Carbon sources and sinks from an Ensemble Kalman Filter ocean data assimilation

Abstract

[1] We quantify contemporary and preindustrial net air-sea CO2 fluxes by an Ensemble Kalman Filter assimilation of interior ocean observations and compare results with published estimates in the light of sensitivity to model transport and input data reconstructions. Four different published reconstructions of anthropogenic carbon and the ΔCgasex tracer are assimilated into different versions of the Bern3D ocean model. The two tracers represent the components of dissolved inorganic carbon due to the anthropogenic perturbation and the air-sea gas exchange of preindustrial CO2. Contemporary air-sea fluxes for broad latitudinal bands are consistent with those from earlier ocean inversions and the observed air-sea CO2 partial pressure differences. We infer modest meridional transport rates of up to 0.5 GtC yr−1 for the preindustrial and the contemporary ocean and a small carbon transport across the equator. The anthropogenic perturbation offsets the preindustrial net sea-to-air flux yielding a weak contemporary carbon sink in the Southern Ocean (south of 44°S) of 0.15 ± 0.25 GtC yr−1. Preindustrial Southern Ocean outgassing varies by almost a factor of 2 among the four ΔCgasex reconstructions. Large differences in regional fluxes are found between an earlier ocean inversion using Green's function and this study for the same model and input data calculation. Systematic differences in assimilated and optimized ΔCgasex fields are large in both inversions, and the contemporary, anthropogenic, and preindustrial air-sea CO2 flux in the high-latitude and midlatitude Southern Hemisphere remains uncertain.