Climate developments in October 2007
The South Pacific Convergence Zone (SPCZ) was active,extending from Papua New Guinea toward Fiji and further east toward central French Polynesia. It also remained more southwest than normal for the time of year. An elongated region of suppressed convection continued to persist along the equator affecting Nauru, Western and Eastern Kiribati,and the region further east (north of the Equator) to the coast of South America. Suppressed convection also extended to Tuvalu.
Rainfall was extremely high in areas under the active SPCZ with over 200% or more of normal in parts of Vanuatu, Fiji,central French Polynesia, and also well above normal in parts of New Caledonia, Niue, and parts of Samoa. Rainfall totalling 150–200 mm occurred in parts of New Caledonia over 22–23 October. Heavy rainfall and flooding occurred in parts of Vanuatu at the end of the month, Aneityum recording 375 mm over the 30–31 October. In contrast rainfall was 50% or less of normal over much of Kiribati and parts of the Cook Islands. Rainfall was near normal in most other regions.
October mean air temperatures were 1.5 ºC or more above normal in parts of Tonga and the Southern Cook Islands, and 1.0 ºC or more above normal in New Caledonia and parts ofFiji (the warmest October on record at Nadi, with records at several other sites), Fua’amotu, and Rarotonga. Temperatures were also above normal in Vanuatu and Samoa.
Tropical Southwest Pacific mean sea-level pressures were below average about and west of the Date Line, especially in the Vanuatu–Fiji region. Higher than normal pressures occurred over the north Tasman Sea, and southern French Polynesia in the east.
Equatorial surface easterlies have strengthened at Tarawa since September, occurring in almost 95% of observations.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Vanuatu | Lamap | 319 | 280 | Well above average |
| Vanuatu | Aneityum | 444 | 467 | Extremely high |
| New Caledonia | Poindimie | 226 | 343 | Well above average |
| Fiji | Nausori Airport | 467 | 228 | Well above average |
| Fiji | Ono-I-Lau | 226 | 262 | Well above average |
| Niue | Hanan Airport | 562 | 300 | Extremely high |
| Niue | Liku | 424 | 291 | Extremely high |
| French Polynesia | Bora Bora | 308 | 305 | Extremely high |
| French Polynesia | Takaroa | 468 | 407 | Well above average |
| Country | Location | Mean temp.(°C) | % of average | Comments |
|---|---|---|---|---|
| Fiji | Nadi Airport | 26.6 | +1.2 | Highest on record |
| Tonga | Fua’amotu Aero | 24.9 | +2.4 | Highest on record |
| Cook Islands | Rarotonga Airport | 25.0 | +1.6 | Highest on record |
Soil moisture in October 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 October 2007 soils were dry (for the 4th consecutive month) at Rarotonga (Southern Cook Islands). In contrast, soils continued to be moist (at field capacity) for the time of year at Hanan (Niue). Soil moisture was moderate at Nadi (Fiji).
El Niño/Southern Oscillation (ENSO)
The La Niña event is now well established in the central and eastern Pacific, where moderate conditions are prevailing in the ocean. These are more consistent in the ocean than in the atmosphere.
Sea surface temperatures (SSTs) in the tropical Pacific now exhibit a clear La Niña pattern, with a well- developed cold anomaly extending from the South American coast to the Date Line, (extending much further west than during September) whilst a warm ‘horseshoe’ extends from tropical to temperate latitudes in both hemispheres. This is also very evident in the sea surface height anomalies. The NINO3 anomaly was –1.3 °C in October (ASO average–0.8) and the NINO4 was –0.4 °C for October(ASO –0.1 °C). Sub surface equatorial temperature anomalies show strong negative anomalies(–3 °C), reinforcing the surface cold tongue. Positive anomalies (+1 °C) in the same layer occurred in the western Pacific.
The trade winds remained enhanced both west and east of the Date Line. The SOI in October was still in the neutral range (+0.4) with an ASO average of +0.2.
Tropical OLR anomalies continue to show suppressed convection in the equatorial region from 150° E eastwards, in the region of the Intertropical Convergence Zone. Farther west there is no clear region of enhanced convection. The TRMM-based ENSO precipitation index was 0.8 in October, in the weak La Niña range. The SPCZ was active and again displaced southwest of its normal position. The Madden Julian Oscillation continues to be weak.
Most models indicate weak–moderate La Niña conditions into early 2008, easing during March to May. The dynamical models give stronger projections of strengthening La Niña conditions, with the event dissipating at the normal time of year from March to May.
The NCEP synopsis suggests La Niña conditions will continue into early 2008. Based on the latest observations and forecasts, the IRI synthesis gives aprobability of 90% maintaining La Niña conditions over the coming three months. The probability of El Niño conditions re-emerging during the forecast period remains at or below 10% until mid 2008, and the probability of returning to ENSO-neutral much below 50% until early 2008.
Forecast validation: August to October 2007
Enhanced convection and above average rainfall wereexpected over Samoa, with a large area of near or above average rainfall forecast from Papua New Guinea southeastto southern French Polynesia, including the Solomon Islands,Vanuatu, New Caledonia, Fiji, Tonga, Wallis and Futuna,Niue, and the Cook Islands.
Suppressed convection with below average rainfall was expected over Western and Eastern Kiribati and Tuvalu,with near or below average rainfall on Pitcairn Island. Near average rainfall was expected elsewhere.
The rainfall outlook for the August–October 2007 period was very similar to what was forecast, the ‘hit’ rate of 90%, easily the highest since the commencement of the Island Climate Update!
Suppressed convection and below average rainfall occurred as expected in the equatorial region about and east of the Date Line, including Western and Eastern Kiribati, and the Northern Cook Islands. Rainfall was above average as expected from Papua New Guinea to Vanuatu, including New Caledonia, Fiji, Niue, and central French Polynesia.Tuvalu rainfall was near average and higher than expected.