Investigating the mechanisms and variability of water mass transformation in the Arctic Ocean

The Arctic Ocean is experiencing dramatic surface warming and freshening due to anthropogenic climate change, which is anticipated to continue throughout the 21st Century [1]. This change is evident from rising air surface temperatures, a substantial decline in summer sea-ice extent and thickness, melting of the Greenland ice sheet, an accumulation of freshwater in the Beaufort gyre, and Atlantification of the Eurasian basin [1, 2, 3]. Water masses modified in the Arctic by air-sea interaction and mixing are ultimately exported into the North Atlantic, and play a significant role in the large-scale ocean circulation [4]. It is unclear how the projected increase in Arctic warming and freshening will alter these water mass ransformations, and what the implications will be for global climate. These changes pose potentially severe global consequences such as a reduction in poleward heat transport, an intensified hydrological cycle, and changes to the ocean's ability to sequester carbon [1, 4]. It is thus fundamental to investigate these dynamic transformation processes, to accurately assess the impacts of future change on local and global climate.

The Arctic Ocean is fed by warm, saline Atlantic Water (AW) entering via the Fram Strait and Barents Sea Opening (BSO), and relatively cool and fresh Pacific Water (PW) flowing through the Bering Strait [4, 5]. The inflow of PW constitutes approximately one third of all freshwater entering the Arctic; freshwater results in a cold halocline and near-surface stratification that shields Arctic sea ice from the warmer underlying AW [6]. Within the Arctic Ocean, AW is transformed into two distinct outflow components; one that is fresh and buoyant (termed Polar Water), and another that is cold and dense [5, 7]. The mechanisms that lead to these transformations are depicted in Figure 1.

Despite sustained monitoring programmes such as NABOS [9] and the Ice-Tethered Profiler programme [10], the Arctic Ocean is historically under-observed, hence formation, modification and transport of these water masses, as well as their spatial and temporal variability, are poorly constrained. Recent advancements in the application of inverse models to obtain heat and freshwater flux estimates from mooring-based observational datasets have greatly aided in constraining transport estimates on seasonal to inter-annual timescales [2, 11, 12]. However, the processes of water mass formation and transformation within the Arctic, and the longer term spatial and temporal variability of gateway transports remain unclear. Given the rapidly changing state of the Arctic Ocean, and its importance for the large scale ocean circulation and global climate, it is vital to develop a clearer understanding of water mass transformation (WMT) in this region, specifically the relative roles of surface buoyancy forcing and interior mixing. This is the first step to determining how WMT will be altered in the future.