Hydrographic Characteristics and Oceanic Heat Flux in the Upper Arctic Ocean over the Alpha Ridge Observed by the DTOP Platform in 2018 and 2021

In 2018 and 2021, the Drift-Towing Ocean Profilers (DTOP) provided extensive temperature and salinity data on the upper 120 m ocean through their drifts over the Alpha Ridge north of the Canada Basin. The thickness and temperature maximum of Alaska Coastal Water (ACW) ranged from 20 m to 40 m and −1.5℃ to −0.8℃, respectively, and the salinity generally maintained from 30.2 to 32.5. Comparison with World Ocean Atlas 2018's climatology manifested a 40 m-thick and warm ACW roughly exceeding the temperature maximum by 0.4 – 0.5℃ in June – August 2021. This anomalously warm ACW was highly related to the expansion of the Beaufort Gyre in the negative Arctic Oscillation phase. During summer, the under-ice oceanic heat flux F_w^\textOHF was elevated, with a maximum value of above 25 W m⁻². F_w^\textOHF was typically low in the freezing season, with an average value of 1.2 W m⁻². The estimates of upward heat flux contributed by ACW to the sea ice bottom F_w^\textOHF were in the range of 3 – 4 W m^{−2 in June} – August 2021, when ACW contained a heat content of more than 80 MJ m⁻². The heat loss over this period was driven by a weak stratification upon the ACW layer associated with a surface mixed layer (SML) approaching the ACW core. After autumn, F_w^\textOHF was reduced (< 2 W m⁻²) except during rare events when it elevated F_w^\textOHF slightly. In addition, the intensive and widespread Ekman suction, which created a violent upwelling north of the Canada Basin, was largely responsible for the substantial cooling and thinning of the ACW layer in the summer of 2021.