What does offshore wind do to a wave?
It is always nice when research validates what surfers already know. Recent publications by Falk Feddersen and colleagues (Paper 1 and Paper 2, both open access) explain how offshore versus onshore winds shape breaking waves. Their research combined modeling with experiments at an ideal location - the Kelly Slater Wave Company's Surf Ranch. Color me jealous.
The Surf Ranch's design makes it perfect for studying wind effects. A figure from their 2023 paper reveals the pool's layout, with panel (a) showing its mirror-image design split at the y = 0 line. Wave generation here differs from the ocean - the face opens in the "offshore" direction. A hydrofoil system, often called the train, generates waves. Northward movement creates rights (panel b), southward creates lefts (panel c).
Two features made this spot perfect for wind research. Ocean waves rarely reproduce exactly, but this pool creates near-perfect copies. More importantly, northerly winds act as onshore winds for lefts and offshore for rights. With identical pool geometry in both directions, researchers gathered clean data by running the wave machine back and forth under consistent winds.
Their setup combined a high-resolution drone camera with a pool-mounted LIDAR system. While RADAR maps with radio waves, LIDAR uses lasers for better resolution at close range. This LIDAR unit tracked wave faces with 2cm accuracy, measuring how wind conditions altered wave shape evolution. A few examples of this data is shown below, with the top panels being the offshore case and bottom being the onshore case. (For this figure, the authors chose to show both the left and right waves, so they had to wait for the wind to shift from the south to get this data).
The second paper validated the first paper’s findings through ocean and atmospheric modeling. Strong onshore winds create turbulent air flow in front of waves (panels a and c below), while offshore winds generate turbulence behind them (panels b and d below). This air movement significantly influences wave shape and breaking patterns. How people keep their eyes open through 15 knot offshore winds is still a mystery to me.
Both studies reached three key conclusions. Onshore winds accelerate wave breaking by adding momentum to the wave crest, creating crumbly waves. Offshore winds delay breaking through a combination of direct air flow and pressure gradients, allowing waves to maintain their form into shallower water.
Offshore winds also increase the size of potential barrel sections. Though bathymetry primarily controls wave transformation, this effect appears consistently at slabby breaks. Offshore conditions create larger hollow sections compared to onshore conditions. The research quantified these differences across both pool and model data and used a special almond-shaped fit to describe the tubes.
I may be alone in this, but what I find most fascinating is that the research reveals that air pressure gradients, rather than surface friction, drive most changes in wave shape during wind-wave interactions. Regardless of if you think that’s interesting or not, the model and data validate what surfers have known about offshore winds, while explaining why those afternoon sessions in Southern California are often supremely mediocre compared to the morning.
Further Reading:
Okay tell me more about the air pressure gradients! Like some sort of Bernoulli's principle thing? And how does that hollow it out? I also think it's the most interesting, so flesh it out for me if you can! 🧐 Love the blog, keep posting!