Georgina Griffiths: High noon for El Nino

The dominance of Highs in the New Zealand region is forecast to continue at the end of October and at the start of November. Photo / Glenn Taylor

When I've been talking with farmers lately, there has been a lot of interest in "what's going on with the El Nino." However, given the mis-firing of the climate system on that particular front, my main interest for the New Zealand farmer instead lies a lot closer to home - namely, in the continued persistence of Highs over our neck of the woods. Here is a little more discussion about what has been catching my attention lately, while I've been monitoring both weather and climate in the New Zealand.

The global climate system
The atmospheric indicators of El Nino remain in the neutral range. The Southern Oscillation Index (SOI) briefly dipped into El Nino territory during September, but more recently has returned to neutral levels. Tropical cloud patterns and the trade winds have also showed brief El Nino tendencies over the last six months, but have not sustained them.

The tropical Pacific Ocean remains warmer than average, but still falls short of El Nino thresholds. Because of the persistent warmth in the tropical seas over the last six months, global climate models continue to suggest an El Nino remains possible by December. The model forecasts indicate roughly a 50 per cent chance of an El Nino development over the coming quarter, about double what is normal at this time of year. The latest model runs clearly indicate that any El Nino event is likely to be relatively weak.

Given the lack-lustre global climate situation, the climate patterns dominant in the New Zealand region take on much greater significance.

The national situation
Highs dominated over New Zealand during August, and across the South Island for most of September. Relatively dry conditions were experienced in the north and east of the South Island during August, and over most of the South Island (excluding Nelson) during September. After a wild and windy start to October, highs subsequently controlled the weather over the country as we moved further into the month - with the ridging being relatively strong over the South Island. Not surprisingly, dry conditions prevailed in many areas around the middle of October. Soils have dried out sharply in the east of the South Island, at the time of writing.

The data above shows the rainfall accumulation for early spring at selected eastern South Island locations. The relatively dry conditions this year, when compared to what is normal, show up clearly.

Locally, it will be the persistence (or otherwise) of highs that will become critical as we move towards Christmas. If the highs persist over southern latitudes for the remainder of the year, fending off incoming rain bands, then soils will not get much chance to replenish in the traditionally dry regions (Canterbury, Marlborough and Central Otago). As we progress into summer, that chance diminishes for most regions of the country, as increasing temperatures and lengthening sunshine hours exacerbate soil moisture loss.

Looking into November
The dominance of highs in the New Zealand region is forecast to continue at the end of October and at the start of November. However, the signal is forecast to be much weaker than was seen around mid-October. Slightly more southwesterlies than usual are expected across New Zealand over this period, and it looks to continue somewhat drier than normal for the north and east of both Islands.

Given the recent weather patterns and drier soils in some regions, it will pay to keep an eye on the updated outlook at http://www.metservice.com/rural/seasonal-outlook.

• Georgina Griffiths - MetService meteorologist