Ocean Science 101: South Pacific Island Shadowing

It is time for another installment of Armchair Forecasting 101. Where I give you random information about surf forecasting that will probably have zero impact on your ability to get waves but will make you a way more interesting conversationalist at parties.

In this post I will cover a term that you will probably see in a lot of my forecasts as we head into the spring, summer, and fall. This term is South Pacific Island Shadowing and is sometimes known as the Tahitian swell shadowing.

South Pacific Island Shadowing occurs for Southern California when we get SW swells that are coming in from a 205- to 220-degree swell angle. This shadowing process can greatly effect the size and consistency of a SW swell and is generally responsible for turning what should have been a great swell event into a mediocre (and sometimes very frustrating) one.

Swells generated in the South Pacific storm track that are approaching from these angles have to pass through a series of island chains on their path to SoCal. This is where the shadowing occurs.

You might ask “Hey I have been to a bunch of those islands…they are really small and far apart. How can they have such an impact on something as large and powerful as a swell?” Which would be a pretty good question.

In this case the answer is: It isn’t what you see above the surface that causes all of the problems but what is below.

A little Background info:
Swells are actually three-dimensional objects…but we spend a lot of time thinking about them in two-dimensions. Swell actually has a component of energy that extends “down” below the ocean surface. The longer the swell period the deeper this energy goes.

It is actually this “below the surface” energy that helps shape and facilitate the breaking waves that eventually hit our spots in SoCal. (I will go over the basics of this in another post…in the meantime you will just have to trust me.)

The thing to remember is that swells coming from the South Pacific end up having some very long swell periods (generally 16-17 seconds and longer), this is because they travel such long distances that much of the shorter swell period energy decays away leaving the more powerful long-period energy.

The Problem Occurs:
The problem occurs when this long-period energy moves through the island chains. Even though on the surface the islands are small they have a very well developed sea-mount supporting them…in fact there is, for all intents and purposes, an undersea mountain range underneath the island chains.

Long-period swells of around 18-seconds start feeling the sea-floor at about 850 feet…as they encounter the bottom they start to slow and change direction. If it was only a single obstruction this wouldn’t be a problem but there are hundred of overlapping islands spread over a couple thousand miles. It is sort of like sending the swell through a really messed up game of bumper pool.

The swell does eventually shuffle out of the other side of these islands but it is greatly diminished and has lost a lot of energy, both in size of the waves and the consistency of the sets.

What this means (or AKA why you should care):
Adam this is making my head hurt.

I know…it makes mine hurt too. I will sum it up for you.

If you see a big red blog on the swell model, but it is located in a position where the swell will pass through the South Pacific Island Shadow, then count on it being smaller and less consistent than if it had a clear shot.

Sometimes the swell models (like NOAA’s WavewatchIII) will “overcall” these swells, which in turn means that many of the surf forecast websites will overcall them too, (because their swell models are basically WavewatchIII with a different skin…).

The trick is to not to get caught up in the hype…yeah there is a swell coming, but it is likely going to be smaller and less consistent that most people think. Personally in these cases I start thinking about my free time…and is it really worth battling a horde of people to get inconsistent sets or should I save my time for a really good swell where I can get plenty of waves. It is usually a pretty easy call for me to make.

Anyway I hope this helps…I put together a cool graphic that will help illustrate the process and it shows both the open and shadowed swell directions for SoCal.