Observations indicate that symmetric instability is active in the East Greenland Current during strong northerly wind events. Theoretical considerations suggest that baroclinic instability may also be enhanced during these events. An ensemble of idealised numerical ocean models, forced with northerly winds show that the short time-scale response (from two to four 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 four 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 parameterisations 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. The duration of a wind event leads to a saturation of mixing rates which means increasing the peak wind stress in an event leads to no extra mixing. Using ERA5 reanalysis data we estimate that between 1.5Sv and 1.8Sv of East Greenland Coastal Current Waters are produced by mixing with lighter surface waters during wintertime by down-front wind events. Similar amounts of East Greenland-Irminger Current water are produced at a slower rate.
Density staircases are observed in an idealised model of a deep western boundary current upon crossing the equator. We propose that the staircases are generated by the excitement of symmetric instability as the current crosses the equator. The latitude at which symmetric instability is excited can be predicted using simple scaling arguments. Symmetric instability generates overturning cells which, in turn, cause the inhomogenous mixing of waters with different densities. The mixing barriers and well mixed regions in density profiles coincide, respectively, with the boundaries and centres of the overturning cells generated by the symmetric instability. This new mechanism for producing density staircases may require us to re-evaluate the origins of some of the density staircases observed in the Tropical Atlantic.