Climate developments in September 2008
The South Pacific Convergence Zone (SPCZ) continued to weaken in September relative to previous months. Only a small region of enhanced rainfall was observed in satellite data during September 2008, centred over Indonesia and Papua New Guinea. A localised region of suppressed convection expanded south of the Equator, and extends from east of the Solomon Islands to Tokelau, and southeast to Tuvalu and Tokelau. The regional circulation was characterised by more frequent anticyclones to the east of New Zealand and lows to the north of the Marquesas Islands.
Rainfall was near average to below average for many countries in the South Pacific region during September 2008. However, there were a few monthly rainfall records, with 307.2 mm falling at Lakeba, Fiji (304 % of normal), and a 24 hour precipitation total of 147 mm at that site. 385.1 mm of rainfall also fell at Salote Pilolevu Airport, Tonga (347% of normal) and 275 mm was recorded at Raoul Island (250% of normal). Near normal to well above normal rainfall also occurred in the Solomon Islands, Tonga, and Vanuatu during September.
Southern French Polynesia experienced low rainfall totals during September, with rainfall totals registering a deficit of 25% – 75% of normal for the month. However the northern portion of French Polynesia had 108 % of normal rainfal, recorded at Atuona. New Caledonia received above average rainfall on the east coast, but below normal rainfall for offshore islands during September. In Samoa, many locations also recorded well–below normal rainfall, with Apia and Faleolo receiving 40–60% of normal precipitation. The dry conditions also were severe enough to produce forest fires and drought warnings.
Western Kiribati also had high rainfall in places, including a one-day rainfall total of 58.8 mm recorded at Butaritari on 18 September. Islands. Eastern Kiribati recorded below normal rainfall for the month, with 15.1 mm and 10.6 mm falling at Christmas and Kanton Islands, respectively.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Tonga | Salote | 385 | 347 | Record high |
| New Zealand | Raoul Island | 275 | 250 | Record high |
| Fiji | Lakeba | 307 | 304 | Record high |
| Australia | Townsville | 1 | 5 | Very low |
Soil moisture in September 2008
Estimates of soil moisture shown in the map (right) are based on monthly rainfall for one station in each country. Currently there are not many sites in the water balance model, but it is planned to include more stations in the future.
The information displayed is based on a simple water balance technique to determine soil moisture levels. Addition of moisture to available water already in the soil comes from rainfall, and losses via evapotranspiration. Monthly rainfall and evapotranspiration are used to determine the soil moisture level and its changes.
Please note that these soil moisture calculations are made at the end of the month. For practical purposes, generalisations were made about the available water capacity of the soils at each site.
Nadi (Fiji) and Tarawa (Western Kiribati) project dry soil moisture conditions. Soils continued to be moist (at field capacity) for the time of year at Port Vila (Vanuatu) and Hanan (Niue) while moderate soil moisture is projected for Apia (Samoa) and Rarotonga (Southern Cook Islands).
El Niño/Southern Oscillation (ENSO)
During August, September, ENSO conditions continued to remain near neutral in the equatorial Pacific. A warm water tongue off the coast of Ecuador that developed last month continues to be prominent. The SOI remains positive, having strengthened somewhat during September to +1.4 with the 3-month mean at +0.8.
The positive SST anomalies in the eastern Pacific weakened during September (NINO3 fell to +0.6 °C, with 3-month mean of +0.8 °C), and the accompanying subsurface warm anomaly has also weakened. NINO4 remains close to normal at 0.2 °C in September (3-month mean around 0.2 °C). The strongest subsurface anomaly in the tropical Pacific now is a cold anomaly of about 3 °C below normal located just below 100m midway between the Date Line and the South American coast.
Low-level easterly anomalies remain stronger than normal in the western equatorial Pacific. The OLR anomaly field has lost the La Niña signature it displayed in past months and lacked coherence in the Pacific in September, though convection remains weakly suppressed near the Date Line and enhanced over Papua New Guinea. The TRMM ENSO precipitation index is –0.87 for September (down from the strong La Niña value of –1.25 in August). However, the upper-ocean heat anomaly in the equatorial Pacific, and the thermocline slope index, are both near zero, indicative of ENSO-neutral conditions. The MJO reappeared in late August, with active convection in the near-equatorial regions of Southeast Asia (east of Indonesia). The SPCZ was very inactive during September.
All models indicate neutral ENSO conditions continuing from late spring 2008 through to the late autumn 2009. The NCEP discussion of 11 September indicates ENSO-neutral conditions during September –November, and considers this is most likely to continue, although also indicates a small possibility of a return to weak La Niña conditions. The IRI summary of 18 September projects a 90% probability of ENSO-neutral conditions for September to November, and the probability of an El Niño or La Niña occurring at only 5%. The Australian weekly tropical summary of 26 August suggests ENSO-neutral conditions now exist, and will persist through the austral spring.
Forecast validation: July to September 2008
A large region of suppressed convection was forecast in the southwest Pacific, extending southward from the Northern Cook Islands to the Austral Islands and eastward to the Marquesas, encompassing the Tuamotu Archipelago and the Society Islands. Below average rainfall was forecast for these islands. Enhanced convection was forecast for Papua New Guinea and from Western Kiribati extending southward to Tonga, Niue, and the Southern Cook Islands, including Fiji, Tuvalu, Tokelau, and Vanuatu. Near-to-above or above average rainfall was expected in those countries for the July–September period. Normal rainfall was forecast for Samoa and New Caledonia. No clear precipitation guidance was offered for Eastern Kiribati, Pitcairn Island, the Solomon Islands, and Wallis & Futuna.
The rainfall outlook for the July–September 2008 period was mixed compared to what was forecast, the ‘hit’ rate being 51%, 13% lower than average. Rainfall totals were overestimated for the north-central part of the South Pacific, including Tuvalu and Tokelau, Western Kiribati, Niue, and for the northeastern fringe of French Polynesia. Many island groups also had prominent north-south or west-east rainfall anomaly splits during the past three month period.
Forecast statistics compiled over the last nine years indicate the multi-ensemble strike rate will improve with the shift to springtime.