The magnitude of rate of change of length of day ( deviations from standard year of 86400 seconds ) shows medium and long term patterns.
The long period is about 37/2 years though spectral analysis show a fairly broad peak to be centred, unsurprisingly, near 18.6 years. The shorter pattern shows pulses at about 4.4 years on average in this sample. That is an amplitude modulation of about 8.8y. Likely the lunar apsidal period of 8.85 years.
The early part of the record displays this pattern more clearly. This may reflect constructive interference of short term periodicity or climatic effects disrupting the regular pattern in the warmer part of the record.
This can be seen more clearly in the Comb2003 series, which has the tidal effects removed:
There is no obvious link to variation in solar activity as witnessed by sun spot area.