Adam Wright
(Tuesday) 6.12.12

South Pacific Storm Strength and Swell Decay

categories: Solspot Q&A

Matt Asked:

Looking at the forecast charts…if the mild storm passing New Zealand gets a good shot, and the high pressure lifts due to a push by WSW winds, could it pick up intensity?

Adam Wright – Solspot Forecaster:

definitely…but a couple of other pieces need to move into place to really have produce anything. Storms by New Zealand are about as “realistically far” distance wise where we can still receive a significant swell, but the storm has to be on the strong side of the bell curve to get that much energy all way from one side of the Pacific to the other. So while a moderate storm can produce a swell for us, a good portion of that energy decays away as it travels.

The general rule of thumb, is that a swell will lose roughly 1/2 to 1/3rd of its swell size as it travels the equivalent “mileage” of its swell period (measured in feet). So to get that number you basically do the math of how many feet are between peaks in a swell travelling through deep water. (I have a chart somewhere in my desk but honestly it is the weekend and hunting for the exact number so lets work with rough numbers).

A 16-second swell travels about 25-mph, which is 36.6 feet per second. So to get the distance between peaks you multiple the swell-period…in this case 16…so 16 by 36.6 and get about 623 feet between wave peaks. Now you basically take the same number of miles (623-miles in this case) and then divide the distance a swell has to travel. Lets say 1800 miles for the easy math. That means if you start out with a 20-foot swell @ 16-seconds then after the first 600 miles the deepwater wave heights will have dropped to 13.6-feet, after another 600 miles the wave heights would be down to 8.8-feet, and then the final 600 miles would drop it to 5.9-feet @ 16-seconds. The decay is a cascade effect and is a constant state of dropping once the swell has left the storm area.

0-miles = 20-feet @ 16 seconds at the swell’s starting location.
600-miles = 13.6-feet @ 16-seconds
1200-miles = 8.8-feet @ 16-seconds
1800-miles = 5.9-feet @ 16-seconds

So you can see how fast a swell loses energy as it travels. The distance from that area of NZ South Pacific waters that we are talking about is roughly 6000 miles so 20-feet of swell down by New Zealand would only amount to about 0.6-feet of swell @ 16 seconds when it arrives in Socal, which is why we need a storm that generates at least 35-40-foot seas to send us much in terms of rideable waves from that area…really we would like to have heights breaking the 40-45-foot mark.

To get back to your question, yes it is conceivable to get a mild storm to send us waves from that area but the resulting swell will be small at best.

Right now, what I am hoping is that these first mild storms can punch a hole in the high-pressure, or at least push it further out of the way so that if/when we see a stronger storm develop in the wake of this first system that it has a better position in the swell window and is able to capitalize on the weaker high-pressure. Now the first storm could always get lucky and pull in some warm air, or link up with a tropical system that is jumping latitudes with a crapload of extra-latent heat energy, which would cause it to flare into something much bigger, but right now that high-pressure is set up a wall to keep that from happening. Personally I would rather have the 2nd-3rd storms (when they form, push further into the South Pacific (and our S-SSW swell window) before doing something similar, but it is mostly for selfish reasons since most of my local spots have trouble with shadowing from our nearshore islands if the swell is too SW in swell direction, so if they can be in our S swell window my spots see bigger waves.

Sorry for the long winded answer, be thankful I didn’t put in anything about shadowing from the SPAC Islands, but hopefully you can see how even a moderate increase in storm activity doesn’t help out a ton…we really need to have the storm go nuclear before we can expect a marked jump in swell and wave heights from that region. One thing to keep in mind, it is why storms in the NPAC can produce so much bigger swells for California…is the storm distance, the closer the storm, the less decay, the bigger the waves.

  • Declan

    That’s actually really cool to know. I didn’t know the rate of decay was so rapid. Well maybe not that rapid considering how far away NZ is.

  • C2C

    Good stuff, fire the bogeys south pacific

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