Backgrounder

Backgrounder

2002 rainfall to date

Rainfall so far this year has been lower than average in much of the north of the North Island and in parts of the north and west of the South Island. Eastern areas of the South Island have been wetter than average.

Total rainfalls for 1 January to 7 May 2002, shown according to decile rankings of all rainfalls for this period from 1972.

Update on the SOI

The Southern Oscillation Index (SOI) for April was -0.5, with the three month average at about -0.2. A weak to moderate El Niño is expected to develop by spring.

The Southern Oscillation Index (SOI), a measure of changes in the atmospheric pressures across the Pacific, smoothed over three months.

El Niño and Marlborough spring rainfall

Signs remain in place that a weak to moderate El Niño may develop by spring (see the Climate Outlook). The best indication of the effect that might have on spring rainfall can be found in the historical record of previous El Niño springs. Rainfall observations at Sevenoaks, Marlborough, show that an El Niño spring in Marlborough tips the balance in favour of drier than normal conditions.

Marlborough spring rainfall

Spring rainfall in Sevenoaks, Marlborough, 1950 to 2001.

Rainfall in Marlborough during spring typically amounts to 150-200 mm across much of the region, with less along the coast, and over 300 mm in parts of the hill country.

At Sevenoaks the rainfall record indicates that spring rainfall is highly variable (see figure left). Notable in the data shown here is that the lower tercile spring rainfalls, shown in orange, occur across almost the full range of values of the SOI. Droughts in Marlborough are not confined to El Niño years, and have in fact occured three times in strong La Niña springs.

El Niño years

Rainfalls occuring during El Niño springs are shown in the figure below. The number of El Niño spring rainfalls occurring in the lower, middle, and upper terciles, is 5, 3 and 3 respectively, suggesting a 45% chance of drier than average conditions during an El Niño spring. The probability of above average rainfall is about 25%.

Note that one of the wettest springs since 1950, 1994, was also an El Niño year. This was in spite of the fact that conditions along the east coast of both the North and South Islands were drier than normal, as would be expected during an El Niño. This highlights the uncertainty of predicting rainfall patterns in El Niño years, because the typically wetter conditions in the west of the country in those years can spread at times into eastern regions.

First dusting of snow in central Otago. (Photograph: David Turner)