Tides in the Florida Keys

Zeblon, NC

After some discussion with friends about sailing the inside passage between Marathon and Biscayne Bay, I decided to do a little research on the tides in that region.  What I learned is very interesting. All the data below pertains to the tide station at Vaca Cut, Florida.

The pie chart shows the components of tidal variation.  The repeated pattern 1/2 to 1 is very interesting.   1/2 day, 1 day, 1/2 lunar month, 1 lunar month, 1/2 year, 1 year.  The proportions of the pie vary according to location.   For example, in Pensacola, Florida there is almost zero 1/2 day component of tide.  They have only one high and one low tide per day.

Below shows the tides for the past year.  The monthly and biannual components are easily visible. We traveled the inside passage around December 1.  As you can see, the water levels are much higher then than in the spring.  The lesson, sail the inside passage southward in Nov/Dec but the outside passage Apr/May.

Below is the sea level trend.  Does this mean global warming?  I don't know. But the rate of 3mm/year is pretty slow in any case.

Below is the tidal trends for the past 11 years.  As you can see, the 2010-2011 winter season was exceptionally low.

I can also see a definite component with a 9 year period.  I posted a question about this on an oceanographer forum.  I learned that there is a 9 year period to the precession of the moon's perigee, and a 18 year period of the precession of the moon's plane.  Wow, imagine observing such sophisticated astronomical things just by keeping records of tides.  Amazing.