Climate developments in October 2008
The South Pacific Convergence Zone (SPCZ) activity strenghtened in October relative to previous months. A consolidated region of enhanced rainfall was observed in satellite data during October 2008, extending southeast from southern Papua New Guinea toward Vanuatu and New Caledonia. A localised region of suppressed convection expanded during the month to the south of the Equator, encompassing northeast of the Solomon Islands, and extending through the central southwest Pacific to Pitcairn Island. The regional circulation for the month was characterised by more frequent anticyclones over eastern Australia, with a ridge of high pressure extending over the Tasman Sea and the southwest Pacific to the north of New Zealand. Lower than normal pressure also developed to the north of the Marquesas Islands.
Rainfall was below average for many countries in the Southwest Pacific during October 2008. A new monthly low rainfall total was recorded at Bora Bora in the Society Islands, which received 14 mm of rainfall (14% of normal). Elsewhere in the eastern half of the Southwest Pacific, the stations in the Northern and the Southern Cook Islands, the Tuamotu archipelago, and the Austral Islands all recorded 20–50% of normal rainfall, with the exception of 175.8 mm (105% of normal) occurring at Rapa on the southern margin of the Austral Islands. Many stations in Fiji and Samoa also received between 25–75% of normal rainfall.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Vanuatu | Bauerfield | 327 | 461 | Record high |
| Vanuatu | Sola | 548 | 151 | Highest monthly total in the region |
| New Caledonia | La Roche | 215 | 256 | Very high |
| Society Islands | Bora Bora | 14 | 14 | Record low |
| Western Kiribati | Kiritimati | 5 | 8 | Very low |
| Australia | Townsville | 4 | 15 | Very low |
Soil moisture in October 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 were made at the end of the month, and 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. 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), Nuku’alofa (Tonga), and Tarawa (Western Kiribati).
In Samoa, the continuation of well–below normal rainfall has meant water restrictions are still in place in Apia, which experienced the lowest monthly rainfall total for the past five years. Western Kiribati also received lower than normal rainfall, with 37% of normal recorded at Tarawa. Eastern Kiribati recorded well below normal rainfall during the month. 5.1 mm (8% of normal) fell on 4 October in Kiritimati, which accounted for the entire monthly total for that island.
In the western Southwest Pacific, rainfall totals were near to above normal in the southwestern Solomon Islands, and also in northern and western New Caledonia, with a high monthly rainfall total recorded at La Roche (215 mm, 256% of normal). A new monthly record was established at Bauerfield, Vanuatu, which recorded 327 mm (461% of normal) rainfall.
El Niño/Southern Oscillation (ENSO)
During October, ENSO-neutral conditions were well established in the equatorial Pacific ocean, but the Tropical Pacific atmosphere continued to exhibit La Niña-like characteristics. The SOI remains positive at around +1.5 (an August-October mean of +1.2), consistent with enhanced trade winds about and west of the Date Line.
SST anomalies in October were close to neutral across the Equatorial Pacific: The NINO3 anomaly for October was +0.2°C (3-month mean +0.6°C), and the NINO4 anomaly was zero (ASO mean 0.1°C). Equatorial subsurface temperature anomalies are now relatively weak, with a -1 to -2°C anomaly centre near 140°W (weakened from -3°C in September).
Tropical Pacific precipitation patterns continue to show a La Niña signal, with the TRMM ENSO precipitation index at -1.26 for October, after weakening earlier in the month. OLR anomalies show suppressed Equatorial convection about and west of the Date Line. A convective pulse associated with the Madden-Julian Oscillation is passing through the Indonesian region during late October, and is expected to move over the Western Pacific in early November. This is on average associated with negative MSLP anomalies over New Zealand and positive anomalies to the north. The SPCZ was more active during October than last month. The Southern Annular Mode (SAM) was predominantly positive in October, and is predicted to continue along a positive trend into November.
Almost all models indicate neutral conditions continuing through to the end of autumn 2009. The NCEP discussion of 9 October indicates ENSO-neutral conditions through to early 2009, with a small possibility of a return to La Niña conditions. The IRI summary of 16 October gives a 90% probability of ENSO-neutral conditions in the October to December season, and the probability of El Niño 2% and La Niña only 8%. The Australian weekly tropical summary of 21 October suggests ENSO-neutral conditions persisting through the austral autumn.
Forecast validation: August to October 2008
A large region of suppressed convection was forecast over Tuvalu, Tokelau, the Northern Cook Islands, Tuamotu, the Society Islands, Pitcairn Island, and the Marquesas during August to October 2008, with average–to–below or below average rainfall expected for those countries. Enhanced convection for the same period was projected from Papua New Guinea extending in a band southeast toward Vanuatu and through Tonga, with above average rainfall forecast for those countries. Near–to–above average rainfall was forecast for Fiji, Niue, the Solomon Islands and New Caledonia. No clear precipitation guidance was offered for Eastern Kiribati, Wester Kiribati, the Southern Cook Islands or the Austral Islands.
The rainfall outlook for the August–October 2008 period was very good compared to what was forecast, the island group global station ‘hit’ rate being 77%, 19% higher than average for forecasts made during August and 16% higher than the average for all months combined. Rainfall totals were overestimated for the west-central part of the South Pacific, including Niue and Tonga and for the eastern fringe of French Polynesia.
Forecast statistics compiled over the last nine years indicate the multi-ensemble strike rate will continue to improve toward the end of the year.