The effect of overturning meridional background circulation on long equatorial Kelvin and Rossby waves

While interactions between atmospheric waves and mean zonal flows have been the focus of extensive research, the same cannot be said of the dynamics of waves in the presence of purely meridional background flow. In the deep tropics, though, where the mean zonal flow nearly vanishes, such dynamics are of central importance to planetary-scale waves. In the Intraseasonal Multiscale Moist Dynamics (IMMD) theory of Biello and Majda (2010), a simplified set of equations describing the behavior of equatorial atmospheric waves in the presence of a planetary-scale overturning meridional-vertical circulation are derived. The IMMD's scaling describes zonally-long anomalies around a mean climatology and thus provides a framework to study long Rossby waves, long Kelvin waves, and the Madden-Julian oscillation. In this poster we study a two-layer equatorial shallow water system arising from IMMD, the solutions of which--planetary and equatorially trapped Rossby waves--do not interact in the absence of the background circulation but resonate under certain meridonal/vertical background flows. This coupling provides a mechanism for energy exchange between the tropical atmosphere and the middle latitudes. In the absence of diffusion the coupled system is skew self-adjoint, and therefore all solutions are neutrally stable; in contrast to wave instability in the presence of zonal shear flow this point is particularly noteworthy. Zonally-symmetric meridional circulation produces three striking effects on the classical Matsuno modes. It creates a conduit through which equatorial Rossby waves leak into midlatitude Rossby waves. It generates easterly barotropic wind in the maximum of the equatorial Kelvin wave's lower tropospheric convergence. For sufficiently strong background circulation or low wave frequency it causes the breakdown of all wave profiles at the latitude where background meridional flow vanishes. This suggests a low frequency cutoff for the Matsuno modes in the presence of such a flow.