Internal Waves in Stratified Ocean

Core Idea

Internal waves oscillate along density interfaces (thermoclines or pycnoclines) within the ocean where stratification provides the restoring force. These waves are much slower than surface waves but have much larger amplitudes for the same energy. Internal waves are important drivers of vertical mixing and transport of nutrients across strong stratification barriers.

Explainer

From your study of ocean layering and stratification, you know that the ocean is not a uniform body of water — it is organized into layers of different density, with lighter, warmer water sitting above denser, colder water. The boundaries between these layers, especially the pycnocline, act like elastic membranes. When something disturbs this boundary — tidal currents flowing over a submarine ridge, for example — the interface oscillates up and down, and these oscillations propagate horizontally as internal waves.

The physics of internal waves differs dramatically from the familiar surface waves you see at the beach. Surface waves occur at the boundary between air and water, where the density contrast is enormous (water is about 800 times denser than air). Internal waves occur at boundaries where the density contrast is tiny — perhaps a fraction of a percent difference between adjacent water layers. Because the restoring force (gravity acting on the density difference) is so much weaker, internal waves travel far more slowly than surface waves, often just a few centimeters per second compared to meters per second for surface waves. But this same weak restoring force means that for a given amount of energy, the wave displacement can be enormous — internal wave amplitudes of 50 to 100 meters are common, compared to the few meters typical of ocean surface waves.

Internal waves matter to the ocean's overall functioning because they are one of the primary mechanisms for breaking down stratification. A strongly stratified ocean resists vertical mixing — nutrients trapped in the deep cannot easily reach the sunlit surface where phytoplankton need them. When internal waves steepen, become unstable, and break (much like surface waves breaking on a shore), they generate turbulence that mixes water across density interfaces. This diapycnal mixing transports nutrients, heat, and dissolved gases vertically, connecting the deep ocean to the surface in ways that would not occur in a perfectly stratified, quiescent ocean.

The generation and behavior of internal waves depend on the local stratification structure and the forcing mechanisms. Tides interacting with rough bottom topography — seamounts, ridges, continental slopes — are the dominant source, producing what are called internal tides. Wind-driven near-inertial oscillations also generate internal waves in the upper ocean. These waves can propagate thousands of kilometers from their source before breaking, meaning that mixing in one part of the ocean can be driven by tidal forcing at a distant ridge. This non-local character makes internal waves a critical but challenging component of ocean circulation models, which must account for their generation, propagation, and dissipation to accurately represent how the deep ocean is mixed and ventilated.