On the influence of a surface coastal front on near‐inertial wind‐induced internal wave generation

Abstract

[1] A numerical model is used to examine the wind-induced near-inertial internal waves in the coastal region in the presence of a surface front. Initial calculations with the front separating a well-mixed coastal region from a stratified offshore area in an infinite sea domain showed that away from the front, surface inertial oscillations are dominant. Near the front on the positive vorticity side, superinertial internal waves are generated and propagate offshore in an analogous manner to that found when stratification intersected a coastal boundary. However, in the case of a coastal boundary, the no-flow condition at the coast produced inertial oscillations at depth phase shifted by 180°from those at the surface. On the negative vorticity side of the front, internal waves at the subinertial frequency are trapped between the well-mixed region and the front. Some inertial energy is found at depth in the frontal region. Although the large-scale features of the distribution of inertial energy are independent of vertical eddy viscosity parameterization, its magnitude depends upon it. In the case of a stratified nearshore region and an offshore front, near-inertial waves generated at the coast were trapped between the coastal boundary and the front, which results in a surface region of enhanced inertial energy. This gradually decreased with time as inertial energy diffused to depth. This timescale depended upon vertical eddy viscosity magnitude.

A model study of spin-down and circulation in a cold water dome

“…They showed that nonlinear effects arising from the change in vorticity in the region of a frontal jet, (such as that associated with a dome) in combination with wind forcing of frequency x f (Xing and Davies 2003), produced internal wave generation on the stratified side of the front. If x f is super-inertial, then internal waves can propagate away from their generation region, although they can be trapped at depth due to sloping isotherms (Mooers 1975;Xing and Davies 2004a). At the sub-inertial frequency, they are trapped in their generation region (Davies and Xing, 2002).…”

Sharp near‐equatorial transitions in inertial motions and deep‐ocean step‐formation

“…This energy can be transferred to gravity waves in the ocean interior at frequencies (σ) just larger than f (1.00f < σ < 1.09f) when they propagate towards lower latitudes [ Garrett , 2001], or, when considered on a spherical shell, just smaller than f (σ ≈ 0.99f) towards higher latitudes [ Maas , 2001]. Near‐inertial waves can also propagate into the interior when generated via surface fronts [ Xing and Davies , 2004]. Internal gravity waves exist between f < σ < N, with f ≪ N = (−g/ρ∂ρ/∂z) 0.5 the buoyancy frequency of stable density (ρ) stratification and g denoting the acceleration of gravity in the negative z‐direction.…”

Modelling non‐linear interactions between wind forced flows in surface frontal regions

“…An along frontal jet with velocity v having a maximum of order 35 cm s −1 gave a region of positive vorticity η = 0.4 × 10 −4 s −1 to the west of the front with a maximum negative vorticity η = −1.0 × 10 −4 s −1 to the east. This choice of frontal parameters is arbitrary and taken to be consistent with Xing and Davies [2004].…”

“…In Craig's case there is no difference between sub‐ or super‐inertial frequencies which have the same value of ∣ω − f∣. However, in frontal regions non‐linear effects associated with the along front jet, lead to significant penetration of the wind's energy to depth [ Xing and Davies , 2004] and internal wave generation. Associated with these internal waves is the possibility of enhanced non‐linear interaction between them.…”