Modeling the Recent Changes in the Arctic Ocean CO2 Sink (2006-2013)

The Arctic Ocean (AO) and its associated marginal seas have recently experienced rapid climate and environmental changes, most notably sea-ice area (SIA) loss and warming potentially impacting its uptake of carbon dioxide (CO2). We used the state-of-the-art ECCO2-Darwin coupled ocean-biogeochemistry model to simulate the 2006-2013 period and investigate the impact of changing SIA on the CO2 uptake of the AO. We find that the mean annual CO2 sink of the AO is 153 +/- 14 TgC a(-1) and the CO2 sink decreased at a rate of 3.6 TgC a(-1) even though SIA decreased by 8 x 10(4) km(2) a(-1) over the same period. Extreme SIA loss in 2007 resulted in a 185.4 TgC CO2 sink, an increase similar to 20% over the 2006-2013 mean. In contrast, extreme SIA loss of 2012 resulted in a CO2 sink of the AO of only 146.3 TgC due to two main factors: (1) increased both wind speed and stratification in the Eastern Siberian Sea absorbing less CO2 and (2) decreased primary production and area of air-sea gas exchange in the Chukchi and Nordic Seas. Our model captures a trend of decreasing CO2 sink in most of the Chukchi Sea during fall but does not show the changes in winter CO2 sink in the Nordic and Barents Seas as previous independent studies have suggested. Our results indicate that future AO-atmosphere CO2 exchange will be determined by complex interplay of SIA and other environmental drivers. Plain Language Summary The seasonal process of melting and formation of sea-ice area (SIA) in the Arctic Ocean (AO) is a key factor to control the physical, chemical, and biological processes of the upper ocean. We used a numerical model to understand the potential consequences of two drastic events of SIA reduction in both 2007 and 2012. Our model simulations show that the biological and chemical responses of the AO were different when we compared 2007 versus 2012. In 2007, the severe reduction of SIA was followed by a large uptake of carbon dioxide (CO2 , as also expected from theory) due to the increase in ice-free area and to the enhanced photosynthetic activity. However, in 2012, the response was different due to several factors: (1) increased wind speed and decreased primary production in the Eastern Siberian Sea in the summer causing less CO2 uptake and (2) the increase in the SIA in the fall in the Chukchi sea and in the summer in the Nordic Seas (close to Greenland) inhibiting the air-sea gas CO2 exchange. All these processes, when combined together, yielded a reduced CO2 sink of the AO in 2012 when compared to 2007.