Climate developments in April 2008
The South Pacific Convergence Zone (SPCZ) extended from Papua New Guinea southeast to Vanuatu and New Caledonia and southwest of Tonga and Fiji to the northeast of New Zealand, with an overall displaced position much further south and west than normal for April. A large region of very suppressed convection persisted along the Equator extending from Western to Eastern Kiribati and included Tuvalu, the Northern Cook Islands, Tuamotu, Pitcairn Island and the Marquesas Islands. Rainfall was well below average in Nauru and western Kiribati.
Rainfall was well above average in the Solomon Islands, parts of New Caledonia, and Vanuatu as a result of a southwestdisplaced SPCZ. New high monthly rainfall totals were recorded for three stations in New Caledonia, and 1139 mm was recorded at Yate, while Bauerfield, Vanuatu recorded near record rainfall for the month (411.5 mm). New Caledonia also reported an average station average rainfall of 220 % from normal. Rainfall at Falelolo, Samoa (259.9 mm) was the seventh highest recorded since 1956. Norfolk Island also received heavy rainfall (336.4 mm), reporting 240 % of normal for the month.
In contrast, April rainfall was near or below normal over much of Kiribati, French Polynesia, the Northern Cook Islands, the Austral Islands and the Marquesas. Rainfall has been below average for each of the past 11 months in Kiribati. French Polynesia reports this is the eighth consecutive dry month, with 39 - 53% normal rainfall in Tuamotu and Tahiti. Fiji experienced high intensity and frequency rainfall, but overall the month was dry. Very low rainfall (19 % of normal) was also recorded at Brisbane, Australia, with Queensland experiencing the driest summer since 1951.
Tropical Southwest Pacific mean sea-level pressures were below average in the north Tasman Sea and to the south of New Caledonia. This pressure pattern produced more north easterlies with abundant rain into northern New Zealand, and heavy rainfall in New Caledonia and the Solomon Islands.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| New Caledonia | Ouloup | 825.1 | 859 | Record high |
| New Caledonia | La Tontouta | 201.2 | 330 | Record high |
| New Caledonia | Moue | 603.2 | 490 | Record High |
| French Polynesia | Rapa | 599.2 | 258 | Very High |
| Australia | Brisbane | 16.8 | 19 | Very Low |
| Vanuatu | Bauerfield | 411.5 | 198 | Near Record |
Soil moisture in April 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.
French Polynesia reports important soil moisture deficits equal to or greater than – 50 % exist at multiple sites. Soils continued to be moist (at field capacity) for the time of year at Nadi (Fiji), Hanan Airport (Niue), and in Tonga. In the Southern Cook Islands conditions are dry in the north, while wetter in the southern part of the island group.
El Niño/Southern Oscillation (ENSO)
During April, La Niña conditions weakened significantly. In the equatorial Pacific Ocean the east-west dipole is still evident in the temperature anomalies, but the cold pool near the South American coast has weakened due to the presence of a warm water tongue off the coast of Ecuador.
Across the equatorial Pacific, ocean surface temperature anomalies have eased dramatically. The NINO3 (eastern equatorial Pacific) anomaly was around 0.1 °C in April, compared to 1.5 °C in February (3-month mean 0.6 °C), while NINO4 (west-central equatorial Pacific) was near 1.0°C in April (3-month mean around 1.3 °C). SST anomalies are positive near the South American coast. At the subsurface, negative temperature anomalies have all but gone from the top 200 m, while a large positive anomaly remains west of the Date Line.
The near-equatorial trade winds remain enhanced west of the Date Line, but there are positive (westerly) zonal wind anomalies east of the Date Line. Consequently, there is a region of strong subsidence about and just west of the Date Line. The Southern Oscillation Index (SOI), also eased further in April to near +0.5 (3-month mean +1.2).
OLR anomalies show a coherent region of suppressed convection extending from east of Papua New Guinea to eastern Kiribati. The region of enhanced convection that was in the western Pacific has consolidated this month, and intensified to the southeast of Vanuatu and New Caledonia. Overall, the SPCZ is displaced well to the southwest of its normal position.
The TRMM ENSO precipitation index was near –2 for April. The MJO is weak at present, but there is an MJO pulse over the Indian Ocean which may progress eastward over the next couple of weeks.
Most climate models indicate conditions in the ENSO neutral range during May – July, or easing to neutral by the beginning of spring. The NCEP discussion of 10 April indicates La Niña becoming weak but persisting through July. The IRI summary of 16 April gives a 60% chance of La Niña conditions continuing through to June, with a 50% of neutral conditions thereafter.
Forecast validation: February to April 2008
A La Niña-like pattern was expected, with a large region of suppressed convection along the equator encompassing Kiribati and including Tuvalu the Northern Cook Islands, the Marquesas, Tuamotu, and the Society Islands. Near average or below average rainfall was expected for the Solomon Islands and Samoa. Enhanced convection was anticipated along a southwest-displaced SPCZ extending from Vanuatu and New Caledonia eastward across Fiji, Tonga, Niue, the Southern Cook Islands, and the Austral Islands of French Polynesia, with average or above average rainfall.
The rainfall outlook for the February – April 2008 period was very similar to what was forecast, the ‘hit’ rate being 68%, 6% higher than average. Rainfall higher than expected in the Solomon Islands and Samoa.