4 Basic Forces and Tides Flashcards
what ‘real’ forces can be found in the ocean?
gravity
pressure gradient forces
friction
which way doas pressure gradient force go
from high pressure to low pressure
what two ways does friction act on the ocean basically
slows down fast moving fluid
speeds up the slow moving fluid
what causes apparent forces?
non-inertial frames of reference
what are the units and direction of centrifugal force?
N/m^3, away from axis of rotation
since centrifugal force is dependent on distance from axis of rotation, what effect does this have on magnitude on earth?
largest at the equator, smallest at the poles
causes a bulge at the equator
do we consider centrifugal force in balance equations?
no, it is too tiny compared to gravity which is in the same direction
what is the coriolis force?
apparent force happening because of a rotating reference frame that makes movig in a stright look like we’re turning
what axis msut be consider for the coriolis force?
the local axis, not where the axis is
how do we label positive x,y,z as earth centric coordinates?
y = North
z = local up against gravity
x = east
what does f* represent?
the upwards directed force associated with the northwards vector component of coriolis force
what does f represent?
the southwards directed force associated with the upwards vector component of coriolis force
which component (f or f*) is important for ocean motions? why?
f, because f* upward force is dwarfed by gravitional force, it has no effect
how does f act on the horizontal plane?
acts on the tangent to the earth’s surface and is directed 90 degrees to the right of flow
where is f negative?
the southern hemisphere
what name does f have?
coriolis parameter
what shape do moving parcels make because of coriolis in the absence of other forces? what are they called?
circles
inertial oscillations
what drives horizontal flow in the ocean surface?
friction
coriolis
net force of acceleration
what drives horizontal flow in the ocean interior in the long term?
mainly:
coriolis
horizontal pressure gradient
what is geostrophic balance?
balance between the pressure gradient force and Coriolis force
what is geostrophic velocity?
velocity who’s associated coriolus force matches the pressure gradient force exactly
isobars definition
lines on maps of constant pressure
where is the horizontal pressure gradient relative to isobars?
perpendicular from high to low pressure
what determines the magnitude of the horizontal pressure gradient?
how fast the pressure is changing
inversely proportional to distance between isobars
what is the geostrophic flow?
flow occuring proportional to the coriolis force, which is always 90 degrees from it
which side is coriolis on in the northern hemisphere?
right
which side is coriolis on in the southern hemisphere?
left
where is the geostrophic flow relative to isobars?
along the isobars to keep high pressure to the right/left
what drives horizontal flow in the ocean interior in the short term?
tide generating force
what is TGF
tide generating force
what balances tide generating force?
local accelerations
coriolis and pressure gradient is too small comparatively
what causes the tides?
gravitational attraction to the moon/sun
centripetal force larger than lunar gravity flung away
how many tidal bulges and dips are there?
two each
what is a sidereal month and how long is it?
rotation of the earth and moon arounf their joint center of mass
27.3 days
what is the residual force?
the tide generating force
fgrav - Fcentri = TGF
where is the residual force balanced?
at the earth’s center of mass
what is the zenith?
point on earth directly under the moon
what is the nadir?
spot directly across the earth from the zenith
how often do the tides repeat? why
every 24.9 hours
moon and earth are rotating at the same time
in one day, the moon rotates 1/27.3 of it’s orbit, so the earth has to catch up
how long is a lunar day
24.9 hours
what is diurnal inequality?
presence of one more intense and one less intense low/high tide
what causes diurnal inequality?
the moon is not directly above the equator because of the earth’s tilt
what is the tropic/equatorial cycle?
two week long cycle where the moon becomes in line with the equator
what is the perigean/anomalistic cycle?
27.3 day cycle where the moon is at one end of elliptical orbit in the northern or southern hemisphere
what are spring tides
strong tides from moon and sun TGF adding together
what are neap tides
weak tides caused by the moon and sun TGF working against each other
what moons are seen at spring tides?
full and new
what moons are seen at neap tides?
first and second quarter
how long is the sydonic month? why?
29.5 days
because earth is also rotating around the sun, the moon has to move slightly farther to make up for the rotation
how many spring/neap cycles occur in a sydonic month?
2 each
why do we need a more dynamic theory for tides?
because technically the speed of the tides can’t keep up with the speed of the TGF
what is the M2 tide?
semi diurnal consituant with a period of 12.4 hours
what is the K1 tide?
diurnal constituent with period of 23.9 hours
what causes tides to be elliptical?
coriolis effects
what happens to the tide in narrow shallow water?
tide moves quicker because of mass conservation
what happens when we have stronger/faster tides?
more mixing occurs
what is a tidal mixing front
point where stratification is destroyed by mixing
