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Severe Tropical Cyclone Pam
Category 5 severe tropical cyclone (Aus scale)
Category 5 tropical cyclone (SSHWS)
Map plotting the track and intensity of the storm according to the Saffir–Simpson hurricane wind scale
FormedMarch 6, 2015
DissipatedMarch 22, 2015
(Extratropical after March 15, 2015)
Highest winds10-minute sustained: 250 km/h (155 mph)
1-minute sustained: 270 km/h (165 mph)
Lowest pressure896 hPa (mbar); 26.46 inHg
(Estimated; second-lowest in the South Pacific basin)
Areas affected
Part of the 2014–15 South Pacific cyclone season

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

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Four concurrent tropical cyclones on March 11: Olwyn near Western Australia, Nathan near Queensland, Pam approaching Vanuatu, and Bavi near the Federated States of Micronesia

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.[1] 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.[2] 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.[3]

On March 4, the Joint Typhoon Warning Center (JTWC) began monitoring an area of disturbed weather, dubbing it Invest 93P, some 635 km (395 mi) west-northwest of the South Pacific nation Tuvalu.[4][nb 1] 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.[6] Numerical forecast models at this time, including the GFS and ECMWF, indicated that an exceptionally powerful cyclone could develop.[7] Throughout March 6 convection steadily increased around the low,[8] prompting the Fiji Meteorological Service (FMS) to designate the system as Tropical Disturbance 11F at 09:00 UTC.[nb 2] At this time, the system was situated roughly 1,140 km (710 mi) northwest of Nadi, Fiji.[10] Subsequent organization resulted reclassification as a tropical depression on March 8 as the system remained nearly stationary.[11] 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.[12]

A consolidating Cyclone Pam at 02:30 UTC on March 11

With environmental conditions expected to improve, the JTWC issued a Tropical Cyclone Formation Alert at 23:00 UTC on March 8.[12] 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.[13] 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.[14] Steering currents soon shifted as a subtropical ridge to Pam's east built,[15] 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.[16] 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).[17] 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).[16]

Pam steadily grew in both size and strength on March 10,[18] reaching severe tropical cyclone status – a storm with winds of 118 km/h (73 mph) or higher – by 12:00 UTC.[19] Although obscured by deep convection, SSMIS microwave satellite imagery revealed the formation of an eye feature that evening.[20] 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.[21] Despite a resurgence of strong shear that day, the cyclone's vigorous outflow counteracted this and allowed for continued deepening.[22] Pam reached Category 4 status on the Australian scale by 12:00 UTC before turning back to the southeast.[23][24] Hours later, the powerful system passed close to or directly over the remote island of Anuta, part of Temotu Province in the Solomon Islands.[24] 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.[25][26]

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.[1][2] 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.[2][27][28]

Peak strength and Vanuatu impact

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Enhanced VIIRS infrared imagery of Cyclone Pam at 14:42 UTC on March 13 as it passed just east of Efate Island

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 (22 mi) and becoming sharply defined. Convective features also deepened and wrapped tightly into the storm's core.[29][30][31] 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).[32] 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 (17 mi), 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 (208 mi) north-north-northeast of Port Vila, the capital city of Vanuatu.[33] This marked the presence of the first SSHWS Category 5 in the South Pacific since Cyclone Ului in 2010.[34] 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.[35] 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.[36] The JTWC assessed Pam to have achieved its peak intensity around this time with one-minute sustained winds of 270 km/h (165 mph).[37]

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.[3] Coinciding with its passage of Efate, Pam's winds reached their record maximum of 250 km/h (155 km/h) according to the FMS.[38] While fortunate for Efate, the aforementioned turn resulted the powerful storm making landfall on Erromango hours later.[3] Pam's landfall on the island as a Category 5 on the SSHWS was the second such event since 1970 in the South Pacific.[39] The nearby island of Tanna suffered a direct hit from the cyclone's eyewall hours later.[3] 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).[40] 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).[39]

Extratropical transition and dissipation

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See also

[edit]
Wikimedia Commons has media related to Cyclone Pam.

