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A linear two-layer model is used to elucidate the role of prognostic moisture on quasigeostrophic (QG) motions in the presence of a mean thermal wind ( ). Solutions to the basic equations reveal two instabilities that can explain the growth of moist QG systems. The well-documented baroclinic instability is characterized by growth at the synoptic scale (horizontal scale of ~1000 km) and systems that grow from this instability tilt against the shear. Moisture–vortex instability—an instability that occurs when moisture and lower-tropospheric vorticity exhibit an in-phase component—exists only when moisture is prognostic. The instability is also strongest at the synoptic scale, but systems that grow from it exhibit a vertically stacked structure. When moisture is prognostic and   is easterly, baroclinic instability exhibits a pronounced weakening while moisture vortex instability is amplified. The strengthening of moisture–vortex instability at the expense of baroclinic instability is due to the baroclinic ( ) component of the lower-tropospheric flow. In westward-propagating systems, lower-tropospheric westerlies associated with an easterly   advect anomalous moisture and the associated convection toward the low-level vortex. The advected convection causes the vertical structure of the wave to shift away from one that favors baroclinic instability to one that favors moisture–vortex instability. On the other hand, a westerly   reinforces the phasing between moisture and vorticity necessary for baroclinic instability to occur. Based on these results, it is hypothesized that moisture–vortex instability is an important instability in humid regions of easterly   such as the South Asian and West African monsoons.

journal of the atmospheric sciences

Interactions between Water Vapor, Potential Vorticity, and Vertical Wind Shear in Quasi-Geostrophic Motions: Implications for Rotational Tropical Motion Systems