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Severe Tropical Cyclone Pam was among the most intense tropical cyclones ever observed in the Southern Hemisphere. Part of a quartet of tropical cyclones that developed across the Pacific and Indian Oceans in early March 2015, the incipient disturbance that became Pam was first identified on March 4.

Genesis and intensification
During early March 2015 the Madden–Julian oscillation (MJO) – an eastward moving equatorial wave of energy that enhances Atmospheric convection and increases the probability of tropical cyclone formation – traveled across the western Pacific Ocean. Multiple tropical disturbances developed on both sides of the equator owing to this feature, two of which later became tropical cyclones alongside Pam: Cyclone Nathan near Australia and Tropical Storm Bavi near the Federated States of Micronesia. A fourth concurrent system, Cyclone Olwyn, was present over the eastern Indian Ocean. An unusually strong westerly wind burst also took place near the equator, aiding in the formation of low pressure systems. The MJO cycle later rose to its strongest magnitude since 1974, when records of such events began, reaching 4.67 units on the Wheeler-Hendon MJO index on March 16. This surpassed the previous record of 4.01 units on February 14, 1985. Sea surface temperatures across the area where Cyclone Pam developed were 2°C (3.6°F) above average, a factor which boosts the maximum potential intensity of storms.

On March 4, the Joint Typhoon Warning Center (JTWC) began monitoring an area of disturbed weather, dubbing it Invest 93P, some 635 km west-northwest of the South Pacific nation Tuvalu. Over the following two days, a broad area of low pressure slowly consolidated underneath scattered convection in this region. Though located in an area of low to moderate wind shear, efficient poleward outflow would aid in allowing tropical cyclogenesis over the subsequent days. Numerical forecast models at this time, including the GFS and ECMWF, indicated that an exceptionally powerful cyclone could develop. Throughout March 6 convection steadily increased around the low, prompting the Fiji Meteorological Service (FMS) to designate the system as Tropical Disturbance 11F at 09:00 UTC. At this time, the system was situated roughly 1140 km northwest of Nadi, Fiji. Subsequent organization resulted reclassification as a tropical depression on March 8 as the system remained nearly stationary. The majority of convection was displaced to the north of the circulation due to increased wind shear throughout the day. A scatterometer pass revealed winds of 45–55 km/h (30–35 mph) directly associated with the depression and winds up to 75 km/h (45 mph) farther north.

With environmental conditions expected to improve, the JTWC issued a Tropical Cyclone Formation Alert at 23:00 UTC on March 8. Following the contraction of banding features toward the depression's center early on March 9, the system intensified into a category 1 tropical cyclone on Australian tropical cyclone scale and was assigned the name Pam by the FMS. An expansive and elongated system, Pam tracked slowly southeast in response to a near-equatorial ridge to its north. At 09:00 UTC the JTWC followed suit with the FMS and began issuing advisories on Pam, designating it as Tropical Cyclone 17P. Steering currents soon shifted as a subtropical ridge to Pam's east built, prompting a turn to the south. Rapid consolidation of the low ensued throughout March 9 into March 10 with a central dense overcast feature persisting over the center of circulation. Pam subsequently reached Category 2 status on the Australian scale by 00:00 UTC on March 10 as winds increased to 95 km/h (60 mph). The JTWC estimated Pam to have been slightly stronger by this time in accordance with Dvorak satellite intensity estimates, reporting one-minute sustained winds of 120 km/h (75 mph) or the equivalent of a low-end Category 1 hurricane on the Saffir–Simpson hurricane wind scale (SSHWS).

Pam steadily grew in both size and strength on March 10, reaching severe tropical cyclone status – a storm with winds of 118 km/h (73 mph) or higher – by 12:00 UTC. Although obscured by deep convection, SSMIS microwave satellite imagery revealed the formation of an eye feature that evening. The moderate wind shear previously slowing development abated by March 11 and allowed for rapid intensification to take place. Intense convection blossomed around Pam's eye and the JTWC estimated it to have attained one-minute winds of 185 km/h (115 mph), equivalent to a major hurricane on the SSHWS, by 03:00 UTC. Additionally, the forward motion of the temporarily shifted to the south-southwest. Despite a resurgence of strong shear that day, the cyclone's vigorous outflow counteracted this and allowed for continued deepening. Pam reached Category 4 status on the Australian scale by 12:00 UTC before turning back to the southeast. Hours later, the powerful system passed close to or directly over the remote island of Anuta, part of Temotu Province in the Solomon Islands. Severe damage took place on there as well as Tikopia, located roughly 140 km (85 mi) to the southwest. Contact with Anuta was lost for more than a week; 5,000 people were directly affected by the storm with 1,500 homes damaged or destroyed.

The simultaneous presence of Cyclone Pam and Tropical Storm Bavi strengthened westerly surface trade winds along the equator, a byproduct of their complementary circulations, on March 11 to the strongest levels since 1997. Normally, trade winds in this region flow west to east. These anomalous winds enhanced the borderline El Niño Southern Oscillation, which was declared about a week earlier by the National Oceanic and Atmospheric Administration, and increased the probability of an El Niño event persisting through much of 2015.

Peak strength and Vanuatu impact
Marked improvement in the overall structure of Pam took place on March 12, with the cyclone's eye contracting from 55 km (35 mi) to 35 km and becoming sharply defined. Convective features also deepened and wrapped tightly into the storm's core. Dvorak intensity estimate rose accordingly and at 12:00 UTC the FMS estimated Pam to have attained Category 5 status on the Australian scale with ten-minute winds now reaching 220 km/h (140 mph). The powerful system soon adopted a south-southwesterly track for a second time, with forecasts bringing it dangerously close to Vanuatu. Following further contraction of the eye to 28 km, the JTWC classified Pam as a Category 5 on the SSHWS with one-minute winds of 260 km/h (160 mph). At this time the storm was situated roughly 335 km north-north-northeast of Port Vila, the capital city of Vanuatu. This marked the presence of the first SSHWS Category 5 in the South Pacific since Cyclone Ului in 2010. Steady intensification ensued thereafter as Pam neared Vanuatu, with the system achieving ten-minute sustained winds of 240 km/h (150 mph) by 06:00 UTC on March 13. The cyclone's core began raking the northern Shepherd Islands shortly thereafter, with the eye passing near or over Tongariki and Buninga around 08:00 UTC. The JTWC assessed Pam to have achieved its peak intensity around this time with one-minute sustained winds of 270 km/h (165 mph).

After devastating the northern Shepherd Islands, Pam took a fortuitous turn to the south-southeast which ultimately spared the heavily populated island of Efate of a direct hit. However, the storm's eyewall still raked the island as it passed just to the east around 12:00 UTC on March 13. Coinciding with its passage of Efate, Pam's winds reached their record maximum of 250 km/h (155 km/h) according to the FMS. While fortunate for Efate, the aforementioned turn resulted the powerful storm making landfall on Erromango hours later. Pam's landfall on the island as a Category 5 on the SSHWS was the second such event since 1970 in the South Pacific. The nearby island of Tanna suffered a direct hit from the cyclone's eyewall hours later. Once clear of Tafea Province, Pam deepened slightly and reached its peak intensity at 00:00 UTC on March 14 with a minimum barometric pressure of 896 mbar (hPa; 26.46 inHg). This ranked Pam as the second-strongest storm on record in the South Pacific, only behind Cyclone Zoe in 2002 which attained an estimated minimum pressure of 890 mbar (hPa; 26.28 inHg).