A new mechanistically-based method for reducing uncertainty in detection of anthropogenic oceanic DIC using Repeat Hydrography

A central goal of Repeat Hydrography is to quantify the rate of uptake of anthropogenic CO2 by the ocean. However, this goal of detection is complicated by the fact that natural variability of DIC is as large as the anthropogenic transient signal as it evolves over decadal timescales. Here we describe a new method for reducing uncertainty in the quantitative assessment of Repeat Hydrography measurements. The method uses satellite altimetry measurements to provide estimates of the natural variability component of DIC changes measured in the ocean interior through Repeat Hydrography, and applies them to the detection of anthropogenic changes. This method is based on our previous modeling study which demonstrated that interannual variability in the positions of the wind-driven gyres of the upper ocean imposes a first-order control on the natural variability of DIC in the upper ocean. Due to the close proximity of the pycnocline and the nutricline in the mean state, variations in the pycnocline depth associated with interannual to decadal variations in circulation are reflected in both sea level height and DIC inventories, with these changes being highly correlated over much of the ocean. Preliminary application of the new method will presented for the North Pacific, North Atlantic, and the Southern Ocean.