Climate developments in November 2008
The South Pacific Convergence Zone (SPCZ) activity was weaker in November relative to last month. A consolidated region of enhanced rainfall was observed in satellite data during November 2008 over southern Papua New Guinea as well as over southeast Australia and southern New Caledonia. A localised region of suppressed convection intensified during the month to the south of the Equator, which encompassed the southern Solomon Islands and Nauru and extended across the southwest Pacific to central French Polynesia. The regional circulation for the month was characterised by more frequent cyclones over eastern Australia, with ridges of high pressure to the east of northern New Zealand. Lower than normal pressure also developed to the north of the Marquesas Islands and east of Eastern Kiribati.
Rainfall was near normal or slightly above normal for many countries in the Southwest Pacific during November 2008. The exception occurred in a swath that extended from French Polynesia to Nauru and the eastern Solomon Islands including Tuvalu, Tokelau, and Vanuatu, that had normal or below normal rainfall. Lower than normal rainfall totals were recorded at Tahiti in the Society Islands, which received 27 mm of rainfall (21% of normal). Elsewhere in the eastern half of the Southwest Pacific, the stations in the Tuamotu archipelago and the Austral Islands all recorded 70–80% of normal rainfall, with the exception of 207 mm (159% of normal) falling at Tubuai.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Tonga | Lupepau’u | 380 | 266 | Record high |
| Solomon Islands | Taro | 474 | 201 | Highest monthly total in the region |
| New Caledonia | Poindimie | 373 | 201 | Very high |
| Society Islands | Tahiti | 27 | 21 | Very low |
| Eastern Kiribati | Kanton | 1 | 2 | Very low |
| Tuvalu | Nanumea | 49 | 28 | Very low |
Soil moisture in November 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), Hanan (Niue), and Apia (Samoa) project moist (at or near field capacity) soil moisture conditions. Soils are moderate for the time of year at Tarawa (Western Kiribati) , while dry soil moisture is projected for Rarotonga (Southern Cook Islands).
Many stations in Fiji received above normal rainfall, with five sites exceeding 200% of normal rainfall for November. At Penang, 296 mm or rain fell in a 24 hour period on 28 November, breaking an 80-year-old record. Nearby in New Caledonia, most stations recorded above average rainfall particularly along the east coast, with a record monthly total recorded at Aoupinie (448 mm).
In Samoa, abundant rainfall finally fell in November, helping to ease drought conditions. In Niue, abundant rainfall was recorded, with a global station average of 150% of normal for the month. Even more rain fell in neighboring Tonga, which had many stations recording above 175% of normal, and a total at Lupepau’u of 380 mm (266% of normal) that broke a 13-year-old record. In the western Southwest Pacific, rainfall totals were near to above normal in the Solomon Islands and well above normal for portions of central and eastern Australia.
El Niño/Southern Oscillation (ENSO)
During October, ENSO–neutral conditions are 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 +1.7 (a September–November mean of +1.5), and easterly trade winds have strengthened during November about and west of the Date Line.
SST anomalies in November were close to neutral across the equatorial Pacific: The NINO3 anomaly for October was +0.0°C (3-month mean +0.3°C), and the NINO4 anomaly was –0.1°C (SON mean 0.1°C). Equatorial subsurface temperature anomalies are relatively weak, but a –2°C anomaly remains centred near 140°W below 100 m.
Tropical Pacific precipitation patterns continue to show a La Niña signal, with the TRMM ENSO precipitation index at –1.1 for November. Conversely, OLR anomalies show suppressed equatorial convection about and west of the Date Line for November as a whole. A convective pulse associated with the MJO was present over Indonesia during late November (probably associated with the strengthening easterlies between the Date Line and the convection), but this is predicted to dissipate by early December as the convective region approaches the Date Line. The development of a La Niña, which would be unusual but not unprecedented (e.g., Dec 1967) at this time of year cannot be ruled out.
Several climate models appear to be responding to the enhanced easterlies and have strengthened the cool SST anomaly: five models out of 14 assessed by NIWA indicate a return to La Niña conditions could occur over the coming summer season. However, the majority indicate neutral conditions continuing through to the end of autumn 2009. The IRI summary of 20 October projects a 85% probability of ENSO–neutral conditions in the November to January season, and the probability of El Niño 2% and La Niña only 13%. The Australian weekly tropical summary of 25 November likewise expects ENSO-neutral conditions to persist through into the austral autumn. The NCEP discussion of 11 December indicates ENSO–neutral or La Niña conditions are equally likely through 2009.
Forecast validation: September to November 2008
A large region of suppressed convection was forecast to encompass Western Kiribati and Tuvalu, and also Tokelau, the Northern Cook Islands, Tuamotu, the Society Islands, Pitcairn Island, and the northeastern half of French Polynesia from September to November 2008, with average–to–below or below average rainfall expected for those countries. Enhanced convection for the same period was projected to be centralised near Vanuatu and north of New Caledonia and east of the Solomon Islands with above normal rainfall. Near–to–above average rainfall was forecast for Fiji, Niue, Tonga, Papua New Guinea, the Solomon Islands, and New Caledonia. No clear precipitation guidance was offered for Eastern Kiribati, Tokelau, Samoa, the Southern Cook Islands, or the Austral Islands.
The rainfall outlook for the September–November 2008 period was very good compared to what was forecast, although this is calculated for only 10 island groups (six islands had a forecast of ‘Climatology’, which is unscoreable).The global station ‘hit’ rate was 91%, 28% higher than average for forecasts made during September and 30% higher than the average for all months combined. Rainfall totals were slightly overestimated for parts of Niue, northern Vanuatu, and Honiara (Solomon Islands). This is the highest validation score for the ICU to date.
Forecast statistics compiled over the last nine years indicate the multi-model ensemble strike rate will continue to have good skill throughout the summer.