Discrepancies in temporal pCO2 variability from Earth System Models and pCO2-products related to high-latitude mixed layer dynamics and equatorial upwelling

The air-sea CO2 flux FCO2 is an important component of the global carbon budget and understanding its response to climate change is crucial to adjust mitigation pathways. Multi-linear regression supports the expectation that the balance between the CO2 partial pressures of air and the sea surface (pCO2) is the most important driver of temporal FCO2 variability. Discrepancies between state-of-the-art Earth System Models (ESMs) and gridded pCO2-products suggest that systematic biases exist across an ensemble of ESMs. In the equatorial regions, upwelling variability of carbon-rich water is biased in ESMs as modeled and observed sea surface temperature are generally uncorrelated. In the high latitudes, the climate change induced trend towards lighter sea water is overestimated in ESMs, which yields - in contrast to observations - shallower mixed layers over the contemporary period and hence a suppressed carbon supply from depth. While mixed layer depth variability and trends appear biased throughout the global ocean, this is not a determining factor for pCO2 variability in subtropical gyres. The results highlight the importance of accurately modeling hydrographic properties to obtain robust estimates of FCO2 and its variability.