7.1 Tidal forces and tidal patterns

Gravitational forces from the Moon and Sun create Earth's tides, causing water to rise, fall, and flow horizontally. These forces result in different tidal patterns: spring tides during new and full moons, and neap tides during quarter moons.

Tidal patterns vary globally, with diurnal, semidiurnal, and mixed types. Factors like coastal topography, continental shelf width, latitude, and resonance affect tidal range and currents, creating unique conditions in different locations worldwide.

Gravitational Forces and Tidal Patterns

Gravitational forces of tides

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• Moon's gravitational pull primarily influences Earth's tides creates two tidal bulges facing the moon and opposite side of Earth
• Sun's gravitational pull secondarily influences tides enhances or diminishes lunar tides depending on alignment (new moon, full moon)
• Effects on Earth's oceans include vertical movement of water (rise and fall) and horizontal movement of water (tidal currents)
• Tidal range measures difference between high and low tide levels varies globally (Bay of Fundy, Nova Scotia: 16m)

Spring vs neap tides

• Spring tides occur during new and full moon phases when Sun, Moon, and Earth align (syzygy) resulting in higher high tides and lower low tides
• Neap tides occur during first and third quarter moon phases when Sun and Moon form right angle with Earth resulting in lower high tides and higher low tides
• Lunar phase relationship links spring tides to new moon and full moon (0° and 180°) and neap tides to first quarter and third quarter moon (90° and 270°)

Types of tidal patterns

• Diurnal tides feature one high tide and one low tide per day in approximately 24-hour cycle common in Gulf of Mexico and parts of Southeast Asia (Thailand)
• Semidiurnal tides have two high tides and two low tides per day in approximately 12-hour cycle most common globally prevalent along Atlantic coasts (East Coast USA)
• Mixed tides combine diurnal and semidiurnal patterns with two high tides and two low tides of different heights common along Pacific coasts of North America (California)

Factors influencing tidal range

• Coastal topography affects tidal range narrow bays and estuaries amplify range (Bay of Fundy) while wide, open coastlines have smaller ranges (Florida)
• Continental shelf width impacts tidal range broad shelves increase range (North Sea) narrow shelves decrease range (West Coast Africa)
• Latitude generally correlates with tidal range higher latitudes experience larger ranges (Alaska) compared to lower latitudes (Caribbean)
• Resonance effects in enclosed basins can amplify tidal range when natural period of oscillation matches tidal forcing (Gulf of Mexico)
• Bathymetry influences tidal currents shallow areas experience stronger currents (English Channel) while deeper areas have weaker currents
• Coriolis effect deflects currents to the right in Northern Hemisphere and to the left in Southern Hemisphere affecting tidal circulation patterns
• Coastal geometry modifies current patterns headlands and bays create complex flow structures (San Francisco Bay)
• Tidal range generally correlates with current strength larger ranges produce stronger currents (Strait of Magellan)
• Location-specific influences:
  1. Bay of Fundy experiences extreme tidal range due to resonance and funnel shape
  2. Mediterranean Sea has minimal tides due to narrow connection to Atlantic Ocean
  3. Cook Strait, New Zealand features strong tidal currents due to narrow passage between islands