Contrasting high-latitude mixed layer depth trends in Earth System Models and data products and their impact on temporal pCO2 variability

Abstract The air-sea CO2 flux FCO2 is an important component of the global carbon cycle 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 in FCO2 variability between state-of-the-art Earth System Models (ESMs) and gap-filled pCO2-products suggest that systematic biases exist across an ensemble of ESMs. In the high latitudes, the climate change induced trend towards lighter seawater 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 the strongly stratified subtropical gyres. The results highlight the importance of accurately modeling circulation and hydrographic properties to obtain robust estimates of FCO2 and its variability.