The science behind King Tides: What are they and how do they happen?

The science behind King Tides: What are they and how do they happen?
Scott Sutherland
·5 min read
The science behind King Tides: What are they and how do they happen?
The science behind King Tides: What are they and how do they happen?

Vancouver Island and the BC South Coast, as well as the coastlines of Washington and Oregon, were 'on alert' over the past week. Exceptional high water levels known as king tides impacted these regions and even caused disruptions along some areas of infrastructure. So, what exactly are these 'king tides' and what causes them to occur?

Tides are a regular fact of life for Canada's coastal regions. The waters along our shores advance and retreat twice every day. These tides also swell even higher and drop even lower twice every month, and there is even a longer cycle that requires a yearly calendar to track.

So, what's the source of these tidal cycles? It's the gravitational 'dance' of the Earth, the Moon, and the Sun.

Due to the Moon's presence, the Earth itself and everything on its surface feel a slight pull of gravity in its direction. It is a fairly weak force compared to the Earth's gravitational pull, so it isn't anything we would consciously notice. However, for the water in Earth's oceans, the Moon's gravitational pull tugs the molecules of this fluid towards the spot on Earth that directly faces the Moon. This results in the oceans 'bulging' at that point, which raises the water level around that area. This same effect draws water on the other side of the planet towards the spot exactly opposite the Moon, resulting in a corresponding bulge there, as well.

Tidal-bulges-3-panel
Tidal-bulges-3-panel

The tidal bulge of Earth's oceans moves along with the Moon as it orbits the planet. A second, smaller tidal bulge points towards and away from the Sun. Diagram is not to scale!. Credit: NASA/Scott Sutherland/Tyler Hamilton

As the Earth rotates through its 24 hour day, the Moon only changes position by a relatively small amount. So, each location on the planet passes through both of these bulges and experiences two high tides and two corresponding low tides in between. Since the Moon does change location each day, though, the timing of the high and low tides advances by a bit each day.

Twice every month, at the Full Moon and the New Moon, the tides tend to be higher than usual due to the influence of the Sun. This influence isn't as strong as the Moon's because while the Sun may be bigger, it is a lot farther away. Still, it results in a second, smaller tidal bulge that always points directly toward and away from the Sun. So, as the Moon orbits around the Earth, this solar tidal bulge adds to the lunar tidal bulge at some times, while subtracting from it at others.

At the Full and New Moons, the Sun, Earth, and the Moon all line up in a pattern known as syzygy.

Tidal-bulge-Syzygy
Tidal-bulge-Syzygy

During syzygy, the lunar and solar tidal bulges combine. Although a New Moon syzygy is shown here, this also occurs during the Full Moon. Diagram is not to scale. Credit: NASA/Scott Sutherland/Tyler Hamilton

When this occurs, the lunar and solar tidal bulges line up as well, resulting in the spring tides. Near the first quarter and third quarter Moons, when the two tidal bulges are perpendicular to one another, high tides are lower (neap tides). Note that they aren't called spring tides because they exclusively happen in spring. Instead, the name refers to the 'springing forth' of the tide.

All tides are not equal, though. As the Moon traces out its elliptical orbit around Earth, sometimes it is closer to us (exerting a stronger gravitational pull), while at other times, it is farther away (exerting a weaker gravitational pull). When the Moon is closest to Earth (perigee), its gravitational pull is strongest, which produces the highest high tides and the lowest low tides. If a Full or New Moon occurs at perigee, this is when we see the perigean spring tides.

Moon-Orbit-2021
Moon-Orbit-2021

Lunar orbit may look circular, but its slight elliptical shape means the Moon's distance changes by thousands of kilometres as it revolves around Earth. Credit: NASA/Scott Sutherland

It just so happens that Earth also traces out an ellipse, rather than a circle, as it travels through its orbit around the Sun. This means that there is a point when we are closest to the Sun, called perihelion. Each year, it occurs sometime between the 2nd to the 5th of January. Aphelion, Earth's farthest point from the Sun, happens between the 4th and 7th of July. Thus, the Sun's tidal bulge grows and shrinks throughout the year, as well, and it is highest near perihelion.

The highest of all high tides during the year, when the greatest number of these factors line up the best, is the King Tide.

Add to all of this the fact that the Moon's perigee distance changes month to month. For example, the closest perigee Moon of 2021 is the May 26 Full Moon (aka, the Super Blood Flower Moon). Earth's perihelion distance changes, year to year, as well. 2021's was closer than usual, but 2020's was the closest of the entire 21st century.

So, if the year's closest perigee Full or New Moon happened simultaneously with perihelion, we would definitely see the highest tides possible. Apparently, the last time that everything lined up that well was on January 4, 1912.

Things won't line up that well again until 2045, but watch at the end of this year. The New Moon on December 4 is the closest perigee Moon since February 2019. Plus, the January 2, 2022, perigee New Moon occurs just a few days before perihelion! The combination of syzygy at perigee and their proximity to perihelion should produce some exceptional king tides!