Nonlinear theory of geostrophic adjustment. Part 1. Rotating shallow-water model

Abstract

We develop a theory of nonlinear geostrophic adjustment of arbitrary localized (i.e. finite-energy) disturbances in the framework of the non-dissipative rotating shallow-water dynamics. The only assumptions made are the well-defined scale of disturbance and the smallness of the Rossby number Ro. By systematically using the multi-time-scale perturbation expansions in Rossby number it is shown that the resulting field is split in a unique way into slow and fast components evolving with characteristic time scales f⁻¹₀ and (f₀Ro)⁻¹ respectively, where f₀ is the Coriolis parameter. The slow component is not influenced by the fast one and remains close to the geostrophic balance. The algorithm of its initialization readily follows by construction.

The scenario of adjustment depends on the characteristic scale and/or initial relative elevation of the free surface ΔH/H₀, where ΔH and H₀ are typical values of the initial elevation and the mean depth, respectively. For small relative elevations (ΔH/H₀ = O(Ro)) the evolution of the slow motion is governed by the well-known quasi-geostrophic potential vorticity equation for times t [les ] (f₀Ro)⁻¹. We find modifications to this equation for longer times t [les ] (f₀Ro²)⁻¹. The fast component consists mainly of linear inertia–gravity waves rapidly propagating outward from the initial disturbance.

For large relative elevations (ΔH/H₀ [Gt ] Ro) the slow field is governed by the frontal geostrophic dynamics equation. The fast component in this case is a spatially localized packet of inertial oscillations coupled to the slow component of the flow. Its envelope experiences slow modulation and obeys a Schrödinger-type modulation equation describing advection and dispersion of the packet. A case of intermediate elevation is also considered.