Lecture 12 - The Oceanic Mixed Layer

Question 1

What is the mixed layer in the ocean?

Answer 1

A surface layer (10–100 m deep) where temperature, salinity, and density are nearly uniform due to turbulence.

Question 2

How is the depth of the mixed layer (MLD) commonly defined?

Answer 2

By the depth at which temperature is 0.5°C colder than at the surface.

Question 3

What causes the mixed layer to deepen?

Answer 3

Wind-driven turbulence and surface heat loss (convection).

Question 4

What causes the mixed layer to become shallower?

Answer 4

Solar radiation, which stabilizes thermal stratification.

Question 5

How does the MLD vary seasonally in the Southern Hemisphere?

Answer 5

Shallowest in late summer; deepens in autumn and winter; becomes shallow again in spring.

Question 6

How is the spring phytoplankton bloom related to MLD?

Answer 6

A shallower MLD in spring with high light and nutrients triggers the main bloom.

Question 7

What triggers the autumn phytoplankton bloom?

Answer 7

MLD deepening brings up nutrients from below, sparking a smaller bloom.

Question 8

What processes can change the temperature of the mixed layer?

Answer 8

Heat exchange with the atmosphere, entrainment from below (exchange of heat), and lateral advection.

Question 9

What is the formula for temperature change in the mixed layer?

Answer 9

ΔT = Q / (ρ × C × MLD) × Δt

Where:
Q = net heat flux,
ρ = seawater density,
C = specific heat,
MLD = depth)

Question 10

What happened during the 2017/18 Tasman Sea marine heatwave?

Answer 10

Low wind reduced mixing, so surface heat flux caused a 4°C warming of the mixed layer.