Vertical Mixing in Stratified Fjords Near Tidewater Outlet Glaciers Along Northwest Greenland. Issue 8 (5th August 2021)

• Record Type: Journal Article
• Authors: Bendtsen, Jørgen; Rysgaard, Søren; Carlson, Daniel F.; Meire, Lorenz; Sejr, Mikael K.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 8(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-08-05
• Subjects: Greenland -- fjord -- vertical mixing -- tidewater outlet glacier -- turbulence -- subglacial discharge; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2020JC016898
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Vertical mixing of upper water masses in Arctic fjords is important for circulation and transport of nutrients and heat. However, the distribution of turbulent mixing is poorly known. Here we present hydrographic and microscale turbulence measurements from six fjords in Northwestern (69°N–75°N) Greenland. Water mass distributions showed the presence of warm bottom water of Atlantic origin in all fjord transects and a significant modification of upper water masses in fjords with tidewater outlet glaciers. Spatial and temporal distribution of turbulence in the fjords showed, in general, low rates of turbulent kinetic energy, ε (∼10 −9 W kg −1 ), and strong stratification implies that vertical exchange in the upper ∼200 m was relatively weak. However, measurements within 2 km of the terminus of a tidewater outlet glacier showed values of ε and inferred turbulent diffusion coefficients >10 −4 m 2 s −1 that were about two orders of magnitude larger than elsewhere in the deep fjords. This elevated mixing corresponded to comparatively large heat and salt fluxes. These observations suggest that vertical mixing near tidewater outlet glaciers results in localized mixing hot spots, likely due to subglacial discharge and near‐glacial current shear, that contribute significantly to vertical exchange in the deep fjords. Plain Language Summary: The Greenland Ice Sheet (GIS) is in direct contact with warm ocean water via glacier tongues that enter directly into fjords. Meltwater from the GIS and direct melt of glacial ice in the fjords generate dynamic areas near the glacier front with increased turbulence. Turbulent mixing brings warm and nutrient‐rich water at depth up to the surface and, therefore, it is a key process influencing currents, submarine melting and biological productivity. However, we don't know how strong this vertical mixing is, since it has rarely been measured. We studied fjord systems in Northwestern Greenland, an area that is responsible for about a quarter of the current mass loss from GIS as either meltwater or solid ice. In general, mixing near the surface in these deep fjords was weak. However, mixing within about 2 km of a tidewater outlet glacier was about 100 times larger than elsewhere in the deep fjords. The turbulence measurements combined with profiles of temperature and salinity showed that vertical mixing from the small areas in front of tidewater outlet glaciers were mixing hot spots that bring heat and nutrients to the surface. Key Points: Turbulence measurements in central parts of deep fjords in Northwest Greenland show weak turbulent vertical mixing in the upper 200 m Vertical mixing modified water masses in the inner part of fjords with tidewater outlet glaciers A vertical mixing hot spot in the upper ∼100 m was observed in front of Store Gletscher