- #141
DaveC426913
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Yeah. Density waves. Where they bunch up together, like sound waves.
WHT said:What this implies is that the fast +sideband of the moon's declination cycle is amplified by the stronger lunar force as the moon nears the Earth during its perigee/apogee cycle. The slower -sideband amplification results in the 8.85 year cycle, which is called the apsidal precession cycle. So again this -sideband cycle has a name but the 4.53 year cycle doesn't.
Like the Bay of Fundy example I referenced above, the consensus theory says 4.4 years, but the measurements are closer to 4.5 years. The behavioral difference between the two is that 4.5 years includes both declination and perigee wheres 4.4 years is just perigee.The role of long-period tides in modulating NF frequency
In addition to secular changes in NF frequencies due to tidal changes, we also detect a ~4.5-year cycle in the time series of additional/reduced NF days (fig. S3), which is close to the half perigee cycle that modulates tides at a period of 4.4 years. The perigean (4.4 years) and nodal (18.6 years) modulations of tides affect high water levels (10). We quantify the impacts of these cycles on NF events by removing them from the tidal prediction and recalculating the number of NF days (assuming that these cycles would not exist); this is only done for the dataset with the observed tides, not the one with historic tides, as we assume that changes in the amplitudes of the 4.4- and 18.6-year cycles were negligible. The oscillations in NF days due to the low-frequency tidal modulations are evident, and their influence increases over time (Fig. 4D). The 4.4-year cycle adds up to 20 NF days across all locations (Fig. 4D) when it peaks under present-day sea level, whereas the 18.6-year cycle causes an additional 30 NF days during its peak compared to average conditions (Fig. 4D)