Inertial Instability and Rossby Wave Breaking in a Numerical Model

Abstract

A mechanistic model of the middle atmosphere is used to study the interaction between Rossby waves forced at the extratropical tropopause and inertial instability in the equatorial lower mesosphere. The impact of cross-equatorial shear strength and Rossby wave forcing amplitude is explored. Model results support the hypothesis, based on satellite temperature observations, that Rossby waves organize regions of equatorial inertial instability into coherent large-scale circulations. Horizontal convergence and divergence maxima are found stacked over the boundaries of regions of anomalous potential vorticity (PV). This supports observational diagnoses and theoretical expectations that parcel inertial accelerations arise in regions of anomalous PV, with divergence and convergence occurring at the boundaries. Although cross-equatorial shear determines the initial volume of inertially unstable air, Rossby waves arriving from the winter hemisphere deform PV contours such that zonally confined regions of anomalous PV extend well into the winter hemisphere and somewhat into the summer hemisphere. Significant parcel inertial accelerations are diagnosed to occur in anomalous PV tongues to 30° latitude in the winter hemisphere. Using a smaller cross-equatorial shear delays the penetration of anomalous PV into the winter hemisphere, while increasing the Rossby wave forcing amplitude causes a more rapid evolution of the low-latitude flow. These results suggest that inertial instability is intimately involved in Rossby wave breaking in the subtropical winter mesosphere. The horizontal scales of inertially unstable regions coevolve with those of PV anomalies, and so affect the breaking process from inception. The vertical scale of inertial circulations is much smaller than that of Rossby waves; hence, Rossby wave PV anomalies are eroded by vertical mixing as well as horizontal mixing. Gravity waves radiate away from the inertially unstable regions, which effectively convert energy from rotational to divergent modes. The interplay between inertial and gravitational instabilities and their role in causing irreversible mixing in the winter subtropics is explored. The effects of inertial instability on the seasonal evolution of PV is discussed from the point of view of ?PV thinking.?