Unveiling the Depths: Exploring the Upsurge in Vertical Diffusivity in Oceanic Depths

1. Getting Started

Vertical diffusivity plays a critical role in the transport of heat, salt, and other properties in the ocean. It refers to the rate at which these properties mix vertically due to turbulent processes. Understanding the factors that influence vertical diffusivity is essential for accurately predicting ocean circulation patterns, heat distribution, and the vertical distribution of nutrients and dissolved gases. An observed pattern in the ocean is that vertical diffusivity tends to increase with depth. In this article we explore the reasons for this phenomenon.

2. The role of turbulence

Turbulence is a key driver of vertical mixing in the ocean. It results from the interaction of currents, tides, and other dynamic processes. Turbulence produces eddies and turbulent gyres, which act as vehicles for mixing water masses vertically. As we move deeper into the ocean, the energy available to drive turbulence decreases due to the decreasing intensity of surface winds and solar radiation. Consequently, the intensity of turbulence also decreases with depth.

While the overall intensity of turbulence decreases with depth, the size of the turbulent eddies tends to increase. This is because the small-scale turbulence generated near the surface cascades into larger scales as it propagates downward. The larger eddies are capable of transporting larger volumes of water, resulting in increased vertical mixing. Therefore, the increase in vertical diffusivity with depth can be attributed to the larger eddies that dominate mixing processes in deeper layers of the ocean.

3. Stratification and Vertical Diffusivity

Stratification refers to the layering of water masses based on their density. In the ocean, stratification results from variations in temperature and salinity. The presence of stratification has a profound effect on vertical diffusivity. In the upper layers of the ocean, where solar heating and wind-driven mixing are most intense, stratification is weak. As a result, turbulence and mixing are relatively strong, resulting in high vertical diffusivity.

However, as we descend into deeper layers, stratification becomes more pronounced. The colder and denser waters tend to sink below the warmer and less dense surface waters. This vertical density gradient acts as a barrier to vertical mixing, inhibiting the transfer of heat, salt, and other properties. Consequently, vertical diffusivity decreases in the presence of strong stratification.

4. Bottom Boundary Effects

The bottom boundary of the ocean, often in contact with the seafloor, also influences vertical diffusivity. Near the seafloor, turbulent mixing is enhanced by the interaction of tidal currents, waves, and the irregular topography of the seafloor. This enhanced mixing near the bottom can contribute significantly to vertical diffusivity, especially in shallow coastal regions.

In addition, the presence of sediments and benthic organisms can also affect vertical diffusivity. Sediments act as a source of turbulence that enhances mixing near the seafloor. Benthic organisms, such as burrowing and filter-feeding organisms, can modify sediment properties and alter the flow of water near the bottom. These bioturbations can have a cascading effect on vertical diffusivity, affecting the exchange of nutrients and oxygen between the sediment and the overlying water column.
In conclusion, the increase in vertical diffusivity with depth in the ocean can be attributed to the decreasing intensity of turbulence and the dominance of larger-scale eddies in deeper layers. The presence of stratification and interactions near the bottom boundary also play an important role. Understanding these mechanisms is critical for accurate modeling and prediction of oceanic processes with implications for climate, marine ecosystems, and resource management. Ongoing research and advances in observational and computational techniques are advancing our understanding of vertical diffusivity and its role in shaping ocean dynamics.

FAQs

Why does vertical diffusivity increase with depth in the ocean?

The increase in vertical diffusivity with depth in the ocean can be attributed to several factors. One of the primary reasons is the presence of turbulence generated by the interaction of oceanic currents with various topographic features and density gradients. Turbulence enhances the mixing of water masses, resulting in a greater exchange of properties such as heat, salt, and nutrients between different layers of the ocean.

How does temperature affect vertical diffusivity in the ocean?

Temperature plays a crucial role in determining vertical diffusivity in the ocean. As water temperature decreases with depth, the density of water increases, leading to the formation of stable stratification. This stratification inhibits vertical mixing and reduces the vertical diffusivity. However, in regions where the water column is unstable due to strong gradients in temperature or salinity, vertical mixing and diffusivity can be enhanced.

What is the impact of wind on vertical diffusivity in the ocean?

Wind has a significant influence on vertical diffusivity in the ocean. Strong winds generate surface waves and induce turbulent mixing in the uppermost layers of the ocean. This mixing can enhance the vertical diffusivity near the surface, promoting the exchange of heat, gases, and momentum between the atmosphere and the ocean. The effect of wind on vertical diffusivity diminishes with increasing depth as the energy from the wind-driven turbulence dissipates.

How does the presence of marine organisms affect vertical diffusivity in the ocean?

Marine organisms, such as zooplankton and fish, can have a notable impact on vertical diffusivity in the ocean. Their vertical movements, known as diel vertical migrations, involve daily migrations between surface waters and deeper layers. These vertical movements can enhance the mixing of water masses, contributing to increased vertical diffusivity. Moreover, the excretion of waste products and the feeding activities of marine organisms can introduce additional turbulence and promote mixing in the ocean.

What role does density play in vertical diffusivity in the ocean?

Density is a fundamental factor influencing vertical diffusivity in the ocean. In regions where the water column is characterized by strong density gradients, such as in coastal areas or at the boundaries of ocean currents, the resulting instability can lead to enhanced vertical mixing and diffusivity. The presence of density-driven currents, such as upwelling and downwelling, can also contribute to the vertical exchange of properties and increase diffusivity in specific regions of the ocean.