Early summer water mass transformation in the eastern Chukchi Sea

Pacific water entering the Chukchi Sea through Bering Strait can be modified by physical processes such as surface heating and cooling, and internal mixing. Here we document the existence of a previously unobserved water mass in the eastern Chukchi Sea during the early-summer using hydrographic and velocity data from the June–July 2010 ICESCAPE survey. The water mass in question is as salty as Pacific Winter Water (PWW), but is much warmer with temperatures typical of summer waters. We label the water mass Dense Chukchi Summer Water (dCSW) which is dense enough to ventilate the upper halocline of the western Arctic Ocean, normally ventilated by the much colder PWW. Satellite sea surface temperature data, long-term mooring data from Bering Strait and the Beaufort Shelfbreak, and atmospheric reanalysis fields are used to characterize the spatial and temporal variability of the dCSW and explore potential forcing scenarios that can lead to its formation. In 2010 the dCSW did not enter the Chukchi Sea through Bering Strait, hence it had to be formed locally on the shelf. Two distinct forcing mechanisms are considered: (1) mixing of very warm Alaskan Coastal Water with the cold PWW when the two water masses come in contact in the energetic Alaskan Coastal Current; and (2) warming of PWW when it experiences solar heating within the coastal polynya in the northeastern Chukchi Sea during the spring melt season. Finally, we explore the conditions under which the springtime polynya has formed over the past three decades as it relates to the formation of dCSW.