Science reporter Jamie Morton talks to climate scientist Dr Brett Mullan of the National Institute of Water and Atmosphere (NIWA) about our holiday season weather and warming world.
Our summer climate is looking to be characterised by average or above average temperatures and near normal rainfall for most of New Zealand. What are the big-picture processes that are driving these conditions and how do they work?

The seasonal climate Outlook for summer that NIWA compiled at the beginning of December was a particularly challenging one, because of a lack of clear "climate drivers" or factors that will steer weather patterns over the summer season.

Into January and especially February, we expect that New Zealand's weather patterns may be more significantly influenced by local drivers, such as the temperature of the seas that surround the country.

Given that New Zealand is an island nation, the water that surrounds the country plays a major role in the overall flavour of the weather.

Sea surface temperatures are most likely to be average or above average during the second half of the summer season, which gives some confidence to the idea that January and February air temperatures may be average or above average.

Advertisement

In terms of rainfall, while much of the country is expected to have near-normal rainfall during the summer as a whole, the east of the South Island has about an equal chance for normal or below-normal rainfall because of expected westerly winds.

Westerly winds warm and dry as they descend the leeside of the Southern Alps.

However, parts of the eastern and northern North Island have received below-normal rainfall during the first half of December.

Trends in these regions will be monitored as the risk for below-normal rainfall may continue into January and February.

In addition, La Nina, which tends to bring persistent east or northeast winds to the country is not expected to be much of a climate driver at all; thus, westerly-quarter winds are forecast to prevail much of the time.

What's the difference between a La Nina climate system and an El Nino system?

El Nino and La Nina are two climate terms that are very commonly mentioned but often misinterpreted or misunderstood.

So let's break it down.

El Nino and La Nina are opposite phases of a naturally occurring global climate cycle known as the El Nino-Southern Oscillation (ENSO).

They disrupt normal patterns of wind and rainfall, in different ways, in many parts of the world - including New Zealand.

El Nino is characterised by unusually warm sea surface temperatures in the eastern and central equatorial Pacific Ocean.

In New Zealand, El Nino's impacts are typically most pronounced during the summer, when winds have an increased tendency to blow from the southwest, leaving parts of the eastern North and South Island drier than normal and the rest of the country with mostly near normal rainfall and cool spells.

La Nina, on the other hand, is characterised by unusually cool sea surface temperatures in the eastern and central equatorial Pacific Ocean.

Much of the eastern and northern North Island become susceptible to above normal rainfall and parts of the southern and western South Island susceptible to below normal rainfall.

Neutral conditions occur when sea surface temperatures in the tropical equatorial Pacific Ocean are near normal - neither El Nino nor La Nina. Sometimes, the ocean can show El Nino or La Nina characteristics but the atmosphere does not follow along, or vice versa.

According to NIWA's latest outlook, the equatorial Pacific Ocean exhibits mainly neutral conditions.

While some oceanic and atmospheric signals are consistent with weak La Nina conditions, others are not.

Over the next three months, international guidance suggests that neutral conditions are about equally as likely as La Nina conditions in the equatorial Pacific.

Thus, during summer 2016-17, New Zealand's climate will most likely not have much of a La Nina flavour or influence.

In other words, winds from an east or northeast direction are not expected to be more common than usual this summer.

Climate forecasters rely on El Nino or La Nina signals to help drive a long-range outlook, as they account for almost 25 per cent of the year-to-year variance in seasonal rainfall and temperature around New Zealand.

But it's important to note that every El Nino and La Nina event is different and the "average outcome" is not to be expected every time.

Impacts will be different if the El Nino or La Nina event is weak, moderate or strong.
Other climate drivers must be considered and sometimes, especially during a weak El Nino or La Nina event, will have more of an influence on weather patterns.

How favourable do you think the summer's overall flavour will be for holiday-making and beach-going?

It's a tough call this far out as this is more related to a weather forecast and not a climate outlook that covers multiple months.

For example, our expectation for this summer is for about equal chances for temperatures to be average or above average for most of the country.

In other words, a cooler than average summer is unlikely.

But even if an average or a warmer than average summer occurs it doesn't mean a cold snap won't occur on the week someone is on holiday at the beach... and trying to pick that particular stretch of time two or three months in advance is next to impossible.

In addition, the outlook we provide covers a three month period as a whole, not individual months.

Campers at Morrison's Bush in the Wairarapa had a wet time in summer 2014. Photo / File
Campers at Morrison's Bush in the Wairarapa had a wet time in summer 2014. Photo / File

Thus, December could be average or even cooler than average, January might be average, and February may be a scorcher and when you average the whole three months, it ends up being average or above average.

So people taking their holiday in December might have a completely different experience from those holidaying in February.

That said, it's not unreasonable to think that there may be an opportunity for more "beach weather" days this summer, with temperatures more likely to be average or above average through to February than below average.

And do you see the summer's climate having any particular effect on agriculture?

It really depends how the day-to-day weather unfolds.

For example, if normal summer rainfall were to occur in a particular region of the country, how did the rainfall occur?

Did the majority of the rain occur during one or two major rain events and was the rest of the summer abnormally dry?

Or was the rainfall more evenly distributed?

Think of it in terms of diet or food budget.

If you're meant to consume 2000 calories a day, do you eat an exceptionally large breakfast, followed by a light lunch and light dinner, or are the three meals more even in size?

Cows on a Waikato farm eat what little grass remains during the 2010 drought. Photo / File
Cows on a Waikato farm eat what little grass remains during the 2010 drought. Photo / File

Short of significant dryness or wetness, the detail of the summer climate, which is the day-to-day weather, plays a significant role on what kind of impacts are realised for agriculture - or any other sector.

