Climate developments in July 2008
The South Pacific Convergence Zone (SPCZ) exhibited diminished activity in July relative to previous months. Only a small region of enhanced rainfall was observed in satellite data during July 2008, centred near Papua New Guinea and the Solomon Islands. A region of suppressed convection expanded along the Equator relative to last month, and extends from Western Kiribati to Eastern Kiribati including Tuvalu, Tokelau, and the Northern Cook Islands. The regional circulation was characterised by more frequent anticyclones to the northeast of New Zealand and lows to the southwest of Pitcairn Island.
Rainfall was well above average in northern New Zealand and in northeastern Australia. A near-high monthly rainfall total was recorded at Townsville, with 95 mm (680% of normal) while a rainy end of the month created 150–200% normal rainfall in the northern North Island of New Zealand. Honiara and Henderson, Solomon Islands, also received significant rainfall, recording 204 mm and 188 mm for July, respectively.
French Polynesia experienced low rainfall totals in the northeastern island groups, with Bora Bora recording 47% normal rainfall and 76% of normal recorded in Tuamotu. In Tahiti, 66 mm of rain fell in two days, and this accounted for 94% of the monthly total. In New Caledonia, many locations also had below normal rainfall, with many stations recording 25–75% of normal precipitation. Record low rainfall was recorded at Lupepau’u, Tonga (5 mm, 5% of normal), and very low rainfall also occurred across the rest of the country and to the east in Niue. Low rainfall also occurred in the Southern Cook Islands, with 30 mm (29% of normal) recorded at Rarotonga Airport.
Fiji also had a drier than normal conditions, due to deterioration and northward displacement of the SPCZ coupled with ridges of high pressure that frequently occurred during the month. Parts of the country received 28 - 70% of normal rainfall during July; however the western parts of Viti Levu and the outer of island of Rotuma experienced above normal to well above normal rainfall (140 to 249% of normal).
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Tonga | Lupepau’u | 5 | 5 | Record low |
| Niue | Hanan | 26 | 22 | Very low |
| New Caledonia | Ouanaham | 29 | 27 | Very low |
| Vanuatu | Lamap | 31 | 33 | Very low |
| Australia | Townsville | 95 | 680 | Very high |
| New Zealand | Whangarei | 300 | 185 | Very high |
Soil moisture in July 2008
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 there are still important soil moisture deficits in the Marquesas Islands. Nadi (Fiji) projects dry soil moisture conditions which is a contrast to last year at this time. Soils continued to be moist (at field capacity) for the time of year at Hanan Airport (Niue), while below normal soil moisture was recorded at Rarotonga Airport.
El Niño/Southern Oscillation (ENSO)
During July, ENSO conditions continued to remain near neutral in the equatorial Pacific. Overall the tropical Pacific temperatures are close to neutral, although some remnants of La Niña persist west of the Date Line. A warm water tongue off the coast of Ecuador that developed last month is still prominent.
Across the equatorial Pacific, ocean surface temperature anomalies have continued to ease from previous months. The SST anomaly in the NINO3 region rose sharply in July to around +0.9°C (3-month mean +0.5°C), while the NINO4 anomaly remains slightly negative at –0.2°C (3-month mean around –0.5°C). At the subsurface, a region of strong positive anomalies persists in the top 100 m near the South American coast, but the positive anomalies below 100 m depth in the western Pacific have all but died out.
The near-Equatorial trade winds are slightly enhanced about and west of the Date Line but are near normal elsewhere. The OLR anomaly field has lost the La Niña signature it displayed in past months and lacked coherence in the Pacific in July, though convection remains weakly suppressed near the Date Line. However, the TRMM ENSO precipitation index is still around –1, suggesting La Niña-like rainfall anomalies persist in the Equatorial Pacific. The Madden - Julien Oscillation remains weak at present.
All ENSO models except one (UKMO) indicate neutral conditions until the end of October. The LDEO5 model shows a warming over the austral summer, but all other models remain in the neutral range through to austral autumn 2009. The NCEP discussion of 10 July indicates ENSO-neutral conditions continuing through November. The IRI summary of 16 July projects a 75% chance of ENSO-neutral conditions continuing through September (15% chance of El Niño, 10% for La Niña). The Australian weekly tropical summary of 22 July suggests ENSO-neutral conditions persisting through the November and into 2009.
Forecast validation: May to July 2008
A La Niña-like pattern was expected, with a large region of suppressed convection forecast in the southwest Pacific, encompassing the Northern Cook Islands, Tuamotu, the Society Islands, the Austral Islands, and the Marquesas. Average rainfall was expected for New Caledonia and Samoa. Enhanced convection was forecast from Papua New Guinea extending southeastward toward Vanuatu, through to the Southern Cook Islands, including Fiji, Niue, and Tonga. Above average rainfall was expected in those countries for May–July period. No clear precipitation guidance was offered for Eastern Kiribati, Pitcairn Island, the Solomon Islands, and Wallis & Futuna.
The rainfall outlook for the May–July 2008 period was very similar to what was forecast, the ‘hit’ rate being 61%, 8% higher than average. Rainfall totals were overestimated for New Caledonia and the Southern Cook Islands, and slightly overestimated for the southwestern fringe of French Polynesia.