**5. Summary and conclusion**

Planetary‐scale atmospheric and oceanic conditions of the western Atlantic basin were analyzed to understand the unique TCG and intensification mechanism of Hurricane Wilma in 2005, using NCEP/NCAR reanalysis data, NOAA optimum interpolation (OI) ¼ degree daily SST V2 data, NOAA/OAR/ESRL PSD interpolated OLR data, and global WRF model simulation. An anomalous development of the 850 hPa circulation pattern in the North Atlantic was triggered by Hurricane Vince (October 8–11, 2005) in the eastern North Atlantic. Circula‐ tion around the southeastern fringe of the North Atlantic subtropical anticyclone during the period had been interrupted by the presence of Vince, causing a perturbation in the down‐ stream flow around the entire southern edge of the North Atlantic subtropical high. On the southwestern flank of the subtropical high, the perturbation contributed to the development of a large‐scale 850 hPa vortex, which would eventually allow for Wilma's TCG in the eastern Caribbean Sea. Due to the change in the low‐level circulation by the deformed subtropical anticyclone, weakened low‐level easterly winds allowed southeasterly winds from the Southern Hemisphere and westerly winds from eastern North Pacific to become relatively important, generating an anomalously prominent low‐level cyclone over the western Atlantic about a week before TCG.

The anomalously large low‐level cyclone over the western Atlantic matured over the warm ocean before it was separated into two cyclones in a north‐south alignment (a northern cyclone and a southern cyclone). The separation was caused by the advance of northerly winds from a mid‐latitude trough over central Canada one day before TCG. By 1200 UTC October 15, the high‐latitude trough merged with the northern cyclone, resulting in a strengthened northern subtropical low. The enhanced subtropical low eventually played a role in sustaining the low‐ level circulation in the Caribbean Sea by preventing a significant interference from the zonally propagating tropical waves (**Figure 2c** and **d**). The southern cyclone became more concentrated in the Caribbean Sea, near Jamaica by October 14, growing into a tropical depression by 1800 UTC October 15.

The unusual but persistent meridionally oriented circulation conditions allowed the tropical depression over the Caribbean Sea to strengthen slowly between the northerly winds associ‐ ated with the trough in the northeastern US and the southeasterly winds from the South Atlantic (**Figure 2c** and **d**). Wilma became a tropical storm at 0600 UTC October 17. Over October 17–18, as the North Atlantic subtropical high strengthened to produce more vigorous low‐level easterly winds in the central tropical Atlantic toward the Caribbean Sea (**Figure 2e**), Wilma drifted toward the west‐northwest and strengthened into a hurricane at 1200 UTC October 18.

The unprecedented rapid intensification of Hurricane Wilma during the next night took place under anomalously warm SST conditions when Wilma was trapped between the northerly winds from the mid‐latitude trough and synoptic‐scale southeasterly winds. During Wilma's explosive deepening, a synoptic‐scale anticyclone over Texas and the North Atlantic subtrop‐ ical high seems to have caused Wilma to intensify between them by advecting angular momentum to the outer radii of Wilma.

The global WRF reproduced planetary‐scale atmospheric circulations at 1500 m (about 850 hPa) very closely and effectively, including the deformation of the low‐level circulation by Hurricane Vince (October 8–11) and the subsequent development of the anomalous low‐level cyclone over and northeast of the Caribbean. However, the error growth after seven days from the model initialization changed the steering circulations, resulting in a failed forecast of Wilma's TCG and its subsequent development into a hurricane over the western North Atlantic. It seems that the major error resulted from misrepresentation of the interactions between mid‐/high‐latitude systems and tropical circulations.

In contrast, the second simulation that was initialized at 0000 UTC October 14 successfully simulated Wilma's TCG and subsequent development (**Figure 12**). The WRF global model reproduced every major vortex and circulation at the 850 hPa level, not only over the North Atlantic but also at least over the neighboring ocean basins. With the successful simulation of the merger of the subtropical cyclone with the mid‐latitude trough off the east coast of the US around October 16, the subsequent forecast of the global WRF model was maintained suc‐ cessfully, reproducing the unique low‐level, large‐scale circulation development in the case of Wilma (see **Figures 2c**–**f** and **12**). The result of the global WRF model suggests that the role of mid‐latitude systems in TC activity is more important than previously considered, and that every large‐scale vortex and circulation component at least in the immediately neighboring region of the storm developing area is important for TCG forecasting. More in‐depth analyses are warranted to better understand the unusual development of Hurricane Wilma.
