COLA Report 19
Wave-CISK, The Evaporation-Wind Feedback, and the Intraseasonal
Oscillation: Phase Propagation and Scale Selection
Ben P. Kirtman and Anandu D. Vernekar
August 1995
Abstract
The scale selection and phase propagation characteristics of the wave-CISK and the E-W
feedback instability mechanisms are examined in a simple five layer model with and without
positive only heating. With wave-like internal heating, the model gives classical wave-CISK
vertical mode coalescence where shallow heating yields the slowest propagation speeds and the
smallest horizontal scales are the most unstable. When the E-W feedback is introduced into the
model with linear heating, the phase propagation and the scale selection characteristics change.
The E-W feedback modes have larger phase speeds and, in the case of shallow heating, the most
unstable mode propagates as much as 15 m s-1 faster than the corresponding wave-CISK mode.
The growth rate of the wave-CISK modes increases linearly with increasing zonal wavenumber,
whereas the growth rate of the E-W feedback modes increases with increasing zonal
wavenumber but at a significantly slower rate. In addition, the growth rate of the E-W feedback
modes is bounded from above as the spatial scale becomes small; in contrast, the growth rate of
the wave-CISK modes is unbounded. A number of numerical simulations with positive definite
heating confirm the phase propagation characteristics of the model with linear heating. The scale
selection characteristics, however, of the model with positive definite heating are distinct from
the model with linear heating. It is shown that, while the a single precipitation/circulation cell
wave-CISK mode is the most unstable configuration, any number of precipitation/circulation
cells can equilibrate. The E-W feedback modes, on the other hand, always select a zonal
wavenumber dominated spatial structure with a single precipitation/circulation cell regardless of
initial condition.
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last update: 31 August 1995
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