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Thunderstorms of Oceania: Climatology

10th June 2021

Welcome to the third instalment to the climatology of thunderstorms series. This time we’re taking on the vast continent of Oceania. This is a region of enormous contrasts, spanning moisture-laden tropics around Indonesia, to parched Australian outback, to the green and pleasant lands of New Zealand.

Similarly, thunderstorms vary greatly in their numbers depending on where you look. In fact, the differences are starker than I had ever imagined!

Winter: Quiet Outside the Tropics

During the southern hemisphere (austral) winter, lighting is a rare sight across most of Australia, New Zealand and even the relatively warm and humid New Caledonia. At this time of year, Australia and New Caledonia are situated right in the middle of a ‘subtropical high pressure belt’. This is quite literally a belt around the world in which high pressure areas predominate.

High pressure brings dry, subsiding air, which is very detrimental to thunderstorm development. Where moisture is readily available from the ocean, surface heating could overcome this, but it’s rarely strong enough in the winter season.

In New Zealand, high pressure is more intermittent, but there’s even less surface heating. The few thunderstorms that do occur tend to be ‘forced’ by particularly vigorous low pressure systems.

To the north of Australia, ‘winter’ doesn’t really apply. The tropics are warm and humid year-round. This suits thunderstorms very well – so much so that single locations can easily see multiple thunderstorms in a day. It’s worth noting that the analysis performed here doesn’t take that into account.

In Jun-Aug, it’s the north-western reaches of Oceania that are most electric, so to speak. Many areas there see thunderstorms on at least half the days in the period.

Spring: Australia Sparks Up

The austral spring sees the subtropical high pressure belt start to shift southward. Meanwhile, temperatures begin a substantial climb toward the summer highs. Especially surface temperatures, as the strengthening sun heats the land. These two factors facilitate more uplift of air, increasing the likelihood that thunderstorms will initiate if there’s enough moisture available.

I emphasised that because for much of inland Australia, that’s not readily the case. Away from the far north, it’s predominantly arid, with sandy or dusty terrain. This keeps thunderstorm frequency on the low side, even as the surface heating becomes intense later in the season.

To the east, most of the islands are small enough that they experience exclusively ‘maritime’ climates. Winds off the ocean limit the daily rise of surface temperatures in response to surface heating. Hence, thunderstorms are infrequent for the likes of New Caledonia, Vanuatu, and Fiji, to name a few.

New Zealand’s islands are considerably larger, mainly 100-200 miles wide. Yet even here, thunderstorms are scarce. In the north, this is due to the arrival of the subtropical high pressure belt. In the south, it’s mainly down to weaker sunshine and higher typical wind speeds.

Up in the tropics north of Australia, it’s more or less ‘as you were’ from Jun-Aug.

Summer: Tropical Expansion

In the austral summer season, the subtropical high pressure belt spans the far south of Australia to New Zealand’s North Island. Here, thunderstorm frequency remains low, despite plenty of surface heating during the long summer days.

Further north, high pressure is far less frequent. Conditions are often unstable, which means that whenever enough moisture is available, thunderstorms kick off easily.

Generally, that moisture must come from either the ocean or tropical forests. No coincidence, then, that thunderstorms tend to be more frequent near to coastlines. At this time of year, parts of northern Australia rival the quieter (least thundery) parts of Indonesia.

There, the peak zone for electrical displays is shifted south compared to Jun-Nov. Essentially, there’s a ‘most unstable atmosphere belt’ (tropical low) which follows the subtropical high pressure belt southward.  Dec-Feb, it spans from the Timor Sea to Solomon Sea.

Autumn: A Lot Like Spring

Away from the tropics, autumn tends to be clearly distinct from spring, as the land and sea have accrued much warmth during the summer. Yet when we look at thunderstorm frequency, there’s not much to tell them apart. The only marked differences are seen in the vicinity of Vanuatu and Fiji.

There, the warm seas provide ample moisture, facilitating frequent thunderstorms, especially early in the season when the sunshine is still strong. The area continues to experience a tropical climate, akin to that of Indonesia.

South and west of there, the high pressure belt becomes the dominant player, as it returns northward.

Climate Trends: Cause for Alarm?

Given the capacity for thunderstorms to bring flooding rainfall, damaging wind gusts and destructive lightning strikes, trends over time are of great interest to insurers.

On this, I bring mixed news. Analysis of the past 15 years compared to the past 30 shows little trend in thunderstorm frequency for Australia or the islands to the east (including New Zealand).

On the other hand, there’s a striking upward trend where they are already most frequent, to the north of Australia (e.g. Indonesia; Papua New Guinea). All months are typically seeing 1-2 extra thunderstorms. That’s at least a dozen more days each year with a high peril risk.

Ocean Versus Atmosphere

To explain these trends (or lack of), we must consider how climate change has played out in both the atmosphere and the ocean.

Across much of Oceania, the ocean has warmed but the atmosphere has become more stable. Which is to say, high pressure has become more frequent.

That means we have oceans providing more moisture (via evaporation) to ‘feed’ thunderstorms, but the atmosphere delivering more days with dry, sinking air. The two have generally ‘cancelled each other out’.

A major exception lies to the northwest of Australia. Here, high pressure has become less frequent. So, we have more available moisture and instability, both contributing to an increase in thunderstorm frequency.

In the decades to come, some further warming of the oceans is expected. Should pressure trends also continue in a similar vein, much of Indonesia will have to adapt to experiencing thunderstorms almost every other day of the year, on average.

Elsewhere, it appears that increasing frequency and intensity of heatwaves, droughts, and floods (yes, it can be both!) will be more pressing concerns.

 

James Peacock MSc
Head Meteorologist at MetSwift

Cover Photo by Unknown Author (but accredited to Daily Mail at the link) is licensed under CC BY

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