Influence of Amplifying Rossby Waves on Tropical Cyclone Intensity,

We re-visit the Tropical Cyclone, TC-trough interaction problem and provide evidence that some types of amplifying Rossby Waves provide favourable settings for interaction. This can occur via injection of environmental air with high potential vorticity (PV) into TC circulations through midlevels.

Ex-TC Oswald produced extreme rainfall over northeast Australia. We demonstrate that as an upper-level Rossby Wave amplified and propagated towards the tropics, a potential vorticity anomaly descended to midlevels, moved equatorward and then was axisymmetrized and merged with the midlevel TC circulation. Using trajectory calculations we illustrate that the midlevel circulation was affected by the injection of high PV from the environment during the heavy rain.

The controversial rapid intensification of hurricane Opal in 1995 is also reviewed in light of these new insights. In addition to previously-suggested processes, it is found that the rapid intensification coincided with the merger between the storm and an environmental PV anomaly through midlevels. From a synoptic-scale perspective, we suggest that the rapid intensification of Opal was associated (a) with enhanced vertical motion ahead of an approaching upper front, and (b) with an enhancement of midlevel PV from the westerly trough. At the planetary scale, it appears that these processes were part of an amplifying Rossby wave with large group and small phase speeds.