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

Tropic

Moon

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