# How often do king tides (perigean spring tides) occur?

Recently, there has been a lot of information about "King Tides" (i.e, perigean spring tides) on the news with a number of groups relating the peak king tide to future normal conditions (2050) under sea level rise scenarios along the east coast of the United States.

The effects on the magnitude of the tides are obvious when we compare normal tides (OSU model results for a day in 2010)

with the recent tides for the entire planet (spring tides): .

While most of the difference is associated with the spring tides, about 2-3% is associated with the relative position of the Moon.

My question is related to the recurrence interval of perigean spring times. Some sources mention that they occur 3-4 times a year. If that is the case, then why such a focus on the mid-October 2016 event?

Another related question, which constituent of the tide is associated with the perigean/apogean oscillations?

• As you can see here there is no sygyzy on that date. – user6402 Oct 25 '16 at 6:11
• spring tides do not happen exactly at sygyzy. There is always a delay. – arkaia Oct 25 '16 at 12:22
• Who said it was only a few hours? – arkaia Oct 26 '16 at 12:23
• earthscience.stackexchange.com/questions/8956/… – arkaia Nov 8 '16 at 2:48

With regard to the effect on tides, the combination of the $M_2$ and $S_2$ tidal constituents gives rise to the spring/neap cycle. Meanwhile, the $N_2$ tidal constituent arises from the fluctuating distance of the Moon from Earth and its frequency is 1 cycle/month less than that of $M_2$ and 3 cycles/month less than that of $S_2$.
We can resynthesize a time series of tidal elevation based on only these three tidal constituents ($M_2$, $S_2$, and $N_2$). The resulting time series of amplitudes has an envelop that shows the size of the spring tides. As can be seen in the figure, even including only these three constituents, the resulting variability in the size of the spring tides (12 years shown) is pretty high. The problem is how to define "king tides". There are some spring tides that are larger than others, but even among the largest there is variability. This is where the uncertainty on the definition of the recurrence interval of the "king tides" comes from. Ultimately, it seems that having ~2 peaks ("king tides") per year is a good approximation. Including 3 constituents is a significant simplification and the entire series gets more complicated when additional constituents are included and defining a "king tide" is also more difficult.