By Leigh Matheson, MetService

The 2015/2016 El Nino is already at very strong levels in the tropical Pacific Ocean. It is expected to strengthen further, peaking around December. So what does this mean for your summer in the mountains?

Firstly, for those who are not familiar, what is ‘El Nino’?

The single biggest climate effect globally is the change from summer to winter. The next biggest, is El Nino, or ENSO (El Nino Southern Oscillation) to give its full scientific name. In simple terms, the atmosphere can be quite profoundly affected by temperature changes within the ocean, and one significant effect is the position and strength of heat islands, which can build up around equatorial parts of the Pacific Ocean. The extreme case, ENSO, involves a significant increase in sea temperature along the equatorial Pacific and along the South American coast. This effect was first ‘labelled’ by the Peruvians who receive higher than average rainfall, and found their anchovy fishery hugely affected, all around Christmas – hence the name ‘the Christ Child’, or ‘El Nino’.

So what about New Zealand?

The first thing to be said about El Nino, is that one strong event is not necessarily like another. Previous events in 1982/83 and 1997/98 were vastly different. The summer of 1997/98 was unusually hot and dry, with frequent west to northwest winds. In contrast, the 1982/83 summer was unusually cold, with low pressures and frequent southwest winds – the conditions in which mountaineers Mark Inglis and Phil Doole were trapped for 14 days in a snow cave on Aoraki/Mt Cook during a relentless blizzard.

What do you need to watch for this summer? Expect extremes, either unusually cold and wet, or hot and dry. When planning the big summer trip, factor in the possibility of persistent westerlies. This has been a facet of both previous strong El Ninos, although there’s no guarantee that this coming one will behave in a similar vein.

Don’t let the weather stand in your way of an enjoyable summer in the hills; be prepared, and keep updated with the latest forecast. And remember, the long-term average conditions won’t necessarily dictate the weather in the short term.

This column was originally published in the November 2015 FMC Bulletin. FMC thanks Leigh for her valued contribution to the FMC Bulletin weather column. If you have mountain weather questions, email them to backcountry@fmc.org.nz, and we will ask Leigh to answer them in future weather columns.