TECHNICAL
STILL RATHER DRY IN THE EASTERN SOUTH ISLAND Words by Georgina Griffiths : MetService Meteorologist
At the time of writing (mid-March), the eastern South Island (Marlborough through to south Canterbury) year-to-date rainfall accumulations were running below normal.
El Niño events, AND drier than normal conditions while persistent Highs sit over the South Island during La Niña phases. Marlborough is a little less clear-cut, but La Niña summers can often by drier than usual, due to the influence of frequent Highs over the South Island.
Figures 1 – 4 show year-to-date rainfall accumulation at Blenheim, Culverden, Christchurch and Ashburton, comparing the 2021 tally with the average year-to-date rainfall accumulation, as well as comparing to the last five years. All four locations continue drier than normal so far in 2021, with the dryness being most marked in Marlborough and north Canterbury, but less unusual for Christchurch and Ashburton.
Together, El Niño and La Niña are known as El Niño-Southern Oscillation (or ENSO). The ENSO pattern exerts an important influence on New Zealand’s climate during its stronger phases. However, overall, ENSO only accounts for around 25% of the year-to-year variance in seasonal rainfall and temperature at most locations.
Some years are drier than others – because of other climate drivers
Other factors, such as the Southern Ocean storminess, and what the Tasman Sea is getting up to, are also very important.
Non-linear response to El Niño Southern Oscillation (ENSO)
Here at MetService, we routinely monitor a number of climate drivers that influence the New Zealand weather maps over the coming weeks and months. One of the reasons that the South Island has been ‘relatively’ quiet this summer is a relative lack of vigorous Southern Ocean weather systems (Figure 5).
Research in the early 1990s by one of our most highly respected climate researchers, Dr Brett Mullan, investigated the effects of both La Niña and El Niño events on seasonal rainfall across New Zealand. He identified a ‘non-linear’ response on rainfall in some regions of the country. Put simply, that means that the typical effects of La Niña are NOT equal and opposite to the normal outcomes during El Niño phases, for some regions of New Zealand.
Looking ahead The waning La Niña should be gone for winter 2021. Under neutral ENSO conditions, the state of both the Tasman Sea weather systems and the Southern Ocean storms, will drive our winter weather patterns.
Canterbury and Otago are the standouts – often experiencing dryness or drought under persistent westerly regimes during
Figure 5: The Southern Annular Mode (Southern Ocean storminess) 4
Figure 5: A plot of observed Southern Annular Mode (SAM, also known as Southern Ocean storminess) from 1 June 2019 to 16 March 2021, with forecast SAM values shown for the period 16–29 March 2021.
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NZGROWER : APRIL 2021
12 2019
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Note the persistence of the positive phase since October 2020. The positive phase of the Southern Annular Mode implies a ‘quiet’ Southern Ocean in the New Zealand region. In other words, a lack of Southern Ocean storms washing up and over the country. The day-today weather maps often show High pressure near the Chatham Islands or lying across the South Island.
