Extended primitive equations in isentropic coordinates

The primitive equations for atmospheric motion in isentropic coordinates were extended to include an implicit dependence on water vapor content. In tropical regions where there is abundance of water vapor, the additional horizontal forces arising from this new perspective were found to be comparable to those in the traditional set of equations. This finding seems to suggest an additional mechanism by which moisture gradients drive the tropical atmospheric motion quite apart from the effect of moisture convergence and latent heat release by condensation. We recommend the use of this extended set of isentropic equations for quantitatively accurate and theoretically consistent global isentropic analysis.