Does The #SuperMoon Affect Surf Conditions?

15 Nov 2016 2

Ben Macartney

Chief Surf Forecaster

A Super Spectacular Moon, Photo by Murray Fraser from SPROUT DAILY

A Super Spectacular Moon, Photo by Murray Fraser from SPROUT DAILY

COASTALWATCH | FORECASTER BLOG

Issued by Ben Macartney, 15 November 2016

After listening to the media hyper, surrounding the Supermoon on Monday, I have to admit it was an anticlimax to see it that evening – undoubtedly big and bright – but not exactly the giant glowing orb of Independence Day scale that my expectations had blown it up to. Along with a good excuse to go the beach at night and partake in full-moon inspired lunacy like midnight nudie runs, the Supermoon is the producer of extreme tides – both today and over the next few days.

SEE ALSO: The 2016/17 Tropical Cyclone Outlook

Big tides can have also sorts of beneficial effects on the surf-zone - you just need to be there when it happens. Photo: CW user Pete20.

Big tides can have also sorts of beneficial effects on the surf-zone - you just need to be there when it happens. Photo: CW user Pete20.

It’s not unusual to experience big tidal swings during a full moon, but the term Supermoon is only used when a full moon coincides with extreme perigees – or its closest approach to the earth. On Monday 14 November, the moon came the closest its been to the earth since January 26, 1948.

It’s the moon’s elliptical orbit that produces the wide variances in its distance from the earth (according to Wikipedia, a distance roughly ranging from 357,000 and 406,000 kilometers). It’s this fluctuating distance that ensures we see constant variances in the tidal extremes. As the moon approached its closest point to the earth in 68 years on Tuesday it was 356,508 kilometers away – which apparently is within 137 kilometres of its closest possible approach – so this definitely amounts to an extreme event on the Supermoon scale.

Good for Surf?

While checking the surf today, someone asked me today if the Supermoon was the source of the 4ft S swell we were looking at. The answer, of course, was an unequivocal no.

Rule 1: Swell is generated by wind at the sea-surface, but that doesn’t mean the moon doesn’t influence surf.
As most of us know, the extreme tidal variations produced by the Supermoon have a profound impact on intra-day conditions, via several mechanisms. The first, and most visible, is the extreme changes in the depth of local bathymetry (the contours of the seafloor). Prima facie, that means changes in depth of the sand bank or reef you’re surfing over. A tidal range of 2 metres combined with, say 1 to 2 metres of swell, means the same bank your surfing at high tide you’ll be walking across at low tide. The low tide can also cause breaks that usually only light up in big swells to break when its head high – and these kind of nuances are unique to each break.

What’s less apparent is the deeper-water influences on the nearshore zone. Extreme low tides also make offshore features, like deepwater bombies and seamounts, much shallower than usual. Extreme low tides can exacerbate their modifying effects on swell, by causing relatively small swells to wrap and refract in ways not usually seen. The exposure of such offshore features can also have a much stronger blocking influence on inbound swells, by effectively expanding their footprints. This is one reason why you might see a beach-break that’s firing at 3 to 4ft at high-tide rapidly fading to 1 to 2ft dribble by dead-low – only to switch back on again with the return of the tide. 

Sydney sees low tides of 0.1 metres and highs of 2 metres on Tuesday and Wednesday – which is about as extreme as you’re likely to see.

Extreme low tides of 0.1m in Sydney on Tuesday and Wednesday will expose much of the nearshore zone.

Extreme low tides of 0.1m in Sydney on Tuesday and Wednesday will expose much of the nearshore zone.

Conversely, extreme high tides can dilute the influence of these offshore features, temporarily allowing for more swell to penetrate breaks in their lee. The other, less visible impact is the extreme tidal influences on currents. At one level, that means big changes in the tide will enhance the strength of rips - and more generally water movement through the surf-zone – and this can result in choppier, more agitated surface conditions.

Then there’s big waterways like Pittwater and Botany Bay in Sydney. As the tide drops, the nearshore evacuation of many gigalitres (one gigalitre equals 1000 million litres) of briny can generate powerful currents; easily strong enough to put a temporary halt on the propagation of inbound wave-trains at breaks adjacent to these waterways.  

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