<p>Arkaia's answer gives good information on the primary cause, which is the multitude of different astronomical changes that operate on various different timescales.</p>

<p>However, there's an additional consideration, which is "storm surge" or, more colloquially, "what the weather is doing". </p>

<p>Differences in atmospheric pressure between different areas can lead to water flowing to those areas with low pressure, and this can make a significant difference (metres) to sea level. The same is true of heavy winds and wave action. Usually this is considered in terms of flooding, and it isn't a tidal process - but it does still affect the elevation of any given high tide.</p>

<p>Storm surge is often discussed with respect to hurricanes, but the more modest pressure differences elsewhere can still have considerable effects.</p>

CC BY-SA 4.0
2019-09-01T00:04:31.567Z
Storm surge is not a tide! This only needs to confusion. Water level variability is affected mainly by tides (astronomical), surge (both caused by wind setup and atmospheric pressure), and other factors like steric anomalies (temperature, salinity effects) and wave setup and runup. Calling surge a tide is a misnomer and leads to all kinds of confusions!
arkaia
CC BY-SA 4.0
2019-09-01T04:09:22.660Z
@arkaia that's correct, and that's why I wrote "it isn't a tidal process". But, as I said, it does still affect the height of any given tide.
Semidiurnal Simon
CC BY-SA 4.0
2019-08-31T21:15:58.430Z
2019-08-31T21:15:58.430Z
Semidiurnal Simon
<p>The dynamics of the tides are quite complex. The main idea is that gravity from the Moon and the Sun affect water (and everything else) on Earth. The issue is that there are several motions that alter the distance between the 3 systems and those motions cause interactions between the different frequencies involved. The <a href="https://en.wikipedia.org/wiki/Theory_of_tides" rel="nofollow noreferrer">Equilibrium Theory of Tides</a> separates the different effects into a set of constituents by conducting a harmonic analysis. The relevant periods are:</p>

<ul>
<li>the lunar day (period of lunar rotation), 24.84 mean solar hours.</li>
<li>the <a href="https://en.wikipedia.org/wiki/Orbit_of_the_Moon" rel="nofollow noreferrer">sidereal month</a> (period of lunar declination), 27.32 mean solar days.      </li>
<li>the <a href="https://en.wikipedia.org/wiki/Tropical_year" rel="nofollow noreferrer">tropical year</a> (period of <a href="https://en.wikipedia.org/wiki/Position_of_the_Sun#Declination_of_the_Sun_as_seen_from_Earth" rel="nofollow noreferrer">solar declination</a>), 365.24 mean solar days.</li>
<li>the period of the lunar perigee, 8.85 years (1 year = 365.2421988 days).</li>
<li>the period of the <a href="https://en.wikipedia.org/wiki/Lunar_node" rel="nofollow noreferrer">lunar node</a>, 18.61 years.</li>
<li>the period of the solar perihelion, 20940 years.</li>
</ul>

<p>The explanation of each constituent can be rather complex (<a href="https://earthscience.stackexchange.com/questions/8407/definitions-of-tidal-harmonic-constituents">some examples in this other answer</a>). The different amplitudes in a day and the <a href="https://en.wikipedia.org/wiki/Tide#Range_variation:_springs_and_neaps" rel="nofollow noreferrer">spring-neap cycle</a> are related to the combination of the main lunar and solar effects. </p>

<p>The differences in high/low tide from month to month are related to the next two main frequencies of oscillation. Mainly, the variations in Earth-Sun distance associated occurring in a period of <a href="https://en.wikipedia.org/wiki/Tropical_year" rel="nofollow noreferrer">tropical year</a>. The lunar distance also contributes to these differences, but its cycle is much longer (~9 years). Also, the spring-neap cycle (with a frequency of half a lunar month ~13.5 days) will occur at different times of the month and can lead to differences in tidal amplitude if you compare the tides measured the same day of consecutive months.
<a href="https://i.sstatic.net/yvFmm.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/yvFmm.jpg" alt="spring-neap tide from www.niwa.co.nz"></a>
(Source www.niwa.co.nz)</p>

CC BY-SA 3.0
2017-01-29T06:12:38.340Z
So if I understand you right, you're suggesting the greatest interannual influence is due to sun angle?  So it's different times of year in different hemispheres?
JeopardyTempest
CC BY-SA 3.0
2017-01-29T15:37:42.590Z
The interannual variability is more affected by the Moon-Earth distance. The intra-annual variability is affected by the relative positions between the Sun and the Earth that varies with the same period as the solar declination. I have tried to clarified this point.
arkaia
CC BY-SA 3.0
2017-01-27T14:51:46.960Z
2017-01-29T15:37:19.400Z
2017-04-13T12:50:30.540Z
arkaia
<p>I've noticed that some ‘highest‘  high tides in one month are bigger than the highest high-tide of previous months. Why is this so? </p>

CC BY-SA 3.0
2017-01-27T13:50:54.527Z
An informed guess would be it is heavily based on the lunar distance (https://www.timeanddate.com/astronomy/moon/distance.html), but I don't know enough about the topic to give any certainty to that.
JeopardyTempest
CC BY-SA 3.0
2017-01-27T14:17:11.680Z
Not only related to the Earth-Moon distance. In reality it is a combination of lunar and solar effects. The main factor for monthly variability being the solar declination.
arkaia
<oceanography><sea-level><tides>
Why do high tides vary month to month?
lithium123
# Why do high tides vary month to month?

