TECHNICAL
METSERVICE UPDATE
Still rather dry in the eastern South Island By Georgina Griffiths : Meteorologist, MetService 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.
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.
Figure 2 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.
Some years are drier than others – because of other climate drivers Together, El Niño and La Niña are known as El NiñoSouthern 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.
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.
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 1).
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.
Figure 1: The Southern Annular Mode (Southern Ocean storminess) Figure 1: 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.
4 3 2 1 0 -1 -2 -3
03 2021
02 2021
01 2021
12 2020
11 2020
10 2020
09 2020
08 2020
07 2020
06 2020
05 2020
04 2020
03 2020
02 2020
The ORCHARDIST : APRIL 2021
01 2020
12 2019
11 2019
10 2019
09 2019
08 2019
62
07 2019
06 2019
-4
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.
