The Changing CO2 Sink in the Western Arctic Ocean From 1994 to 2019

The Arctic Ocean has turned from a perennial ice-covered ocean into a seasonally ice-free ocean in recent decades. Such a shift in the air-ice-sea interface has resulted in substantial changes in the Arctic carbon cycle and related biogeochemical processes. To quantitatively evaluate how the oceanic CO2 sink responds to rapid sea ice loss and to provide a mechanistic explanation, here we examined the air-sea CO2 flux and the regional CO2 sink in the western Arctic Ocean from 1994 to 2019 by two complementary approaches: observation-based estimation and a data-driven box model evaluation. The pCO(2) observations and model results showed that summer CO2 uptake significantly increased by about 1.4 +/- 0.6 Tg C decade(-1) in the Chukchi Sea, primarily due to a longer ice-free period, a larger open area, and an increased primary production. However, no statistically significant increase in CO2 sink was found in the Canada Basin and the Beaufort Sea based on both observations and modeled results. The reduced sea ice coverage in summer in the Canada Basin and the enhanced wind speed in the Beaufort Sea potentially promoted CO2 uptake, which was, however, counteracted by a rapidly decreased air-sea pCO(2) gradient therein. Therefore, the current and future Arctic Ocean CO2 uptake trends cannot be sufficiently reflected by the air-sea pCO(2) gradient alone because of the sea ice variations and other environmental factors.