What Does This Super Indo Swell Mean For Australia? – Forecaster Blog

24 Jun 2015 4 Share

Ben Macartney

Chief Surf Forecaster

Issued Wednesday, June 24, 2015

The fact that the Southern Hemisphere is roughly comprised of four parts ocean to one part landmass (80% ocean to 20% land) is one of those curious facts that usually only comes to light when sitting around the dinner table discussing round-the-world yacht races or missing passenger jets. However, when it comes to gargantuan low-pressure systems like the one that’s been churning it’s way across the southern Indian Ocean this week, these geographical characteristics have more profound implications – at least for surfers. Over the last few days this nameless synoptic beast has spawned one of the larger, high interval groundswells we’re likely to see through the 2015 winter – and although the vast coastlines of Australia and Indonesia will feel the brunt of the groundswell, it’s reverberations will be felt far and wide into late June and the first week of July.

This Wave Tracker image depicts peak wave period, showing the leading edge of the groundswell spreading north and east across the Indian Ocean at peak intervals in excess of 20 seconds on Friday.

This Wave Tracker image depicts peak wave period, showing the leading edge of the groundswell spreading north and east across the Indian Ocean at peak intervals in excess of 20 seconds on Friday.

In Summary

  • A deep extratropical low traversing the southern Indian Ocean is the source of a large, long period groundswell currently fanning out north and east towards Indonesia and Western Australia.
  • The leading edge of the groundswell initially begins show across Western Australian coasts on Thursday at peak intervals of 24 seconds, ahead of the bulk of swell arriving from 230 degrees at 18 to 20 seconds on Friday, producing surf in the 10 to 15ft range across exposed breaks and larger at deepwater breaks.
  • Indonesia is next, with leading forerunners materialising late Friday/ early Saturday, leading in a steep rise in SSW groundswell from 210 to 215 degrees at peak intervals around 20 seconds. The bulk of the size fills in across Bali and surrounding island early on Sunday and Monday, producing sets in the 10 to 12ft range across exposed reefs under light to moderate ESE tradewinds.
  • Victoria and South Australia also receive the swell over a similar time-frame. Victoria’s Surf Coast is likely to be one of the better regions handling the deepwater energy, producing long-lined 6 to 8ft sets as it peaks on Sunday and Monday.
  • The groundswell continues to spread east into the Pacific Ocean to impact New Zealand, Fiji and even Australia’s East Coast into the final days of June and early July.


This synoptic giant maintained a central pressure of 922 hPa throughout on Monday and Tuesday before beginning to fill throughout Wednesday. Source: BOM.

This synoptic giant maintained a central pressure of 922 hPa throughout on Monday and Tuesday before beginning to fill throughout Wednesday. Source: BOM.

This Wave Tracker image depicts significant wave height. The storm produced a 30 to 50ft wave-field that's now spreading rapidly out towards Indo, Australia and beyond.

This Wave Tracker image depicts significant wave height. The storm produced a 30 to 50ft wave-field that's now spreading rapidly out towards Indo, Australia and beyond.

Overview
The lifecycle of this system commenced with the evolution of a series of deep, polar lows forming well below South Africa and Madagascar early to mid last week. These systems eventually amalgamated into a larger, complex storm on Friday before consolidating over he southwest Indian Ocean last weekend. By Monday morning the system had intensified into a single-celled, 922 hPa extratropical beast located over a remote polar region near 57S, 60E (verging on 4,000 nautical miles south-southwest of Indonesia). The system maintained this central pressure as it continued to move slowly eastward on Tuesday before starting to weaken throughout Wednesday.

During the height of the system’s lifecycle from Sunday to Tuesday satellite passes consistently confirmed a gale to storm-force WSW fetch exhibiting core wind speeds of 40 to 60 knots. This was flanked by a broader area of 30 to 40kt winds that, in its entirety encompassed most of the southern Indian Ocean on Monday and Tuesday. The exceptional length and duration of this fetch supported an even larger area significant wave heights ranging from 30 to 50ft plus over this time frame. While the swell heights are impressive it’s the sheer dimensions of the storm and the vast surface coverage of the resulting sea-state that are truly exceptional. The end result is a large, long-period SW groundswell – primarily impacting Western Australia and Indonesia, but also producing large to extra-large surf across South Australia and Victoria.

The phenomenal breadth and depth of the wave-field has a knock-on effect that extends well beyond the direct targets of Indonesia and WA. The early stages of the storm’s lifecycle has produced a SSW groundswell that’s already arriving across more western targets like Mozambique and Madagascar and it will continue to spread north to impact a the tropical coastlines of India, Sri Lanka, the Maldives. The eastward propagation of the storm carries the wave-field below the Australian mainland over the coming days to deliver smaller, albeit exceptionally long-interval groundswell to locations throughout the Pacific Ocean. The groundswell traces Great Circle paths to make landfall across New Zealand and Fiji on Monday and Tuesday respectively; the leading of the groundswell exhibiting peak intervals of 26 seconds as it arrives on Fiji’s doorstep early on Tuesday. Although the Eastern Seaboard will also feel the influence of the system it will only come in at a relatively small fraction of the size of most of the aforementioned targets.


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