Where is the Monsoon - ABC Kimberley
Monsoons are commonly associated with torrential downpours that bring widespread flooding to parts of Asia and the Indian subcontinent.
But Australia has its own monsoon and it is important too, despite its lower profile.
This year it is late.
Pieter Classen, a meteorologist with the Bureau of Meteorology in Darwin, said we're not in uncharted waters just yet but rainfall has been below average.
"Darwin has received 388 millimetres so far this wet season, while its average to the end of January is almost 900mm, so we have quite a lot of catching up to do."
Last year the true monsoon didn't arrive until January 20, but Mr Classen said it brought a "ripping" burst that transformed the season from one that was below average to being well above average.
He said there were "good indications" that the latest monsoon trough could deepen and "potentially" move south towards the end of next week.
So while we wait expectantly, here is a crash course in monsoons.
Why does the monsoon happen?
A monsoon is the general name given to the phenomenon of widespread and heavy seasonal rain.
The Australian monsoon and "the wet" (details later) are triggered by the same process that governs most of our weather — the relationship between Earth and the Sun.
There are some key Sun-Earth relationship points:
1. Earth is a sphere
Yes, it is a no brainer (flat-earthers notwithstanding) but it really matters.
Because Earth is a sphere, the incoming energy from the Sun is concentrated near the equator. As you go north and south, the equivalent amount of energy is spread over a larger surface area.
This means around the equator is very hot, causing the moist air to rise like steam from a kettle. This forms the Intertropical Convergence Zone (ITCZ).
The ITCZ is an area of buoyant, moist air which sits around the equator, ready to drop buckets of rain at the slightest opportunity. It encourages the moist air down from the north to form the monsoon trough.
Main point being: there is a band of rainy conditions around the equator.
2. Earth is on a tilt
The tilt of Earth's rotational axis as it orbits is just enough so where the Sun's energy is concentrated changes throughout the year, dictating our seasons.
In December and January, when the Southern Hemisphere is pointed more directly at the Sun, the energy is focused further south. This causes the Southern Hemisphere summer and the southward movement of the ITCZ, bringing the rainy zone over the north of Australia.
In June and July, the north of the planet is pointed towards the Sun, bringing summer in the Northern Hemisphere and the ITCZ's shift north of the equator, taking the rain with it.
It would be nice if that was the end of the story, but there's a lot more going on out there.
Complication 1: Land and oceans heat up differently
When the amount of energy changes, the temperature of the air over the land changes quickly in response. However, because the oceans are able to mix and store the energy, it takes much longer to respond.
This means that when summer comes around and there is more energy available over Australia, the land warms up more quickly than the surrounding oceans.
The hot air over the land then rises, causing a vacuum at surface level which helps suck the ITCZ further south. It is the same process which creates ocean breezes.
But there is a delay in the time between when it starts warming up and when the full-blown ITCZ or monsoon trough moves over Australia.
This nasty period between when the temperature and the humidity increase and before the sustained rains arrive is known as the "build-up" — it's said to make some people in the Top End go troppo.
Complication 2: Wind
Global wind patterns are a beautiful product of Earth's spin, gravity, friction and the Coriolis effect.
Generally, in the north of Australia the winds blow from the south-east and are known as the south-easterly trade winds. In the Top End these south-easterlies have come across the continent so bring with them dry air.
But when the ITCZ moves down, the south-easterly trades are replaced by north-westerlies. These north-westerly winds have come across the Indian Ocean and pump moist air into the already primed system.
The monsoon is defined by this wind shift — a particular monsoon event is signalled by a sustained 120-degree (or more) swing in the source direction of the normal easterly winds.
The wet season does not strictly align with the onset of the monsoon.
The BOM defines the northern rainfall onset, or the start of "the wet", as when at least 50mm has fallen since September 1, although most people view the northern wet as simply the period from November to April.
In reality, the traditional monsoon only reaches a small part of northern Australia and can come and go; the Indian monsoon is stronger and more sustained.
As a consequence, "the wet" is the broader term in the sense of both the physical land area affected and duration of the season.
Complication 3: Breaks
Even when the monsoon is present, it can be interrupted by monsoon breaks or an inactive phase, when the ITCZ weakens or retreats north and rainfall lessens.
Monsoon breaks are often associated with changes in the Madden-Julian Oscillation, an atmospheric wave that travels around the equatoreither encouraging or discouraging lift and rain.
When and for how long?
Perhaps the most consistent thing about the monsoon is its inconsistency.
The 50mm-of-rain-since-September wet season trigger is generally passed in Darwin by the start of November whereas the monsoon arrives later, typically in late December as the winds swing to come from the west.
There is a large amount of variability in both start dates.
The variability in the rainfall onset has largely been linked to the El Nino Southern Oscillation (ENSO) with it starting earlier in La Nina years and later in El Nino years.
The duration of the monsoon can also be inconsistent, ranging anywhere from two weeks to four months.
How much does it rain?
All of these stops and starts mean that total wet season rainfall is highly changeable from year to year, but northern Australia has experienced an increase in summer rainfall over the past few decades.
With all this inconsistency, links between climate change and the Australian monsoon are still largely uncertain.
The wet is wet and the dry is dry
Darwin's mean annual rainfall is 1,728.8mm — 87.5 per cent of it is recorded between November and March; 97.4 per cent between October and April.
The Australian monsoon is an integral part of life for those living in the north.
In Tiwi country, Jamutakari is the name of the wet season which falls between December and February.
It is the time when it rains every day and Wunijaka (the north-west wind) blows.