Climate developments in December 2006
Weakly enhanced convection occurred in the Southwest Pacific during December, from the north of Vanuatu east to Northern Tonga, including parts of Fiji. A large region of suppressed convection existed in the east, particularly between Tokelau and the Northern Cook Islands, extending towards Northern French Polynesia. Convection was weakly suppressed over Papua New Guinea, and parts of the Solomon Islands and New Caledonia. The Intertropical Convergence Zone (ITCZ) was rather active north of the Equator. The South Pacific Convergence Zone (SPCZ) was south of its normal location.
Very low rainfall for the time of year, totalling only 10% of less of normal, occurred throughout much of New Caledonia in December. Rainfall was also relatively low, being 50% or less of normal in the northwest of Fiji’s Viti Levu Island, as well as parts of the Northern Cook Islands, and northern and southern French Polynesia. In contrast, rainfall was above average (at least 150% of normal) on Fiji’s Rotuma Island and the southeast of Viti Levu Island, and in parts of Tonga. Suva recorded 272 mm in 24 hours about the middle of the month.
Rainfall in New Caledonia has been about 50% of normal during the past 7 month period, which is very unusual.
December mean air temperatures were about 1.0°C above average in Tuvalu, 0.5 °C or more above average throughout much of French Polynesia, and 0.5 °C below average in New Caledonia.
Tropical Southwest Pacifi c mean sea-level pressures were above average over the southern half of Australia, extending northeast to Southern Tonga. Pressures tended to be below average in equatorial areas east of the Date Line.
Equatorial surface westerlies weakened in December, occurring in about 20% of observations at Tarawa, 15% less than November, and about 30% less than October.
| Country | Location | Rainfall (mm) | % of average | Comments |
|---|---|---|---|---|
| French Polynesia | Hiva Hoa, Atuona | 11 | 11 | Well below normal |
| New Caledonia | Koumac | 2 | 2 | Extremely low |
| New Caledonia | Ouanaham | 6 | 4 | Extremely low |
| New Caledonia | La Tountouta | 3 | 4 | Extremely low |
| Cook Islands | Penrhyn | 39 | 12 | Well below normal |
Soil moisture in December 2006
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 December 2006, Nadi soils were relatively dry soil while Rarotonga soils were at field capacity (full). Hanan soil moisture levels were moderate.
El Niño/Southern Oscillation (ENSO)
Although the tropical Pacific atmosphere and ocean are in a moderate El Niño state, evidence suggests that the current episode has peaked.
Sea surface temperature anomalies near the Date Line and in the eastern Pacific peaked in early December. Sea surface temperature anomalies in the subtropical South West Pacific are cooler than normal, and the typical "horse shoe" pattern, associated with an El Niño in the subtropics has fragmented. The NINO3 and NINO4 anomalies were +1.5°C and +1.3°C respectively in December, with the respective 3-month means +1.4°C and +1.3°C.
Westerly wind anomalies continued weakening during December, with easterly trade winds developing to near normal strength at the Date Line by the end of the month. The sub-surface warming along the Equatorial thermocline eased during December and moved east with areas above +4°C at 100m now east of 140°W, whilst there is a strengthening of the cool signal at depth further west.
The SOI remained steady during December averaging -0.5, with the 3-month mean -0.8. Although OLR and tropical rainfall anomalies still show the El Niño–like pattern, this has weakened.
Suppressed convection occurred over Indonesia/northern Australia south east into the subtropical South Pacific, and enhanced convection at 5°N from just west of the Date Line into the central Pacific during December.
The Madden-Julian Oscillation (MJO) is currently active over Indonesian/Australian longitudes, and this is expected to propagate to the Date Line by mid January. The South Pacific Convergence Zone was weaker and further south than normal, whilst the Intertropical Convergence Zone (ITCZ) north of the equator was particularly active in December.
Many models show an El Niño state, observations and models have retained their consensus of El Niño conditions peaking December 2007, weakening during Southern Hemisphere autumn 2007. Seven out of the eleven models predict an easing to neutral for the April–June 2007 period: the remaining four models retain El Niño conditions into late Southern Hemisphere autumn.
The IRI give a 95% chance of maintaining El Niño conditions through to February 2007, with a less than 10% chance of becoming neutral during that time. NCEP predict a continuation of El Niño conditions through to May 2007. The World Meteorological Organization update forecast the El Niño event as expected to continue until at least the first quarter of 2007.
Forecast validation: October to December 2006
Rainfall patterns consistent with a developing El Niño were expected. Enhanced convection with above average rainfall was expected over Western and Eastern Kiribati, and Tuvalu, with average or above average rainfall in Tokelau, the northern Cook Islands, and Pitcairn Island. Suppressed convection with below average rainfall was expected over New Caledonia, with average or below average rainfall over Papua New Guinea, Vanuatu, Fiji, Tonga, Niue, and the Marquesas Islands. Near average rainfall was expected elsewhere in the region.
Enhanced convection and/or above average rainfall affected the region west of the Date Line between Western Kiribati and Vanuatu, and parts of Eastern Kiribati north of the Equator. Suppressed convection and/or below average rainfall occurred over Papua New Guinea, and also from the Northern Cook Islands to Northern French Polynesia. Rainfall was well below average in New Caledonia. Seasonal rainfall anomalies turned out as forecast for many Islands. Rainfall was higher than expected in the Solomon Islands, and lower than expected in Tuvalu. The ‘hit’ rate for the October-December 2006 outlook was about 75%.