(Asked by: username_0 on 2017-01-27)

I've noticed that some ‘highest‘ high tides in one month are bigger than the highest high-tide of previous months. Why is this so?

**Tags:** oceanography, sea-level, tides

> **Comments:**
> **username_1**: An informed guess would be it is heavily based on the lunar distance (https://www.timeanddate.com/astronomy/moon/distance.html), but I don't know enough about the topic to give any certainty to that.  
> **username_2**: Not only related to the Earth-Moon distance. In reality it is a combination of lunar and solar effects. The main factor for monthly variability being the solar declination.  

---

## 2 answers

### Answer by username_2 on 2017-01-27. Score: 9 | Accepted answer

The dynamics of the tides are quite complex. The main idea is that gravity from the Moon and the Sun affect water (and everything else) on Earth. The issue is that there are several motions that alter the distance between the 3 systems and those motions cause interactions between the different frequencies involved. The [Equilibrium Theory of Tides](https://en.wikipedia.org/wiki/Theory_of_tides) separates the different effects into a set of constituents by conducting a harmonic analysis. The relevant periods are:

* the lunar day (period of lunar rotation), 24.84 mean solar hours.
* the [sidereal month](https://en.wikipedia.org/wiki/Orbit_of_the_Moon) (period of lunar declination), 27.32 mean solar days.
* the [tropical year](https://en.wikipedia.org/wiki/Tropical_year) (period of [solar declination](https://en.wikipedia.org/wiki/Position_of_the_Sun#Declination_of_the_Sun_as_seen_from_Earth)), 365.24 mean solar days.
* the period of the lunar perigee, 8.85 years (1 year = 365.2421988 days).
* the period of the [lunar node](https://en.wikipedia.org/wiki/Lunar_node), 18.61 years.
* the period of the solar perihelion, 20940 years.

The explanation of each constituent can be rather complex ([some examples in this other answer](https://earthscience.stackexchange.com/questions/8407/definitions-of-tidal-harmonic-constituents)). The different amplitudes in a day and the [spring-neap cycle](https://en.wikipedia.org/wiki/Tide#Range_variation:_springs_and_neaps) are related to the combination of the main lunar and solar effects.

The differences in high/low tide from month to month are related to the next two main frequencies of oscillation. Mainly, the variations in Earth-Sun distance associated occurring in a period of [tropical year](https://en.wikipedia.org/wiki/Tropical_year). The lunar distance also contributes to these differences, but its cycle is much longer (~9 years). Also, the spring-neap cycle (with a frequency of half a lunar month ~13.5 days) will occur at different times of the month and can lead to differences in tidal amplitude if you compare the tides measured the same day of consecutive months.
[![spring-neap tide from www.niwa.co.nz](https://i.sstatic.net/yvFmm.jpg)](https://i.sstatic.net/yvFmm.jpg)
(Source www.niwa.co.nz)

> **Comments:**
> **username_1**: So if I understand you right, you're suggesting the greatest interannual influence is due to sun angle?  So it's different times of year in different hemispheres?  
> **username_2**: The interannual variability is more affected by the Moon-Earth distance. The intra-annual variability is affected by the relative positions between the Sun and the Earth that varies with the same period as the solar declination. I have tried to clarified this point.  

---

### Answer by username_3 on 2019-08-31. Score: 1

username_2's answer gives good information on the primary cause, which is the multitude of different astronomical changes that operate on various different timescales.

However, there's an additional consideration, which is "storm surge" or, more colloquially, "what the weather is doing".

Differences in atmospheric pressure between different areas can lead to water flowing to those areas with low pressure, and this can make a significant difference (metres) to sea level. The same is true of heavy winds and wave action. Usually this is considered in terms of flooding, and it isn't a tidal process - but it does still affect the elevation of any given high tide.

Storm surge is often discussed with respect to hurricanes, but the more modest pressure differences elsewhere can still have considerable effects.

> **Comments:**
> **username_2**: Storm surge is not a tide! This only needs to confusion. Water level variability is affected mainly by tides (astronomical), surge (both caused by wind setup and atmospheric pressure), and other factors like steric anomalies (temperature, salinity effects) and wave setup and runup. Calling surge a tide is a misnomer and leads to all kinds of confusions!  
> **username_3**: @username_2 that's correct, and that's why I wrote "it isn't a tidal process". But, as I said, it does still affect the height of any given tide.  

---

