If you read the East Coast Forecast Notes you may be aware of the increasing use of the term 'trough block', though you may not understand exactly what a trough block is.

Well fear not, dear readers, let us shine a light into the darkness with this short Q&A.

The reason we bring this to you now is because much of the swell on the East Coast this past week, and also the ramping up in size this weekend, is due to a trough block.

So let's not waste any time...

Who invented the term?
Though there was speculation in the Swellnet office, a Google search shows Ben to have first used the term in July 2021. Even deployed a trademark™️ symbol throughout the forecast notes, though the inconvenience of that gave it a short life.

First known use, confused everyone

What is a trough block?
Trough blocks are used on farms to unblock stock troughs from algae growth, and same goes at particularly busy nightclub troughs. That's not what we're talking about here.

For our purpose, the trough refers to an area of low pressure being blocked by a high pressure system. Why it's worth naming and writing about is all because of it's positioning: the trough moves across the East Coast bringing light winds, then it squeezes against the outer edges of a high creating a tightening pressure gradient and an increase in wind strength - and hence waves.

What causes a trough block?
Highs (especially strong ones), are stubborn and when approaching frontal systems or troughs try and push in from the west, they’re usually hit by a fairly substantial brick wall - either deflected away or weakening on approach. It’s also why they’re given the name ‘blocking high’.

Generally the setup is best when the high sits over New Zealand, opening up Australia’s East Coast to a broad area of easterly trade-winds while at the same time, inland instability moves offshore, feeding off the warm East Australian Current.

The classic set-up: slack isobars over the East Coast indicate light wind, while a trough line just offshore abuts a large high pressure system. The convergence increases the pressure gradient which creates wind which creates swell.

What is the result?
Swell! And from an easterly angle, which is welcomed by most surfers on the East Coast. In terms of swell production, troughs are much better than frontal systems as the troughs move slower and can feed off off the warm East Australian Current, resulting in a slow, gradual increase in the fetch and winds speeds around the high.

Who benefits?
The main beneficiaries are initially the East Coast beaches as low-period tradeswell slowly builds, while the point and reef breaks come to life when the trough-block reaches maturity, bringing larger, longer-period easterly swell energy.

Depending on how broad the high is, we generally see quality surf from Queensland all the way down to Tasmania’s East Coast. There’s also the longevity, and this comes from the fact that the trough is blocked. It pushes very slowly to the east, resulting in a slow easing trend and long-tail of the swell.

The cons for such a setup fall mostly with Victoria, thanks to blocking highs reducing swell-generation, but it does bring periods of favourable winds to exposed beaches which is generally not the norm in winter.

It's not all about the beaches. The swell that arrived on 06/06/16 - the Black Nor'Easter - was created by a trough block. Beau Mitchell's first wave at Queensie Bommie during that mighty swell (Agnes Durlik)

Are they becoming more common?
In our eyes they are, especially over the past decade or so with less frontal activity penetrating the southern states, resulting in weaker, troughier disturbances and mid-latitude lows across the continent.

Is it thanks to a changing climate?
Maybe. Predictions for a warming planet and expanding tropics identify an increase in the size of stalling high-pressure systems - that'd be our blocking high - and less frontal intrusion from the Southern Ocean. This is in line with what we’ve been seeing in recent years, though the background La Niña signals have exacerbated the presence of high pressure across the mid-latitudes.