Saturation of destratifying and restratifying instabilities during
down-front wind events: a case study in the Irminger Sea
Abstract
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.