Large Scale Processes (L1-6)

Question 1

What is the pycnocline?

Answer 1

The density increase between the surface ocean and deep waters, confined to a thin layer
Can be permanent or seasonal
It takes a lot of energy to mix through the pycnocline, therefore is an effective barrier to vertical mixing

Question 2

Why is the pycnocline important to phytoplankton?

Answer 2

They require light from the surface and nutrients from depth, but these are separated by the density stratification of the pycnocline

Question 3

Why is ocean stratification becoming stronger and what are the impacts of this?

Answer 3

More than 90% of the excess atmospheric heating is going into the ocean and causing it to become more stratified (increase by 18% in upper 150m)
This leads to less vertical mixing and more heat in the upper ocean (usually in the form of heatwaves)
This impacts many marine organisms, due the increase in heat, as well as the reduced supply of nutrients and the alteration of productivity, biodiversity, and food webs, and an increase in sedimentation

Question 4

What does sea water density depend on?

Answer 4

Temperature, salinity, pressure

Question 5

What is cabling?

Answer 5

Where water masses have different temperatures and salinities, yet still arrive at the same density

Question 6

How does temperature affect density?

Answer 6

Density increases with decreasing temperature

Question 7

How does salinity affect density?

Answer 7

Density increases with increasing salinity

Question 8

How does pressure affect density?

Answer 8

Density increases with pressure (increasing pressure squeezes water into a smaller volume)

Question 9

What is Archimede’s Principle?

Answer 9

An object submerged in a fluid will experience an upward buoyant force that is equal to the weight of fluid it displaces

Question 10

What is stable stratification and the consequence of this?

Answer 10

This occurs when each layer is less dense than the one below it
Buoyancy forces tend to preserve stable stratification
A consequence of this is that internal waves are formed which propagate horizontally along the pycnocline

Question 11

What are the impacts of internal waves?

Answer 11

They drive cold water into shallow oceans/coastal waters, injecting nutrients into the euphotic zone, transporting marine larvae into coastal areas, and delivering cooler water to coral reefs

Question 12

What is unstable stratification?

Answer 12

This occurs when a layer is denser than the once below it
Buoyancy triggers convection
Cannot persist as buoyant forces will always try to return it back to stable stratification
This could happen near the poles where it cools dramatically in winter

Question 13

What is hydrostatic pressure?

Answer 13

The pressure at any depth in the ocean is the weight of the water that is acting on a unit area
Pressure acts equally in all directions

Question 14

What is advection?

Answer 14

The transport of properties by the flow of a fluid

Question 15

What is diffusion?

Answer 15

The redistribution of properties from an area of higher to lower concentration (spreading)
Requires a concentration gradient

Question 16

Do advection and diffusion occur together or separately?

Answer 16

It is very rare that only one of these processes is occurring at a time

Question 17

What is volume flux?

Answer 17

The volume of fluid passing through a prescribed area per unit time
The flow rate of an ocean current

Question 18

What is a Sverdrup?

Answer 18

A measure of the amount of water
1 million m³/s

Question 19

What is molecular diffusion?

Answer 19

Where properties are redistributed due to random motion of water molecules
Usually a slow process as it relies on the movement of individual particles
It is always at the same constant random motion due to thermal energy

Rate of diffusion is k = 1x10⁻⁷ m²/s

Question 20

What is eddy diffusion?

Answer 20

Where properties are redistributed as a result of stirring by eddies
Large range of spatial scales (mm-km)
Stirring speeds up the rate of mixing, and is much faster than molecular diffusion as more kinetic energy is introduced

Rate of diffusion is k = 500 m²/s

Question 21

What is the principle of continuity?

Answer 21

Volume in the ocean must be conserved, volume flux into an area must equal the volume flux out of an area

Question 22

Why is it important that seawater is nearly incompressible?

Answer 22

It means a flow that is converging in one direction must be diverging in another direction
Converging leads to downwelling
Diverging leads to upwelling

Question 23

What is Newton’s Second Law of Motion?

Answer 23

The rate of change of velocity (acceleration) of an object is equal to the sum of the forces acting upon it, and inversely proportional to the mass of the object

Question 24

What happens if the forces in the ocean are unbalanced?

Answer 24

The parcel of water will move in the same direction as the forces
The opposite will occur if the forces are balanced, and the water may not move, or continue at a constant velocity (steady-state), dependent on the magnitude of the force

Question 25

What are the four forces that affect motion in the ocean?

