Get Your MJO On
Get Your MJO On
While the southern states are shivering under relentless polar intrusions, to the north of the country it looks like we'll see increasing tropical activity into the final week of the year.
It's time for a little refresher on the bringer of tropical instability - the Madden-Julian Oscillation (MJO).
As the Southern Hemisphere transitions from spring into summer, the sub-tropical high pressure belt shifts further south. This is in reaction to the warming and expanding air in the tropics, and the westerly storm track retreating towards the pole.
Meanwhile, Central Australia also heats, creating areas of low pressure (because hot air rises) which draw in north-west winds from the tropics. This reversal/switch of winds from south-east trades - linked to the seasonal position of the sub-tropical high - to the north-west is known as the monsoon. Also, the area of convergence between the south-east trades and north-west breezes is called the monsoon trough.
The north-west winds usually contain moisture, transporting it south from the tropics, while the south-east trades are comparatively dry. Hence the arrival of tropical moisture during late spring/early summer to northern regions is linked to the north-west monsoon.
The bulk of the moisture falls along and north of the monsoon trough, however it isn't consistent; there are active and inactive phases. These phases are linked to a wave of tropical activity that circles the globe called the Madden-Julian Oscillation (MJO).
When the MJO becomes active, it activates the monsoon trough, bringing increased instability, moisture, and tropical storms. If conditions are favourable, we'll also see the formation of tropical cyclones. Favourable conditions being sea surface temperatures over 26.5°C and weak upper atmospheric winds (wind shear).
Currently the MJO is inactive but it's forecast to increase in intensity while passing through the Indian Ocean next week before moving towards the north of Australia and the Coral Sea around Christmas time.
Below is a forecast chart for the MJO. The further away the lines are from the centre, the greater the intensity of the MJO. The multiple shaded lines indicate varying model forecasts (ensemble), with the mean of the ensembles shown in black.
Making sense of the above MJO forecast, we'll likely see a couple of weak tropical systems forming in the Indian Ocean during the coming week ahead of a more significant system in the Coral Sea into the last week of the year.
The formation of any tropical storm relative to swell potential depends on a lot of wide ranging factors, but keen weather watchers will have plenty of time to peruse the long-range charts over the coming fortnight.
Hopefully the MJO behaves as forecast, gaining strength while moving to a position north-east of the country. If it does, the chance of a late Christsmas present are very high.