Monthly climate

Climate developments in May 2009

The South Pacific Convergence Zone (SPCZ) was displaced southwest of its normal position last month, but exhibited less coherent rainfall concentrations than in previous months. A region of enhanced rainfall, partly due to intensified convection, was observed over part of Vanuatu, New Caledonia, and Fiji during May. Suppressed convection existed northeast of the Solomon Islands during May, in a zone including Western Kiribati, Nauru, and to the north of Tuvalu. There was also suppressed convection along the equator including Eastern Kiribati, and from the Northern Cook Islands to the Austral Islands. The regional circulation in May was characterised by more frequent low pressure near Pitcairn Island and higher than normal pressure in the northwestern portion of the southwest Pacific.

High precipitation totals were concentrated in the western half of the southwest Pacific during May, and also the far east and south of French Polynesia. Drier than normal conditions occurred over much of the Society Islands and the Marquesas, which also experienced more frequent easterly winds than normal during the month. Nearby in the Southern Cook Islands, well below normal rainfall occurred at Rarotonga and Aitutaki (50 and 100mm, respectively). Most stations in Fiji received well above normal rainfall during May. The exceptions were Laucala Bay, Lakeba, and Vunisea, which received well below to below normal rainfall (25 – 75% of normal). In addition, much drier than normal conditions occurred in the North Tasman region this past month, with Norfolk, Raoul, and Lord Howe Islands recording 25 – 30% of normal rainfall.

There were some localised high rainfall totals in the southwest Pacific during May, particularly in the northern regions of New Caledonia and its eastern offshore islands. Koumac and La Roche received more than 275% normal rainfall, a contrast to the near normal totals for the southern part of New Caledonia, and below normal rainfall levels observed nearby in the North Tasman Sea.

In Tonga, normal or well below normal rainfall was recorded, with a low of 46mm at Lupepau’u (24% of normal). Monthly rainfall reports also inidcated drier than normal conditions occurred in Tuvalu, which had 30 – 90% of normal rainfall in May. Northern New Zealand had a wetter than normal month, with 130 – 170% of normal rainfall recorded in the northern part of the country.

Island Group	Location	Rainfall (mm)	% of average	Comments
Fiji	Nacocolevu	322	379	Record high
Vanuatu	Sola	680	183	Highest monthly total in the region
North Tasman	Raoul Island	37	30	Record low
North Tasman	Norfolk Island	37	26	Very low
North Tasman	Lord Howe Island	48	30	Very low

Soil moisture in May 2009

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 were made at the end of the month, and for practical purposes, generalisations were made about the available water capacity of the soils at each site.

Hanan (Niue), Apia (Samoa), Port Vila (Vanuatu), Nadi (Fiji) and Fua’amotu (Tonga) project moist soil moisture conditions. Soil moisture is moderate at Rarotonga (Southern Cook Islands), and dry at Tarawa and Kiritimati (Kiribati).

El Niño/Southern Oscillation (ENSO)

During May, the equatorial Pacific Ocean exhibited ENSO-neutral characteristics. Sea surface temperatures (SSTs) in that region have been rising steadily since February, and are now warmer than normal across the entire Pacific basin. NINO3 and NINO 4 SST anomalies are +0.8°C and +0.3°C, respectively, for May (and +0.1°C and –0.0°C, respectively, for MAM). SSH anomalies still remain positive in the tropical Pacific west of the Date Line, but show a north-south banded structure in the east, with positive anomalies along the Equator east of the Date Line. Positive equatorial subsurface ocean temperature anomalies strengthened during May in the uppermost 100m near the South American coast, and now exceed +3°C. The upper ocean heat content, averaged over the top 300m, is positive right across the equatorial Pacific.

Westerly anomalies are now evident in the trade winds west of the Date Line. The TRMM ENSO precipitation index remains negative at –1.0, about the only indicator remaining of the recent La Niña. At the same time, the SOI fell rapidly to a value of –0.5 in May (but averaged +0.1 over MAM). Tropical Pacific convection (from OLR) was enhanced over Indonesia and north of Australia in late May, but was still suppressed along the Equator, east of 150°E, MJO activity was heightened at the end of May and convection was expected to propagate at least as far as the Date Line during the first week of June.

The global climate model ensemble assessed by NIWA show statistical models are signaling ENSO–neutral conditions into early 2010. However, seven of the 10 dynamical models that are monitored indicate reaching an El Niño threshold by the end of Austral winter. Thus, the ENSO discussion has shifted from whether La Niña will continue through winter to whether current neutral conditions will transition into El Niño before the end of the year. Most forecasts still expect Austral winter to be ENSO–neutral. The Australia Bureau of Meteorology has noted that signs indicating the possible development of El Niño have strenghtened in early June. NOAA also indicates conditions are ripe for El Niño development if the atmospheric response catches up to the ocean anomalies. The IRI indicates a 48% chance of neutral ENSO conditions for the coming season, and a 45% chance of an El Niño.

Tropical Cyclone Activity Summary

Eight tropical cyclones occurred in the Southwest Pacific during the 2008–09 season, which was one less than the long term average (1969/70-2007/08). This year was the most active TC season since 2005/06. ICU tropical cyclone guidance produced in October 2008 projected average TC activity (8–10 TC’s) for the 2008/09 season. The first tropical cyclone formed on January 28, making this the latest start to the Southwest Pacific TC season since 2000/01. Four TC’s developed west of the Date Line and five formed east of the Date Line. Peak activity occurred in March (four TC’s), and December and February were unusually quiet, and only one TC reached hurricane intensity (sustained wind speeds of at least 118 km).

Forecast validation: March to May 2009

A region of suppressed convection was forecast for the central and eastern Southwest Pacific, extending from Tuvalu southeast to the Northern Cook Islands, including Tokelau, with an expectation of below normal rainfall. Near–to–below normal rainfall was expected for Samoa, Eastern Kiribati, and Western Kiribati. Enhanced convection was expected to extend southeast from Papua New Guinea, and include New Caledonia, Vanuatu, Tonga, the Southern Cook Islands and the Austral Islands, where above normal rainfall was forecast. Near or above normal rainfall was forecast for Fiji and Niue. Normal rainfall was anticipated for the Marquesas, Pitcairn Island, and also the Society Islands. No clear precipitation guidance was offered for the Solomon Islands, Tuamotu, or Wallis & Futuna for the forecast period.

The March – May 2009 forecast validation was calculated for 13 island groups (four countries did not report rainfall values; three were forecast as climatology and were unscorable). The global island group ‘hit’ rate was 64%, 1% higher than average, and 3% higher than the average for all months combined. Rainfall was overprojected for Vanuatu, Tonga and Samoa.