Physical and Chemical Oceanography

Section 7 Part II: Tides, Currents and Upwelling

Tide• Regular rise and fall of sea level• Due to gravity of Sun, Moon, Earth and

rotation of Earth• Cycle of 12.5 hours– Most coasts: 2 high and 2 low tides daily

To understand…

NEW MoonFULL Moon

1st QUARTER

3rd QUARTER

Spring Tide• Earth, Moon and Sun are aligned– Amplitude is greatest

• New Moon

• Full Moon

Neap Tide

• Tidal range is smaller– Due to Moon being perpendicular to Earth/Sun

Conclusion

Spring Tide vs. Neap Tide (Moon has about 70% effect on tides)

Tidal Amplitude

• Tidal range– Difference in height between low and high water

• Varies considerably– 12 m tall to 0.1 m

Influences on Tides

1. Shape of coastline– Tide enters tapering river mouth or bay tide

increased by opposite sides of channel

2. Wind and air pressure changes– Strong onshore wind and low pressure tidal

surge (exceptionally high tide)

Tidal Amplitude

• Ocean TA Average = 0.6 m– Increases as tide enters shallow continental margins

• Lowest TA: small bodies of water– Ex: Mediterranean Sea, Red Sea, Baltic Sea– Small tidal ranges in large lakes, but often masked

Ocean Currents

• Continuous movement of water– Forces like waves, wind, Coriolis effect,

temperature, salinity, tides– Sea bed contours influence direction and strength

Warm, less dense, less salinity

Cold, more dense, more salinity

Surface Ocean Currents• Driven by wind• Clockwise spiral in N. hemisphere and vice-

versa in S.

Coriolis Effect

• Earth rotates, causes water to move to the right– Vice versa in S. hemisphere

• Surface currents not entirely driven by wind direction (deflected by the Coriolis effect)

Deep Ocean Currents

• Driven by temperature and density gradients• Recall: upwelling – significant vertical movement of

water occurs– Mid-ocean ridge: deflects deep water currents upward