Interaction between a Midlatitude Upper-level Trough and Tropical Cyclone Dora (2007) over the Southwest Indian Ocean

A question remains among the scientific community in the context of tropical cyclone (TC) intensity prediction: will an approaching upper-tropospheric synoptic-scale trough produce intensification or decay of a given TC? Many observational studies and numerical modeling have documented TC-trough interaction. Hanley et al. (2001) introduced the “good trough/bad trough” terminology to sort troughs according to TC intensification. An approaching trough can have contradictory effects on the environmental ingredients that are crucial for TC intensification. On the one hand, it may induce significant vertical wind shear which is usually detrimental to TC intensity. On the other hand, it may increase upper-level divergence and enhance outflow poleward of the storm as well as import cyclonic eddy angular momentum (Molinari et al. 1995), all positive contributions to TC intensification. From a potential vorticity (PV) perspective, TC-trough interaction can be associated with PV advection from the trough toward the TC, which is beneficial below the level of the outflow anticyclone (“PV superposition principle”, Molinari et al. 1998). The triggering of convection by upward velocities associated with the PV anomaly, as well as dry air intrusion from the low stratosphere, are other interesting processes that might have a role to play in TC-tough interactions. Other studies (WMO 2010) have demonstrated that internal processes that modify the vortex structure such as an “eyewall replacement cycle” (ERC) are responsible for rapid intensity changes under favorable environmental conditions. The remaining work to get a comprehensive insight into TC-trough interaction is therefore to investigate the TC inner core during such interactions, and see whether exter-