Properties of Seawater and Pressure Gradient Forces
Reading
Things to know
Salinity
- How is salinity defined? (briefly - Emery et al go into a lot of
detail I don’t expect you to be responsible for)
- How is salinity measured (again, briefly)?
Temperature
This should be review of 110 and common knowledge, but there are
a few things to remind yourself of.
- What is the difference between “temperature” and “heat”
- What is the accuracy of an oceanic thermistor?
- What is “potential” temperature, versus “in-situ” temperature?
Density differences
- What three properties of seawater (primarily) determine its
density?
- How does the density vary with those properties?
- If you change the temperature of seawater by 1 degree C, what is
the change of density (approximately)?
Sound in the ocean
- What is the approximate speed of sound in the ocean?
- What does the sound speed depend on?
- What is the SOFAR channel?
- People use the SOFAR channel - do any other mammals?
Light in the ocean
- What transmits better in the ocean, blue or red light?
- For clear water, how deep might you still see light?
- What factors affect the clarity of the water?
Ice
- How does salt affect the freezing temperature of water?
- What is meant by “brine rejection”
- Once formed, how does ice affect the interaction of the ocean with
the atmosphere?
- What is a Polynya?
What is pressure?
- What is pressure from a molecular point of view?
- What happens to the pressure if the temperature is increased?
- What are the units of pressure?
Hydrostatic Pressure
- Consider a column of water 10m deep, 1m x 1 m in the horizontal.
How much does this column of water weigh if it is all at one
density 1000 kg/m^3?
- How much force must be exerted on the bottom of the column to stop
it from falling?
- In an ocean at rest that is much deeper than 10 m, what provides the force that holds the upper 10 m from falling (i.e. keeps it in hydrostatic equilibrium)?
- What is the pressure of seawater at this depth?
- If the water column was 20 m deep, what is the pressure?
- NOTE We get tired of carrying around all these extra digits, most
of which are not likely to be significant, so pressures are often
written as decibars: 1 dbar = 10^-1 bar = 10^4 N/m^2. Why do
you think oceanographers like to use this unit? (Hint, how many
bars would the pressure at the bottom of a 20-m deep column of
water be?)
- What would the pressure be under a column of water where 5 m was
at 1022 kg/m^3 and the next 5 m was at 1024 kg/m^3?
Pressure gradients
- If you tilt the water in a bathtub what way does the water flow
and why?
- Imagine a parcel of water (say 1m x 1m x 1m). Consider the “x”
direction only. If the average pressure on both sides is 98000 N/m^2 (9.8
dbar), then what is the net force in the x-direction on the block?
- Now imagine that the water on the right side of the block is 2 cm
deeper than on the left side of the block. What is the pressure
difference across the block?
- What is the net force on the block?
- According to Newton’s second law what direction will the block
tend to be accelerated (in the absence of other forces)?
- What is the sign of this compared to the sign of the horizontal pressure
gradient dP/dx?
Internal Pressure Gradients.
- Imagine the seasurface is flat, but there is a two-layer fluid.
The upper layer has a density of 1000 kg/m^3, and the lower has a
density of 1010 kg/m^3. Imagine that you have a box, 1 m wide and
that the interface between the two fluids is 10 cm from the
seasurface on one side and 12 cm deep on the other. Sketch this
situation.
- Consider two points at either side of the box 5 cm from the
surface. What is the horizontal pressure gradient at this location?
- Consider two points on either side of the box at 15 cm depth.
Approximate the horizontal pressure gradient at this depth.
- What direction will the deep layer be accelerated?
- The upper layer will soon start to move as well. Why?
Exercise
Get it here.
Last Modified: 09 July 2018 Licence: Creative Commons Attribution required, non-commercial uses (CC BY-NC 4.0)