Climate developments in July 2007
The South Pacific Convergence Zone (SPCZ) affected the Solomon Islands (closely linked to the monsoon trough north of Papua New Guinea) and Vanuatu, with little activity east of the Equator. Convergence was weakly enhanced south of Fiji and over New Zealand’s North Island, due to frequent depressions in the Tasman Sea. An elongated region of suppressed convection persisted along the equator from the region north of the Solomon Islands across Western and Eastern Kiribati and further east (north of the Equator to South America).
Rainfall was about 300% of normal throughout much of Vanuatu. Several parts of Fiji experienced well above average July rainfall, with some significant rain towards the end of the month. Floodproducing rainfall occurred in parts of Vanuatu on 21 July. Rainfall was also well above normal in northern New Zealand, due to floodproducing rains on 10 July, some locations recording more than 400 mm for the month.
July rainfall was 50% or less of normal in parts of Southern French Polynesia, and also below normal over much of Kiribati and New Caledonia. It was extremely dry in parts of Queensland, Australia, some locations recording less than 1 mm for the month.
July mean air temperatures were 1.0 °C or more above normal in parts of Fiji (where there were some record high temperatures, including 33.6 °C measured on Viwa Island on 29 July), as well as parts of Tuvalu, Wallis & Futuna, and the Southern Cook Islands. It was also warmer than usual in Northern French Polynesia. Temperatures were near average in New Caledonia.
Tropical Southwest Pacific mean sea-level pressures were above average over Australia, and generally below average in the equatorial Southwest Pacific from Papua New Guinea to the region east of the Date Line as well as in the Tasman Sea (due to frequent cyclogenesis there).
Equatorial surface easterlies were steady, occurring in about 85% of observations at Tarawa, similar to that of June.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Vanuatu | Pekoa | 233 | 275 | Well above average |
| Vanuatu | Lamap | 273 | 288 | Well above average |
| New Zealand | Whangarei Airport | 456 | 282 | Extremely high |
| Australia | Townsville Airport | <1 | 1 | Extremely low |
| Australia | Brisbane Airport | 1 | 1 | Extremely low |
| New Caledonia | Ile Art | 17 | 14 | Well below average |
| Cook Islands | Rarotonga EWS | 14 | 14 | Record low |
| French Polynesia | Tubuai | 36 | 25 | Extremely low |
Soil moisture in July 2007
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.
At the end of July 2007, Rarotonga soils were dry, Nadi soils were at moderate capacity and Hanan soils were at field capacity.
El Niño/Southern Oscillation (ENSO)
Conditions in the tropical Pacific are currently neutral, but with an incipient La Niña state.
Sea surface temperature (SSTs) anomalies in the equatorial Pacific have become more strongly negative east of 140°W during July, but the monthly anomaly fi eld shows only a weak La Niña pattern, with a ‘cold tongue’ evident near the South American coast and a partial ‘warm horseshoe’ in the extratropics of both hemispheres (also evident in SSH anomalies).
The NINO3 anomaly was around –0.4°C in July (May–July average –0.3) and the NINO4 anomaly was +0.3 °C (May–July mean also +0.3 °C).
Equatorial zonal wind anomalies have been variable in July and the Southern Oscillation Index (SOI) fell to –0.4, suggesting no clear atmosphere–ocean coupling as yet.
Tropical OLR anomalies show an east-west dipole from the Indonesian region to near the Date Line, somewhat suggestive of a La Niña convective anomaly and the TRMMbased ENSO precipitation index is –1.6, also suggestive of La Niña.
Equatorial subsurface temperatures for July showed a signifi cant negative anomaly in the top 150 metres in the eastern Pacific, although this feature appeared quite weak in the last few days of the month.
The amplitude of the Madden-Julian Oscillation is currently very weak.
There has been a general weakening in ENSO forecasts since June, though around half the dynamical models still show apparent La Niña conditions through the rest of 2007.
The Scripps model indicates a warm event developing during the southern summer and autumn, but differs from the other models
The NCEP synopsis suggests ENSO-neutral conditions at first with a 50% chance of La Niña conditions during spring.
The IRI synthesis continues to give a probability of 55% for a La Niña by November. The probability of El Niño conditions re-emerging during the forecast period remains below 10%.
Forecast validation: May to July 2007
Enhanced convection and above average rainfall was expected over the Solomon Islands and also the Southern Cook Islands, with near or above average rainfall in Papua New Guinea, Wallis and Futuna, Samoa, the Northern Cook Islands, central and southern French Polynesia, and Pitcairn Island. Suppressed convection and below average rainfall was expected over Eastern Kiribati, with near or below average rainfall in Western Kiribati, Tuvalu, and the Marquesas Islands. Near average rainfall was expected elsewhere in the region.
A region of weakly enhanced convection and above average rainfall existed over Samoa (as expected), while suppressed convection and below average rainfall occurred in the equatorial region about and east of the Date Line, including Western Kiribati (as expected). Rainfall was below average (and lower than expected) in Wallis and Futuna and the Austral Islands, and higher than expected in Vanuatu. The ‘hit’ rate for the May–July 2007 rainfall outlook was about 75%.