3 minute read
Still rather dry in the eastern South Island
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.
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.
Non-linear response to El Niño Southern Oscillation (ENSO)
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. Canterbury and Otago are the standouts – often experiencing dryness or drought under persistent westerly regimes during 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.
Some years are drier than others – because of other climate drivers
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. Other factors, such as the Southern Ocean storminess, and what the Tasman Sea is getting up to, are also very important. 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).
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.
Figure 5: The Southern Annular Mode (Southern Ocean storminess)
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-4 06 2019 07 2019 08 2019 09 2019 10 2019 11 2019 12 2019 01 2020 02 2020 03 2020 04 2020 05 2020 06 2020 07 2020 08 2020 09 2020 10 2020 11 2020 12 2020 01 2021 02 2021 03 2021 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. 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.
Rainfall accumulation plot for Blenheim
1 Rainfall accumulation plot for Culverdon
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Figure 1: Blenheim annual rainfall accumulation (mm) for the last five years (2017 to 2021). The annual average rainfall accumulation is shown in black. Figure 2: Culverden annual rainfall accumulation (mm) for the last five years (2017 to 2021). The annual average rainfall accumulation is shown in black. Figure 3: Christchurch annual rainfall accumulation (mm) for the last five years (2017 to 2021). The annual average rainfall accumulation is shown in black. Figure 4: Ashburton annual rainfall accumulation (mm) for the last five years (2017 to 2021). The annual average rainfall accumulation is shown in black.
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