New Zealand’s National Institute of Water & Atmospheric Research (NIWA) and New Zealand MetService have issued a tropical cyclone outlook on behalf of collaborating organisations from the southwest Pacific, including Australia, the USA, the Pacific Island National Meteorological Services, French Polynesia, and New Caledonia.
Climate and weather forecasting organisations across the Pacific are predicting near normal tropical cyclone numbers for the 2011 – 12 season as a whole. On average, ten named tropical cyclones occur in the southwest Pacific (between 135°E and 120°W) each season (November – April). The TC forecast update indicates that a total of 6 – 8 named cyclones are expected for the late season (February – April). Two named storms have occurred since the start of the season in November 2011 through the end of January 2012 (TC Fina and TC Grant). As such, the November–April seasonal total is likely to be in the range of 8 – 10 named storms. A small area of higher than normal activity is expected to be focused well south of the Southern Cook Islands and Austral Islands. For most island groups, TC activity is expected to be normal or below normal for the remainder of the season.
Despite normal or below normal activity, most countries west of the International Date Line, including Papua New Guinea, the Solomon Islands, Vanuatu, New Caledonia, and New Zealand (from ex–tropical cyclones) are likely to experience an interaction or two with a tropical cyclone because of La Niña that is currently in place. It should be recognised that the expectation of activity (and associated risk) ascribed for each island group is subjective, but these expectations are based on the style of La Niña that exists at present, along with international climate forecast centres that indicate the high likelihood of La Niña diminishing from mid-autumn. Note that the forecast of normal or below normal activity for islands like New Caledonia, Vanuatu, Fiji, and Tonga indicates two or three tropical cyclones can still be expected during the season as a whole. In addition, most of the tropics and sub–tropics can be severely affected by at least one tropical cyclone during the remainder of the season, and as such all nations should remain vigilant.
Outlook in more detail
La Niña conditions exist at present, and sea surface temperature anomalies across the central and eastern Equatorial Pacific Ocean are negative relative to normal. More frequent high pressure anomalies have occurred over French Polynesia during late austral spring and through mid-summer, with lower pressures over eastern Australia. The style of La Niña that has developed is moderately coupled between the ocean and atmosphere, and the South Pacific Convergence Zone has been displaced south and west of normal during the past four months. The oceanic component of ENSO is expected to weaken over the coming months, and as such the style of the current La Niña is anticipated to degrade into an atmospheric-dominated event from mid-autumn. Past situations that describe a similar scenario (analogues) indicate that normal or below normal tropical cyclone (TC) activity is likely for most islands in the southwest Pacific during the remainder of the TC season, with slightly elevated activity well to the south of the Southern Cook Islands and Austral Islands from an extra-tropical storm. Although some reduced activity is indicated to the west of the International Date Line during this time, all communities should remain alert and prepared.
TC forecast suggests average overall numbers of 8 – 10 named TCs for the southwest Pacific basin can be expected by the end of April 2012. On average, ten named tropical cyclones occur each year for the southwest Pacific region. Southwest Pacific TCs are grouped into classes ranging from 1 to 5, with 5 being the most dangerous. For the coming season, at least one cyclone is forecast to reach at least Category 3, with mean wind speeds of at least 64 knots or 118 km/h and one system may reach at least Category 4 strength, with mean wind speeds of at least 86 knots or 159 km/h.
Each year, tropical cyclones have a significant impact on the southwest Pacific. Places like Vanuatu and New Caledonia typically experience the greatest activity in the region, with an average of about 3 or 4 TCs passing close to those countries each year. The past years selected as being similar to this season are 2000/01, 07/08 and 08/09. New Zealand usually experiences more frequent ex-tropical cyclone interactions during La Niñas, and is also vulnerable to a close TC passage during seasons where the Southern Oscillation Index is low or neutral. However in only one of these selected years did an ex-tropical cyclone come close to the country. While TC activity is generally reduced for islands to the east of the International Date Line during La Niñas, historical cyclone tracks (see Figure 2) indicate that TCs can affect parts of southwest French Polynesia, including the Society and Austral Islands, and the southern Cook Islands during La Niñas, especially late in the season. All islands should remain vigilant as the current La Niña continues to evolve with progression into Austral summer. During moderate La Niñas with a strong atmospheric component (strong SOI) but slightly weaker equatorial SST anomalies(see explanation of analogue selection below), ex-tropical cyclones are known for transitioning into the mid-latitudes within ±15° of the Dateline and they typically have a strong southeasterly or southerly slant to their trajectory.