Understanding the causes of intense tropical cyclones
One of the many effects of human activity and global warming appears to be the generation of tropical cyclones (cyclogenesis) that can have catastrophic consequences including material damage and loss of life. Previous research supported tropical cyclogenesis and intensification by a process of axisymmetrisation (imposing symmetry around a given axis) of vortices. EU-funded researchers set out to study the ‘Emergence and intensification of tropical cyclones’ (EITC) as related to axisymmetrising and vortex Rossby waves (VRWs) given that axisymmetrising disturbances on a vortex are accompanied by outward propagating VRWs, or forced asymmetric eddies. Scientists identified three main objectives of the research. First, they sought to confirm that axisymmetrisation can in fact transform a small depression into an intense tropical cyclone by investigating eddy-mean vortex dynamics that may lead to such intensification. In the second step, investigators evaluated whether axisymmetrisation can maintain the stability of the formed cyclone in the face of processes that tend to disrupt it, again with a focus on eddy-mean vortex dynamics. Finally, they proposed to study the effects of thermal excitation on such dynamics. Development of novel analytical tools enabled demonstration of two competing mechanisms that control the emergence of a zonal jet out of small-scale non-zonal eddies. Results and tools were then used to study the specific problem of the interaction of VRWs with a circular vortex. The same mechanisms were found to apply, leading to identification of a threshold excitation above which a minimal vortex (tropical depression) is amplified or intensified. Continued research will focus on the third objective, effects of thermal excitation, as well as effects of divergent motions on emergence and maintenance of intense tropical cyclones. Understanding the dynamics of both tropical cyclone emergence and subsequent stabilisation is of critical importance in predicting tropical cyclone intensity and thus invaluable to organisations making environmental policy decisions.
