Department of Earth System Science

The authors consider quasi-stationary planetary waves that are excited by localized midlatitude orographic forcing in a three-dimensional primitive-equation model. The waves propagate toward subtropical regions where the background flow is weak and the waves are therefore likely to break. Potential vorticity fields on isentropic surfaces are used to diagnose wave breaking. Nonlinear pseudomomentum conservation relations are used to quantify the absorption–reflection behavior of the wave-breaking regions. Three different three-dimensional flow configurations are represented: (i) a barotropic flow, (ii) a simple baroclinic flow, and (iii) a more realistic baroclinic flow. In order to allow the propagation of large-scale waves to be studied over extended periods for the baroclinic flows, the authors apply a mechanical damping at low levels to delay the onset of baroclinic instability.

For basic states (i) and (ii) the forcing excites a localized wave train that propagates into the subtropics and, for large enough wave amplitude, gives rise to a reflected wave train propagating along a great circle route into midlatitudes. It is argued that the reflection is analogous to the nonlinear reflection predicted by Rossby wave critical layer theory. Both the directly forced wave train and the reflected wave train are quite barotropic in character and decay due to the damping. However, the low-level damping does not inhibit the reflection. The authors also consider the effect of thermal damping on the absorption–reflection behavior and find that, for realistic wave amplitudes, reflection is not inhibited by thermal damping with a timescale as low as 5 days.

For the third basic state it is found that the small-amplitude response has the character of a longitudinally propagating wave train that slowly decays with distance away from the forcing. The authors argue that part of this decay is due to low-latitude absorption and show that at larger amplitudes the decay is inhibited by nonlinear reflection.

The authors also compare for each basic state absorption–reflection behavior for isolated wave trains and for waves forced in a single longitudinal wavenumber.