Federated Farmers: El Nino about to fire up

The proportion of historical El Nino events that were dry (*below normal rain), using the NINO3.4 Index, post 1950. El Nino conditions have recently strengthened significantly in the tropical Pacific Ocean.

April and May are the traditional months when the oceans 'switch over' -- meaning it is the usual time of year that existing El Nino or La Nina events decay, or new events form.

Seas along the equator have become abnormally warm during May, with seas now more than two degrees above normal near South America, and about one degree above normal elsewhere. This so-called 'warm tongue' of water along the equator is the classic El Nino signature.

The atmosphere is playing ball, too, with weaker trade winds observed along the equator. You need both the seas and the atmosphere to couple-up, in order to get a full-blown El Nino.

What about last year?

About this time last year, climate commentators around the globe predicted a weak El Nino for the second half of 2014. There was significant debate from then on, about whether El Nino thresholds had been crossed, or not.

The atmosphere failed to join in during 2014, and the El Nino failed to fire. From a practical (farming) point of view, if El Nino is so ho-hum that scientists cannot agree, then adopt a pragmatic approach. It won't be a major player for you on the farm.

This time around, it's worth keeping an eye on with all global commentators agreeing that El Nino is underway in autumn 2015. The latest predictions indicate a solid chance (around 70 per cent) that El Nino conditions will continue through the southern winter and spring.

At this stage it is too early to tell whether this El Nino will be a strong event, as the models still show a fairly wide range of outcomes.

But what does El Nino really mean for us?

Historically, El Nino is only one of a number of players that influence our weather. An active Tasman Sea (with frequent lows spawning over the Tasman, as seen in May 2015) typically produces a wet month for many regions of New Zealand. The Southern Ocean also affects us, and we monitor the Southern Annular Mode for storminess across the country (blog.metservice.com/2015/02/the-southern-annular-mode-or-sam), as was seen in November 2014 when the storms tracked further north than usual.

Overall, El Nino may explain up to 40 per cent of what is going on with our seasons. But during strong events, some regions see prolonged dry or wet spells across multiple seasons. And that is useful to know about.

Historical El Nino impacts for New Zealand

Looking back at previous El Nino events (the last one occurred in 2009/10), we see more southerly winds over New Zealand in winter, and more southwesterlies in spring, with cooler seas around our coastline. The signal is typically amplified during spring, with a much stormier, windier, and colder spring than usual. During El Nino summers, enhanced westerlies mean drier conditions in the east.

The image shows the historical proportion of El Nino events that were dry (below normal rain) during winter and spring.

The chances of a relatively dry winter/spring increase during El Nino in the western North Island (Northland, Auckland, Waikato, Waitomo, Taumarunui and Taranaki), as well as for Nelson and Marlborough.

But in the North Island, we may not notice this effect very much, since winter and spring usually yield enough rain.

For the west and south of the South Island, the chances of a dry winter and spring decrease during El Nino.

For places like Canterbury and the eastern North Island, El Nino often (but not always) results in drier conditions once we head into summer.

These regions are impacted by El Nino towards Christmas and into the New Year (in fact, during the driest time of year).

In the short-term, weather sequences, rather than El Nino, will affect June.

See what's likely in the latest issue of the MetService rural outlook at www.metservice.com/rural/monthly-outlook.