It's a question summer-deprived Wellingtonians couldn't be blamed for asking: Why is Australia sweltering in a heat wave while their own city hasn't broken 25C all season?

The answer is a complex one, but meteorologists can offer one quick and positive take-away: Some of that Aussie heat is now making its way across the Tasman Sea to us.

To understand the dismal summer much of the country has so far experienced is to understand two big driving factors: A huge, blocking, high-pressure ridge parked to the north-west of the country, and what's called the Southern Annular Mode, or SAM.

The high-pressure ridge has been reaching out toward Northland, keeping the region dry to the point that it's now in drought, while other weather has had to move around it, pushing a south-westerly flow over New Zealand.


That pattern was also pulling cold air up from the south and keeping temperatures down, especially in large areas of the South Island.

At the same time, the rest of the country had been stuck in an extensive negative phase of SAM, something typically seen more in spring.

Scientists know the SAM as a ring of climate variability, filled with huge ridges and troughs, that swirls around the South Pole and extends out to latitudes of New Zealand.

When the SAM is in its negative, or stormy, phase, the Southern Ocean storms wash right up and over the country.

Usually, when the warmer season arrives, the amount of normal summer ridging over the country keeps the SAM from having a big effect.

This year, however, the negative phase had remained stuck in gear.

Some places, especially areas in the lower North Island, had fared much worse than others.

Wellington, which just suffered its cloudiest January on record, hasn't been able to break 25C all summer, according to instruments monitored at the capital's airport.

In the past 55 years, there have only been six other instances of that occurring. If that threshold isn't crossed by the end of February, the cool summer would be officially marked down as a one-in-nine year event.

But now that the SAM pattern was kicking into a neutral phase, our summer was beginning to escape the cooling influence.

The summer-long trend hasn't come without some disruptions.

A triple-hit of lows that moved across the country in late January caused flooding and landslips, the first such systems all season, while, later this week a ridge of high pressure is forecast to move across both islands.

"With this, we will actually see a shift in the pattern that we've had all through the summer so far," said MetService meteorologist Lisa Murray said.

"And, in this particular case, it's come across from Australia, which has had these massive temperatures where you could quite literally fry an egg on the pavement."

Murray said that travelling heat, which was flowing across the Tasman along with smoke from huge bushfires now alight in Australia, was much of the reason Wellington was today recording temperatures of 20C, even with gale-force wind and rain.

For eastern areas, such as Gisborne and Hawkes Bay, the heat was arriving in a different form: through dry foehn winds, that came without the moisture that hadn't made it across the ranges which typically sheltered that part of the country from the wet.

Hawkes Bay farmer Bruno Chambers sits in a dry paddock on January 15. Eastern areas of the country have been deprived much of the season's normal rainfall. Photo / File
Hawkes Bay farmer Bruno Chambers sits in a dry paddock on January 15. Eastern areas of the country have been deprived much of the season's normal rainfall. Photo / File

"So we are actually seeing heat now ... but we are seeing it New Zealand-style."

What was driving Australia's scorching temperatures, hitting the high 40s in some places, was a different story altogether.

Meteorologists cited a range of influencing factors for the huge ridge sitting across the centre of the country.

One was Australia being affected by a particular, ridge-driving phase of the Madden-Julian oscillation (MJO), the largest element of the intra-seasonal variability in the tropical atmosphere.

"When the MJO is in certain phases, it can lead to certain weather patterns and the tendency for certain weather patterns to exist," said Ben Noll, a forecaster at the National Institute of Weather and Atmosphere.

"This drives things like big ridges in one place and big troughs in another; right now, the MJO is creating an easterly pulse of thunderstorms and cloud along the equator, and, basically, the rising and sinking motions in the climate there is disrupted, which in turn disrupts the rising and sinking in mid-latitudes further south."