Title: Circulation pathways, time scales, and water mass composition in the Arctic Ocean
Description: The Arctic is a hotspot of global change. For example, changes caused by global warming are both amplified and are seen more rapidly in the Arctic (e.g., Serreze & Francis, 2006; Bekryaev et al., 2010; Serreze & Barry, 2011; Overland et al. 2015; Macdonald et al., 2015). Thus, the Arctic is an indicator of the state of the planet. Among the strongest changes that have been observed in the Arctic Ocean are changes in circulation regimes, hydrographic properties and freshwater content and composition. These changes have the potential of global impact through interaction with the deep-water formation regions of the North Atlantic Ocean, a major source of deep and bottom water in the global ocean. Although significant progress in understanding the signals of change in the Arctic Ocean and their causes has been made during the past decades there are still some fundamental questions unanswered. They include the stability of the circulation of the upper waters and changes in the freshwater budget and how these changes are connected to changes in the composition of the freshwater lens that covers the Arctic Ocean.

In this thesis, we address these two topics using measurements of isotopes obtained during over three decades of Arctic Ocean section work.This dissertation is composed by three parts and its structure mimics the layered vertical structure of the Arctic Ocean water column. Chapter 1 is dedicated to the Atlantic waters, Chapter 2 to the halocline waters, and Chapter 3 to the freshwater sources and their distribution and variability in the surface layer. In the first two chapters, we present transient tracer (³H/³He) and hydrographic data from over 25 years of Arctic oceanographic campaign ranging from 1987 to 2013 to evaluate flow rates and circulation pathways in the Upper Halocline Water (UHW), Lower Halocline Water (LHW), and Atlantic Layer on a pan-Arctic scale. In agreement with previously established circulation schemes, tracer data show that the flow paths in the LHW and the Atlantic layer are typically topographically steered with the presence of a cyclonic boundary current along the continental shelf and separate circulation branches tracking major bathymetric features, such as the Lomonosov Ridge.

Tracer data suggest that the general circulation of UHW is decoupled from the cyclonic regime observed in the deeper layer, and strongly influenced by surface stress forcing, such as the anticyclonic Beaufort Gyre. Within the limits of our method, tracer data show that the mean flow paths and spreading velocities have been more or less constant over the past three decades despite dramatic shifts in the Arctic system heat and freshwater balances from anthropogenic climate change over imposed to a high natural variability. The third and final chapter discusses the water-mass composition and the distribution of freshwater sources in Canadian Basin, the western section of the Arctic Ocean. Results are produced by performing a water-mass decomposition on the water samples collected during the 2015 Arctic GEOTRACES (GN01) oceanographic expedition. Stable isotope measurements (H₂¹⁸O/H₂¹⁶O and DHO/H₂O ratios) are used in combination with salinity and nutrients data to calculate the water-mass components for the upper 500 m Arctic Ocean (mixed layer through Atlantic Water layer).

The sources of liquid freshwater into the Arctic Ocean include Pacific water, sea ice meltwater, river discharge and net precipitation. The topmost 50 meters of Canadian Basin contain the large fraction of freshwater from sea ice meltwater and meteoric water. Pacific water dominated the freshwater budget along the 2015 GN01 transects from 100 to 250 m. These depths are also characterized by a strong brine rejection signal, reflecting an enhanced annual sea ice cycle with more ice refreezing and melting each year, and an overall loss of multiyear ice. The 2015 results are compared with the overlapping 1994 and 2005 Arctic Ocean Sections (AOS94 and AOS05) and discussed in the context of regional and temporal variability of liquid freshwater and its components distribution. Our findings show significant increases in the Canadian Basin total liquid freshwater reservoir both compared to the 1994 and 2005 transects confirming a freshwater accumulation in the Canadian Basin already established by numerous observations and modeling studies (Gilles et al., 2012; Carmack et al., 2016; Proshutinsky et al. 2019; Solomon et al., 2021).

The total freshwater reservoir increased by ca. 12,500 km³ from 1994 to 2015, of which ca. 5,000 km3 are within the Beaufort Gyre. Meteoric and Pacific freshwater components were the largest sources of the observed freshwater accumulation in the upper 500m of the western Arctic Ocean. An intensified Ekman transport in the Beaufort Gyre and increased availability of freshwater for accumulation are the two primary drivers for freshwater accumulation in the Canadian Basin. Within the limits of our analysis, it is not possible to quantitatively estimate the relative importance of the each forcing nor to resolve the seasonal to year‐to‐year variability. Our tracer-based analysis suggests that there is a significant variability in the freshwater components and UHL distribution while the major features of the circulation patterns and spreading velocities of the AW and the LHW have remained largely stable over the past decades. Future research should address whether in a fast changing Arctic, the dynamics of the surface layer will expand to the halocline and Atlantic layer substantially destabilizing the current Arctic Ocean water column with potentially dramatic consequences.