Although the SOI can bounce around month to month because of local weather, sustained trends in the SOI reflect the state of the tropical atmospheric circulation.
So, the tropical Pacific Ocean is now effectively a bathtub of lukewarm water. This upsets the normal balance of winds and weather around the tropics.
And like ripples spreading outwards in a pond, there will be lesser effects on the edge of the tropics, in places such as New Zealand. What is important to note is a major El Nino in the tropics does not always have major effects elsewhere.
International climate models predict this El Nino will likely continue into early 2016, and there is every indication it will remain strong (NINO3.4 index greater than 1.5C) during the second half of 2015.
El Ninos traditionally peak towards the end of the calendar year, but it does pay to remember that effects felt in New Zealand may not peak at the same time.
Is El Nino already starting to bite?
So far this winter, we have already seen solid signs of El Nino in the New Zealand region.
In June and July, we have seen more frequent southerly outbreaks than usual -- standard El Nino winter fare for us. These southerlies have resulted in below average temperatures, overall, for the South Island in June and July and for the North Island since the mid June. Several of these southerlies produced heavy snowfalls, and were followed by a ridge in behind, which resulted in widespread and severe frosts. The week June 20-28 was extraordinarily frosty across the country, with Omarama in the Mackenzie Basin recording -21C during this week.
What's in store for August and beyond?
The detailed August forecast can be found at www.metservice.com/rural/monthly-outlook. And while the weather in August will reflect a variety of drivers (since what's going on in the Tasman Sea and to the south of the country affect us most strongly), the longer-term guidance clearly focuses on what is happening in the tropics, and shows a strong El Nino flavour.
Prepare for a stormy, windy and unusually cold spring (September-November), with frequent southwest winds across New Zealand. This is 'textbook' El Nino.
As we head towards the New Year, November onward, those winds should tilt westerly, and dry out eastern parts of both the main islands.
Taking Christchurch and Napier as examples, we see no real change in the odds of a dry spring under El Nino in both regions, but a doubling in odds for summer (once the winds have swung westerly).
