Abstract
Abstract 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, fo...
Highlights
Understanding the low-frequency longitudinal variations in the tropospheric circulation remains an important problem
Observations of the potential vorticity (PV) field (e.g., Hsu et al 1990; Kiladis and Weickman 1992) show the subtropical upper troposphere to be a region of strong Rossby wave breaking, analogous to the ‘‘surf zone’’ in the midlatitude winter stratosphere
We argue that the accumulation of waveactivity in the primary region has saturated and the secondary wave-breaking region arises from breaking of the wave train that was reflected from the primary region
Summary
Understanding the low-frequency longitudinal variations in the tropospheric circulation remains an important problem. Observations of the potential vorticity (PV) field (e.g., Hsu et al 1990; Kiladis and Weickman 1992) show the subtropical upper troposphere to be a region of strong Rossby wave breaking, analogous to the ‘‘surf zone’’ in the midlatitude winter stratosphere. In order to determine the significance of the simple Rossby wave propagation models, and to improve them where necessary, it is important to understand the interaction between the nonlinear regions and the rest of the flow. One model problem in which this interaction is clear is the nonlinear Rossby wave criticallayer problem describing the behavior of small-amplitude waves on a basic-state shear flow containing a critical line
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