Climate developments in August 2008
The South Pacific Convergence Zone (SPCZ) continued to exhibit diminished activity in August relative to previous months. Only a small region of enhanced rainfall was observed in satellite data during August 2008, centred near Papua New Guinea. A region of suppressed convection expanded south of the Equator, and extends from Nauru and Western Kiribati to Eastern Kiribati, and southeast to the Southern Cook Islands and Society Islands. The regional circulation was characterised by more frequent anticyclones to the northwest of New Zealand and highs to the southwest of Pitcairn Island.
Rainfall was well above average for northern New Zealand and Takaroa, French Polynesia, but these sites were the exception for the South Pacific region during August 2008. Most countries had global rainfall averages below 80% of normal rainfall. No monthly record high monthly rainfall totals were recorded for any reported climate monitoring site.
French Polynesia experienced low rainfall totals in the southern island groups, with Bora Bora and Tahiti recording 40% and 31% of normal rainfall, respectively. The southern fringe of the Austral Islands and Tuamotu archipelago also had low monthly rainfall totals. In Samoa, many locations also had well–below normal rainfall, with many stations recording 10–30% of normal precipitation. Record low rainfall was recorded at La Roche, New Caledonia (9 mm, 11% of normal), and very low rainfall also occurred in Niue. Low rainfall also occurred in the Northern Cook Islands, with 82 mm (43% of normal) recorded at Penrhyn.
Fiji also had a drier than normal conditions. A localised high pressure frequently occurred during the month, which generated southerlies onto the country. A weak SPCZ displaced to the northeast of the country and the circulation created fine conditions that helped monthly temperatures to reach +0.7 to 2.9 °C above average with a record high at Nacocolevu of 29.3 °C, breaking a 71–year–old record established in 1938. Warmer than normal temperatures were also recorded for many other Pacific islands during August.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Tonga | Salote | 7 | 7 | Record low |
| New Caledonia | La Roche | 9 | 11 | Record low |
| Samoa | Faleolo | 10 | 6 | Very low |
| Vanuatu | Tanna | 8 | 14 | Very low |
| Fiji | Nadi | 3 | 4 | Very low |
| Australia | Cairns | 2 | 8 | Very low |
Soil moisture in August 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 Rarotonga (Southern Cook Islands) project dry soil moisture conditions, which is similar to last year at this time. Soils continued to be moist (at field capacity) for the time of year at Port Vila (Vanuatu), while moderate soil moisture is projected for Hanan (Niue).
El Niño/Southern Oscillation (ENSO)
During August, 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 positive NINO3 anomaly increased during August to +1.1 °C (3-month mean +0.8 °C), and is a continuation of the progressive warming from 1.5 °C in February 2008. However, the warming at depth has weakened. NINO4 remains slightly below normal at 0.1 °C in August (three–month mean around 0.3 °C). The negative anomaly continues to weaken at the surface but has strengthened below 100m.
The near-Equatorial trade winds are slightly enhanced throughout the entire equatorial Pacific. Convection continues to be largely suppressed along the equatorial regions from eastern Papua New Guinea to the central Pacific, but enhanced over central Indonesia. The OLR anomaly field has lost the La Niña signature it displayed in past months and lacked coherence in the Pacific in August, though convection remains weakly suppressed near the Date Line. The SPCZ was moderately active from near the Solomon Islands towards the Southern Cook Islands, but with a noticable gap over Vanuatu, Fiji, and Tonga. The TRMM ENSO precipitation index was –1.25 at the end of August (weakening from a strong La Niña value of –2 in April). The Madden Julien Oscillation is non-existent at present.
All ENSO models indicate neutral conditions during spring, and continuing neutral conditions through to the May 2009. The NCEP discussion of 7 August indicates ENSO-neutral conditions during September – November. The IRI summary of 19 August gives an 80% probability of ENSO-neutral conditions in the August – October season, and the probability of El Niño only 15%. The Australian weekly tropical summary of 26 August suggests ENSO-neutral conditions now exist, and persisting through the austral spring.
Forecast validation: June to August 2008
A La Niña-like precipitation pattern was expected, with a large region of suppressed convection forecast in the southwest Pacific, from Tuvalu and Tokelau, extending south and east to Pitcairn Island and the Marquesas and encompassing the Northern Cook Islands, Tuamotu, and the the Society Islands. Average rainfall was expected for the Austral Islands. Enhanced convection was forecast from Papua New Guinea extending southeastward toward the Solomon Islands and Vanuatu, through to Niue, including Fiji and Tonga. Near-to-above or above average rainfall was expected in those countries for the June–August period. No clear precipitation guidance was offered for Eastern and Western Kiribati, Samoa, and the Southern Cook Islands.
The rainfall outlook for the June–August 2008 period was mixed compared to what was forecast, the ‘hit’ rate being 51%, 11% lower than average. Rainfall totals were overestimated for much of the southern pat of the South Pacific, including New Caledonia and the Southern Cook Islands, and for the southwestern fringe of French Polynesia.
Forecast statistics compiled over the last nine years indicate the multi-ensemble strike rate will improve with the shift to springtime.