In the Southern Ocean, mesoscale eddies contribute to the upwelling of deep waters along sloping isopycnals, helping to close the upper branch of the meridional overturning circulation. Eddy energy is not uniformly distributed along the Antarctic Circumpolar Current (ACC). Instead, ‘hotspots’ of eddy energy that are associated with enhanced eddy-induced upwelling exist downstream of topographic features. This study shows that, in idealized eddy-resolved simulations, a topographic feature in the ACC path can enhance and localize eddy-induced upwelling. However, the upwelling systematically occurs in regions where eddies grow through baroclinic instability, rather than in regions where eddy energy is large. Across a range of parameters, along-stream eddy growth rate is a more reliable indicator of eddy upwelling than traditional parameterizations such as eddy kinetic energy, eddy potential energy or isopycnal slope. Ocean eddy parameterizations should consider metrics specific to the growth of baroclinic instability to accurately model eddy upwelling near topography.

Major gaps exist in our understanding of the pathways between internal wave generation and breaking in the Southern Ocean, with important implications for the distribution of internal wave-driven mixing, its sensitivity to change, and the necessary ingredients of mixing parameterizations. Here we assess the dominant processes in internal wave evolution by characterizing wave and mesoscale flow scales based on full-depth measurements in a Southern Ocean mixing hot spot and a ray tracing calculation. The exercise highlights the importance of Antarctic Circumpolar Current (ACC) jets as a dominant influence on internal wave life cycles through advection, the modification of wave characteristics via wave-mean flow interactions, and the set-up of critical layers for both upward- and downward-propagating waves. Our findings suggest that it is important to represent mesoscale flow impacts in parameterizations of internal wave-driven mixing in the Southern Ocean.

Quantifying mixing rates in the Arctic Ocean is critical to our ability to predict heat flux, freshwater distribution, and circulation. However, turbulence measurements in the Arctic are sparse, and cannot characterize the high spatiotemporal variability typical of ocean mixing. Using year-round temperature and salinity data from Ice-Tethered Profiler (ITP) instruments between 2004 and 2018, we apply a finescale parameterization to obtain pan-Arctic estimates of turbulent dissipation and mixing rates at unprecedented space-time resolution. Building on previous work that used ITP data to identify double-diffusive staircases and analyze the associated convective mixing, we apply the finescale parameterization only where these step-like thermohaline structures are not present and mixing is expected to be internal wave-dominated. We find that the inferred wave-driven dissipation and mixing rates are generally low, but highly variable in both space and time, displaying significant regional differences between the shelves and central basins, as well as a small seasonal cycle. We detect no statistically significant interannual trend in mixing rate estimates over the period examined, with the exception of a small increase in the Canada Basin immediately below the mixed layer. The joint consideration of turbulent dissipation rates and stratification imply varied Arctic Ocean mixing regimes, which are most often not appropriately characterized as isotropic turbulence. Where justified, we infer turbulent heat fluxes out of the Atlantic Water layer that are mostly small, but also exhibit a distinct regional dependence.

As glaciers melt, a range of glacier processes modify and export freshwater and sediments to the ocean. This glacial runoff may influence biological productivity in coastal ecosystems by supplying essential nutrients and labile carbon. Previous studies of glacial meltwater export to the ocean have primarily been conducted on rivers draining land-terminating glaciers, or in fjords with large tidewater glaciers. These studies speculate about downstream effects (river studies) or upstream causes (fjord studies) of differing carbon and nutrient availability and biological productivity, but do not measure them. Here, we conduct the first ice- to-ocean study at a marine-terminating glacier in the Canadian Arctic Archipelago (CAA). We characterize the nutrient and carbon content of ice and meltwater collected on the glacier surface, at its margins, and in the near-shore coastal ocean, all within 1 to 25-km of the glacier terminus. Results demonstrate that while meltwater from a shallow tidewater glacier did not directly increase downstream carbon and nutrient concentrations, it can induce upwelling of deeper nutrient-rich marine water. Also, although carbon concentrations in meltwater were low, results show that this carbon is potentially more bioavailable than marine carbon. Glacially-mediated delivery of labile carbon and upwelling of nutrient-rich water occurs in summer, when surface waters are nutrient-limited. Collectively, these processes could benefit surface marine plankton, potentially stimulating production at the base of the food web. Shallow tidewater glaciers are commonly retreating in Arctic regions like the CAA and Svalbard, and understanding how increased meltwater output from these systems impacts marine ecosystems is critical.

The Canadian Arctic Archipelago (CAA) is vulnerable to climate warming, and with over 300 tidewater glaciers, is a hotspot for enhanced glacial retreat and meltwater runoff to the ocean. In contrast to Greenlandic and Antarctic systems, CAA glaciers and their impact on the marine environment remain largely unexplored. Here we investigate how CAA glaciers impact nutrient delivery to surface waters. We compare water column properties in the nearshore coastal zone along a continuum of locations, spanning those with glaciers (glacierized) to those without (non-glacierized), in Jones Sound, eastern CAA. We find that surface waters of glacierized regions contain significantly more macronutrients (nitrogen, silica, phosphorus) and micronutrients (iron, manganese) than their non-glacierized counterparts. Water column structure and chemical composition suggest that macronutrient enrichments are a result of upwelling induced by rising submarine discharge plumes, while micronutrient enrichments are driven directly by glacial discharge. Generally, the strength of upwelling and associated macronutrient delivery scales with tidewater discharge volume. Glacier-driven delivery of the limiting macronutrient, nitrate, is of particular importance for local productivity, while metal delivery may have consequences for regional micronutrient cycling given Jones Sound’s important role in modifying water masses flowing into the North Atlantic. Finally, we use the natural variability in glacier characteristics observed in Jones Sound to consider how nutrient delivery may be affected as glaciers retreat. The impacts of melting glaciers on marine ecosystems through both these mechanisms will likely be amplified with increased meltwater fluxes in the short-term, but eventually muted as CAA ice masses diminish.

Tidal mixing is recognized as a key mechanism in setting water properties in coastal regions globally. Our study focuses on Canada's British Columbia coastal waters, from Queen Charlotte Strait to the Strait of Georgia. This area is bisected by a region of exceptionally strong mixing driven by some of the strongest tidal currents in the world. We examine the influence of this tidal mixing on regional differences in water properties and nutrient ratios. Our results quantify a spatially-abrupt and temporally-persistent lateral gradient in temperature, salinity, and density co-located with the region of strongest mixing. The distributions of density on either side of this front remain largely distinct throughout the spring-neap tidal cycle, year-round, and for over 70 individual years for which data are available. Additionally, nutrient molar ratios north of the front are statistically distinct from those to the south. Seasonal changes driven by the arrival of upwelled water differ in both timing and magnitude on either side of the front. Taken together, these results indicate limited exchange of water through the region of strongest tidal mixing, and suggest that Queen Charlotte Strait and the Strait of Georgia are largely isolated from each other. As such, this area provides a valuable case study for the degree to which the reduction of estuarine exchange by tidal mixing can maintain abrupt and substantial regional differences in physical and biogeochemical water properties. Further, it demonstrates the potential of tidal mixing to modify nutrient transport pathways, with implications for marine ecosystems.