Trade vs cyclone swell, a real time example
The northern NSW south-east Queensland coast are currently days into an extended run of easterly trade swell and favourable winds, with a larger, stronger increase in cyclone swell due to make landfall tomorrow.
Excluding the coming energy from Tropical Cyclone Lucas (now extratropical), the run of swell seen through the week, (even in the lead up to it) has been generated by a broad, elongated and persistent fetch of easterly trade winds blowing for days on end. A fully developed sea state ensues, with great radial spread, reaching from Queensland to southern NSW, albeit with less size.
Such setups, as seen into summer and early autumn develop when a strong high pressure ridge sitting either across the Tasman Sea or New Zealand, is squeezed ever so slightly on its northern flank by tropical depressions and instability throughout the Coral Sea and Western Pacific Ocean.
Without too much effort at all, a persistent fetch of 25-30kt winds will whip up a fully developed sea state, creating surprisingly well organised, and consistent easterly trade swell sets between 3-5ft which on any other coast (such as those open to the Southern Ocean), would hardly rise an eyebrow.
These patterns, once established see days of surf and any further intensification, say from a deepening tropical low or cyclone, will super-charge the active sea state, generating even larger, more consistent surf again.
The beauty of the setup is that the high pressure systems cradle and support any tropical developments forming to the north, as is the case right now with extratropical cyclone Lucas and a high in the Tasman Sea.
Now, fast forward to Monday next week, where another, much more significant tropical cyclone is forecast to form around Fiji, drifting south through Queensland’s and northern NSW’s swell window over the following days.
This cyclone looks much more impressive on the Mean Sea Level Pressure charts, but what’s missing? The supporting high pressure ridge.
With lower pressure to its south and a high-pressure ridge to the south-east, the cyclone will ‘fall’ south through the swell window, but more importantly, the strongest winds are always where the cyclone squeezes the high, and that’ll be on its eastern and south-eastern flank.
Instead of an easterly fetch, we’ll see a northerly fetch, aimed straight south towards the pole.
And therein lies the theme commonly pushed by us on Swellnet. That being, tropical cyclones alone don’t necessarily equal great swell and you’re better off keeping an eye out for days of consistent, all-encompassing trade-swell rather than a more focussed, inconsistent, one day wonder.