Abstract
The nonlinear behavior of quasi-stationary planetary waves excited by midlatitude orographic forcing is considered in a three-dimensional primitive equation model that includes a representation of the Hadley cir- culation. The Hadley circulation is forced by Newtonian cooling to a zonally symmetric reference temperature and vertical diffusion on the zonally symmetric component of the flow. To quantify the effect of the Hadley circulation on wave propagation, breaking, and nonlinear reflection, an initial state with no meridional flow, but with the same zonal flow as the Hadley state, is also considered. In order to allow the propagation of large- scale waves over extended periods, Rayleigh friction is applied at low levels to delay the onset of baroclinic instability. As in the absence of a Hadley circulation, the waves in the Hadley state propagate toward low latitudes 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 region. In the presence of a Hadley circulation representative of winter conditions, the nonlinear reflection requires more forcing to get established, but a reflected wave train is still present in the numerical simulations, both for a longitudinally symmetric forcing and for the more realistic case of an isolated forcing. The effect of the thermal damping on the waves is more severe in the current three-dimensional simulations than in the shallow water case considered in an earlier study. Both the directly forced wave train and the reflected wave train are quite barotropic in character; however, in the shallow water case one is essentially assuming an infinite vertical scale.
Highlights
Recent modeling studies have presented evidence that large-amplitude planetary waves propagating from the midlatitudes toward the Tropics are, after an initial period, reflected back toward the midlatitudes (Brunet and Haynes 1996; Magnusdottir and Haynes 1999, hereafter MH99; Magnusdottir and Walker 2000, hereafter MW00; Esler et al 2000, hereafter EPP)
Wave breaking is shown in observational studies by, for example, Hsu et al (1990), who examined potential vorticity (PV) on isentropic surfaces in the lowlatitude upper troposphere, and found the overturning of PV contours associated with breaking waves
We have shown that in the presence of a Hadley circulation, planetary waves propagating toward low latitudes in three-dimensional tropospheric flow are reflected back to midlatitudes if the forcing amplitude is large enough
Summary
Recent modeling studies have presented evidence that large-amplitude planetary waves propagating from the midlatitudes toward the Tropics are, after an initial period, reflected back toward the midlatitudes (Brunet and Haynes 1996; Magnusdottir and Haynes 1999, hereafter MH99; Magnusdottir and Walker 2000, hereafter MW00; Esler et al 2000, hereafter EPP). As waves excited in midlatitudes propagate toward the equator, they encounter regions where their zonal phase speed matches the background zonal flow (the critical line) and, if dissipation is small enough, the waves must break. Brunet and Haynes (1996) used a shallow water model and found that reflection can occur in flow that is not balanced and when the wave train is not periodic in longitude. MH99 generalized the results of Brunet and Haynes (1996) to three
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