>A two year delay differencing can act as a very granular derivative, so it may be a measure of the speed at which the sea-level is rising in the harbor.
Why don't you cook up a time series so I could wavelet transform it and see this 2 year periodicity.
I have a question: Why do you think these periodicals need to have a cognitive or rational explanation e.g. sloshing. Maybe the dynamical system has 'beats' like waves canceling some parts of other waves.