As a new long period ENE groundswell from Tropical Cyclone Victor (TC Victor) begins to hit our shores in Sydney (a rare treat, especially for the Sydney goofy footer), I thought it would be useful to reflect upon the difference between Groundswell vs. Windswell and Why it Matters to Surfers.
Different but the Same
Here is the kicker:
from a scientific standpoint, windswell and groundswell are the same thing.
Energy transfers from wind to water as wind blows across the water’s surface. In general, the stronger the winds, the longer the duration and larger the distance of the fetch (fetch, also called the fetch length, is the length of water over which a given wind has blown), the more energy is transferred to water and the larger the resulting sea state will be.
Distance from the source of the swell to the place where the swell makes landfall in the form of the waves we ride matters. As the energy in the swell organises and consolidates over time and distance into more defined swell trains, the power in each peak of the swell increases. It’s important to note that this consolidation also can cause swell trains to become further apart, making surfing conditions ‘inconsistent’.
This act dictates the ‘swell period’ or ‘swell interval’ of the waves that are ultimately produced.
Why It Matters to Surfers
Swell Period is simply the measurement in time between waves in a set. Watch a swell train from a solid, 15 second period groundswell hit a reef in Indonesia and, from wave-to-wave in a given set, the peak (top) of each wave will pass a given point exactly 15 seconds apart. It’s a thing of beauty.
But why swell period matters to surfers is due to the power in the waves of a given swell. This power is at the heart of the difference between Groundswell vs. Windswell and Why it Matters to Surfers.
Take two examples:
Example 1: a localised Northeast wind swell (typical in Sydney summers) is generated by a high pressure system just off of the coast. The winds may be strong but short lived and the distance from the wind source to the place where waves break is relatively short.
RESULT: While the result may be head high waves, the swell period will likely be quite low, say an 8 second period, and the waves may be weaker / slower / flatter (or more ‘average’ as we refer to them often here) , depending on the bathymetry where waves break. The waves in this scenario may also be wind affected given the proximity of the wind source to the place where breaking waves occur.
Example 2: back to our Tropical Cyclone Victor (TC Victor), which has produced a long period groundswell hitting Sydney right now (I have to finish this promptly and go surfing…), the always enlightening Coastalwatch chief forecaster Ben Macartney at summarises the swell event:
“a long-period episode generated by Tropical Cyclone Victor (TC Victor) – a category 3 system that developed over the remote South Pacific, east of Samoa last Friday. Although its location places TC Victor just under 2,500 nautical miles away from Australia’s Eastern Seaboard, the system is responsible for several days of mid-sized and occasionally large ENE groundswell across the entire East Coast (of Australia).
Although this fetch south of the cyclone’s eye only exhibits wind-speeds of 20 to 35 knots, it’s slow southward drift is extending the longevity of this fetch – thereby facilitating constant wave-growth over the same area of Southwest Pacific Ocean.
Maximum significant wave height generated by TC Victor was up around 25ft on Monday morning and it reached a maximum of 30ft plus on Tuesday before beginning to subside in line with a slow weakening over the last 24 to 48 hours.
The real upside of TC Victor is its extended, slow moving presence over the Southwest Pacific pointing to an extended run of long-period ENE groundswell. Going on latest model runs, Friday’s first pulse is backed up by a stronger round of ENE groundswell on Saturday – peaking at stronger 4 to 5ft plus levels at breaks predisposed to the ENE direction during the afternoon at long peak intervals of 15 to 16 seconds.”
Read the full swell analysis on Coastalwatch here.
RESULT: Far from flatter, slower, weaker waves, this 15-16 second period ENE groundswell should produce, power-packed, fast and punchy waves (again, bathymetry plays a big role in the character of the breaking waves).
What defines the difference between windswell vs. groundswell? Well, Surfline, for example, put a breakpoint at a 10 second period. Mid to long period groundswell is above a 10 second period and short period wind swell below a 10 second period. The swell’s source, which is usually localised (as in example 1) or created a long distance away (as in example 2), is also a good driver of groundswell vs. windswell.
Regardless, when a long period groundswell meets your local with good conditions it’s game on.
See you in the water!
Swell (ocean): https://en.wikipedia.org/wiki/Swell_(ocean)
Fetch (geography): https://en.wikipedia.org/wiki/Fetch_(geography)
Surf forecasting: https://en.wikipedia.org/wiki/Surf_forecasting
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