[A-OS13] Exploring Variability and Changes in Ocean Biogeochemical Cycles

[AOS13-07] Using historical observations and machine learning algorithms to understand biogeochemical changes under warming and stratifying ocean

★Invited Papers

*Takamitsu Ito¹, Qi Zhang¹, Ahron Cervania¹, Yumi Abe¹, Annalisa Bracco¹ (1.Georgia Institute of Technology)

Keywords:Biogeochemistry, Dissolved oxygen, Ocean deoxygenation, Chemical tracers

Historical shipboard observations show that global oxygen inventory has declined significantly while the ocean heat content increased in the last several decades. Linkages between ocean heat and oxygen contents are unclear. The oxygen-to-heat ratio, which measures the oxygen loss per unit heat uptake, is greater than that predicted by the warming-induced solubility decrease in the global mean. This implies the important roles of changing ocean ventilation and/or biochemical processes with potential linkages to warming and stratifying water columns. At the regional scale, the oxygen-to-heat ratio is highly variable. To better characterize the biogeochemical variability and trends, we developed global, four-dimensional maps of oxygen and nutrients based on the historical shipboard observations and machine learning. We establish linkages between the global and regional oxygen content changes using the quasi-conservative tracer, PO. The observation-based four-dimensional datasets revealed that PO changes are linked to the temperature changes in all ocean basins. Since global phosphate inventory cannot change significantly over the anthropogenic timescale, this provides a clear explanation behind the global oxygen-heat ratio. Thus, the global oxygen decline is primarily driven by the combination of solubility decrease and the increased efficiency of the biological pumps as measured by the depletion of preformed nutrient in the regions of water mass formation. The heterogeneity of the reconstructed oxygen distribution informs about the regional drivers of biogeochemical changes. Oxygen changes in the Southern Ocean primarily reflects the increased upper ocean ventilation likely driven by the intensified and poleward shifted zonal wind, with far-field influences in the mode and intermediate waters. Vertically differentiated changes of Atlantic OMZs indicate distinct influences of upper ocean stratification and sub-surface ventilation. Integration of new observations from autonomous platforms such as BGC-Argo floats will be the crucial step for future development in biogeochemical datasets. However, differences in data QC and sensor bias corrections can have impacts on the reconstructed trends and variability. Community effort is necessary to assess uncertainties in gridded biogeochemical datasets from different data QC, bias corrections, and spatio-temporal interpolation methods.