Synoptic Features Driving the CO2 Sink in the Mediterranean Sea in Winter

The Mediterranean Sea is a weak sink for the atmospheric CO2 with the October-March extended winter season characterized by the occurrence of high CO2 sink events. Here, we analyzed state-of-the-art ocean and atmospheric reanalyses and observational data sets to investigate the variability of the winter sink and its relation with synoptic atmospheric features crossing the region in the period 1999–2020. High CO2 sink events are identified using classical extreme event approach with fixed threshold (95p) based on the CO2 daily flux distribution. First, we showed that these events are driven by large-scale atmospheric configurations that produce stronger-than-average wind speed and colder-than-average 2 m and sea surface temperature patterns in the region. Second, a co-location analysis was applied to assess the probability to detect an extra-tropical cyclone at a fixed distance from the location of the events showing that the larger the event's magnitude, the higher the probability. In most of the cases, these cyclones originate within the Mediterranean region and are usually deeper, bigger in terms of size and characterized by a stronger circulation with respect to the systems that usually cross the region. By establishing a statistical relationship between high CO2 sink events and synoptic atmospheric activity, we emphasize the potential influence of the cyclone activity on the carbon budget of the Mediterranean Sea.