Notes

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  1. ^ The Joint Typhoon Warning Center is a joint United States Navy – United States Air Force task force that issues tropical cyclone warnings for the western Pacific Ocean and other regions.[5]
  2. ^ The Fiji Meteorological Service is the Regional Specialized Meteorological Center for the Pacific Ocean south of the equator between 160°E and 120°W.[9]

References

[edit]
  1. ^ a b Jon Erdman (March 13, 2015). "Four Tropical Cyclones At Once : How Rare Is It?". The Weather Channel. Retrieved March 29, 2015.
  2. ^ a b c Jeff Masters (March 18, 2015). "Strongest MJO Event on Record Boosts El Niño Odds". Weather Underground. Retrieved March 29, 2015.
  3. ^ a b c d Bob Henson (March 17, 2015). "Cyclone Pam's Toll Remains Uncertain; Great Plains Roast in Record Winter Heat". Weather Underground. Retrieved March 29, 2015.
  4. ^ "Tropical Cyclone 17P (Pam) Infrared Imagery Archive". United States Naval Research Laboratory. March 15, 2015. Retrieved March 15, 2015.
  5. ^ "Joint Typhoon Warning Center Mission Statement". Joint Typhoon Warning Center. United States Navy. 2011. Archived from the original on July 26, 2007. Retrieved March 29, 2015.
  6. ^ Significant Tropical Weather Advisory for the Western and South Pacific Oceans. Joint Typhoon Warning Center (Report). United States Navy. March 6, 2015. Archived from the original on March 6, 2015. Retrieved March 15, 2015.
  7. ^ Bob Henson (March 6, 2015). "El Niño Conditions Now Official; Cold, Snow Take a Parting Swipe at East". Weather Underground. Retrieved March 15, 2015.
  8. ^ Significant Tropical Weather Advisory for the Western and South Pacific Oceans. Joint Typhoon Warning Center (Report). United States Navy. March 6, 2015. Archived from the original on March 6, 2015. Retrieved March 15, 2015.
  9. ^ Christopher W. Landsea (2014). "Subject: F1) What regions around the globe have tropical cyclones and who is responsible for forecasting there?". National Hurricane Center. National Oceanic and Atmospheric Administration. Retrieved March 15, 2015.
  10. ^ RSMC Nadi — Tropical Cyclone Centre (March 6, 2015). "Tropical Disturbance Summary March 6, 2015 09z". Fiji Meteorological Service. Archived from the original on March 7, 2015. Retrieved March 15, 2015.
  11. ^ RSMC Nadi — Tropical Cyclone Centre (March 8, 2015). "Tropical Disturbance Advisory Number A5". Fiji Meteorological Service. Archived from the original on March 8, 2015. Retrieved March 15, 2015.
  12. ^ a b "Tropical Cyclone Formation Alert". Joint Typhoon Warning Center. Unisys. March 8, 2015. Retrieved March 15, 2015.
  13. ^ RSMC Nadi — Tropical Cyclone Centre (March 9, 2015). "Tropical Disturbance Advisory Number A9". Fiji Meteorological Service. Archived from the original on March 9, 2015. Retrieved March 15, 2015.
  14. ^ "Tropical Cyclone 17P (Seventeen) Warning Nr 001". Joint Typhoon Warning Center. United States Navy. March 9, 2015. Archived from the original on March 9, 2015. Retrieved March 15, 2015.
  15. ^ "Tropical Cyclone 17P (Pam) Warning Nr 002". Joint Typhoon Warning Center. United States Navy. March 9, 2015. Archived from the original on March 9, 2015. Retrieved March 30, 2015.
  16. ^ a b "Tropical Cyclone 17P (Pam) Warning Nr 003". Joint Typhoon Warning Center. United States Navy. March 10, 2015. Archived from the original on March 10, 2015. Retrieved March 30, 2015.
  17. ^ RSMC Nadi — Tropical Cyclone Centre (March 10, 2015). "Tropical Disturbance Advisory Number A12". Fiji Meteorological Service. Archived from the original on March 10, 2015. Retrieved March 30, 2015.
  18. ^ "Tropical Cyclone 17P (Pam) Warning Nr 004". Joint Typhoon Warning Center. United States Navy. March 10, 2015. Archived from the original on March 10, 2015. Retrieved March 30, 2015.
  19. ^ RSMC Nadi — Tropical Cyclone Centre (March 10, 2015). "Tropical Disturbance Advisory Number A14". Fiji Meteorological Service. Archived from the original on March 10, 2015. Retrieved March 30, 2015.
