BIOL 331

Lecture 5

Tides & Currents

Primary Causes of Tides

• Gravitational pull of moon
• Gravitational pull of the sun
• Relative positions of sun and moon
• Centrifugal Force (opposite moon)

Average Tidal Amplitude Variation

• Spring Tides
• Neap Tides

Other Factors Affecting Tidal Amplitude

• Basin morphology
• Wind
• Storm Waves
• Barometric Pressure

Geographic Variation in Tidal Amplitude

• Greatest extremes on West coast are 15-20 ft amplitude in Puget Sound and Cook Inlet, AK.
• CA coast, 10-15 ft maximum amplitude.
• Bay of Fundy has up to 50 ft.
• Lowest amplitudes of less than 3 ft occur in Jamaica and Mediterranean.

Periodicity of Tides

• Semidiurnal: 2 high and 2 low each day, highs and lows of nearly same amplitude. E coast of U.S.
• Diurnal: 1 high and 1 low each day. Vietnam/China coast.
• Mixed: 2 highs and 2 lows of different amplitudes. Pacific Coast of U.S.

Ocean Currents

Ultimately Caused by Atmospheric Circulation

1. Equator receives more solar radiation per unit area.
2. Hot air rises and moves north (and south) leaving a low pressure zone at equator.
3. As it moves north it loses heat and sinks to the ocean and spreads north and south near water (forms high pressure subtropical calm).
4. South moving air are the NE trade winds (deflected to their right in N hemisphere).
5. North moving air meets polar air moving south forming polar front where they meet.
6. North moving air is deflected right to form the disturbed westerlies.
7. Three cells of circulating air are thus formed.
8. Overall wind patterns cause similar ocean current patterns.

Coriolis Affect

• The earth rotates W to E.
• As water moves north it is deflected right (east), and as it flows south it is deflected left (east).
• Ocean currents are caused by wind and are also affected by the Coriolis effect.
• Ocean current patterns therefore have a general clockwise direction (called gyres) in the Northern Hemisphere and a counter-clockwise pattern in the Southern Hemisphere.

Convergence and Divergence Zones

• Water piles up in convergence zones
• Water “thins” in divergence areas
• Causes some ocean surface topography
• Difference in height between Western Convergence Zones and Antarctic Divergence Zone is 2 m

Boundary Currents

• Near Continents & are strongest of ocean currents
• Currents are strongest where sea slope topography is steepest
• Western boundary currents stronger than eastern
• The Florida Current approaches speeds of 3 mph

Factors Making Western Boundary Currents Stronger

1. Earth’s rotation displaces gyres to west making sea slope steeper
2. Trade winds blow toward West and pile up water even more
3. Westerlies cause mid-latitude water to flow to equator over broad area not confined to just the eastern boundary currents
4. The earth rotates fastest at the equator & water maintains momentum going north

Upwellings & Downwellings

• Water moving away from the coast cause upwellings
• Waters moving toward the coast cause downwellings (causes currents to parallel coast)

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