Monthly climate
Climate developments in May 2008
Outgoing Long-wave Radiation anomalies for May 2008. (Click for enlargement and detail)
The South Pacific Convergence Zone (SPCZ) extended from Papua New Guinea southeast to northern Vanuatu, across Fiji and Niue to the Southern Cook Islands and Austral Islands, with an overall displaced position southwest of normal for May. A large region of very suppressed convection persisted along the Equator extending from Western Kiribati and included Nauru, Tuvalu, Tokelau, the Northern Cook Islands, and Eastern Kiribati.
Rainfall was well above average in parts of New Caledonia, Vanuatu, Tonga, and Niue due to a southwest-displaced SPCZ. New high monthly and near-record rainfall totals were recorded at Fua’amotu Airport, Tonga, with 343.5 mm(254% of normal), and Maota, Samoa with 584.6 mm (257% of normal). Fiji stations had high rainfall totals ranging from 150 – 225% of normal. Very high rainfall and flooding also occurred in Efate, Vanuatu in late May.
Solomon Islands rainfall varied across the country during the past month. Honiara and eastern Kira Kira had above normal, and Lata and Taro below normal rainfall. Aitutaki reported near-record low monthly rainfall, whle Rarotonga Airport was above normal, indicating a north-to-south split in rainfall over the Southern Cook Islands.
Near or below normal rainfall fell over much of French Polynesia, Tuamotu, the Northern Cooks and all of Kiribati. Rainfall has been below average for the past 11 months in Western Kiribati. Record low rainfall was recorded at Nui Island, Tuvalu (20% of normal). French Polynesia reports this is the ninth consecutive dry month, with 72% of normal rainfall recorded in Northern Tuamotu and the Society Islands. Very low rainfall (2% of normal) was also recorded at Townsville, Australia, and a monthly record low (4 mm, 3% of normal) occurred in Sydney. The Australia Bureau of Meteorology reports this was the driest May since 1900.
Tropical Southwest Pacific mean sea-level pressures were below average to the northeast of New Zealand and above average over eastern Australia, producing more south westerly winds across the southwestern south Pacific than in previous months.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Tonga | Fua’amotu | 343.5 | 254 | Record high |
| Samoa | Maota | 584.6 | 257 | Very high |
| Niue | Liku | 353.7 | 218 | Very high |
| French Polynesia | Bora Bora, Motu | 26 | 23 | Very low |
| Australia | Sydney | 3 | 3 | Record low |
| Tuvalu | Nui Island | 43.7 | 20 | Record low |
Soil moisture in May 2008
Estimated soil moisture conditions at the end of May 2008, using monthly rainfall data. (Click for enlargement)
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 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)
Sea surface temperature anomalies (°C) for May 2008. (Click for enlargement)
Mean sea surface temperatures (°C) for May 2008. (Click for enlargement)
During May, La Niña conditions have continued to weaken and overall the tropical Pacific is close to neutral, although some remnants of La Niña persist. 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 continued to ease from previous months. The NINO3 anomaly became positive during May at +0.2 °C (3-month mean of +0.1 °C), a progressive warming from 1.5 °C in February 2008. NINO4 remains below normal at 0.8 °C in May (3-month mean around 1.0 °C), but this negative anomaly at the surface does notextend below about 100m. A positive sub-surface temperature anomaly persists west of the Date Line, and a new positive anomaly has developed near South America above 100m.
The near-equatorial trade winds have weakened and at the end of May were near normal around and westof the Date Line, with positive (westerly) zonal wind anomalies east of about 160 °W. Convection was enhanced over the Philippines during May and the SPCZ was active from Papua New Guinea through Fiji, slightly south of its normal position. The Southern Oscillation Index (SOI), also shifted to a negative value in May for the first time in ten months, and is – 0.5 (3-month mean +0.4).
OLR anomalies show a coherent region of suppressed convection (but weaker than in previous months) extending from 130 °E across the date line toward the Northern Cook and Marquesas Islands. The area of relatively enhanced convection last month along the SPCZ is more consolidated in a narrow band across the Solomons and Vanuatu. In the past monthm the region to the east of the Date Line around the equator had the most visible OLR changes from the previous month. Overall, the SPCZ is displaced to the southwest of its normal position.
The TRMM ENSO precipitation index was near –1 for May (weakening from a strong La Niña value of –2 in April). The MJO strengthened during May, but current projections suggest little enhancement of subtropical convection east of the Date Line.
Most climate models indicate ENSO neutral conditions continuing through to the end of 2008. The NCEP discussion of 8 May indicates a transition from La Niña to ENSOneutralconditions during June/July. The IRI summary of 19 May suggests a 60% chance of a return to ENSO-neutral conditions coming three-month period, with only a 35% chance of La Niña returning. The Australian weekly tropicalsummary of 27 May suggests ENSO-neutral conditions now exist, but with a few weak La Niña signals still present.
Forecast validation: March to May 2008
A La Niña-like pattern was expected, with a large region of suppressed convection forecast along the equatorial Pacific from Western to Eastern Kiribati, including the Solomon Islands, Tuvalu, the Northern Cook Islands, the Tuamotu archipelago, the Marquesas, and the Society Islands. Average rainfall was forecast for Papua New Guinea, Samoa and Tokelau. Enhanced convection was forecast from Vanuatu through to Pitcairn Island, including New Caledonia, Fiji, Wallis & Futuna, Tonga, Niue, and the Southern Cook Islands, with above average rainfall expected. Near or above average rainfall was forecast for the Austral Islands of FrenchPolynesia, which had a prominent northeast-southwest disparity in the rainfall distribution.
The rainfall outlook for the March–May 2008 period was very similar to what was forecast, the ‘hit’ rate being 68%, 6%higher than average. Rainfall higher than expected occurred in parts of the Solomon Islands and in Samoa.