18 October 2011

Southwest Pacific Tropical Cyclone Outlook: Below average numbers likely, but increased activity in the late season near North Queensland and French Polynesia

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 below average tropical cyclone occurrence for the 2011 – 12 season. On average, nine named tropical cyclones occur in the southwest Pacific (between 135°E and 120°W) each season (November – April). The forecast indicates that 5 – 8 named cyclones are expected for the November 2011 – April 2012 season. Activity is expected to be below normal to the west of the Dateline, except near North Queensland where near normal or slightly above activity may occur. During the second half of the season, higher than normal activity is also likely to the east of the Dateline near the Society Islands and Austral Islands.

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 close to normal or lower than normal activity because of the style of La Niña that is currently developing. It should be recognised that the expectation of activity (and associated risk) ascribed for each island group is subjective, but expectations are that increased activity to the east of the Dateline is likely as the season progresses. 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. In addition, most of the tropics and sub–tropics can be severely affected by at least one tropical cyclone during the season, and as such all nations should remain vigilant.

Outlook in more detail

La Niña conditions are currently re-developing in the region. Sea surface temperature anomalies across the central and eastern Equatorial Pacific Ocean are becoming more negative in recent weeks. More frequent high pressure anomalies have developed over French Polynesia (continuing from earlier in the year) with lower pressures over eastern Australia for the lead-in to Austral spring. The expectation is that below normal tropical cyclone (TC) activity is likely for most islands west of the International Date Line in the southwest Pacific during the November – April season, with normal or slightly elevated activity near the Gulf of Carpentaria and North Queensland, particularly late in the season. Tropical cyclone activity is likely to be near normal to the east of the International Date Line. Although reduced activity west of the International Date Line is likely, all communities should remain alert and prepared.

TC activity is expected to be below average with 5 – 8 named TCs expected over the November 2011 – April 2012 period for the southwest Pacific. On average, nine 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. Projections show near normal to slightly increased tropical cyclone activity for the 2011–12 season over North Queensland, but lower than normal activity for most other regions to the west of the Dateline. New Zealand usually experiences more frequent ex-tropical cyclone interactions during La Niñas; however in only half of the past years selected as being similar to this season 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. The Austral Islands and Society Islands are likely to experience slightly increased activity this year, particularly during the latter half of the TC season (see Figure 4). 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.

Background

In the Southwest Pacific, tropical cyclones (TCs) usually develop in the wet season, from November through to April, but occasionally occur in October and May, and have even occurred in June.

Peak cyclone occurrence is usually from January to March. In seasons with similar background climate conditions to present, several tropical cyclones occur over the Gulf of Carpentaria as well as in the Coral Sea region between the Solomon Islands and New Caledonia, and near Vanuatu, Fiji and Tonga, while a few affect other areas. On average, nearly half of the tropical cyclones that developed since the 1969-70 season have reached hurricane force with mean wind speeds of at least 64 knots (118 km/h).

To find past analogues to describe the current climate state (a re-development of La Niña) leading into the TC season, the autumn (MAM) and winter (JJA) conditions were examined for the tropical Pacific for 1969 to Present. For the past autumn, the ENSO system was well-coupled. However during winter, both the ocean and atmosphere anomalies slackened toward neutral, but with the latter component persisting a bit longer in the range of La Niña. We used a joint ENSO index that combines the atmospheric index (SOI) with the most widely-used oceanic index (NINO3.4), as described in Gergis and Fowler (2005) who call this the “Coupled ENSO Index” (CEI) to select analogue TC seasons for the 2010-11 forecast. Using the CEI, we highlighted years when the equatorial SSTs were below normal and atmospheric signals were in synch with the ocean (i.e. what Gergis & Fowler term a well coupled or “NINA-style” La Niña) for the autumn preceding the TC season, followed by a winter where either neutral ENSO conditions existed or conditions where the ocean was neutral and the atmosphere indicated La Nina (an SOI NINA) highlighted four analogue tropical cyclone seasons (1976/77; 1989/90; 2000/01; 2008/09). Note that the small number of analogue years relates to the brevity of the high-quality TC data set (only 41 years) and the limited number of similar analogues to this season.

Global climate models also indicate re-development of La Niña is likely into the Austral summer. Despite close to normal conditions existing at present, analysis of past tracks using the South Pacific Enhanced Archive of Tropical Cyclones (SPEAr-TC) data set (Diamond et al., 2011) suggests it is possible that there increased incidence of a tropical cyclone transitioning through or developing near North Queensland and the Gulf of Carpentaria. In addition, there may be elevated activity near the Society Islands and Austral Islands. TC genesis regions are expected to be close to normal this season (usually in a band between 10 – 12°S), and there is reduced likelihood of a Category 5 system occurring. A total of 6 storms on average is expected, and the range of variation between analogue years suggests 5 – 8 for the total TC count is probable. A strong bias towards southeasterly track orientations during ex-tropical transition (ETT) are expected in the region (mean bearing of ~145°) with ~50% of all historical TC tracks for the selected analogue seasons having made ETT within ±15° of the International Dateline. This projection is consistent with the subtropical jet and the South Pacific Convergence Zone (SPCZ) being displaced southwest of normal during La Niña years (see Figure 3).

A split of the analogue tropical cyclone seasons into early (November – January) and late (February – April) periods suggests TC activity could be more concentrated during the latter half of the season this year, with a change in the foci of activity as the season progresses (particularly for islands identified to the east of the Dateline).

TC intensity is related to how long developing systems reside in the deep tropics and feed on warm waters for their growth, and how the subtropical jet and SPCZ mutually interact and contribute to shear during ex-tropical transition. It should also be noted that the interplay of the hemispheric-scale atmospheric circulation with the timing of a Madden-Julian Oscillation passage can also significantly contribute to TC activity in the region.

ENDS

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