Recent ‘king tides’ caused by a combination of factors

‘King tides” is a colloquial term for the highest occurring tides. They are a type of spring tides, which have nothing to do with spring, but rather the position of the sun and moon.

Spring tides occur twice each month in conjunction with the new and full moons.

All tides result from a combination of the gravitational forces of the sun and moon. Most of the tidal force is due to the moon, but the sun contributes about half as much.

During the new and full moons, the near alignment of the sun and moon adds the two forces.

In contrast, when the sun and moon are at quadrature — when they are at right angles to each other, as seen from Earth — we see a quarter moon (colloquially called a “half moon”), and the pull of the sun and moon are at 90 degrees to one another producing a neap tide, or a moderate tide.

Observed spring-tide water levels have been several inches above predicted tidal heights since early 2016, so there must be other factors that also influence the height of the tides.

The most reliable and constant are the highest and lowest spring tides that occur when the moon is closest to Earth at perigee and Earth is closest to the sun at perihelion.

Perihelion happens in January and perigee happens every month. Either of these events causes higher-than- average spring tides; when they happen at the same time, they produce extremely high tides.

The declination of sun and moon, atmospheric pressure, water temperature, sea level changes. offshore winds, ocean currents, waves and storm surge can all contribute to the height of the tides.

“Declination” is the term used to describe the location of an astronomical object on the celestial sphere.

From Earth the declination of the sun appears to change as we orbit due to the tilt of Earth’s axis. The tilt of 23.4 degrees causes the sun’s declination to shift back and forth between the Tropic of Cancer and the Tropic of Capricorn at solstices in December and June.

The moon’s declination changes monthly, and when its declination lines up with the sun, it produces higher tides below that point.

Hawaii is the only state where the sun can be directly overhead at noon. In astronomical terms this means the declination of the sun is the same as the observer’s latitude. Locally we call it “Lahaina Noon,” and it happens in Honolulu on May 26 and July 15.

This brings the tidal bulge directly in line with the islands, causing higher tides. Since the tidal bulge is symmetrical, there is a similar effect Jan. 13 when the sun is in the Southern Hemisphere at Hawaii’s latitude.

Low pressure and high water temperature raise the water surface; we should always expect higher tides in the summer months. On top of that, oceanographers predict a strong El Nino this summer, during which warm water accumulates in the Eastern Pacific and contributes to overall higher sea level.

In April and May the highest tides were more than 9 inches above predictions and were the highest daily mean water observed over the 112-year record.

According to the University of Hawaii Sea Grant Program, these record tides were due to “an unusual combination of ocean eddies with high centers, Pacific-wide climate and sea level variability associated with recent El Nino events and the Pacific Decadal Oscillation (another pattern of Pacific climate variability), and sea-level rise from global warming.”

Chip Fletcher, associate dean of the School of Ocean and Earth Science and Technology, noted in the news article posted on UH’s website May 19, “Within a few decades, this will be the new normal. Hawaii should consider this a practice run, and reevaluate policies and development practices accordingly.”

Richard Brill is a professor of science at Honolulu Community College. His column runs of the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.