Dynamics of oceanic processes
Due to the upwelling of more saline water, estuarine circulation, and tidal exchange, a density anomaly forms in the fjord—a non-stratified layer of water from the surface down to 50 metres, with salinity and temperature similar to those of ocean water at the lower edge of the curtain. During ebb tides, water from the fjord, including that accumulated through the rise of mixed water, exits into the ocean.
Typically, fjords supply highly freshened water that spreads across the surface, a phenomenon well documented in scientific literature and modeling. In this case, the water exiting the fjord is denser than the surface layer of the ocean. This flow creates a density front (Flynn & Linden, 2006). In contact zones with the surrounding fresher water, turbulence and Kelvin-Helmholtz instabilities develop, and filament detachment occurs (Smyth & Moum, 2012).
Since the surrounding freshened water is typically colder, these filaments, due to the mechanism of double diffusion, cool more quickly than they lose their salinity and sink into the saline layer even to a greater depth (Schmitt, 1994). This results in a local thickening of the saline water layer, the extent of which depends on the volume of a single water discharge from the fjord during ebb tide. For fjords with an area of 100–200 sq km, the volume of the tidal prism is 0.2–0.4 cubic km. If this water is spread over the surface of the stratified layer in the ocean at a depth of 50–70 metres as a lens 10 metres thick, it forms a spot 5–7 kilometres in diameter, even without considering the entrainment of less saline surface water.
This size corresponds to the Rossby radius in the Labrador Sea (Rieck et al., 2019), which facilitates the formation of an anticyclonic vortex through baroclinic instability (Feng et al., 2021). Importantly, the saline lens does not rotate in isolation within its 10-metre thickness but interacts hydraulically with the underlying homogeneous saline layer of the ocean. The surface disturbance generates a horizontal pressure gradient that extends downward through the entire homogeneous layer (Yankovsky et al., 2022; Zhang et al., 2024). During barotropization, baroclinic energy converts into barotropic energy, allowing the vortex to develop a vertically coherent structure (Meunier et al., 2023). The upper boundary of the saline layer is likely situated beneath the fresh layer, at approximately 50 metres depth. The lower boundary is defined by the depth of the involved column of homogeneous water. Rotation elevates the centre of the 5–7 kilometre diameter lens, reducing its separation from the surface to about 20–30 metres, which facilitates wave mixing.
The subsurface anticyclonic vortex also generates Ekman convergence in the overlying freshwater layer through viscous coupling at the interface (McGillicuddy, 2016). Fresh water is drawn horizontally toward the vortex center and undergoes downwelling, where it mixes with the saline water of the vortex core. This process gradually thins and destabilizes the surface freshwater cap, creating conditions favorable for deep winter convection and breakthrough of the convective vortex to the surface, where its cooling rate and internal convection become maximal.
Furthermore, the repeated discharge of saline water from the fjord may cause the formation of closely spaced vortices with similar density and size. At a certain distance between vortex centers, a vortex merger occurs — a process in which two co-rotating vortices combine into a larger, more stable vortex (Griffiths & Hopfinger, 2018; Le Vu et al., 2018). Regarding a large-scale project involving 20 fjords, we consider the possibility of 40 such vortices forming each day, although not all will necessarily be stable.
Of course, not every ebb tide will lead to the formation of a new vortex, and this process itself requires multiple validations both through models and in field conditions. However, the aim of this manuscript is to specifically outline the sequence of scientifically supported effects that require thorough verification.
In effect, the introduction of saline water in the manner described promotes the formation of a mesoscale vortex, salinization of the surface layer, a drop in temperature at the vortex core, upwelling of saline waters, and bending of isopycnals. It can also be suggested that there is a weakening of vertical stratification and a reduction in the influence of freshwater runoff. All these factors align with the well-known phenomenon of preconditioning in the Labrador Sea, which is a crucial factor for initiating deep convection.
The successful implementation of this project, which requires independent verification, could help restart deep convection in the Labrador Sea and contribute to stabilising and accelerating the Atlantic Meridional Overturning Circulation. This research does not aim to provide a quantitative description or assess the project's readiness for the proposed approach. Its goal is to identify a physically grounded yet under-researched class of mechanisms related to controlling the regime of freshwater export from fjords and to promote discussion on their potential experimental validation. Given the possible consequences of AMOC degradation, even small-scale field experiments in individual fjords appear justified from both scientific and practical viewpoints.
