ON DIVERGENT BAROTROPIC AND INERTIAL INSTABILITY IN ZONAL-MEAN FLOW PROFILES

Impacts of atmospheric mean zonal flows on equatorial and subequatoria l waves are examined using the generalized Laplace tidal equation, a h orizontal structure equation that explicitly includes the effect of ho rizontal divergence on disturbances via the Lamb parameter epsilon = ( 2 Omega a)(2)/(gh(e)). Two important consequences of the mean zonal fl ow profile are the excitation of barotropically unstable waves associa ted with a reversed meridional gradient of potential vorticity and the excitation of inertially unstable waves related to anomalous potentia l vorticity values. Investigations show that both types of instabiliti es occur in realistic wind fields of the upper troposphere around 200 hPa. The barotropically unstable waves are divided into weakly diverge nt waves located in the middle latitudes and the more important class of strongly divergent waves located at the equatorward flank of the je t stream in the subtropics. The strongly divergent inertially unstable waves are identified by equatorially trapped waves showing horizontal structures of Kelvin waves, Rossby-gravity waves, and gravity waves. For large values of the Lamb parameter epsilon, that is, small values of the equivalent depth h(e), it is possible to obtain barotropically unstable perturbations located in the subtropics with an additional se condary maximum penetrating into equatorial regions. The secondary max imum resembles Kelvin wavelike structures, and it is related to the we ak inertial instability caused by a nonvanishing cross-equatorial hear . This shear usually occurs for barotropically unstable flow profiles with corresponding unstable regions in the subtropics. For inertially unstable flow profiles it is also possible to excite equatorially trap ped Kelvin wavelike perturbations with an additional secondary maximum in the subtropics showing Rossby-like gyres. When the underlying mean flow profile is barotropically and inertially unstable, the structure of subtropical barotropic perturbations and equatorial inertial pertu rbations are ''fused'' forming a new class of strongly divergent modes . This class may be interpreted as a generalization of inertially unst able perturbations for linear wind shear profiles on the equatorial be ta plane. The coupling of barotropic and inertial instabilities is dis cussed for the equatorial middle atmosphere where inertial stability i s influenced by the occurrence of subtropical Rossby waves, and for th e tropical troposphere where the intraseasonal oscillation may be affe cted by barotropically unstable waves and their influence on equatoria lly trapped Kelvin wavelike structures.