Monthly climate

Climate developments in May 2006

The SPCZ was displaced further south than average during May, extending from the Coral Sea east towards Vanuatu, and across the Date Line southeast to the Austral Islands of Southern French Polynesia. Enhanced convection occurred within the SPCZ region, as well as over much of New Caledonia.

Rainfall was at least 150% of average throughout much of central French Polynesia, and near or above average over much of Vanuatu. Rainfall was above average in Niue. Mixed rainfall anomalies occurred in Fiji. Much of New Caledonia’s rainfall occurred during the last 10 days. Rainfall was well above average in the northern parts of New Caledonia and below average in the southern parts. More than 200 mm fell in the mountains on the 29th, with at least 100 mm in many other areas.

A region of suppressed convection continued to affect Nauru and Western and Eastern Kiribati, extending to the region north of the Marquesas Islands. Rainfall was well below average in southern Tonga.

Mean air temperatures were about 1.0 °C above average in parts of Tonga and Southern French Polynesia, at least 0.5 °C above average in central French Polynesia, and slightly below average over New Caledonia.

Tropical Southwest Pacific mean sea-level pressures tended to be above average west of the Date Line, and below average further east. Equatorial surface easterlies weakened along the equator, occurring in 69% of observations at Tarawa, the lowest frequency since February 2005. There were several consecutive days with westerlies from 11 to 14 May, and a few brief periods later in the month.

Country	Location	Rainfall (mm)	% of average	Comments
New Caledonia	Poindimie	380	201	Well above average
Fiji	Nadi Airport	229	258	Well above average
French Polynesia	Tahiti – Faaa	432	420	New record
French Polynesia	Tuamotu Hereheretue	292	214	Well above average
New Zealand	Auckland Airport	191	208	Extremely high
Tonga	Lupepau’u	402	210	Extremely high
Tonga	Fua’amotu Airport	34	25	Well below average

Soil moisture in May 2006

Estimates of soil moisture shown in the map (above) 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 May 2006, Nadi, Apia, Niue, and Rarotonga were at full soil moisture capacity. Tarawa soils are at their average soil moisture capacity. Soils were drier at the same time last year at Nadi and were average at Rarotonga.

El Niño/Southern Oscillation (ENSO)

The tropical Pacific remains in a neutral state, with the few La Niña features that were present a two months ago erased by sustained warming during April and May.

Equatorial Pacific sea surface temperature anomalies have risen rapidly in the past two months and are now positive across most of the basin. The NINO3 SST anomaly for May was about +0.4 °C and NINO4 was about +0.3 °C: both averages were near zero for March-May.

The thermocline has deepened significantly in the east, with a weak positive temperature anomaly at about 50 m depth where a strong negative anomaly had existed in March. The Southern Oscillation Index (SOI) fell to -1.0 in May, from +1.7 in April, with the March-May mean slightly reduced at +0.7.

The trade winds are close to their normal strength across the Pacific. Patterns of convection have weakened during May, but retain the La Niña-like character seen in April, with somewhat suppressed convection along the equator about and east of the Date Line, and some enhanced convection over northern Queensland and Papua New Guinea. The NASA ENSO precipitation index for May was -0.3 (indicative of weak La Niña). The "double ITCZ" structure seen earlier has now disappeared.

Almost all global ENSO forecast models are in the neutral range for June-August, though the Australian POAMA model predicts a short-lived cooling over the next few months. Most models indicate weak warming through the rest of the year, with three approaching an El Niño state by late 2006. NCEP and the IRI both call for neutral conditions through the rest of the year. The IRI gives a 90% chance of neutral conditions through July.

Madden-Julian Oscillation (MJO)

The Madden-Julian Oscillation (MJO), the eastward progression of both enhanced and suppressed tropical rainfall cycling at 30 – 60 days, is an equatorial travelling pattern of anomalous convection that has been operating on a 30 day timescale this season. Based on the recent short periodicity of events, it seems the most likely time for renewed activity of the MJO over the near-equatorial longitudes of India/SE Asia would be around the middle of June, moving later into the western Pacific. However, at this time of year other influences are also operating and any MJO influences are likely to be weak in the South Pacific.

During this period, when an active phase of the MJO enters the South Pacific, it enhances precipitation. The MJO is usually more active in the South Pacific wet season (November-April).