Backgrounder

Backgrounder

ENSO signal approaching neutral

The current El Niño phase of the El Niño-Southern Oscillation is nearing its end. Sea surface temperatures in the equatorial region of the eastern Pacific have fallen, and subsurface temperatures are now close to zero.

Most forecast models of the development of the El Niño-Southern Oscillation are predicting neutral conditions through autumn and winter.

Mean sea surface temperature departures from normal.

Update on the SOI

The mean Southern Oscillation Index (SOI) for February was -0.9, with the three month average now at -0.8. The present El Niño is waning. Further general information on El Niño is available on the World Meteorological Organization web site, www.wmo.ch

The Southern Oscillation Index (SOI).

Online climate graphics

Climate maps and line plots of climate site observations are updated each week on the Climate Now website.

Climate summary

Summer was cooler and drier than normal, as expected

The summer of 2002-03 was the coolest El Niño summer since 1991-92 and 1992-93 (see dark bars in the figure below). Summer was generally drier than normal in many parts of New Zealand.

Rainfall was about 50% of normal in parts of central Hawke’s Bay and Horowhenua (see below rainfall map), and drier than average in parts of Taranaki, eastern Bay of Plenty, Taupo, Kapiti, Nelson, and in many eastern South Island areas from Kaikoura to Central Otago.

Significant soil moisture deficits occurred throughout the summer in much of the eastern South Island, spreading to much of the North Island from January onwards. Extreme fire risk also developed in many areas.

Sunshine hours were above average in most regions, especially from Marlborough to Otago, with Christchurch recording its sunniest summer in over 50 years.

The Pacific El Niño event had some effect on the New Zealand summer climate pattern. Anticyclones (zones of high atmospheric pressure) were more frequent than usual in the Tasman Sea and east of the Chatham Islands, keeping pressures a little higher than normal over New Zealand. More frequent westerly winds occurred south of the country over the Southern Ocean.

Rainfall percent of average for summer, 2002–03.

Summer average temperatures from 1949–50 to 2002–03.


Little Barrier Island capped by orographic cloud. Cirrus clouds emanating from tropical cyclone Zoe cover much of the sky. Zoe, in commen with other tropical cyclones this summer, had little impact on New Zealand weather (note the calm state of the sea). Photograph: Georgina Griffiths