Asymmetric structure and maintenance in Hurricane Juan

When tropical cyclones (TCs) move polarward, mid-latitude circulation plays a very important role in their evolution, and the processes related to extratropical transition (ET), dissipation or reintensification. What makes Juan (September of 2003) an interesting and special case is that as it moved to extratropical latitudes it did not undergo an immediate transformation and reintensification. Rather, it retained a strong tropical hurricane structure with almost sustained intensity as a category 2 hurricane, until landfall. In this study, Juan is simulated by the Canadian Mesoscale Compressible Community atmospheric model, using a vortex initialization. It is shown that when Juan stays south of the Gulf Stream, the cyclone circulation associated with Juan is the main contribution to its maintenance and the structure is basically symmetric. Thereafter (and until landfall), the interaction between the cyclone's circulation and the midlatitude intense high pressure system to the northeast of Juan, dominates the storm's evolution in the form of a low-level strong south-southeastern jet associated with the high system as well as enhanced warm advection ahead of the surface cyclone. Storm structure is quite asymmetric due to the asymmetric convection within the associated deep saturated air mass. Although a broad low system is located to the northwest of Juan, it is relatively far away due to blocking by the intense high pressure system to the northeast, and its direct influence is negligible in this case.