Zelia consistent with tropical cyclone outlook

The expected impact of ex-tropical cyclone Zelia this week reinforces NIWA's 2010-11 season tropical cyclone outlook. The outlook indicates higher than normal risk of ex-tropical cyclones over the Tasman Sea region and New Zealand during the present La Nina event (through to April).

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.

The outlook for the season indicates that average or above average tropical cyclone activity is likely for the southwest Pacific, with 9–12 named cyclones expected for the November 2010 – April 2011 season. 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 at higher risk than normal because of moderate–to–strong La Niña conditions. While an elevated risk has been identified for these places, it should be recognised that most of the tropics and sub–tropics can be severely affected by tropical cyclones during the season, and as such should remain vigilant.

Outlook in more detail

Moderate–to–strong La Niña conditions currently exist in the region. Sea surface temperature anomalies across the central and eastern Equatorial Pacific Ocean are negative, and the atmosphere has shown a classic high pressure anomaly over French Polynesia, with lower pressures over eastern Australia for the lead-in to Austral spring.  The expectation is that average or above average tropical cyclone (TC) activity will occur for most islands west of the International Date Line in the southwest Pacific during the November – April season. Although risk is reduced east of the International Date Line, all communities should remain alert and prepared.

TC activity is expected to be average or above average with 9–12 named TCs expected over the November 2010 – April 2011 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, three cyclones are forecast to reach at least Category 3, and one system is expected to reach at least Category 4, with mean wind speeds of at least 64 knots or 118 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 TCs passing close to those countries each year. Projections show an increased risk of tropical cyclones for the 2010–11 season over the Coral Sea and to the southwest of Fiji, particularly for Papua New Guinea, the Solomon Islands, Vanuatu, and New Caledonia. New Zealand is also at higher risk of experiencing an ex-tropical cyclone interaction this season. While risk is generally reduced for islands to the east of the International Date Line during La Niña, historical cyclone tracks (see Figure 2) indicate that TCs can affect parts of southwest French Polynesia, including the Society and Austral Islands, and the Cook Islands during La Niñas. All islands should remain vigilant as the current La Niña continues to evolve with progression into Austral summer. During moderate-to-strong La Niñas, ex-tropical cyclones are known for crossing the central and south Tasman Sea and it is typical for at least one ex-tropical cyclone to pass within 550km of the New Zealand coastline.

Figure 1: Departure from normal of the number of Tropical Cyclones occurring later in the season (February-June) in the southwest Pacific (135°E to 120°W), in years with similar SST anomalies and SOI to the present situation, and a well coupled La Niña in the lead up and transition to Austral spring (July-September).

In the Pacific Islands, please contact your local national meteorological service for information about how this guidance should be interpreted.