Answer 25

Gravity - from the Earth, Moon and Sun
Pressure Gradient Force - from variations in the weight of fluid
Friction - predominantly seen at ocean boundaries (sea surface, sea floor, coastlines)
Coriolis Force - due to the movement of water/air and Earth’s rotation

Question 26

What are the three types of frictional forces in the ocean?

Answer 26

Wind stress (accelerating)
Viscosity or internal friction (resistant)
Frictional drag at boundaries (resistant)

Question 27

What way does the Coriolis Force act in the different hemispheres?

Answer 27

Acts 90° to the right of motion in the Northern Hemisphere
Acts 90° to the left of motion in the Southern Hemisphere

Question 28

Where is the Coriolis Force the strongest?

Answer 28

It is strongest at the poles and weakest at the equator
The faster the motion, the stronger the effect of the Coriolis Force

Question 29

What does the Rossby number tell us?

Answer 29

Whether the Coriolis Force is important in a system
Ro < 1 signifies a system affected by the Coriolis Force
Ro <1 indicates a system with little influence from the Coriolis Force

Question 30

When is the Coriolis Force most important?

Answer 30

It becomes the most important when travelling relatively slowly over large distances

Question 31

What are the different directions of motion?

Answer 31

x = positive east
y = positive north
z = positive up

Question 32

What are the different velocities of motion?

Answer 32

u = positive east
v = positive north
w = positive up

Question 33

What are the different accelerations of motion?

Answer 33

du/dt = positive east
dv/dt = positive north
dw/dt = positive up

Question 34

Why are ocean surface currents important?

Answer 34

They play a role in controlling climate, transporting dissolved and floating material (nutrients, gasses, larvae) and the determine the fate of pollutants in coastal waters

Question 35

What is geostrophic balance?

Answer 35

It the horizontal force balance, where the horizontal pressure gradient force and Coriolis force balance each other
It is responsible for large-scale ocean circulation

Question 36

What is occurring if a fluid is in geostrophic balance?

Answer 36

The PGF acts from higher toward lower pressure
The PGF and CF act in opposite directions but with equal magnitude
The direction of the geostrophic flow is at 90° to the PGF (R in NH, L in SH)
The velocity of the geostrophic flow is proportional to the strength of the PGF

Question 37

What does satellite altimetry measure?

Answer 37

The sea surface height and topography of land

Question 38

Why does the pressure gradient force change with sea surface height?

Answer 38

Due to the changes in pressure between the different heights
Higher/taller water column has more density, while a shorter/lower water column is less dense
Gradients in the height of sea surface height produce horizontal pressure gradients

Question 39

What does a circle with a cross mean?

Answer 39

Movement into the page

Question 40

What does a circle with a dot mean?

Answer 40

Movement out of the page

Question 41

What is Ekman transport?

Answer 41

The net movement of fluid within a wind-affected layer (the Ekman layer)
The direction of Ekman transport it at 90° to the wind (R in NH, L in SH)

Question 42

Why is Ekman transport important?

Answer 42

It is responsible for the horizontal fluxes of water properties (temperature, salinity, nutrients, gasses, dissolved material, larvae, and marine debris)
Convergences and divergences produce vertical water movement (upwelling and downwelling)

Question 43

What is wind stress?

Answer 43

Wind blowing across the sea surface producing a force per unit area

Question 44

How is the direction of wind stress described?

Answer 44

Wind stress on the x-axis is τₓ
It is positive towards the east, negative towards the west
Wind blowing west to east (westerly) would have a τₓ > 0

Wind stress on the x-axis is τᵧ
It is positive towards the north, negative towards the south
Wind blowing north to south (northerly) would have a τᵧ < 0

Question 45

What is sheared flow?

Answer 45

Where one part of the ocean is moving faster than others causing turbulence

Question 46

What is the Ekman depth?

Answer 46

Momentum at the surface from wind stress cause turbulence, which acts as a gearing mechanism and transfers wind stress force down into the water column
As the wind imparts on the surface, turbulence acts to transfer down into the ocean
The depth to which the effects of wind stress are felt is the Ekman depth

Question 47

How long does force balance between wind stress and Coriolis force take to develop?

Answer 47

About 36 hours
Varies with the Ekman depth and the strength of the Coriolis force

Question 48

How do the velocities of Ekman transport and wind stress differ?

Answer 48

The velocity of flow in the Ekman layer is proportional to the strength of the wind stress
The direction of flow in the Ekman layer is at right angles (R in NH, L in SH) to the direction of the wind stress

Question 49

What happens when the wind stress causes divergences and convergences?

Answer 49

Upwelling and downwelling of seawater occur