Sensitivity of 21st Century Ocean Carbon Export Flux Projections to the Choice of Export Depth Horizon

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 (MLDmax) 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 MLDmax 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 MLDmax, 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 MLDmax, 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.

Automated summary

This study compared the changes in global ocean carbon export flux and export efficiency under different depth horizons (MLDmax and 100m) for the 21st century. The enhanced stratification makes it more difficult for export efficiency to decrease in deep winter mixing regions. The choice of different depth horizons can significantly impact the spatial patterns of simulated results.