Rapid intensification of tropical cyclones in the context of the solar wind-magnetosphere-ionosphere-atmosphere coupling

Abstract

Rapid intensification of tropical storms is examined in the context of solar wind coupling to the magnetosphere-ionosphere-atmosphere system. Tropical cyclone “best tracks” in the southern and northern hemispheres are used in the superposed epoch analysis of time series of solar wind parameters. The results indicate that rapid intensification of tropical storms tends to follow arrivals of high-speed streams from coronal holes or interplanetary coronal mass ejections, which can trigger geomagnetic storms. The ensuing auroral and polar cap activity including ionospheric currents and ionospheric convection generates atmospheric gravity waves that propagate from the high-latitude lower thermosphere both upward and downward. If ducted in the lower atmosphere, they can reach tropical troposphere. Despite significantly reduced wave amplitudes, but subject to amplification upon reflection in the upper troposphere, these gravity waves can trigger/release moist instabilities to initiate convective bursts, with the latent heat release leading to intensification of storms. Convective bursts have been linked to rapid intensification of tropical cyclones. Cases of tropical cyclone intensification closely correlated with the solar wind structure are found to be preceded by atmospheric gravity waves generated by the solar wind magnetosphere-ionosphere-atmosphere coupling process. The gravity waves are observed in the ionosphere as traveling ionospheric disturbances. Their propagation in the lower atmosphere is examined by ray tracing in a model atmosphere to show that they can reach tropical cyclones. It is suggested that the interaction of aurorally-generated gravity waves with the tropical cyclone vortex and the inner primary eyewall could play a role in the intensification process. Assuming that quasi-periodic convective bursts lead to vortex waves, a two-dimensional barotropic approximation is used to obtain asymptotic solutions representing propagation of vortex waves and their absorption by the mean flow in the critical layer.