Climate developments in January 2007
The South Pacific Convergence Zone (SPCZ) was located further north and east of its normal location in January, which is characteristic during an El Niño. Enhanced convection occurred extending from the region north of the Solomon Islands, southeast to Central French Polynesia, including Nauru, Tokelau, and the Northern Cook Islands. The Inter-Tropical Convergence Zone (ITCZ) continued to be more active than normal north of the Equator. A region of suppressed convection and low rainfall existed south of the SPCZ, centred near Niue, extending over Tonga and toward the Southern Cook Islands. Another region of suppressed convection occurred in the Coral, extending over the Solomon Islands.
January rainfall was 25% or less of normal throughout Tonga and Niue, as well as on Lord Howe Island, and Raoul Island. Rainfall was also low, being 50% or less of normal, in parts of New Caledonia (especially in the west), Fiji, and the Marquesas Islands. In contrast, rainfall was above average (at least 150% of normal) in parts of Western Kiribati, the Northern Cook Islands, and Central French Polynesia (as much as 450 mm in 10 days at Tahiti-Faaa) and Southern French Polynesia. Rainfall in the last 8 month period has only been about 50% of normal in parts of New Caledonia.
January mean air temperatures were about 1.0 °C above average north of Fiji, 0.5 °C or more above average throughout much of Tonga and French Polynesia, and 0.5 °C below average throughout much of New Caledonia.
Tropical Southwest Pacific mean sea-level pressures were above average in the North Tasman Sea, and tended to be below average in the tropics areas east of the Date Line, with a -3 hPa anomaly centred near the Cook Islands, enhancing southeasterlies over Tonga. Equatorial surface westerlies were only sporadic in January, occurring in about 5% of observations at Tarawa, 10% less than December, and about 25% less than November.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| Fiji | Nausori | 102 | 28 | Extremely low |
| Niue | Liku | 43 | 17 | Extremely low |
| Tonga | Lupepau’u | 71 | 25 | Well below normal |
| Tonga | Ha’apai | 13 | 8 | Extremely low |
| Australia | Lord Howe Island | 18 | 16 | Well below normal |
| New Zealand | Raoul Island | 20 | 15 | Well below normal |
| Cook Islands | Penrhyn | 680 | 262 | 2nd highest for January |
| French Polynesia | Tahiti-Faaa | 665 | 244 | Well above normal |
Soil moisture in January 2007
Estimates of soil moisture shown in the map 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 January 2007, Fua’amotu and Hanan soils were relatively dry soil while Apia and Tarawa soils were at field capacity (full). Nadi and Rarotonga soil moisture levels were moderate.
El Niño/Southern Oscillation (ENSO)
The current El Niño episode in the tropical Pacific is weakening rapidly. Conditions are expected to return to neutral over the coming months.
Sea surface temperature anomalies near the Date Line and in the eastern Pacific peaked in early December. The NINO3 and NINO4 anomalies were +1.°C and +1.0°C respectively in January (down from +1.5°C and +1.3°C in December), with the respective November–January 3-month means +1.3°C and +1.2°C.
Easterly trade winds returned briefly to near normal strength at the Date Line at the end of December, hastening the decline of equatorial sea temperatures, and contributing to the rapid eastward movement of a cold sub-surface anomaly.
The sub-surface warming along the Equatorial thermocline weakened substantially during January and is now confined mainly to the top 75 metres east of 130°W, whilst the cold anomaly at 150 metres depth has reached 140°W. However, the easterlies weakened again in January, with enhanced Madden-Julian Oscillation (MJO) activity, and this might halt any further immediate decline of tropical Pacific sea surface temperatures.
The Southern Oscillation Index (SOI) showed little change with a value of -0.8 in January (-0.5 in December), with the 3-month mean at -0.5.
OLR and tropical rainfall anomalies continue to show enhanced convection east of the Date Line in the Intertropical Convergence Zone and also, this month, enhanced activity in the South Pacific Convergence Zone at 10°S, 160°W.
The focus of anomalous warmth near the Date Line may still generate increased convection for a month or two, leading to the persistence of the classical OLR-ENSO dipole. However, the intensity of the dipole should weaken as El Niño decays.
Most ENSO forecast models show a persistence of El Niño conditions through the February-April 2007 forecast period, but further weakening to a neutral state by the end of autumn (May). The IRI synthesis of international models gives a 75% chance of maintaining El Niño conditions through the February-April period, but by April-June the probability of neutral conditions (52%) exceeds that of an El Niño (40%). NCEP (8 February statement) predict a transition from weak El Niño conditions to ENSO neutral conditions through March-May 2007.
Forecast validation: November 2006 to January 2007
Suppressed convection, with below average rainfall was expected over Papua New Guinea and New Caledonia, as well as the Tuamotu and Marquesas Islands, with average or below average rainfall from the Solomon Islands southeast to the Southern Cook Islands, including Vanuatu, Fiji, Tonga, and Niue. Enhanced convection and above average rainfall was forecast for Western and Eastern Kiribati, Tuvalu, and Tokelau, with areas of near or above average rainfall over Wallis and Futuna, Samoa, and the Northern Cook Islands, as well as Pitcairn Island. Near average rainfall was expected in the Society and Austral Islands.
Enhanced convection and/or above average rainfall affected Western Kiribati (near and west of the Dateline), extending well east north of the Equator (within the ITCZ Zone). Suppressed convection and/or below average rainfall occurred in the Coral Sea and over New Caledonia, as well as Fiji, Tonga, Niue, the Northern Cook Islands, and Northern French Polynesia. Seasonal rainfall anomalies turned out as forecast for many Islands. Rainfall was lower than expected in Eastern Kiribati, Tuvalu, and Tokelau, the convective region being slightly further north than forecast. The ‘hit’ rate for the November 2006 – January 2007 outlook was about 65%.