的分层技术掩盖了这一地平线的深度。在这两个深度范围内变化的空间格局不同，表明在观察和建模工作中包括多个出口深度范围以监视和预测潜在的未来出口变化的重要性。