Forecaster Blog: What does a weak low off Vietnam have to do with flat surf along the East Coast?

4 Jan 2018 0 Share

Ben Macartney

Chief Surf Forecaster

COASTALWATCH | FORECASTER BLOG

Issued Thursday, 4 January 2018

If you live anywhere along Australia’s eastern coastal fringes, you might be wondering where in the world this season’s tradewind swell and cyclone season might be hiding. Apart from a weak tropical cyclone (Hilda) that developed off Western Australia’s northwest coast late in December, the monsoon trough and tropical storm-activity in general has remained heavily subdued across the continent. Indeed, according the BOM the onset of the Australian monsoon is yet to officially set in and this lack of activity is also extending to the broader Southwest Pacific, meaning easterly tradewinds are still extremely weak or non-existent inside Queensland’s swell window.

It looks like Queensland won't see any of this kind of wave-action until mid to late January - at the earliest. Photo: Simon Muirhead.

It looks like Queensland won't see any of this kind of wave-action until mid to late January - at the earliest. Photo: Simon Muirhead.

The knock on effect is a virtual absence of the easterly tradewind swell that we’d usually see gracing the region in January. So what’s happened? Is global warming putting our seasons out of whack? Are covert Chinese satellites sucking in all the convective energy out of the tropical Pacific Ocean and depositing it in the South China Sea to generate swell for their Olympic Surfing team? While the former may well be factor, the answer to the latter is, surprise, surprise; probably not.

The reasons are however, inextricably linked to the northern side of the equator. Late-season tropical activity has been inhibiting the cross-equatorial flow we’d normally see coming out of the South China Sea. According to the BOM, this due to a persistent, late-season monsoon trough that’s still in place north of the equator; supporting a weak tropical low (Bolaven) now located near Vietnam and a second low over the Bay of Bengal.

This satellite image depicts a tropical low (Bolaven) off Vietnam. It's one of two storms still feeding off an active monsoon trough over the tropical northern Pacific Ocean, which is inhibiting cross-equatorial flow and delaying the onset of the Australian monsoon. Source: Joint Typhoon Warning Centre.

This satellite image depicts a tropical low (Bolaven) off Vietnam. It's one of two storms still feeding off an active monsoon trough over the tropical northern Pacific Ocean, which is inhibiting cross-equatorial flow and delaying the onset of the Australian monsoon. Source: Joint Typhoon Warning Centre.

At the same time the MJO (Madden-Julien Oscillation) is located out over the tropical western Indian Ocean and probably won’t begin to influence Australian longitudes until later next week. The MJO is a key modulator of tropical activity and it probably won’t be until it propagates across the mainland and into the Southwest Pacific over the course of next week that we’ll see the monsoon finally setting in. The upshot is better chances for new swell of tropical origins to start develop into mid to late January. Fingers crossed it will kick off with something spectacular.

A tropical low developing over Western Australia early next week may be a precursor to the onset of the Australian monsoon mid to next week.

A tropical low developing over Western Australia early next week may be a precursor to the onset of the Australian monsoon mid to next week.


Tags: Forecaster , Blog , Cyclone , Season , Ben , Macartney (create Alert from these tags)

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