Square Waves: What Causes These Unusual Ocean Phenomena?

Last Updated on October 23, 2023 by Matt Hapgood

The ocean’s winds and tides sculpt waves into an endless variety of shapes and sizes.

But occasionally, waves with unusually straight, square-like crests appear. Known as square waves, or a cross sea, these weird anomalies stand out among the typical ‘corduroy’ lined-up swell.

So what causes these odd square waves to occur? Are they dangerous? Let’s explore the science behind square waves and what makes them different.

TL;DR (too long didn’t read)

What Causes Square Waves?

Square waves arise primarily due to how swells interact with different ocean currents and wind conditions. As waves travel through the water, changes in direction or speed alter the orientation of the wave crests.

When swells pass perpendicularly through a strong opposing current, the crest can be pushed into a straighter alignment, forming a squarer face.

Specific causes include:

• Wind Resistance – Strong onshore or offshore winds blowing against wave direction straighten crests perpendicular to the wind flow.
• Currents – When swells meet a rip or areas of shifting currents like eddies, the wave shape morphs in response.
• Ocean Floors – Square waves sometimes form over shallow shelves or steep drop-offs which affect how swells propagate.
• Refraction – Waves change direction when entering shallower water. Refraction can make swells temporarily square before crashing.

The most consistent square waves form where persistent swells collide with incoming swells at an opposing angle.

The uniqueness of these ocean conditions gives square waves their uncommon, geometric appearance.

How Are Square Waves Formed?

Square waves originate like any other wind-driven weather system – storms produce waves that travel across the ocean in groups of similar periods and wavelengths. As these regular swells encounter currents or winds, the leading edge of the wave moves faster than the trailing edge.

This causes the crest to become more perpendicular to the direction of travel.

In ideal conditions, the wave crest flattens and extends straight out along the surface. This gives the approaching wave a squared, box-like shape instead of a typical peak or triangle profile.

While visually striking, true square waves are fairly rare.

Are Square Waves Dangerous?

Square waves are not necessarily more hazardous than other types of waves – but they do present some unique risks:

• Power – The ‘peak’ of a square wave can pack a serious punch if it drives directly into objects or swimmers.
• Visibility – It can be harder for swimmers to keep sight of incoming square waves in choppy conditions.
• Currents – Squares often indicate turbulent currents that create hidden dangers.
• Shorebreak – Their shape drives straight into the shore with concentrated energy.
• Unpredictability – The odd behavior of square waves can catch swimmers and surfers off guard.

While square waves don’t automatically spell disaster, exercising caution and scanning for hazards is advisable when they appear. As with any swell, it pays to respect the ocean’s forces.

What Happens If You Swim in Square Waves in the Ocean?

Braving square waves while swimming introduces a few specific risks and sensations:

• Powerful impacts – Square wave chop hits bathers with dynamic, smacking blows from above.
• Disorientation – Visibility through the straight ledge of water is obscured. swimmers can easily lose their bearings.
• Drag – The flat wall of the square wave produces a dragging effect perpendicular to swimming direction.
• Turbulence – Squares indicate chaotic currents that can pull swimmers off course.
• Shorebreak – Squares smash directly into the shore with little dissipation. This makes near-shore swimming extremely dangerous.
• Exhaustion – The pounding energy and disorienting nature of square waves can quickly sap a swimmer’s energy and confidence.

While an experienced open water swimmer may fair okay in brief square wave conditions, they remain an unpredictable and potentially hazardous environment for most bathers. Square waves are best observed from the safety of land when possible!

Where Are Square Waves or a Cross Sea Found?

Square waves pop up in scattered locations around the globe, usually near strong currents:

• California – Places like Newport Beach’s Wedge see square waves when swells hit the right way.
• Hawaii – Spotty offshore squares form due to complex interacting currents.
• Australia – Sections of the Great Barrier Reef produce squarish waves.
• Japan – Consistent wind and current patterns off some Japanese coasts generate square wave zones.
• UK – The famous Severn Bore tidal wave often spawns transient square shapes.
• Cape Town – Rare standing square waves emerge offshore with massive force.
• Caribbean – Smaller square chop created by crosswinds and tides.
• France – The Northern coast of France can sometimes have two swells – from the North Sea, and the Atlantic. 

Square waves may materialize and then disappear again once wind and current patterns shift. Savvy surfers and ocean photographers have to anticipate the ideal conditions to catch this phenomenon.

Square Waves vs Wedges

It’s common for “wedge” waves that peak and close out quickly to get confused with proper square waves. Though both are steep and powerful, squares and wedges arise through different mechanisms:

• Wedges – Formed by swells hitting shallowing sea floors at specific angles, causing waves to grow and peak rapidly. The crest doesn’t flatten out.
• Squares – Crest flattens perpendicularly due to wind resistance and currents rather than sea floor effects. Face stays vertical as it barrels.
• Shape – Wedges have a triangular, peaked crest. Square waves have a flat and uniform crest.
• Breaking – Wedges plunge directly down the face. Squares break laterally across the front.
• Hazards – Wedges close out powerfully. Squares churn and drag swimmers about.

So while wedges and squares both pack power, their distinct shapes arise for different reasons. Wedges form due to underwater topography, while currents and winds sculpt squares over deeper water.

Appreciating Nature’s Power in the Surf

Square waves are a unique expression of the ocean’s complex movements. They provide visual proof of the hidden dynamics and forces swirling below the water’s surface.

By understanding what causes these remarkable squared-off swells, we gain a deeper appreciation of nature’s vast power.

We’re reminded that winds, currents and tides all flow together in complex harmony, creating stunning phenomena like square waves in the process.

So next time you spot rows of straight-crested swells marching shoreward, take a moment to marvel at their geometry and what it reveals about the changing moods of our oceans.

Stay safe, but allow yourself to be awed by the mix of science, randomness and beauty flowing in nature’s mysterious square waves.