Climate developments in May 2007
Outgoing Long-wave Radiation anomalies for May 2007. (Click for enlargement and detail)
The South Pacific Convergence Zone (SPCZ) extended from Papua New Guinea to the Society Islands, and was slightly further north than average west of the Date Line, and a little further south than average well east of the Date Line. The SPCZ was quite active over Samoa. A region of suppressed convection occurred over Vanuatu and New Caledonia, extending into the Tasman Sea and toward Fiji.
Rainfall was above average (at least 150% of normal) in several parts of Fiji and American Samoa, and at least 125% of normal in parts of Northern French Polynesia. May rainfall was extremely low being 25% or less of normal in the Kermadec Islands, and also below average (75% or less of normal) in parts of New Caledonia, Western Kiribati, and the Austral Islands.
May mean air temperatures were about 1.0 °C or more above average in Southern Tonga, Niue, the Southern Cook Islands, and parts of Central and Southern French Polynesia, and at least 0.5 °C above average throughout Tuvalu. New Zealand recorded its warmest May since reliable measurements commenced in the 1860s, and all eastern Australian States also recorded their warmest May on record.
Tropical Southwest Pacific mean sea-level pressures were above average in the sub-tropics from the North Tasman Sea to the region west of South America, and slightly below average in the tropics about and east of the Date Line.
Equatorial surface easterlies occurred in 67% of observations at Tarawa, a significant decrease from 97% during April.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Fiji | Ono-i-Lau | 285 | 272 | Well above average |
| Fiji | Udu Point | 307 | 184 | Well above average |
| New Zealand | Raoul Island | 28 | 22 | Record low |
| New Zealand | Kaitaia | 29 | 24 | Record low |
| New Zealand | Auckland Airport | 12 | 13 | Record low |
Soil moisture in May 2007
Estimated soil moisture conditions at the end of May 2007, 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. 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 with 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.
At the end of May 2007, Tarawa was experiencing dry soil conditions whilst wet soil conditions prevailed at Rarotonga and Hanan sites. Nadi soils were at moderate capacity.
El Niño/Southern Oscillation (ENSO)
Sea surface temperature anomalies (°C) for May 2007. (Click for enlargement)
Mean sea surface temperatures (°C) for May 2007. (Click for enlargement)
Conditions in the tropical Pacific are currently neutral, although they are suggestive of a slow development towards La Niña.
The pattern of sea surface temperature (SSTs) anomalies is beginning to resemble La Niña conditions with colder than average waters in the far eastern Pacific and slightly warmer than average waters in the western Pacific. SSTs remain slightly above normal to the west of the Date Line, and then below average SSTs extend from 140°W. to the South American coast.
The NINO3 anomaly was –0.3°C in May (–0.1°C in April, and also for March–May average) while the NINO4 anomaly was +0.3°C in May (March–May mean around +0.4°C). Thus, the surface temperatures are decreasing, but only slowly.
The Southern Oscillation Index (SOI) has remained almost constant, being slightly negative although in the neutral range at –0.3 for May. Subsurface data for May continues to show a prominent negative anomaly in the top 150 metres between the Date Line and the South American coast.
The complication is that there have been anomalous westerly winds west of the Date Line during April and May, which are likely to have constrained upwelling of the cool subsurface waters.
Outgoing longwave radiation (OLR) and tropical rainfall anomalies for May indicate suppressed convection along the equator in the Pacific.
The ENSO Precipitation Index of –0.40 indicates weak cold conditions. The global sea surface height (SSH) anomaly shows a classic La Niña pattern with lower SSH anomalies from the Date Line east, surrounded by a horseshoe of higher SSH anomalies in the western Pacific.
The Madden-Julian Oscillation (MJO) is presently weak.
The dynamical models show a transition to La Niña conditions over the next 3 months, with this persisting for the remainder of the year, whilst the statistical models project neutral ENSO states for the next 6–9 months.
No ENSO forecast model retains warm conditions during the remainder of the year. The National Center for Environmental Predication (NCEP) synopsis suggests a transition from ENSO-neutral to La Niña conditions is possible within the next 3 months, while the International Research Institute for Climate Society (IRI) synthesis gives a probability of 55% for a La Niña by mid-year. The probability of El Niño conditions re-emerging during the forecast period remains at or below 5%.
Forecast validation: March to May 2007
Suppressed convection was expected over Tuvalu and the Marquesas Islands, with near or below average rainfall. Enhanced convection and near or above average rainfall was expected over Western Kiribati, the Solomon Islands, and Vanuatu, as well as Samoa, the Society Islands, and Pitcairn Island. Near average rainfall was forecast elsewhere in the region.
A large region of enhanced convection and/or above average rainfall extended from the Solomon Islands southeast over Samoa and toward the Southern Cook Islands, including Fiji, Tonga, Samoa, and Niue. Much of this above average region was further east and hence many islands in this region were wetter than expected. Below average rainfall occurred over Eastern Kiribati, the Northern Cook Islands, and Northern French Polynesia. Rainfall was lower than expected in Eastern Kiribati and the Austral Islands. The ‘hit’ rate for the March – May 2007 rainfall outlook was just over 60%.