tides lecture 2

what causes ocean tides

gravitational pull of the moon & sun combined w/ centrifugal force from the earth-moon rotating around a common center of mass

which has a greater effect on tides: the sun or moon, & why

the moon has a greater effect on tides bc it is much closer to earth, even though it is smaller than the sun

what 2 forces create tides

1. gravitational attraction - btw the moon & sun

2. centrifugal force - from earth & moon rotating as a system

why are there 2 high tides on earth at the same time

• on the side closest to the moon, gravity > centrifugal force → high tide

• on the opposite side, centrifugal force > gravity → high tide

why do low tides occur

water is pulled away from areas btw the 2 high tides, producing low tide where there is no net force

how long is it btw consecutive high tides in a semidiurnal system

about 12 hrs & 25 min btw 1 high tide & the next

why do tides occur about 50min later each day

bc the moon moves in its orbit while earth rotates, earth must rotate an extra 50(ish) min to realign w/ the moon

flood tide

ebb tide

slack tide

flood: incoming tide (sea level rising)

ebb: outgoing tide (sea level falling)

slack: little or no water movement btw flood & ebb

what factors cause tides to vary geographically

• earth’s axial tilt

• landmasses blocking tidal waves

• coriolis effect

these can delay, reduce, reverse, or amplify tides

what determine the tidal pattern at a location

the interference of tidal waves & local geography, especially landmasses

semidiurnal tide

a tide w/ 2 high & 2 low tides per day of roughly equal height

common on atlantic coast of US

diurnal tide

a tide w/ 1 high & 1 low tide per day

common in gulf of mexico

mixed semidiurnal tide

a tide w/ 2 highs & 3 lows per day of unequal height

common on pacific coast of US

tidal amplitude

difference in height btw high & low tide, which varies geographically

spring tide

a tide w/ max tidal range (higher highs & lower lows) that occurs during full & new moons, when the sun & moon align

neap tide

a tide w/ reduced tidal range that occurs during quarter moons, when the sun & moon are at right angles

how often do spring & neap tides occur

about every 2 weeks, bc lunar month is about 27.5 days

why do louisiana & gulf of mexico tides differ from the bay of fundy

gulf of mexico: tides are microtidal (0.3m) & wind & precipitation dominated

bay of fundy: very large tidal ranges (2-4m) driven by lunar tides

why are louisiana tides not strongly lunar-dominated

wind & weather systems (cold fronts, rainfall) have a larger influence than lunar gravity

how do cold fronts affect the gulf tides in winter

winds push water out of marshes, lowering water levels by 0.3m & causing extended low tides that can stress organisms

how do spring rains affect the gulf’s tides

incr rainfall raises water levels, sometimes flooding marshes for several days, which can also be stressful for organisms

how do tides affect intertidal organisms

• air exposure causes desiccation & thermal stress

• feeding & respiration usually occur when submerged

• tidal currents can be strong

how do tides influence beh & life cycles of organisms

feeding, movement, reproduction, & timing of larval release, often synchronized w/ spring tides

why are larve often released during spring tides

spring tides enhance dispersal, incr survival & colonization success

tidal locking

phenomenon where the moon rotates onces on its axis in the same time it orbits earth, so only one side of the moon is visible from earth

how have tidal forces affected earth’s rotation

tidal forces have slowed earth’s rotational velocity, contributing to longer days over geological time