Forecaster Blog: Tropical Cyclones Ava and Irving to Merge Over the Indian Ocean

9 Jan 2018 5 Share

Ben Macartney

Chief Surf Forecaster

COASTALWATCH | FORECASTER BLOG

Issued Tuesday, 9 January 2018

The convergence of two ex-tropical cyclones over a remote part of the southern Indian Ocean looms as the catalyst for a major swell-event for Australian coasts next week. This event is projected to impact both Western Australian and the Southern Australian coasts; taking the form of a high-energy SW groundswell arriving at long peak periods in excess of 20 seconds. The Tropical cyclones (TC’s) in question are TC Irving and the now ex TC Ava – both positioned over disparate parts of the Indian Ocean.

This was the end result of tropical Cyclone Iggy, which formed off Western Australia back in January 2012, generating a rare NW groundswell for the West Coast. This time around, ex-TC's Ava and Irving will combine to deliver a large SW groundswell. Photo: Micheal Vogelslanger.

This was the end result of tropical Cyclone Iggy, which formed off Western Australia back in January 2012, generating a rare NW groundswell for the West Coast. This time around, ex-TC's Ava and Irving will combine to deliver a large SW groundswell. Photo: Micheal Vogelslanger.

At face value, the development of a tropical cyclone usually holds fairly straightforward swell-potential. Along the Eastern Seaboard that usually means some kind of NE or E swell-event aimed at most directly at southern Queensland and northern NSW coasts. On the West Coast a cyclone can, once in a blue moon, produce in a freak NW swell; but otherwise the vast bulk of swell is easterly – and therefore aimed westward at Madagascar and Africa’s eastern coast.

The BOM's southern hemi chart depicts the still active tropical storms, TC Irving and ex TC Ava, still located over disparate parts of the Indian Ocean. Source: BOM.

The BOM's southern hemi chart depicts the still active tropical storms, TC Irving and ex TC Ava, still located over disparate parts of the Indian Ocean. Source: BOM.

What’s usually less-apparent is the latent swell-potential associated with the final stages in a tropical cyclone’s lifecycle; better known as extratropical transition. As TC’s move south into the mid latitudes (usually between 30S and 40S), they encounter the baroclinic zone, where increasing vertical wind-shear and lower sea surface temperatures invariably lead to a rapid weakening and even complete dissipation of the system within a few days. However, as an ex TC move into this environment they also encounter larger and much colder, drier air-masses – and if conditions are right the collision of the systems can culminate in the development of a much larger, supercharged extratropical (cold core) low.

Tropical Cyclone Irving, located about 760 nautical miles SSE of Diego Garcia, is tracking swiftly southwest and will eventually curve poleward, before merging with ex-TC Ava, depicted below. Source: JTWC.

Tropical Cyclone Irving, located about 760 nautical miles SSE of Diego Garcia, is tracking swiftly southwest and will eventually curve poleward, before merging with ex-TC Ava, depicted below. Source: JTWC.

TC Ava hammered Madagascar earlier this week before weakening and moving away to the southeast on Tuesday. It's forecast to merge with ex TC Irving over the southern Indian Ocean on Thursday. Source: JTWC.

TC Ava hammered Madagascar earlier this week before weakening and moving away to the southeast on Tuesday. It's forecast to merge with ex TC Irving over the southern Indian Ocean on Thursday. Source: JTWC.

That’s exactly what’s forecast to occur as the two ex TC’s merge into a deep, mid-latitude storm later this week. Latest model guidance exhibits strong agreement on this scenario; indicating the storm’s central pressure will bomb into the 930’s or 920’s just east of the Kerguelen Islands on Friday and Saturday.

The low is projected to generate deepwater wave-heights of 40 to 50 feet over the Southern Ocean on Friday and Saturday, giving rise to a large, long-period SW groundswell impacting Australian shores early to mid next week. Source: Wave Tracker.

The low is projected to generate deepwater wave-heights of 40 to 50 feet over the Southern Ocean on Friday and Saturday, giving rise to a large, long-period SW groundswell impacting Australian shores early to mid next week. Source: Wave Tracker.

A resulting, severe gale to storm-force westerly fetch generated by the storm is projected to drive deepwater seas and swell to peaks of 40 to 50 knots over the Southern Ocean later Friday through early Saturday; in turn spawning a long-period SW swell-event across Australian shores. Western Australia will be the groundswell’s first port of call; filling on Monday and Tuesday to produce large, powerful surf across the Southwest.

Latest Wave Tracker runs depict peak wave period, showing the leading edge of the groundswell arriving across Australian coasts at peak intervals of 20 to 22 seconds early to mid next week.

Latest Wave Tracker runs depict peak wave period, showing the leading edge of the groundswell arriving across Australian coasts at peak intervals of 20 to 22 seconds early to mid next week.

A smaller version of the groundswell is then projected to hit South Australia and Victoria on Wednesday and Thursday. Given the two storms are yet to merge as they undergo extratropical transition there’s still plenty of scope for changes to the projected size and timing of the swell, so stay tuned to the detailed forecasts and keep an eye out for further updates as the week progresses.


Tags: Ben , Macartney , Forecaster , Blog , Bomb , Low , Indian , Ocean (create Alert from these tags)

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