How does NIWA monitor and report on these patterns? How accurate is the data?

NIWA uses a large range of datasets to keep track of national and global climate anomalies.

NIWA's national database includes more than 450 in-situ stations, recording a range of parameters including rainfall and temperature.

In addition, NIWA maintains the Virtual Climate Station Network (VCSN), which uses statistical techniques to produce climate information for areas that are away from actual monitoring stations.

We also keep a close eye on the state of the overall "climate system", particularly in the Pacific region, notably the El Nino Southern Oscillation phenomenon.

We use direct in-situ observationns as well as datasets derived from satellite remote sensing, detecting things like sea surface temperature, cloudiness and large-scale rainfall.

Climate monitoring necessitates continuous investment and maintenance to be useful, as gaps in datasets hinder the capacity to correctly evaluate long-term trends.

Continuous, long-term climate datasets are an essential component of climate research and operations.

NIWA is also providing a running "Hotspot Watch". How will these be useful?

Hotspot Watch is useful for those in need of an accurate evaluation of current and future soil moisture content across the country.

Our aim is to communicate where the driest soils are located, and identify areas where drought might potentially develop.

Hotspot Watch shows recent soil moisture trends as well as our expectation on how soil moisture will change in the near future.

We hope this will allow readers to make informed decisions and plan appropriately.

Is there anything unique about this summer's climate or weather that may make it memorable in the climate record?

At this time, the expected climate and weather patterns for the 2016-17 summer do not appear to be particularly unusual.

NIWA's seasonal outlook is for temperatures to likely be near average or above average, and the rainfall most likely to be near normal, for most parts of the country.

Of course, the weather varies day to day, and it is always possible that extreme events will occur.

Examples from the past few summers include a few very hot days when daytime temperatures exceed 35C; heavy rainfall and flooding; a close encounter with a severe weather system originating in the tropics; tornadoes or waterspouts; gale force winds bringing down power lines, cancelling flights and ferry sailings, or affecting travel plans; hail - and even snow, which was recorded on January 15, 2012, around Queenstown.

2016 is likely to be the warmest year on record for New Zealand. What is the significance of this?

The saying "one swallow does not a summer make" goes back a long way, to Aristotle in the 4th century BC.

It's the pattern and long-term trends that count rather than a single record, even though people get excited about these things.

On the longer time-scale, New Zealand has warmed by almost one degree Celsius over the past century, and the 2016 record is further evidence of the fact that the globe is warming.

Carbon dioxide levels in Antarctica's atmosphere hit 400 parts per million for the first time this year. Photo / Brian Karl
Carbon dioxide levels in Antarctica's atmosphere hit 400 parts per million for the first time this year. Photo / Brian Karl

We expect that high temperature records will continue to tumble in the future.
Global and regional warming continues to push New Zealand's temperatures up, but such record breakers are irregular.

For example, over the past three summers: in 2013-14, NIWA reported no new records or near-records for mean temperature broken at either the high or low ends; in 2014-15, 8 climate sites produced new high mean temperature records - or near-records - but no low mean temperature records; in 2015-16, 28 sites had new high mean temperature records but, again, no low mean temperature records.

This year, we also saw levels of carbon dioxide in New Zealand's atmosphere cross the threshold of 400 parts per million. Why should this be concerning to Kiwis?

Carbon dioxide (CO2) levels crossing a threshold of 400 parts per million (ppm) present no special threat per se, but are more of a psychological landmark - a reminder that, remote though New Zealand is from the main anthropogenic sources of greenhouse gases, the atmosphere eventually mixes CO2 around and transports it across the globe.

We are not isolated from the actions of other countries; we are already well aware of this in regard to the economy, but it is also true for climate.

NIWA's Baring Head observatory first saw CO2 levels exceeding 400ppm in June 2016, the same month as Australia's observatory in Tasmania recorded it, but three years after the 400ppm threshold was first crossed in Northern Hemisphere clean air - at Hawaii in May 2013.

In September 2016, CO2 levels above 400ppm were recorded in Antarctica.

Rising CO2 is confirmation that mankind's industrial activities continue to change the atmosphere we live in and rely upon.

Carbon dioxide concentration today is 40 per cent higher than in pre-industrial times (at 280ppm), and it's all due to our activities.

More CO2 leads incontrovertibly to higher temperatures, as the atmosphere becomes more opaque in the infra-red part of the spectrum and the Earth emits less energy to space to counterbalance the incoming solar radiation.

A CO2 increase today commits us to a temperature increase in the future.

If the average temperature rises by several degrees by the end of the century, what would this mean for our weather? How might the summer we're experiencing at the moment be different in 2100?

With a rise in temperature of a few degrees by the end of the century, there will still be plenty of day-to-day variability in the weather as anticyclones and not-so-cold "cold fronts" cross the country.

The type of extreme weather events we experience today are still likely to occur, but be more frequent or more extreme - especially heavy rainfall.

On the other side of the coin, longer or more extreme droughts are also to be expected.
There could be double the number of summer days exceeding 25C, with more frequent days above 35C too, although these extremes will remain uncommon.

The Kiwis of 2100 will have acclimatised to warmer conditions.

Their winters will of course remain much colder than our summers now.

However, their cold summers will likely be warmer than the hottest summers we experience today, and their hot summers would seem unprecedented to us.

Kiwis will no doubt still enjoy the country's beaches, although inexorable sea-level rise will make our coastline look rather different from today.