Sensitivity of Steady State, Deep Ocean Dissolved Organic Carbon to Surface Boundary Conditions

We use the transport matrices of a data-constrained circulation model to efficiently compute the steady state distribution of the deep ocean dissolved organic carbon (DOC) at a 1° horizontal resolution by propagating the surface DOC boundary conditions into the ocean interior. An equivalent simulation in the traditional forward modeling approach would be prohibitively computationally expensive. Our model simulates the total DOC as the sum of two DOC pools, the refractory and the semi-labile. The model is able to simulate the large-scale features of the deep ocean DOC without local sources or sinks of DOC in the ocean interior. The deep ocean DOC in the model is sensitive to the preformed DOC concentrations in the formation sites of deep and bottom waters, where observations are lacking. Furthermore, our model experiments indicate that the deep Atlantic DOC gradient is sensitive to the mixing of deep waters with different concentrations of preformed refractory DOC, the transport of semi-labile DOC from the surface North Atlantic, and the decay rate of semi-labile DOC. These, combined with the observation that much of the deep ocean DOC gradient is in the Atlantic, suggests that the semi-labile DOC may be an important component of the deep Atlantic DOC. Finally, we show that DOC export depends substantially on the depth level where it is evaluated.