First Bart, then Debbie and Cook, and now Donna.

While it might seem like we've seen more than our usual share of tropical cyclones this season, meteorologists say, on the whole, it's been ultimately close to normal.

It's simply that our season has been back-loaded with big events, following a quieter-than-usual start to the official November to April period.

"And when you blend it all together, you get something that ends up being not too far from the norm," said Niwa's Ben Noll.


"You can get storms that are evenly spaced apart, or you can get these periods of heightened activity and then times where basically nothing is going on.

"In our case, we've had the latter with this cyclone season - and it's been interesting to observe."

Tropical cyclones are defined as low pressure systems that form over warm tropical waters and have gale force winds - sustained winds of 63 km/h or greater and gusts in excess of 90 km/h - near the centre.

Based on a 30-year average, around 10 cyclones usually form in the southwest Pacific region each season.

Back in October, Niwa predicted a "near normal" risk of New Zealand catching the fall-out of tropical cyclones over the season.

While an ex-tropical cyclone typically edges within 550km of the country around once every season, bringing with it significant wind, waves and rainfall, forecasters had warned of the possibility of more than one rolling in from the southwest Pacific this time around.

It wasn't until late February that short-lived Bart made a brief appearance near the Cook Islands, before the remnants of Debbie hit New Zealand in early April, bringing the deluge that led to a swollen Rangitaiki River breaching a stopbank and flooding Edgecumbe.

Next came the mid-month Cook, making landfall in Bay of Plenty and causing damage across the North Island.

The major storm events of Debbie and Cook resulted in the vast majority of North Island centres receiving well over their normal April rainfall, with some places recording two and three times the usual amount.

Now, just outside the official bounds of the season, meteorologists are waiting to see whether Donna, expected to hit Vanuatu this weekend, will bring more rain to New Zealand next week.

Why so many systems at once?

Noll said it boiled down to La Nina-like patterns in the Pacific that had made for a wet March to April period.

"Even though we don't have a very strong ENSO event - that means either La Nina or El Nino - going on right now, when we have these neutral phases, you can still get periods of time when the weather pattern is either reflecting La Nina-like or El Nino-like conditions.

"They undulate and oscillate over time, and we just so happen to have had such a pattern in March and April."

The other part of the equation was what's called the Madden-Julian oscillation (MJO), the largest element of the intra-seasonal variability in the tropical atmosphere.

When it's in certain phases, the MJO can lead to certain weather patterns and the tendency for certain weather patterns to exist.

Noll described it as a "spark plug" that could help activate tropical cyclones.

"So you have this La Nina-like state sitting in the background, and then you have these MJO pulses - areas of thunder storms in the atmosphere that track across the equatorial region - and they've been in favourable phases that help spark these tropical cyclones."

Noll expected that another just-begun MJO was a big reason for the formation of Donna.

The last two big systems had also been given a helping hand by low pressure systems in the Tasman Sea that had effectively siphoned them down to New Zealand.

"We've also had highs over the South Island, which has blocked the systems from pressing down there and has kept them in the North Island, but, overall, it's been very reflective of that La Nina-like flavour."

Noll also wasn't sure that we won't be in for yet another big rain-maker.

"I actually wouldn't be surprised if there is another northerly rain event in New Zealand - even possibly in late May or early June."

Although October's cyclone forecast proved to be close to the mark, Noll conceded that it had been hard to pick the late-season action.

"If we could have given the detail that the latter half of the season would have been more active than the front end, I think that would have been useful.

"But often, when you are forecasting on seasonal time scales, you are smoothing out humps, and you are trying to find that general flavour."