Climate developments in March 2007
Outgoing Long-wave Radiation anomalies for March 2007. (Click for enlargement and detail)
The South Pacific Convergence Zone (SPCZ) was located near its normal location in March, with enhanced convection extending from the region south of the Solomon Islands east across Fiji and Samoa to central and southern French Polynesia, including Tonga, Niue, and the Southern Cook Islands. A double Inter-Tropical Convergence Zone (ITCZ) pattern existed in March, both north and south of the Equator. A region of suppressed convection and low rainfall existed north of the SPCZ, affecting Nauru, Eastern Kiribati, and the Northern Cook Islands.
Rainfall was extraordinary in Niue. The Liku March total of 1024 mm was the highest for any month in the Niue rainfall records which commenced in 1905. There were 23 raindays at Liku, and 26 rain-days at Hannan Airport. Rainfall was at least 200% of normal in parts of Tonga, the Southern Cook Islands and Fiji, and at least 125% of normal in parts of Vanuatu, Western Kiribati, and in Northland, New Zealand (where severe flooding occurred). Nadi, Fiji recorded 29 days with rainfall during the month. Three lives were lost due to floods in parts of Fiji.
March mean air temperatures were 0.5 °C or more above average throughout much of Vanuatu, Tuvalu, Tonga, and French Polynesia.
Tropical Southwest Pacific mean sea-level pressures were above average northeast of New Zealand, with high pressure extending toward the region south of Niue. Pressures were below average over Australia, and also in equatorial areas especially east of the Date Line. Equatorial surface easterlies occurred in 97% of observations at Tarawa, the highest frequency since April 2006.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Niue | Hanan Airport | 732 | 350 | Highest for March |
| Niue | Liku | 1024 | 449 | Highest for any month |
| Tonga | Salote Airport | 462 | 203 | Well above average |
| Tonga | Lupepau’u | 728 | 243 | Extremely high |
| Cook Islands | Rarotonga EWS | 436 | 256 | Extremely high |
| Cook Islands | Rarotonga Airport | 470 | 277 | Well above average |
| New Zealand | Whangarei Airport | 310 | 244 | Extremely high |
Soil moisture in March 2007
Estimated soil moisture conditions at the end of March 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 March 2007, Tarawa, Apia, Nadi, Nuku’alofa, Hanan, and Rarotongan soils were at fi eld capacity (full).
El Niño/Southern Oscillation (ENSO)
Sea surface temperature anomalies (°C) for March 2007. (Click for enlargement)
Mean sea surface temperatures (°C) for March 2007. (Click for enlargement)
The tropical Pacific is showing some signs of a move towards La Niña, but signals are mixed, suggesting the usual level of uncertainty for the time of year.
SSTs remain above normal to the west of the Date Line, but there is some development of an enhanced "cold tongue" in SSTs off the South American coast: the NINO3 anomaly was near zero in March (January–March average around +0.5°C) while the NINO4 anomaly was +0.5°C in March (January–March mean around +0.6°C).
The Southern Oscillation Index (SOI) is still slightly negative, but in the neutral range.
There is a strong negative anomaly (below -3°C) in the equatorial Pacific Ocean subsurface temperature field centred near 120°W and 100m depth. It is uncertain whether the cold water will remain in place, move eastward, or advance westward.
The easterly trade winds have been slightly stronger than normal during the month of March. OLR and tropical rainfall anomalies for March indicate enhanced convection in the SPCZ and an apparent double-ITCZ structure east of the Date Line. Convection has also been enhanced over Western Australia and Indonesia.
The Madden-Julian Oscillation is presently weak, with the main centre of action over the western Pacific.
Most models show neutral ENSO states for the next 6–9 months, with one or two on the warm side and a few on the cold side.
The NCEP synopsis suggests a possible transition to La Niña conditions over the next 2–3 months, while the IRI synthesis gives a probability of 50% for a La Niña by mid-year. An El Niño is deemed unlikely.
A World Meteorological Organisation ENSO update on 30 March suggested a substantial possibility of La Niña a development within the next 6 months.
Forecast validation: January to March 2007
A region of suppressed convection and below average rainfall was expected over Vanuatu and New Caledonia, with average or below average rainfall affecting Fiji and Tonga. A large region of enhanced convection and above average rainfall was expected over Western and Eastern Kiribati, the Solomon Islands, Tuvalu, and Tokelau, and near or above average rainfall over Wallis and Futuna eastsoutheastwards to Pitcairn Island, including the Northern and Southern Cook Islands, and French Polynesia. Near average rainfall was expected elsewhere.
A large region of enhanced convection occurred along the equator in the Western South Pacifi c. Enhanced convection also occurred, associated with the SPCZ, occurred over Fiji and Samoa, as well as the Southern Cook Islands, and central and southern French Polynesia. Suppressed convection occurred over Northern French Polynesia. Rainfall was higher than expected in Niue, and lower than expected in Tuvalu, Eastern Kiribati and northern French Polynesia. The ‘hit’ rate for the January – March 2007 rainfall outlook was about 60%.