Monthly climate

Climate developments in March 2009

The South Pacific Convergence Zone (SPCZ) was displaced south and west of normal during March, continuing the trend from last month. A region of enhanced rainfall, partly due to intensified convection, was observed over Papua New Guinea and New Caledonia last month. Suppressed convection intensified during near the Equator during March and encompassed the region northeast of the Solomon Islands, including Western Kiribati, Nauru, and Tuvalu. Significant drought has developed for some of the southern islands in Western Kiribati group, particularly at Banaba. There was also suppressed convection localised near the Pitcairn Islands. The regional circulation was characterised by more frequent high pressure in the North Tasman Sea, and lower than normal pressures to the northeast of New Zealand, over the Southern Cook Islands, and near New Caledonia.

There were many high precipitation totals during March, particularly in French Polynesia where all stations recorded between 140–270% of normal rainfall. In New Caledonia, with the exception of Belep (22% of normal) all stations had well above normal rainfall, with record highs at Koumac and Poindimie. These high rainfall totals were due to the position of the SPCZ near New Caledonia, and also because of the passage of Tropical Cyclone Jasper close to the island on 24 March. There was also a record high rainfall total at Hanan Airport in Niue (see table below). In general, the Solomon Islands had near normal or above normal rainfall for the month, except at Lata .

In contrast to last month when Townsville, Australia received a new monthly high rainfall total of 989 mm (339 % of normal), only 18mm of rainfall (8% of normal) fell at that location. In general, normal or below normal rainfall occurred in the central, northwestern, and northeastern parts of the Southwest Pacific during March. Record low rainfall totals were recorded in Tonga and Tuvalu (see table below). Low monthly rainfall totals also occurred in the Northern Cook Islands, Eastern and Western Kiribati, Pitcairn Island, and the northern part of Fiji and Rotuma.

Northern New Zealand experienced a relatively dry month, with the northern part of the country recording normal to below normal rainfall (40–90% of normal). The North Tasman Region, including Raoul Island and Norfolk Island, was also relatively dry during March.

Island Group	Location	Rainfall (mm)	% of average	Comments
New Caledonia	Koumac	397	263	Record high
New Caledonia	Poindimie	891	223	Record high; Highest monthly total in the region
Nuie	Hanan	380	182	Record high
Tonga	Salote	52	23	Record low
Tuvalu	Niulakita	84	23	Record low

Soil moisture in March 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.

Nadi (Fiji), Hanan (Niue), Rarotonga (Southern Cook Islands), and Apia (Samoa) project moist (at or near field capacity) soil moisture conditions. Soils are moderate for the time of year at Kanton (Kiribati) and Fua’amotu (Tonga).

El Niño/Southern Oscillation (ENSO)

During March, previous La Niña conditions seen in the equatorial Pacific since December 2008 are now weakening towards the borderline of neutral. The SOI, which was +1.4 in February, dropped to +0.2 by the end of March (January–March mean +0.9). Easterly trade winds are now close to normal across much of the equatorial Pacific, although a westerly burst appeared late in March west of the Dateline.

SST anomalies across much of the equatorial Pacific have eased a little. NINO3 and NINO4 anomalies are –0.7°C and –0.4°C respectively (both around –0.7°C in February). Sea surface height anomalies still show a clear La Niña pattern with positive heights west of the Dateline. Negative subsurface oceanic heat content anomalies (average temperatures in the upper 300m of the ocean), and temperature anomalies at thermocline depth continue to weaken slowly across the eastern half of the Pacific. Tropical Pacific convection and precipitation was suppressed near the Date Line just south of the Equator, about Indonesia and northern Australia, and was enhanced across the northern hemispheric tropics at the Australian longitudes during March. The TRMM ENSO precipitation index was –1.6 as of 24 March. The MJO is presently weak and is expected to strengthen in early April, resulting in suppressed convection over the eastern Indian Ocean.

Tropical Pacific convection and precipitation was suppressed near the Date Line, just south of the Equator, and was enhanced across the Tropics at Australian longitudes during February. The TRMM ENSO precipitation index was –0.89 as of 24 February. The MJO is weak and slow-moving over the Indian Ocean, and is expected to remain so into early March.

The global climate model ensemble assessed by NIWA indicate neutral ENSO conditions for April –June, and all but two models indicate neutral conditions in July–September. The NCEP discussion of 5 March indicates La Niña conditions will gradually weaken with more than a 50% chance of ENSO–neutral conditions in the coming months. The IRI summary of 19 March indicates a 50 %of La Niña conditions persisting through to end of May reducing to 25–30 percent for May–July as near-neutral conditions become most likely.

Tropical Cyclone Activity and Guidance

Four Tropical Cyclones (TC) affected the Southwest Pacific region during March. TC Hamish formed off the Australian coast in the Coral Sea on March 5. TC Joni followed, forming on March 11 near the Southern Cook Islands and attained a maximum intensity of 55 knots. Damage in the Southern Cooks, if any, was minimal. TC Ken then formed between Niue and the Southern Cook Islands on March 17 and attained a maximum intensity of 50 knots. TC Ken did not affect land areas. TC Jasper entered the RSMC Nadi region of responsibility from the Coral Sea on March 24 as a tropical cyclone with storm intensity and later passed through New Caledonia’s waters, with no significant damage reported.

Forecast validation: January to March 2009

A region of suppressed convection was forecast for January–March 2009 over the central and eastern Southwest Pacific, extending from Western Kiribati to Eastern Kiribati, including Tuvalu, Tokelau, the Northern Cook Islands, and the Tuamotu archipelago. Below normal rainfall was expected for those island groups. Near–to–below normal rainfall was expected for Samoa. Enhanced convection was expected to extend southeast from Vanuatu and New Caledonia, to the Austral Islands, including Tonga, Fiji, the Southern Cook Islands and Niue, with average to above or above average rainfall for those island groups. No precipitation guidance was offered for Pitcairn Island, Papua New Guinea, the Solomon Islands, the Marquesas, or the Society Islands for January–March 2009.

The validation for the January – March 2009 forecast was calculated for 13 island groups (three countries did not report rainfall values; three were forecast as climatology and were unscorable). The global island group ‘hit’ rate was 64%, 7% higher than average for January and 3% higher than the average for all months combined. Rainfall was overprojected for the Southern Cook Islands and Austral Islands.