Climate developments in March 2008
The South Pacific Convergence Zone (SPCZ) extended from Vanuatu and New Caledonia eastward across Tonga and Fiji to the Southern Cook Islands, with an overall displaced position further south and west than normal for March. Alarge region of very suppressed convection persisted along the Equator extending from Western to Eastern Kiribati and included Tuvalu, the Northern Cook Islands, Tuamotu, and the Marquesas Islands. Rainfall was well below average in Nauru.
Rainfall was well above average in parts of New Caledonia, and Vanuatu as a result of a southwest-displaced SPCZ. New high monthly rainfall totals were recorded for Niue, whileAneityum, Vanuatu recorded one of the highest rainfall readings for March in the last six years (507.1 mm). New Caledonia also reported an east coast station average rainfall of 180 % from normal, with rainfall exceeding 1 meter in the mountains.
In contrast, March rainfall was near or below normal over much of Kiribati, French Polynesia, the Northern CookIslands, the Austral Islands and the Marquesas. Rainfall has been below average for each of the past 10 months in Kiribati. Suva, Fiji experienced the third driest month ever recorded in the instrumental period, with monthly rainfall totals between 25 – 75% normal. A near-record low rainfallwas also recorded at Townsville, Australia.
Australia also experienced heatwave conditions in March, with six consecutive days >39 ºC in Melbourne, and 15 consecutive days >35 ºC in Adelaide. March mean airtemperatures were above average in New Zealand, and were 0.5 ºC or more above normal in New Caledonia and Fiji.
Tropical Southwest Pacific mean sea-level pressures were below average in the north Tasman Sea and to the west of New Caledonia. This pressure pattern produced more northeasterlies with abundant rain into northern New Zealand, and heavy rainfall in New Caledonia and parts of northeastern Australia.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Niue | Hanan Airport | 364.3 | 174 | Record high |
| Niue | Liku | 368.1 | 161 | Record high |
| Australia | Townsville | 1.8 | 1 | Extremely low |
| New Caledonia | La Roche | 420.8 | 178 | High |
| Fiji | Nausori | 134.1 | 145 | High |
Soil moisture in March 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.
El Niño/Southern Oscillation (ENSO)
During March, the La Niña event that persisted from previous months has reached maturity, and is now showing signs of weakening. The Southern Oscillation Index (SOI) decreased markedly this past month, indicating deterioration of the event is underway.
Below normal sea surface temperatures (SSTs) extendacross most of the equatorial Pacific, with anomalies of –1.0 °C or lower over the central equatorial region (Date Line to 120 °W, approximately). The oceanic component of La Niña has lost strength over the last few weeks as indicated by the weakening of the cold anomalies and the strengthening of a significant warm anomaly along the South American Coast. The NINO3 anomaly has weakened to –0.4 °C in March (down from –1.5 °C in February) and NINO4 was –1.4 °C (down from –1.6 °C). In the equatorial subsurface Pacific Ocean, the east-west dipole is still evident in the temperature anomalies but the cold pool near the South American coast has contracted considerably since last month due to the presence of warmer surface waters along the South American coast. The 0–300m heat content west of the Date Line has increased further since February.
The trade winds have remained enhanced in March west of the Date Line along the equatorial Pacific, and have all but disappeared near the South American coast. The SOI dropped during March to +1.1 (down from +2.1 in February).
OLR anomalies show an east-west dipole with enhanced convection over Indonesia and suppressed convection from the near Papua New Guinea to east of the Date Line. The region of enhanced convection that was in the western Pacific has continued to shift westward and intensified during the past month. The SPCZ has shifted well to the southwest of its normal position.
The TRMM ENSO precipitation index was –1.3 in March. A moderate MJO episode is evident near Indonesia and is expected to spread into the equatorial Pacific in mid-to-late April.
Most models indicate La Niña conditions easing to neutral by the end of austral winter. The NCEP discussion (of 6 March) indicates La Niña is likely to continue through April–June (though “weaker”). Some of the models suggest La Niña could continue into spring. The IRI synthesis (19 March) suggests aweakening of La Niña conditions during 2008 and a 50% chance of neutral conditions towards the latter part of 2008.
Forecast validation: January to March 2008
A La Niña-like pattern was expected, with a large region of suppressed convection along the equator from WesternKiribati, 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 Tokelau. Enhanced convection was anticipated along a southwest-displaced SPCZ extending from Vanuatu and New Caledonia eastward across Fiji, Wallis & Futuna, Niue, Pitcairn Island, and the Southern Cook and Austral Islands, where average or above average rainfall was expected.
The rainfall outlook for the January– March 2008 period was very similar to what was forecast, the ‘hit’ rate being 68%. Rainfall was lower than expected in northern New Zealand, and Tokelau, and higher than expected in Samoa.