Fig. 2: Hypothesized mechanism of distribution change in low oxygen water.

a 2017–2020: the atmospheric condition was characterized by a positive Arctic Oscillation phase (AO+; a thick red arrow at the top) and a small extent of the Beaufort High (BH; upper blue arrows) with a strongly negative wind curl (thick arrows of the BH). The intrusion of North Atlantic cyclones (Cys; small red arrows) could contribute to AO+. This atmospheric condition caused a shrink of the Beaufort Gyre (BG; lower blue arrows), which transported Pacific water (PW) to the east of the Chukchi Plateau (CP) in the Canada Basin (CB). Lower Halocline Water (LHW) from the eastern Arctic penetrated near the CP (lower red arrows) under AO+. A northward flow along the CP (a yellow allow) caused by the PW/LHW front carried low oxygen (LowOxy) water from the shelf-slope at the north of the East Siberian Sea (ESS) to the CP. b 2008–2016: a negative Arctic Oscillation phase (AO−; a thin red arrow at the top) and a large extent of the BH caused BG enlargement to the west of the CP with PW extension. LHW could not penetrate the CP under AO−. LowOxy water formed on the shelf-slope at the north of the ESS (yellow ellipse) between the oxygen-rich PW and LHW influences. c The 1950s–1980s: AO− and a large extent of the BH with a weakly negative wind curl (thin arrows of the BH). Sea ice cover inhibited the input of wind curl to the ocean. The BG was wide, but its circulation was weak. The BG transported the PW to the west of the CP and the ESS water (ESSW) to the north of the ESS. The penetration of LHW was minimal as the AO was negative and Atlantification had not yet occurred. Thus, LowOxy water occupied the shelf-slope at the north of the ESS.