Tides
Changes in elevation of the ocean
surface
Caused by the gravitational forces
exerted upon the Earth by the
• Moon, and to a lesser extent by the
• Sun
Three tiny moons in independent orbits around Earth:
Moon A distance: near side of Earth’s moon;
Moon B distance: center of Earth’s moon;
Moon C distance: far side of Earth’s moon.
After one orbit, the three moons are no longer aligned. Moon
A, in the lowest orbit, completes an orbit first and passes
Moon B. Moon C, in the outermost orbit, falls behind.
Now imagine that we connect these three moons to each
other, so that they must remain in alignment as they orbit
the Earth.
Moon A: moving more slowly than its orbit calls for, is pulled
toward the Earth;
Moon B: maintains stable orbit;
Moon C: moving faster than it would independently, moves
away from the Earth.
Now we put the real moon back into place; the differential
pull of gravity on the near and the far sides acts to stretch
the sphere into an oblate spheroid.
Idealized tidal bulges on
Earth
Tides
Monthly tidal cycle
• Spring tide
• During new and full moons
• Gravitational forces of Sun, Moon added
together
• Large daily tidal range
Earth-Moon-Sun positions
during the Spring tide
Tides
Monthly tidal cycle
• Neap tide
• First and third quarters of the Moon
• Gravitational forces are offset
• Daily tidal range is least
Earth-Moon-Sun positions
during the Neap tide
Tides
Tidal patterns
• Many factors influence the tides
• Shape of the coastline
• Configuration of the ocean basin
• Water depth
How Tides Move in the Oceans
How Tides Move in the Oceans
Tidal Bore: Salmon River, NS
Tidal Bore: Salmon River, NS
Features associated with
tidal currents
Aerial view of a spit and baymouth bar
along the Massachusetts coastline
High tide in the Bay of Fundy
along the Nova Scotia coast
Low tide in the Bay of Fundy
along the Nova Scotia coast
Tides
Main Tidal patterns
• Diurnal tidal pattern
• A single high and low tide each tidal day
• Occurs along the northern shore of the
Gulf of Mexico
Diurnal
Tides
Main Tidal patterns
• Semidiurnal tidal pattern
• Two high and two low tides each tidal
day
• Little difference in the high and low
water heights
• Common along the Atlantic Coast of the
U.S.
Semi-diurnal
Tides
Main Tidal patterns
• Mixed tidal pattern
• Two high and two low waters each day
• Large inequality in high water heights,
low water heights, or both
• Prevalent along the Pacific Coast of the
U.S.
Mixed
Tides
Tidal patterns
• Tidal currents
• Horizontal flow accompanying the rise and fall
of tides
• Types of tidal currents
• Flood current – advances into the coastal
zone
• Ebb current – seaward moving water
• Sometimes tidal deltas are created by tidal
currents
Features associated with
tidal currents
Tides
Affect
Earth's
Rotation