Tidal Analysis in Florida Keys: Investigating 9 Year Periods & Anomalous Data

[anorlunda]

I've been looking at historical tide data for the Florida Keys at http://tidesandcurrents.noaa.gov/waterlevels.html

I understand harmonic constituents. In fact, I found the following data for this location.

Amplitude in Feet, Period in Days, Description
0.2 1 Principal lunar semidiurnal constituent
0.1 1 Principal solar semidiurnal constituent
0.0 1 Larger lunar elliptic semidiurnal constituent
0.2 1 Lunar diurnal constituent
0.1 0 Shallow water overtides of principal lunar constituent
0.2 1 Lunar diurnal constituent
0.0 0 Shallow water overtides of principal lunar constituent
0.0 0 Shallow water terdiurnal
- 0 Shallow water overtides of principal solar constituent
0.0 0 Shallow water quarter diurnal constituent
0.0 1 Larger lunar evectional constituent
- 0 Shallow water overtides of principal solar constituent
0.0 1 Variational constituent
0.0 1 Lunar elliptical semidiurnal second-order constituent
0.0 1 Lunar diurnal
- 1 Smaller lunar evectional constituent
0.0 1 Solar diurnal constituent
0.0 1 Smaller lunar elliptic diurnal constituent
0.0 1 Smaller lunar elliptic diurnal constituent
0.1 28 Lunar monthly constituent
0.1 183 Solar semiannual constituent
0.3 365 Solar annual constituent
0.1 15 Lunisolar synodic fortnightly constituent
0.0 14 Lunisolar fortnightly constituent
0.0 1 Larger lunar evectional diurnal constituent
0.1 1 Larger lunar elliptic diurnal constituent
0.0 1 Larger solar elliptic constituent
- 0 Smaller solar elliptic constituent
0.0 1 Larger elliptic diurnal
0.1 1 Solar diurnal constituent
- 0 Shallow water semidiurnal constituent
- 0 Lunar terdiurnal constituent
0.0 1 Smaller lunar elliptic semidiurnal constituent
0.0 0 Shallow water terdiurnal constituent
0.0 0 Lunisolar semidiurnal constituent
- 0 Shallow water eighth diurnal constituent
0.0 0 Shallow water quarter diurnal constituent


Clearly there is no constituent longer than a year. Also, the sea level trend for this location is 2.78 mm/year; very small compared to the tidal variations.

I plotted the the monthly average low tides for the period 2000-2014 (see the attached picture), I see a strong component with a 9 year period. I'm struggling to understand that. Could it be El Nino effects in Florida?

The same attached file, shows a very anomalous year from about June 2011 to June 2012. I'm struggling to understand that too. My instincts say that a weather event could only disturb the data for a month or two.

 

[AlephZero]

I think you need to refer your list of components back to the Doodson numbers. http://en.wikipedia.org/wiki/Arthur_Thomas_Doodson#Doodson_Numbers

The precession of the moon's perigee (period 8.8 years) and the precession of the plane of the moon's orbit (period 18.6 years) should show up somewhere, as "beats" between the periods of the components. But since your OP only shows the periods to the nearest day, there isn't enough accuracy to "reverse engineer" where the 8.8 year cycle is coming from, except by checking the Doodson numbers for each constituent.

Another possibility is that a severe weather event caused a long-term change the depth profile of the sea bed, and therefore changed the amplitude of the tidal components.