Atmospheric wave propagation in heterogeneous flow: basic flow controls on tropical-extratropical interaction and equatorial wave modification

It is noted that wave propagation in the atmosphere and error propagation in numerical models appear to have preferred geographical loci. However, these paths appear to be more associated with the low frequency background state of the atmosphere than the location of the wave source. Theoretical and numerical models are used to determine the control the basic state has on wave propagation both between the extratropica and the tropics (and vice versa) and within the tropics. Basic states with horizontal and vertical shear as well as longitudinal stretching deformation are studied. It is shown that strong horizontal shear embedded in equatorial easterlies confines modes close to the equator. Weak shear in westerly tropical flow allows modes to project into the extratropics. That is, the degree of equatorial trapping is diminished. Negative stretching deformation acts to trap modes longitudinally so that the western sides of the westerly ducts are accumulation regions. Negative vertical shear inhibits vertical propagation while positive shear enhances propagation. It is concluded that the basic state is the determining factor in the observed patterns of waves and the locations of errors in numerical weather prediction and climate models. Of particular interest are the westerly duct regions of the eastern Pacific Ocean and the Atlantic Ocean. These regions appear to act as wave attractors for both equatorially trapped modes and extratropical modes propagating towards the equator and also are regions of enhanced vertical propagation. (C) 1997 Elsevier Science B.V.