Lecture 3: Ocean Circulation forces Flashcards
What is a force and what are its two properties?
A force is a pushing or pulling that will result in motion if it is unopposed
Forces have two properties:
* Magnitude (length of arrow)
* Direction (direction of arrow)
What is a gradient?
The change in stuff with respect to another quantity
e.g.
Acceleration: How fast velocity changes with time
What is Newton’s second law?
“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.”
F = m*a
What are the four driving forces for the ocean?
- Gravity – From Earth, Sun and Moon
- Pressure Gradient Force – from variations in weight of fluid
- Friction – predominantly at boundaries of the ocean (sea surface, sea floor, coastlines)
- Coriolis – due to movement of water / air and Earth’s rotation
What is pressure gradient force (PGF)?
= difference in pressure / distance
Flow driven by differences in pressure (horizontal, vertical)
Fluid moves from high to low pressure
What is friction force and what are the three components for it in the ocean?
(Generally) a resisting force experienced by a parcel of fluid moving relative to another or to its surroundings
- Wind stress - *Momentum transferred from atmosphere into the ocean, creates surface currents
- Viscosity (internal friction) - *Momentum transferred between parts of fluid that are flowing at different speeds
- Frictional drag at ocean boundaries - *Momentum extracted from overlying flow as water molecules collide with the seafloor
What is Coriolis Force?
apparent force due to the rotation of Earth
An object moving at constant velocity is seen to deflect when viewed within a rotating frame of reference
The Coriolis force acts on all moving bodies (air, water) in a rotating frame of reference (e.g. Earth)
How does the Coriolis force act in the Northern vs Southern Hemisphere and the equator?
Acts at 90° to the right of the motion in the Northern Hemisphere
90° to the left of motion in the Southern Hemisphere
NO deflection at the equator (=0)
What determines the magnitude of the Coriolis force?
Coriolis force = f x U
U is velocity
f is the Coriolis parameter
f = 2 x Earth Rotation Rate x sin of latitude
f = 2 x Ω x sin (Φ)
Ω = Earth rotation rate = 7.27×10.-5
Φ = latitude
How does the strength of the Coriolis force change with velocity?
The strength of the Coriolis force increases with velocity
How does the strength of the Coriolis force change with latitude?
The strength of the Coriolis force increases with latitude
Where is the Coriolis force weakest?
at the equator where it = 0
How can we find Geostrophic Currents?
Pressure Gradient Force + Coriolis Force
How can we find Ekman Transport?
Friction + Coriolis Force
What is the equations of motion for the ocean to describe fluid dynamics?
Forces on a parcel of water:
acceleration = 1/p * (PGF + Coriolis Force + Gravity + Frictional Forces)
