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Tides and Currents

Tides and Currents. Tides 101. Vertical movement of water Predictable S tanding wave circling an amphidromic point 3 Kinds of Tides Semi-diurnal Two high and two low tides per day (Atlantic Coast) Diurnal One high and one low tide per day (Northern Gulf of Mexico) Mixed semi-diurnal

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Tides and Currents

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  1. Tides and Currents

  2. Tides 101 • Vertical movement of water • Predictable • Standing wave circling an amphidromic point • 3 Kinds of Tides • Semi-diurnal • Two high and two low tides per day (Atlantic Coast) • Diurnal • One high and one low tide per day (Northern Gulf of Mexico) • Mixed semi-diurnal • Two high and two low tides per day (Seattle) • Different amplitudes (MLLW) • Primarily influenced by the position of the Moon and Sun relative Earth

  3. NOAA, 2008

  4. Equilibrium theory of tides Earth water Moon SUN Gravity Centrifugal water

  5. Tidal Forces Earth • Sun and moon are aligned (new & full) • Syzygy • Spring Tides, higher high tides and lower low tides Sun Moon Sun’s gravity + Moon’s gravity = tidal amplitude

  6. Compared to… Moon • Sun and moon are at 90 degrees (first and third quarter) • Quadrature • Neap Tides, less extreme heights Earth Sun BUT WE DON’T HAVE SPRING AND NEAP TIDES!!!!!

  7. Tropic and Equatorial Tides • Occur in Diurnal Systems • Declination of the moon • Elliptical orbit of the moon… Moon Tropic Earth Moon Equatorial

  8. Tidal Questions • Moon to circle Earth • 29.5 days (Lunar Month) • Moon declination above equator max to max: • 29.5 days (Tropical Month) • How long between two successive tropic tides? • Two successive spring tides? • Successive tropic and equatorial tides? • How long will it take to complete one complete diurnal tidal cycle? (HighLowHigh) • Why?

  9. Currents • Horizontal flow of water • Impacted by rainfall, wind, water density (temperature/salinity), tides • VERY IMPORTANT WHEN DIVING!!!! • Do NOT underestimate the power of moving water!!!!!!

  10. Density Currents NOAA 2011

  11. Tidal Currents • When is the best time to dive, with respect to tidal currents? • Slack Water (GOOD FOR DIVING) • Two-three per day (high and low tides) in Mobile Bay and occur when the tides switches directions (from ebb to flood and vice versa) • Maximum Current (BAD FOR DIVING) • Two per day and occur at the midpoint between high and low tides. • Ask Dardeau about the flow directions in the bay… they really don’t make sense… • What else can impact the flow rate of tidal currents?

  12. Seiche • Similar to storm surge • Consistent wind direction causes water to pile up and can negate tides… • What happens when wind direction changes? • When it changes during a low tide?

  13. Online Resources • Tides for Mobile Bay • http://tidesandcurrents.noaa.gov/tide_predictions.html?gid=226 • Currents for Mobile Bay • http://tides.mobilegeographics.com/locations/3842.html • NOAA tidal currents • http://tidesandcurrents.noaa.gov/currents14/cpred2.html#FL • Note the units on the y-axis between the first two…

  14. Diving Relevance • How strong is a 2 knot current? • When should you call the dive? • What can you do to mitigate current? • VIDEO

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