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Tides and Currents
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
NOAA, 2008
Equilibrium theory of tides
EarthMoonGravity Centrifugal
SUN
wat
er
water
Tidal Forces
• Sun and moon are aligned (new & full)– Syzygy– Spring Tides, higher high tides and lower low tides
Moon EarthSun
Sun’s gravity + Moon’s gravity = tidal amplitude
Compared to…
Moon
EarthSun
• Sun and moon are at 90 degrees (first and third quarter)
• Quadrature
• Neap Tides, less extreme heights
BUT WE DON’T HAVE SPRING AND NEAP TIDES!!!!!
Tropic and Equatorial Tides
• Occur in Diurnal Systems• Declination of the moon• Elliptical orbit of the moon…
Earth Moon Equatorial
TropicMoon
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? (HighLowHigh)– Why?
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!!!!!!
Density Currents
NOAA 2011
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?
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?
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…
Diving Relevance
• How strong is a 2 knot current?– When should you call the
dive?– What can you do to
mitigate current?
• VIDEO