Wait, What? The tide you see at the coast can also generate enormous waves that travel invisibly inside the ocean.
These are internal tides. They form when the ordinary astronomical tide pushes stratified seawater across underwater ridges, continental slopes, sills or seamounts. Some of the tide’s energy is converted from a depth-uniform, or barotropic, motion into baroclinic internal waves that move along density layers through the ocean interior.
Scientific Job Claimed by This Manual
This manual owns one Ocean World process: astronomical tide → barotropic tidal current crosses rough topography in a stratified ocean → density surfaces are displaced → baroclinic internal tide is generated → wave energy propagates away → part of that energy later breaks and contributes to mixing. The Tides Learning Manual owns the astronomical forcing. The Internal Waves Learning Manual owns the general wave class. The Diapycnal Mixing Learning Manual owns irreversible cross-density mixing. This article owns the tidal generation and propagation of internal-wave energy.
Primary: How Can a Tide Make a Wave You Cannot See?
The whole ocean rises and falls with the tide, but tidal currents also move water horizontally. If that current meets an underwater mountain or ridge, the water is pushed upward and downward.
If the ocean is layered—with lighter water above denser water—those displaced layers try to return to where they were. That restoring motion creates a wave inside the ocean.
Why Stratification Is Essential
An internal tide needs stable density stratification. Without a vertical density gradient, there is no strong internal buoyancy restoring force to support the wave.
This is why the pycnocline matters. The Pycnocline Learning Manual owns the density structure itself. Internal tides use that structure as the medium in which their hidden vertical displacements propagate.
Secondary: Barotropic Tide Versus Baroclinic Internal Tide
- Barotropic tide: most of the water column moves in roughly the same horizontal direction at once.
- Baroclinic internal tide: different density layers move differently, creating large internal vertical displacements and shear.
Topography performs the conversion. A depth-uniform tidal flow becomes a layered wave because the seabed forces the stratified water column to move vertically.
Why Ridges, Sills and Slopes Are Generation Hotspots
Generation becomes strong where tidal currents cross steep or extensive topography. Major mid-ocean ridges, island chains and narrow straits can therefore radiate internal-tide energy far into the surrounding ocean.
The geometry matters. A shallow ridge may produce little conversion if it barely disturbs the stratified layers, while a steep ridge intersecting strong tidal flow can produce a much larger response.
JC: Energy Conversion From Barotropic to Baroclinic Motion
Internal-tide generation is an energy-conversion problem. The barotropic tide does pressure work against topography. In a stratified water column, that forcing excites internal gravity-wave modes.
Some of the converted energy remains near the generation site and dissipates locally. Some radiates away as low-mode internal tides that can travel hundreds or thousands of kilometres before interacting with other waves, currents or topography.
Low Modes Travel Far; High Modes Break More Easily
Internal tides can be decomposed into vertical modes. Lower modes have broad vertical structure and can propagate long distances efficiently. Higher modes contain finer vertical structure and usually dissipate more readily.
This helps explain why a ridge can generate internal-tide energy in one place while significant mixing happens far away.
How Internal Tides Become Turbulence
A propagating internal tide is organised wave motion, not yet irreversible mixing. Mixing occurs when wave energy cascades toward smaller scales or when the wave becomes unstable and breaks.
NOAA GFDL identifies tides and winds as major energy sources that excite internal waves, which can then cascade toward turbulence and diapycnal mixing.
Why Internal Tides Matter for the Global Overturning
The deep ocean is strongly stratified. Dense water sinks at high latitudes, but long-term overturning also requires water masses to be transformed and ultimately returned toward lighter density classes.
Internal-tide-driven turbulence is one route by which mechanical energy enters the deep ocean and helps support that slow transformation.
Connection to the Indonesian Throughflow
The Indonesian Throughflow Learning Manual owns Pacific-to-Indian transport through the Indonesian archipelago. That region contains strong tides and rough sills, so it is also an important internal-tide and mixing hotspot. One manual owns the gateway transport; this one owns the internal-wave energy conversion.
Connection to the Bottom Boundary Layer
The Ocean Bottom Boundary Layer Learning Manual owns frictional turbulence right above the seabed. Internal tides can be generated by the same topography yet radiate energy far above and away from the bottom boundary layer.
How Do We Know?
Oceanographers use moorings, shipboard current profilers, pressure sensors, temperature chains, gliders, microstructure profilers and satellite altimetry. Satellites can detect the tiny surface-height patterns associated with coherent internal tides, while in-water instruments reveal their vertical velocity and density structure.
High-resolution models then test whether observed tidal currents interacting with measured bathymetry and stratification reproduce the wave beams and energy pathways seen in the data.
Observation Versus Energy Attribution
A mooring can directly record periodic internal vertical motion at tidal frequencies. Calling that signal an internal tide requires its frequency, phase, vertical structure and relationship to astronomical tidal forcing to match the expected dynamics.
Likewise, observing turbulence near a ridge does not by itself prove internal tides caused all of it; mean currents, lee waves and other instabilities may contribute.
Can You Predict It?
- Stronger barotropic tidal currents cross a steep ridge: expect stronger internal-tide generation, all else equal.
- Stratification weakens greatly: internal-wave generation and propagation change because the restoring buoyancy force is reduced.
- A low-mode internal tide reaches another rough ridge: scattering and enhanced dissipation become more likely.
- A region has strong internal-tide energy but little breaking: wave transport can be large even while local irreversible mixing stays modest.
Transfer Test
Two ridges experience the same tidal current. Ridge A lies beneath a strongly stratified ocean; Ridge B lies beneath a nearly homogeneous water column. Which is more favourable for generating a strong internal tide?
Ridge A. Strong stratification supplies the internal buoyancy restoring force needed to convert the tidal disturbance into propagating internal gravity waves.
Model Boundary
Not all tidal energy becomes internal tides, and not all internal-tide energy becomes local turbulence. Conversion efficiency depends on current strength, stratification, bathymetric slope and modal structure. Dissipation can occur locally, remotely or after nonlinear interactions, so “tides cause mixing here” should be treated as an energy-pathway hypothesis requiring evidence.
Useful Misconceptions to Correct
- Internal tides are not surface tides hidden by darkness.
- They require stratification and vertical displacement of density layers.
- Generation and breaking can occur far apart.
- Internal tides are waves; irreversible mixing happens mainly when their energy dissipates.
- Astronomical tide supplies the frequency, while topography and stratification control much of the conversion.
Canonical External Sources
Teaching Method
Begin with the contradiction: “How can the ordinary tide at the surface make a completely different wave inside the ocean?” Have students trace the chain tide → current → ridge → displaced density layer → internal wave.
For Primary learners, use tidal current hitting an underwater mountain. For Secondary learners, distinguish barotropic and baroclinic motion. For JC learners, frame internal tides as an energy-conversion and propagation problem, then ask students to predict where generation, long-range propagation and breaking should each be strongest.