Prediction of average or above average tropical cyclone occurrence for the 2010–11 season.

Southwest Pacific Tropical Cyclone Outlook: Average or above average numbers expected

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 organisations across the Pacific are predicting average or above average tropical cyclone occurrence for the 2010–11 season. On average, nine named tropical cyclones occur in the southwest Pacific (between 135°E to 120°W) each season (November – April). Risk is expected to be higher than normal to the west of Fiji, and elevated activity within the Coral Sea extending into the North Tasman Sea region is anticipated.

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.  

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

Additional background information

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 in the Coral Sea region between the Solomon Islands and New Caledonia, and near Vanuatu, Fiji and Tonga, while a few affect other areas. Activity is more concentrated to the west of the Dateline relative to normal. 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).

Finding past analogues to describe the current climate state (a well-coupled La Niña) preceding the TC season (strengthened oceanic anomalies in the NINO3.4 region (colder than normal) and consistently positive Southern Oscillation Index (SOI) values during the lead-up to the coming TC season) was straight-forward. Use of a joint ENSO index that combines the atmospheric index (SOI) with the most widely-used oceanic index (NINO3.4) has been described in Gergis and Fowler (2005), who call this the “Coupled ENSO Index” (CEI) was used to select analogue TC seasons for the 2010-11 forecast. Using the CEI to highlight 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 November through April identified eight analogue tropical cyclone seasons (1970/71; 1971/72; 1973/74; 1975/76; 1988/89; 1998/99; 1999/00; 2007/08).

Global climate models indicate a moderate-to-strong La Niña is likely through the Austral summer into autumn. It is possible that during this time we will see an elevated risk of cyclone tracking or developing west of the Date Line. TC genesis regions are expected to be further away from the equator than normal this season, in a band between 12-15°S, and there is reduced risk of a Category 5 system occurring. A strong bias towards southeast-to-southerly tracks during ex-tropical transition is expected in the region, which means increased disturbance in areas like the North Tasman Sea and closer to New Zealand. This projection is consistent with the subtropical jet and the South Pacific Convergence Zone (SPCZ) being displaced southwest of normal during typical La Niña years. The recent southwest shift in the SPCZ relative to its normal position highlights the changes that have occurred in the atmosphere during Austral winter and the early part of spring.

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.

References

Gergis, J. and A.M. Fowler (2005), Classification of synchronous oceanic and atmospheric El Niño-southern Oscillation (ENSO) events for palaeoclimate reconstruction. International Journal of Climatology, 25, 1541-1565.

Table 1: The average number of tropical cyclones passing within 5° of the main South Pacific Island groups between November and May.

 

 

Table 2: Analogue seasons and intensity of tropical cyclones that occurred in the Southwest Pacific.

 

 

Analogue guidance summary:

9-12 storms expected for the 2010-11 season

Three expected to reach category 3 status, 1 expected to reach Category 4 status.

At least one ex-tropical cyclone is expected to come within 550 km of New Zealand during this time, slightly elevated risk due to La Niña year.