The final months of New Zealand's summer were brutal, with "unprecedented" rainfalls, from widespread flooding in Auckland at the end of January to ex-tropical Cyclone Gabrielle dumping record rains and causing devastating floods along the east coast of the North Island.
More energy in the atmosphere and ocean
AMOS GUMULIRA/AFP via Getty Images
Some answers have come from the international World Weather Attribution team, which released a rapid assessment today that shows very heavy rain, such as that associated with Cyclone Gabrielle, has become about four times more common in the region, and extreme downpours now drop 30% more rain, as per Phys.org.
The researchers examined weather data from several stations, which revealed an increase in heavy rain. It then used computer models to compare today's climate with the climate of the past, after about 1.2°C of global warming since the late 1800s.
Because of the small size of the analyzed region, the team was unable to quantify the extent to which human-caused warming is responsible for the observed increase in heavy rain in this part of New Zealand, but they concluded that it was the most likely cause.
Many factors contribute to a storm's strength and rainfall intensity, particularly in short bursts. The amount of energy available is always a critical factor.
Climate change raises that amount of energy in two ways. First and foremost, everything is becoming warmer. Rising sea surface temperatures provide additional fuel for tropical cyclone development because they grow by heating from below.
Warmer seas may result in faster tropical cyclone development and stronger, more violent storms overall.
To support the formation of a tropical cyclone, sea temperatures must be at least 26.5°C. As the oceans warm, these storms will be able to travel further from the equator.
Second, warmer air has the ability to hold more water vapor. Every degree of warming adds about 7% to the maximum amount of water vapor. That extra water vapor tends to fall out as extra rain, but it also gives a storm more energy.
Causing Waves To Travel Further Inland
When the vapour condenses back into liquid water, the energy required to evaporate the water from the ocean surface is released, as per Greenpeace.
When clouds and rain form, a moister airmass heats the atmosphere more, making the air more buoyant and able to rise higher. This results in deeper, more vigorous clouds, stronger updrafts, and more rain.
Stronger storm updrafts require more air to be drawn into the storm near the Earth's surface, resulting in more "convergence" of air and moisture (water vapor).
That is why, even though a degree of warming results in 7% more water vapor in the air, extreme rainfall can increase by 20% or more.
All of this extra energy can help to strengthen the storm overall, with stronger winds and lower air pressures in its core.
This appears to have occurred with Cyclone Gabrielle. As the storm passed, several North Island locations recorded record low pressures.
Low pressures act as a vacuum cleaner, sucking the sea surface above normal sea level. Strong winds can then cause waves to travel much further inland. When the sea-level rise is factored in, coastal inundation can worsen much faster.
These processes also apply to thunderstorms. Thunder clouds frequently form as a buoyant mass of air over a warm surface.
As air rises (or convects), it cools and forces water vapour to condense back to liquid water, releasing heat and increasing buoyancy and speed.
Again, this draws more moist air into the cloud, and the convergence of moist air can increase rainfall amounts well above 7% per degree of warming for short bursts of very intense convection.
The stronger the convergence of moisture and the heavier the resulting rainfall, the more intense the convection.
Tropical cyclones have thunderstorm rings around their eyes when they are truly tropical storms.
They change their structure as they move from the tropics to our area, but they retain a lot of the moisture and buoyancy of the air.
An ex-tropical cyclone like Gabrielle, moving over very warm water, has the potential to be devastating.
This causes thunderstorm buoyancy to work even better and more strongly, encouraging very heavy rainfall.
Although the conditions appear to have improved, severe thunderstorms continue to develop. As summer gives way to autumn, the warmest seas move eastward away from us, and La Nia fades in the tropics, the chances of a repeat event diminish. At least for the time being.
Why has New Zealand experienced so much heavy rain in the weeks since late January? It's partly due to the extremely warm ocean waters surrounding New Zealand (up to marine heatwave conditions) and further north into the Coral Sea.
That, in turn, is related to the ongoing La Nia event in the tropical Pacific, which tends to accumulate warm water (and tropical cyclones) in the west.
However, it is also linked to ongoing global warming. Heatwave conditions become easier to achieve as sea temperatures rise. Warmer seas add water vapour to the atmosphere.
Part of the reason for this is that the air over the North Island has been unusually "unstable" recently, with temperatures near ground level being very warm but cooler than usual higher up.
