What is Dynamic Fetch and how Does it Impact Your Forecast?

Tony Butt

by on

Updated 102d ago

Another large, long-period swell is forecast for this week. A big low tracks across the North Atlantic and generates wave heights of over 30 feet off northwest Ireland towards the end of the week. The charts show a text-book example of dynamic fetch.

We all know that swell is generated by wind blowing over the ocean surface. And we also know that the height of that swell depends on the wind – not just the strength of the wind but other factors as well. Wave heights in a storm are dependent on three things: the speed of the wind, the length of ocean over which that wind blows (the fetch), and the length of time for which that wind blows over that stretch of ocean (the duration). It’s pretty intuitive.

Wind charts (top) and swell charts (bottom), at 12-hour intervals for later on this week: The dark red blob on the wind charts shows the main fetch, with winds around force 9, which moves in a straight line across the North Atlantic. Even though the area of force-9 winds never exceeds about 800 km long, the swell generated corresponds to an area of force-9 winds extending well over 2000 km.

Wind charts (top) and swell charts (bottom), at 12-hour intervals for later on this week: The dark red blob on the wind charts shows the main fetch, with winds around force 9, which moves in a straight line across the North Atlantic. Even though the area of force-9 winds never exceeds about 800 km long, the swell generated corresponds to an area of force-9 winds extending well over 2000 km.

In fact, the very first wave models, developed in the 1940s by Harald Sverdrup and Walter Munk, were based on a set of equations for wave height and period as a function of windspeed, fetch and duration. Even though the models we use nowadays give us an infinitely more precise forecast than those first ones, it is still interesting to go back and look at the basic charts to see a large area of strong winds over the ocean and watch how that windfield generates a big swell.

However, sometimes a fairly unremarkable low pressure produces a really big swell, and at other times a much more powerful system – with more windspeed, more fetch and more duration – doesn’t produce the same swell. I wondered about this for ages. There must be something apart from those three factors that influences wave generation.

Walter Munk, a legendary oceanographer and one of the original pioneers of wave forecasting.

Walter Munk, a legendary oceanographer and one of the original pioneers of wave forecasting.

© 2018 - San Diego Union Tribune.

Dynamic fetch, sometimes called captured fetch, is one of those things. This is when the windfield is moving along the same trajectory and at the same speed as the very swell it is producing. The windfield is kept synchronous with its own swell and the waves are continually being pumped with energy.

Dynamic fetch has the same effect on the waves as extending the fetch while maintaining the full strength of the wind over that distance. This will make the wave height bigger because you are putting a longer fetch into the equation.

Dynamic fetch is very common in the North Atlantic, particularly when the general situation favours a train of lows steaming across from west to east, one after the other. The disadvantage, of course, is that the low itself usually just ploughs into the coast of northwest Europe (typically Ireland), adding strong westerly winds to a big, long-period swell. Things in the south are often much better, especially if the Azores high extends eastwards over Iberia and maintains clean conditions over local areas.

Cover shot; macking Mullaghmore by Tim Borrow.