  20. ^ "Tropical Cyclone 17P (Pam) Warning Nr 005". Joint Typhoon Warning Center. United States Navy. March 10, 2015. Archived from the original on March 10, 2015. Retrieved March 30, 2015.
  21. ^ "Tropical Cyclone 17P (Pam) Warning Nr 006". Joint Typhoon Warning Center. United States Navy. March 11, 2015. Archived from the original on March 11, 2015. Retrieved March 30, 2015.
  22. ^ "Tropical Cyclone 17P (Pam) Warning Nr 008". Joint Typhoon Warning Center. United States Navy. March 11, 2015. Archived from the original on March 11, 2015. Retrieved March 30, 2015.
  23. ^ RSMC Nadi — Tropical Cyclone Centre (March 11, 2015). "Tropical Disturbance Advisory Number A19". Fiji Meteorological Service. Archived from the original on March 11, 2015. Retrieved March 30, 2015.
  24. ^ a b "Tropical Cyclone 17P (Pam) Warning Nr 009". Joint Typhoon Warning Center. United States Navy. March 11, 2015. Archived from the original on March 11, 2015. Retrieved March 30, 2015.
  25. ^ "World Vision to provide relief to Temotu". Solomon Star. March 25, 2015. Retrieved March 30, 2015.
  26. ^ "Aid to Solomons Tikopia and Anuta". Radio New Zealand International. March 20, 2015. Retrieved March 30, 2015.
  27. ^ Emily Becker (March 5, 2015). "March 2015 ENSO discussion: El Niño is here". National Oceanic and Atmospheric Administration. Retrieved March 29, 2015.
  28. ^ Michelle L'Heureux (March 25, 2015). "Déjà Vu: El Niño Take Two". National Oceanic and Atmospheric Administration. Retrieved March 29, 2015.
  29. ^ "Tropical Cyclone 17P (Pam) Warning Nr 010". Joint Typhoon Warning Center. United States Navy. March 12, 2015. Archived from the original on March 12, 2015. Retrieved April 4, 2015.
  30. ^ "Tropical Cyclone 17P (Pam) Warning Nr 011". Joint Typhoon Warning Center. United States Navy. March 12, 2015. Archived from the original on March 12, 2015. Retrieved April 4, 2015.
  31. ^ "Tropical Cyclone 17P (Pam) Warning Nr 012". Joint Typhoon Warning Center. United States Navy. March 12, 2015. Archived from the original on March 12, 2015. Retrieved April 4, 2015.
  32. ^ RSMC Nadi — Tropical Cyclone Centre (March 12, 2015). "Tropical Disturbance Advisory Number A22". Fiji Meteorological Service. Archived from the original on March 12, 2015. Retrieved April 4, 2015.
  33. ^ "Tropical Cyclone 17P (Pam) Warning Nr 013". Joint Typhoon Warning Center. United States Navy. March 12, 2015. Archived from the original on March 12, 2015. Retrieved April 4, 2015.
  34. ^ "Tropical Cyclone 20P (Ului) Best Track". Joint Typhoon Warning Center. United States Navy. 2011. Retrieved April 4, 2015.
  35. ^ RSMC Nadi — Tropical Cyclone Centre (March 13, 2015). "Tropical Disturbance Advisory Number A24". Fiji Meteorological Service. Archived from the original on March 13, 2015. Retrieved April 4, 2015.
  36. ^ "Advanced Dvorak Technique listing for Tropical Cyclone 17P". Cooperative Institute for Meteorological Satellite Studies. University of Wisconsin. March 15, 2015. Retrieved April 4, 2015.
  37. ^ "Tropical Cyclone 17P (Pam) Warning Nr 015". Joint Typhoon Warning Center. United States Navy. March 13, 2015. Archived from the original on March 13, 2015. Retrieved April 4, 2015.
  38. ^ RSMC Nadi — Tropical Cyclone Centre (March 13, 2015). "Tropical Disturbance Advisory Number A25". Fiji Meteorological Service. Archived from the original on March 13, 2015. Retrieved April 4, 2015.
  39. ^ a b Jeff Masters and Bob Henson (March 16, 2015). "Category 5 Cyclone Pam Catastrophic for Vanuatu; Western U.S. Heat Records Fall". Weather Underground. Retrieved March 29, 2015.
  40. ^ RSMC Nadi — Tropical Cyclone Centre (March 14, 2015). "Tropical Disturbance Advisory Number A27". Fiji Meteorological Service. Archived from the original on March 14, 2015. Retrieved April 4, 2015.
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