Climate developments in January 2008
The South Pacific Convergence Zone (SPCZ) extended from Papua New Guinea, over the Solomon Islands, Fiji, and toward the Austral Islands, displaced further south and west than normal for the time of year in that region. A large region of very suppressed convection persisted along the Equator, extending from Western to Eastern Kiribati, affecting the regions both north and south of the Equator, including the Northern Cook Islands, Tuvalu, and the Marquesas.
Rainfall was well above average in parts of Vanuatu and Niue (Port Vila recording 649 mm) as a result of tropical cyclone Funa, and also in parts of the Solomon Islands and Fiji. Heavy rainfall, exceeding 200% of normal, occurred in central and western Fiji with the passage of Tropical cyclone Gene and was accompanied by flooding and >1.5 m of standing water in Nadi Rainfall was also above average over much of Samoa as a result of cyclones this month, with extensive flooding in and around Apia.
In contrast January rainfall was near or below normal over much of Kiribati, French Polynesia, and the parts of the Cook Islands, New Caledonia, the Marquesas, and Pitcairn Island. Rainfall has been below average for each of the past 8 months in Kiribati, and above average for each of the past 4 months in Fiji.
December mean air temperatures were about 1.0 ºC below normal in northeast Australia, and were 0.5 ºC or more above normal in New Caledonia, Samoa, and Tonga.
Tropical Southwest Pacific mean sea-level pressures were below average over New Caledonia and a band of the southwest Pacific to the south and west of the SPCZ. Higher than normal pressures occurred across the South Island of New Zealand and extended toward the region southeast of the Cook Islands as well as over all of French Polynesia.
This pressure anomaly produced more frequent mid-latitude equatorial surface easterlies for New Zealand and eastern Australia.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Vanuatu | Port Vila | 649.1 | 234 | Record high |
| Kiribatii | Christmas Island | 6 | 5 | Extremely low |
| New Zealand | Auckland Airport | 15 | 22 | Very low |
| Niue | Liku | 473.6 | 191 | Record high |
Soil moisture in January 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. 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.
Soils continued to be moist (at field capacity) for the time of year at Nadi (Fiji), Hanan Airport (Niue), Apia (Samoa), and in Tonga.
El Niño/Southern Oscillation (ENSO)
During January, a moderate–strong La Niña event persisted from the previous month. and conditions continued to spread into the western tropical Pacific. The Southern Oscillation Index (SOI) continued its strong movement upwards, indicating a further strengthening of the ocean-atmosphere coupling.
Below normal sea surface temperature (SST) anomalies now extend from 160°E to the South American coast, while a warm horseshoe continues in the extratropics of both hemispheres. The NINO3 anomaly was -1.3 °C for January (NDJ average -1.4 °C), while NINO4 continued to strengthen and is now -1.4 °C (NDJ average -1.1 °C). The subsurface temperature anomalies have continued to strengthen: the largest cold anomalies near 100m depth around 120 °W during January were about -4 °C (-3 °C in December), while the warm pool of water in the western Pacific appears to have moved deeper into the ocean.
The easterly trade winds that were strong and persistent during December and the start of January over a wide longitude band have weakened in the western Pacific recent days, probably in response to Madden-Julian Oscillation (MJO) activity. The SOI stabilised during January and is around +1.5 (+1.4 in December), with the NDJ average of +1.3.
Tropical OLR anomalies show suppressed convection in the equatorial region from west of the International Date Line (160 °E) eastwards. The region of enhanced convection that was over Indonesia and parts of northern Australia in December has shifted eastwards into the western Pacific. The South Pacific Convergence Zone is still prominent between New Caledonia and Tonga, well south of its normal position. The TRMM-based ENSO precipitation index was -0.4 in January, has weakened from a value of -1.35 in December. The MJO has displayed a fairly regular periodicity of 40 to 50 days in recent months. A new active MJO in the western Indian Ocean is expected to move eastwards, bringing enhanced convection to Indonesia and northern Australia by mid February.
All models now clearly indicate existing La Niña conditions. Most models indicate La Niña conditions continuing through summer and autumn, before easing to neutral conditions by the end of the austral winter 2008. The NCEP synopsis (of 10 January) indicates La Niña has reached a moderate strength (Niño3.4 index below -1.5 °C), and likely to continue into the austral autumn. The IRI synthesis gives moderately strong La Niña conditions, with a 96% probability of La Niña conditions through the next three months and a gradual weakening there after with the probability of neutral ENSO conditions rising to 50% by mid 2008.
Forecast validation: November 2007 to January 2008
A La Niña-like pattern, with a large region of enhanced convection was anticipated from Vanuatu and New Caledonia eastward across Fiji, Wallis & Futuna, Niue, the Southern Cook and Austral Islands to Pitcairn Island, with average or above average rainfall expected in this region. A swath of suppressed convection was anticipated from West Kiribati eastward across the Date Line to the Marquesas, encompassing Tuvalu, Tokelau, Northern Cook Islands, and the Tuamotu Archipelago.
The rainfall outlook for the November 2007– January 2008 period was very similar to what was forecast, the ‘hit’ rate being 66%. Rainfall was lower than expected in New Caledonia.