Internal waves generate turbulence and mixing in the ocean primarily by breaking, a process initiated by their generation from winds and tides and subsequent nonlinear interactions.
Understanding Internal Waves and Their Crucial Role in Ocean Mixing
Internal waves are gravity waves that propagate within the ocean's stratified layers, where density changes with depth. Unlike surface waves that occur at the ocean-atmosphere interface, internal waves oscillate within the water column, often unseen from the surface. Despite their hidden nature, these waves play an absolutely critical role in the ocean's turbulent mixing processes. This mixing is essential for distributing heat, nutrients, and gases throughout the ocean, profoundly impacting global climate patterns and marine ecosystems.
The Mechanism: How Internal Waves Create Turbulence and Mixing
In the ocean interior, the primary cause of turbulent mixing is the breaking of internal waves. This complex process involves a sequence of energy transfers and physical transformations that ultimately stir the water column.
The mechanism can be broken down into the following stages:
- Generation: Internal waves are predominantly generated by large-scale forces such as winds blowing across the ocean surface and tides interacting with seafloor topography. These forces impart energy into the stratified water column, initiating wave propagation.
- Nonlinear Interaction and Energy Transfer: As these waves propagate, they do not travel in isolation. Instead, they interact nonlinearly with each other and with the surrounding ocean currents. During these interactions, energy is transferred from larger, parent waves to smaller, shorter waves, a process often referred to as an "energy cascade" or "downscale energy transfer."
- Instability: As energy continues to transfer to smaller scales, the internal waves gradually become unstable. This instability means that the wave's shape can no longer be sustained without deforming significantly.
- Breaking and Mixing: Once unstable, the internal waves break. Similar to how surface waves break on a beach, breaking internal waves create localized areas of intense turbulence. This turbulent motion vigorously stirs and mixes the water column, causing denser water to rise and lighter water to sink, effectively homogenizing the water in that region. This continuous breaking action is what drives the turbulent mixing responsible for vertical exchange in the ocean interior.
The following table summarizes the key stages of this vital ocean process:
| Stage | Description |
|---|---|
| Generation | Internal waves are primarily created by the action of winds and tides on the stratified ocean layers. For example, tidal currents flowing over underwater mountains (seamounts) or continental shelves can generate significant internal wave fields. |
| Energy Transfer | These waves undergo nonlinear interactions, causing them to transfer their energy to progressively smaller scales. This is a critical step in preparing the waves for instability, as energy concentrates in smaller, more unstable wavelengths. |
| Instability | Due to the downscale energy transfer, the waves reach a critical point where they become dynamically unstable. Their shape and amplitude can no longer be maintained, leading to a tendency to overturn. |
| Breaking & Mixing | The unstable waves break, analogous to ocean waves breaking on a shoreline but occurring entirely within the water column. This breaking creates intense, localized turbulence, which efficiently mixes the water layers, contributing significantly to vertical ocean mixing. |
The Broader Impact of Internal Wave Mixing
The turbulence and mixing generated by breaking internal waves are fundamental to various oceanographic and climatic processes:
- Nutrient Cycling: It facilitates the upward transport of nutrient-rich deep waters to the surface layers, fueling marine productivity and supporting ecosystems.
- Heat Distribution: It helps distribute heat throughout the ocean, playing a crucial role in the global climate system and oceanic heat budgets.
- Oxygen Supply: Mixing aids in the diffusion of oxygen from the surface to deeper waters, supporting abyssal life.
- Carbon Sequestration: It influences the transport of dissolved carbon, impacting the ocean's role in the global carbon cycle.
Understanding these processes is vital for accurate climate modeling, predicting ocean currents, and managing marine resources.
