Cyclone Ivy - Meteorological History

Meteorological History

In the middle of February 2004, a monsoon trough formed in the southwest Pacific Ocean during the middle of a Madden–Julian oscillation pulse. A low pressure area developed along the trough, and the Fiji Meteorological Service (FMS) designated it Tropical Disturbance 5F on February 21 about halfway between Vanuatu and Fiji. Initially, the system was moving little and had a circulation that was detached from the primary area of convection. On February 22 after the thunderstorms increased, the Joint Typhoon Warning Center (JTWC) initiated advisories on the disturbance with the designation Tropical Cyclone 13P. Shortly thereafter, the FMS reported that the disturbance developed into a tropical depression, although the convection was transient and displaced from the circulation. The system tracked west-northwestward, due to a ridge located to its south. With low wind shear and good outflow, the system gradually organized, with the circulation becoming situated underneath the convection. At 0300 UTC on February 23, the FMS upgraded it to a tropical cyclone and gave it the name Ivy while the storm was about 510 km (315 mi) northeast of Port Vila, Vanuatu.

After being named, Ivy quickly intensified after its convection was drawn into the circulation. Late on February 23, the JTWC upgraded Ivy to the equivalent of a Category 1 hurricane on the Saffir-Simpson scale, and the FMS followed suit the following day by upgrading Ivy to a severe tropical cyclone as continued favorable conditions allowed for rapid deepening. On February 24 the cyclone began a turn to the southwest as it moved around the ridge, and the next day it turned to the south toward Vanuatu as an approaching shortwave trough passed to the south and created a weakness in the ridge. A nearby upper-level low temporarily cut off outflow, although once it moved further away the low enhanced ventilation. Strengthening continued and, late on February 25, the FMS reported that Ivy attained peak 10–minute sustained winds of 165 km/h (105 mph) over the Vanuatu archipelago; this made it a Category 4 on the Australian Region Tropical Cyclone Intensity Scale. Around the same time, the JTWC estimated peak 1–minute sustained winds of 195 km/h (120 mph), after a 19 km (12 mi) eye developed.

After passing slowly through Vanuatu, Ivy began accelerating to the southeast and its eye passed very close to the capital city of Port Vila, after moving over the island of Epi. As the cyclone moved away from the island group, it underwent an eyewall replacement cycle. In addition, the combination of cooler water temperatures, greater wind shear, less outflow, and land interaction with Vanuatu caused gradual weakening. Late on February 27, Ivy entered the area of warning responsibility of the Wellington Regional Specialized Meteorological Center. The convection rapidly diminished and left the circulation exposed, and Ivy transitioned into an extratropical cyclone on February 28. It remained strong and maintained a well-defined circulation as it passed just east of East Cape, New Zealand. Continuing southeastward, the extratropical remnants crossed the International Dateline on February 29, and Ivy dissipated about 1800 km (1100 mi) southeast of New Zealand on March 2.