Transformation of Pacific water masses north of Bering Strait

Hydrographic observations have revealed the presence of Pacific summer water (PSW) throughout the Canada Basin of the western Arctic Ocean. This warm and relatively fresh water mass ventilates the upper halocline and impacts the stratification of the water column in the interior basin. It is presently believed that PSW originates in the Bering Sea and is subsequently advected northward through different pathways across the Chukchi Sea shelf, with relatively little modification before reaching the shelfbreak. Here we use shipboard hydrographic and velocity data from an extensive early-summertime survey of the Chukchi Sea to examine the characteristics and transformation of the Pacific water masses progressing northward from Bering Strait to the shelfbreak. We explore the hypothesis that a significant portion of the PSW is formed locally in the eastern Chukchi Sea due to mixing of the cold resident winter water on the shelf (from the previous winter) and the inflowing warm and buoyant Alaska Coastal Water, with possible contributions from surface melt water. This interpretation is more consistent with the data than the commonly accepted notion that PSW stems entirely from the Bering Sea. The potential vorticity dynamics of the flow is investigated, as well as the role of wind-forcing and diapycnal mixing in the evolution of the Pacific water masses from Bering Strait to Barrow Canyon. Our results imply that local processes in the Chukchi Sea are of primary importance in influencing the PSW that ultimately ventilates the interior Western Arctic Ocean.