Why The Swell Train Is Often Late
What's the biggest swell period you've ever seen recorded at your local wave buoy?
On Tuesday morning the Point Nepean wave buoy, located in Bass Strait, recorded an incredible peak period of 25 seconds. The impressive swell period was associated with the arrival of a long-range groundswell generated by an intense storm in the deep Southern Ocean.
The Cape du Couedic wave buoy in South Australia also registered this swell at a similar time with peak periods of 22 seconds. The interesting thing regarding this swell is that a peak in wave energy was not seen until 24 hours after the initial arrival of the swell when periods had dropped to 15-16 seconds.
So why a 24 hour time lag between the peak wave period and peak of the swell?
We're about to get a bit technical here so try and stay with me...
Storms in the Southern Ocean often have large variations in wind speeds which result in numerous swell trains, each with unique characteristics, travelling out and away from the storm. The general rule of thumb is that the stronger the wind speed, the duration and the fetch length, then the larger the period of the resulting swell.
The storm responsible for this weeks swell produced maximum wind speeds in excess of 60kts initially, but for the majority of its life winds were between 40 and 50kts.
The initial 22-25 second period component of the swell was generated by a short burst of 60kt+ winds, yet due to the storm blowing at 40-50kts for the majority of its life cycle the main body of the swell was of a lesser period. These waves registered in the 15-16 second bracket and provided the bulk of the storms wave energy.
That explains the creation of different swell trains but why such a lag in time between the peak wave period and peak of the swell?
The large time difference is a product of swell trains with higher periods travelling through the ocean faster than their lower period counterparts. This is a common occurrence but in this case the time difference was greatly amplified because the swell had to travel 5000 km's to reach the Southern Australian coast. Therefore there was a significant time difference – 24 hours – between the arrival of the fast-moving, long-period forerunners and the peak in wave energy.
So next time you see a jump in swell periods on the buoy, it may be worth just holding out for a while as the swell train may be running late. //CRAIG BROKENSHA