Climate developments in August 2006
During August, convection associated with the SPCZ has been concentrated in the region west of the Date Line; a pattern that has been evident since June 2006. This month much of the SPCZ activity occurred over the Solomon Islands. There were weak regions of enhanced convection over Vanuatu and Tuvalu. A region of weakly suppressed convection affected the Northern Cook Islands and region north of Tokelau.
Exceptionally high rainfall occurred in parts of Niue. Wet conditions persisted in the Southern Cook Islands and parts of central French Polynesia, where rainfall was 150 to 200% of average. Rainfall was near or above average in New Caledonia (much of the rainfall occurring at the start of August). There were 23 days with rainfall at Nausori Airport in Fiji. Rainfall was 75% or less of average throughout southern Tonga.
Mean air temperatures were about 1.0 °C above average throughout much of Tonga, and about 0.5 °C above average in northern French Polynesia. In contrast, they were about 0.5°C or more below average in parts of New Caledonia,where it has been now been cooler than usual for three months.
Tropical Southwest Pacifi c mean sea-level pressures were above average over Australia, but below average over much of the tropical region east of about Eastern Kiribati and Samoa. Equatorial surface westerlies occurred in 28% of observations at Tarawa, the highest frequency of occurrence there since 40% in February 2005.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Niue | Liku | 305 | 272 | Highest |
| Australia | Townsville | Nil | 0 | Lowest |
| Tonga | Salote Pilolevu Airport | 19 | 19 | Well below average |
| Tuvalu | Funafuti | 466 | 201 | Well below average |
Soil moisture in August 2006
Estimates of soil moisture shown in the map (above) 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 August 2006, Apia, Hanan Airport, Fua’motu and Rarotonga were at field capacity (full). Nadi soil moisture levels were moderate.
El Niño/Southern Oscillation (ENSO)
Current atmospheric and oceanic conditions in the equatorial Pacific Ocean are consistent with a developing phase of warm ENSO event. Monitoring of the atmospheric and oceanic ENSO indicators over next few months will be critical in defi ning this event.
There has been continued warming in the upper layers of the equatorial Pacifi c Ocean over the past three months. Equatorial Pacifi c sea surface temperature (SST) anomalies are now positive across the whole basin, with anomalies in excess of +1°C in places. The NINO3 SST anomaly for August was around +0.6°C (+0.5°C for June–August) and NINO4 was around +0.9°C (+0.7°C for June–August).
Subsurface temperatures show a more coherent structure of warm anomalies this month from the Date Line eastward centred at about 100m depth. The 5-day subsurface temperature ending 30 August 2006 has a warm anomaly exceeding +3°C at 100m depth from 145-115°W.
The Southern Oscillation Index (SOI) has remained consistently negative during the past 3 months, with a value of –1.8 for August, and -1.1 for the June-August mean. There have been signifi cant westerly wind anomalies along the Equator in July and August, west of the Date Line.
Madden-Julian Oscillation (MJO) and Subtropical highs
The Madden–Julian Oscillation (MJO) has not been very active of late, but the Australian Bureau of Meteorology suggests a gradual eastward movement of MJO-associated weather in the western Pacifi c in early September.
A number of models increase SST anomalies (NINO3.4) further to around +0.8°C or above by January 2007, but with an equally large group showing no warming trend, although with positive SST anomalies.
The UK Met Office and Scripps models are warm enough to qualify at the border of a weak El Niño for the coming season. Two of the models, ECMWF and NCEP/CFS, show substantially larger warming by the end of 2006 in their latest runs.
The NCEP synopsis is for weak El Niño conditions likely by the end of 2006.
The IRI (who have a higher threshold for an El Niño) indicate neutral ENSO conditions are favoured with a 60% likelihood for the remainder of 2006, and a 45% probability of El Niño by year-end. However, more recent model forecasts suggest that the probability of El Niño will be increased on the September outlook.
The Australian Bureau of Meteorology ENSO wrap-up (30- August) warns of an “increasing risk of an El Niño”. They comment that most of the ENSO models will not have factored in the (warming) developments that have taken place during August, but also note that sustained warming from August is unusual climatologically, with El Niño events usually beginning to evolve between March and June.