Destratifying and Restratifying Instabilities During Down‐Front Wind Events: A Case Study in the Irminger Sea

• Published in: Journal of geophysical research. Oceans 2024-02, Vol.129 (2), p.n/a
• Main Authors: Goldsworth, F. W., Johnson, H. L., Marshall, D. P., Le Bras, I. A.
• Format: Article
• Language: English
• Subjects: Atlantic Meridional Overturning Circulation; Atlantic Meridional Overturning Circulation (AMOC); Atmospheric models; Baroclinic flow; Baroclinic instability; Baroclinic mode; Baroclinity; Climate; Climate models; Coastal currents; Coastal waters; East Greenland Current; Flow stability; Fresh water; Freshwater; Inland water environment; Instability; Irminger Current; Isopycnals; Mixed layer; mixing; Ocean currents; Ocean models; Ocean surface; Oceans; Potential vorticity; Slumping; Stratification; sub‐polar North Atlantic; Surface stability; Surface water; Symmetric instability; Vorticity; Water circulation; Water column; Water mass transformation; Water masses; Wind; Wind stress; Winds
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2023JC020365

Observations indicate that symmetric instability is active in the East Greenland Current during strong northerly wind events. Theoretical considerations suggest that mesoscale baroclinic instability may also be enhanced during these events. An ensemble of idealized numerical ocean models forced with northerly winds shows that the short time‐scale response (from 10 days to 3 weeks) to the increased baroclinicity of the flow is the excitation of symmetric instability, which sets the potential vorticity of the flow to zero. The high latitude of the current means that the zero potential vorticity state has low stratification, and symmetric instability destratifies the water column. On longer time scales (greater than 4 weeks), baroclinic instability is excited and the associated slumping of isopycnals restratifies the water column. Eddy‐resolving models that fail to resolve the submesoscale should consider using submesoscale parameterizations to prevent the formation of overly stratified frontal systems following down‐front wind events. The mixed layer in the current deepens at a rate proportional to the square root of the time‐integrated wind stress. Peak water mass transformation rates vary linearly with the time‐integrated wind stress. Mixing rates saturate at high wind stresses during wind events of a fixed duration which means increasing the peak wind stress in an event leads to no extra mixing. Using ERA5 reanalysis data we estimate that between 0.9 Sv and 1.0 Sv of East Greenland Coastal Current Waters are produced by mixing with lighter surface waters during wintertime due to down‐front wind events. Similar amounts of East Greenland‐Irminger Current water are produced. Plain Language Summary Symmetric instability is a process that leads to the mixing of waters with different densities. Observations show that in winter, when winds blow from the north, along the coast of Greenland, symmetric instability occurs; however, observations are limited which makes it difficult to understand the effect of the instability on the ocean currents in the region. We test the hypothesis that symmetric instability leads to the production of dense waters which are known to form in the region and contribute to the Atlantic Meridional Overturning Circulation (or “ocean conveyor” (Broecker, 1991, https://doi.org/10.5670/oceanog.1991.07)). We find that symmetric instability makes waters at the ocean surface denser, so that subsequent cooling can make them dense enough to sink