3A Basic Physical Oceanography Flashcards
What is pressure?
force pressing against an object
what happens when pressure is not uniform?
not force goes from the high pressure side to the low pressure side
what makes pressure not as strong in the ocean
liquid is almost incompressible, pressing down on it causes it to push out laterally
which direction does pressure come from in the ocean
all directions
what is hydrostatic balance
pressure below a water parcel is slightly higher to balance pressure and gravity coming from above
what are the two forces in the ocean at rest
gravity
pressure gradient
what is the equation representing the balance of forces in the ocean at rest
Fgravity = - Fpressure
-pg = deritive of p/z
what direction to pressure and gravity work in
vertical
equation for mass
density x volume
equation for weight
mass x g
what is the weight per unit area
integral of density gravity area per vertical depth
what is hydrostatic pressure
weight of the water above per unit area
how does density vary with depth
heavier as we got deeper
what is happening horizontally in terms of pressure gradients
spatial changes in density cause very small gradients
compared to the vertical
what can cause pressure to vary horizontally at a given depth
surface height variation
horizontal density variations
what happens to light availability as we go deeper into the ocean
decreases exponentially
what is the two layer ocean model
a layer where photosynthesis is significant and one underneath that is not significant
2 weakly stratified layers
does the layer boundary in the two layer model change?
yes, depending on processes we’re considering
what is one way to choose an interface depth in a two layer approximation from a sigma t and depth graph?
choose spot where density is changing rapidly with depth
benefits of the two layer model
simplify conceptually
what causes the pressure to increase in a two layer model
raising the ocean surface
raising the interface surface
what is the equation for pressure
P = pgh
what is different about the pressure change at from change of height at the surface vs the interface
need to consider the lost/gained amount of pressure from the other layer when the interface height changes
what does a change in density below you cause in water
nothing, density is allows weight of water above you
what causes mixing to work faster?
turbulence
what does moleculatr mixing depend on?
transport of heat due to diffusion
Fd
how fast is molecular mixing
slow, works best over sharp gradients
what does turbulent mixing depend on
transport of heat due to diffusion
transport of heat by turbulent flow
Fd + Fadvective
how fast is turbulent mixing
much faster than molecular mixing
how is turbulence enhancing molecular diffusion
stretching fluid
increase SA
wide range of scales
highly intermittent in the ocean
what is the main thing that causes mixing in the ocean
turbulent eddies
what direction can eddies mix?
both horizontal and vertical
what are the two ways to generate turbulence?
shear
convection
what is shear?
spatial gradient in flow velocity
(fast moving water beside/above/below slow moving water
what are examples of turbulence caused by shear
tides/ internal waves
wind-driven mixing
what is convection?
movement caused by less dense fluid rising and more dense fluid sinking because of gravity
unstable densities cause vertical turbulence
examples of turbulence caused by convection
winter cooling at surface
freezing/evaporation causing high salinity at surface
double diffusion process
what do tides cause
strong mixing near the bottom
internal waves on rough bottoms
what are internal waves
wave that are 5-10 meters down
moving water faster over another neighbour layer
break causes mixing
what are the causes of internal waves
winds
tides
wind driven mixing
wind moving the surface layer faster than below layers
surface wave and top layer wave breaking causes mixing
what is type one of convection driven mixing?
classic convection
what is classic convection
convection due to ‘classic’ convection instability
dense water going under lighter water
what are examples of classic convection
winter cooling at surface
freezing/evaporation causing high salinity at surface
what is type two of convection driven mixing?
double diffusion
what is double diffusion
convection driven by two different density gradients with different rates of diffusion
what are the two types of double diffusion
diffusive instability
fingering instability
diffusive instability
cold, fresh (light) water above warm salty (dense) water
fingering instability
warm salty (light) water above cold fresh water (dense)
what is the basic steps of finger instability
perturbation causes tongues to enter each layer from each layer
water diffuses 100 faster than salt, so water changes temperature and becomes lighter/denser
becomes heavier/lighter than other water until salt has also diffused
what kind of eddies cause stronger/faster mixing
faster
bigger
why does more mixing happen horizontally vs vertically
much more energy to move water across density surfaces (up/down) than acroess density surfaces (sides)
what can cause more mixing to occur in the vertical
when water is less stratified and vice versa
what two processes are happening at the surface mixing layer
wind is mixing surface, less stratified
sun is warming surface, more stratified
what kind of energy does wind change? how?
increase kinetic energy of mixing
indirectly increase potential energy of stratification
by making density difference smaller between surface and depth; moves center of mass up
what kind of energy does cooling/evaporation change? how?
increase potential energy of stratification
by making density difference smaller between surface and depth; moves center of mass up
what kind of energy does solar warming change
decrease potential energy of stratification
by making density differences larger
what kind of energy does rain/runoff change
decrease potential energy of stratification
by making density differences larger
mixing layer
region of the upper ocean that is actively mixing
not necessarily mixed yet
processes that change mixing energy affect depth
surface mixed layer
region of upper ocean that is homogeneous
not necessarily mixing now
why does the mixed layer deepen in the fall
cooling
strong winds
when is maximum depth of mixed layer usually
early spring
why does the mixed layer shallow in the spring
warming
weak winds
