Lecture 11: Vertical Mixing in the Coastal Ocean

Question 1

What are the two types of turbulence?

Answer 1

Convection and velocity shear

Question 2

What is velocity shear? what drives this (2 things)

Answer 2

When the bottom layer of a fluid becomes denser and the top layer becomes less dense, the bottom moves slower, and the top moves faster. This difference in speed between the layers is called velocity shear. It can cause turbulence and mixing between the layers.

Can be driven by wind or friction (bottom-generated shear)

Question 3

What is convection turbulence? What are the two processes in the ocean?

Answer 3

When the surface water of the ocean cools down, it becomes denser and sinks, while the warmer, less dense water from below rises to replace it. This vertical movement creates convection. If the cooling is intense, the sinking and rising currents can become chaotic, leading to turbulence and mixing of the water layers.

Or

When surface water evaporates, it leaves behind salt, making the remaining water denser. The denser water sinks, and lighter, less salty water rises to take its place. This vertical movement is called convection and can lead to mixing in the ocean. (reverse estuary)

Question 4

What are tides comparable to? Deep or shallow waves

Answer 4

shallow water waves

Question 5

Are tidal amplitudes and currents bigger in deep or shallow water?

Answer 5

Tidal amplitudes and currents are larger in shallow water

Question 6

Is tidal turbulence greater in shallow or deep water?

Answer 6

• Tidal turbulence is larger in shallow water

Question 7

How do the seasons effect stratification and turbulence?

Answer 7

• In winter: storms with strong wind and
  heat loss at the ocean surface.

1. Offshore, stratification found at depth.
2. On shelf, well mixed by storms, let alone tides
3. Nearshore, always mixed

• In summer: weak winds and substantial
  heating at the ocean surface.

1. Offshore, well-stratified
2. On shelf, some stratification, some tidal mixing (strong tides close to shore)
3. Nearshore, always mixed

Question 8

What is a tidal mixing front (TMF)?

Answer 8

• Boundary between stratified
  offshore and well-mixed onshore
  creates a tidal mixing front
• This occurs on wide continental
  shelves
• Impacts coastal productivity

Question 9

What are the Spring-neap adjustment of TMFs?

Answer 9

• Tidal mixing fronts move offshore, into deeper waters during spring tides, when mixing is stronger

Question 10

Summarise Convection vs shear turbulence…

Answer 10

• Convection-driven turbulence (no horizontal currents): reduces surface density until stratification reduced
• Shear-driven turbulence (with horizontal
  currents): simultaneously overturns density in both layers