Influence of Convective Momentum Transport on Mixed Rossby-Gravity Waves: A Contribution to Tropical 2-Day Waves

Influences of convective momentum transport (CMT) on tropical waves are analytically studied with an idealized model, which captures the first-order baroclinic structure in the vertical. The CMT has significant influence on the mixed Rossby-gravity (MRG) waves, especially over the Indo-Pacific warm pool. The westward-propagating MRG wave with a small wavenumber becomes unstable because of the CMT. The convergence and geopotential are no longer in a quadrature phase relation, which is different from the classical MRG wave. As a result, there is a net source of mechanical energy within one wave period and there is an upscale momentum transfer that can have impacts on slow variabilities in the tropics, such as the Madden-Julian oscillation. The unstable MRG wave mimics the temporal and spatial features of the observed 2-day waves in tropics. Within this framework, the asymmetric structure of the MRG waves and the 2-day waves with respect to the equator are well captured by both the idealized model and observations. In addition, the CMT is found to be critical for determining the meridional scale of tropical waves. The meridional scale in the two-layer model is wider than the Rossby radius of deformation R-L over the Indo-Pacific warm pool, but narrower than R-L from the central to the eastern Pacific Ocean and over the Atlantic Ocean. Such variation is consistent with observations.