Developing superbomb in the Indian Ocean
As most of the East Coast sits waiting for the first decent swell of the year, it's of interest to cast our eyes over into the Indian Ocean and in particular, a pair of tropical cyclones.
An active phase of the Madden Julian Oscillation (MJO) across the western Indian Ocean has spawned two tropical cyclones, Ava off the Madagascar coast and the more intense Irving which is west-southwest of Cocos Islands.
As Ava stalls off the south-east tip of Madagascar today, Irving will make a bee line south-west but come tomorrow, Ava will be drawn like a magnet east-southeast on a collision course with the then southward tracking Irving.
When two volatile and energy charged cyclones combine in the Southern Ocean along with an infeed of cold air from the upper atmosphere, the end result can only be something of significance.
Regular Swellnet readers will be familiar with the weather term 'bombing low,' that being a low that drops 24hPa or more in central pressure within a 24 hour period.
The exact meteorological definition is that this 24hPa drop in 24 hours occurs at 60degrees latitude (called 1 bergeron), but if the storm is at a lower latitude (more towards the equator) less of a pressure drop is required for it to be classified as a 'bomb'. For example at 25degrees only a drop of 12hPa is needed for it to be classified as a 'bombing low.'
This is because storms in the polar regions usually exhibiting less strength (from a comparable pressure drop) due to the effects of latitude. When looking at a synoptic chart, if we have the same isobar wind spacing across the Gold Coast and Victoria, the winds will be stronger across the Gold Coast due to this latitudinal effect.
The storm in question is forecast to drop a phenomenal 47hPa in 24 hours at a latitude of 40-45 degrees south. At this latitude the low only needs to drop approximately 18hPa in 24 hours to be classified as a bomb, but it will clearly surpass this, intensifying at 2.6 bergerons.
As the intensification will be greater than 2 bergerons we'll see this storm fall into the superbomb/ultrabomb classification which is very rare. The last time I can recall a system like this locally was March 2007 - almost ten years ago - when the BOM issued their first ever hurricane wind warning for Southern Australian waters (read Alex Zadnik’s article here).
But moving away from the geeky weather talk, what most want to know is how much swell it will produce.
Current forecasts have the ultrabomb generating a fetch of storm-force W'ly winds (possibly reaching hurricane force) as it dips south-east through the Indian Ocean.
While the south-east track of the low isn't ideal, the slow moving nature and pure wind strengths will help generate large long-period surf ranging from Sumatra to southern Australia, with smaller very inconsistent energy filtering into New Zealand and Fiji.
A peak in size is expected across Western Australia on Monday afternoon (12-15ft Margaret River) with favourable morning winds, while Indonesia should see the swell mid-week, peaking to 8ft on the sets across swell magnets around Bali (smaller west of Java).
Coming back to the East Coast, and the MJO which is the source of tropical instability (namely cyclones) is forecast to move more towards the Australian region in the coming fortnight though it will be weakening. Ben Matson will provide updates on this in the East Coast forecaster notes over the coming weeks.