Long Wave Trough: The Waves That Create Waves
In the last five years the science of surf forecasting has come an awful long way. No longer is it enough to know that lows spin clockwise, highs counter-clockwise, and they're both reversed in the northern hemisphere.
Of course, if you have no interest in DIY forecasting then you don't even need to know that much, you can simply read our wave graphs to find out what the surf will be like. Yet for those of us that never treat the weather as mere small talk the art of surf forecasting is a continual learning process.
If you live in any of the southern states and read Swellnet's extended forecast notes you'll occasionally see the term 'Long Wave Trough' mentioned. The term often appears in relation to good swells so it's worth knowing what a Long Wave Trough (LWT) is and how it works.
When learning about LWT's the first thing the lay surf forecaster has to understand is that all the world's weather patterns are controlled by the upper atmosphere.
Chart 1 shows a snapshot of the upper atmosphere in the Southern Hemisphere on Thursday the 12th of May, with Australia located in the bottom lefthand corner. If you follow the undulating orange contours around the world, this is the upper level LWT that is often mentioned in our forecasting notes for the southern states.
If you take a look you can see four pronounced areas of blue on the chart: one between Australia and New Zealand; another to the west of South America; a third to the east of South America; and a fourth to the south-east of South Africa. These are called nodes. This particular LWT has four nodes, yet LWT's can have more or less nodes than this. The number of nodes it has controls the speed of the LWT as it circulates eastward around the Southern Hemisphere. The speed increases when there are more nodes (4-5), and decreases when there are less (2-3).
These upper level waves influence surface weather with cold fronts forming in the Southern Ocean following a very similar path to the contour lines shown. You can see that the LWT showed a peak between Australia and New Zealand on Thursday morning, with the node to the west of South America being even more pronounced.
When one of the LWT nodes is positioned between Australia and New Zealand as shown in chart 2, any cold front forming in the Southern Ocean is steered up along the contours towards the southern Tasman Sea producing south-southwest swells for Victoria and Tasmania, with southerly swell energy impacting New South Wales.
Much like pressure gradients at the surface, the tighter the contours on the chart, the stronger and more pronounced the LWT is in that area and the greater the likelihood of cold fronts strengthening as they push up from the Southern Ocean. Chart 3 shows a surface front pushing up from the Southern Ocean across the south-east corner of the country as a result of the positioning of the LWT.
The recent cold weather experienced across the south-east of the country is associated with the LWT moving slowly east across Tasmania (chart 4). A succession of vigorous cold fronts have been projected up, one after the other, across the corner of the country providing plenty of swell for Victoria, Tasmania and New South Wales.
This all came to a peak during the first half of last week with Victoria and Tasmania seeing large waves on Tuesday, with the New South Wales coast experiencing a strong episode of southerly groundswell on Wednesday.
Fiji is also a big beneficiary when the LWT is parked across the Tasman Sea, with swells hitting 2-3 days after they impact New South Wales. This was the case last Friday, with Cloudbreak and other surrounding reefs seeing huge surf.
Swellnet will always keep you informed of the presence or position of the Long Wave Trough in our extended forecast notes, but if you'd like to follow it yourself click here for the charts. //CRAIG BROKENSHA