What is a perigean spring tide?
A perigean spring tide occurs when the moon is either new or full and closest to Earth.
In order to understand the phenomenon called a ‘perigean spring tide,’ you first have to know that the gravitational pull of the moon and the sun cause tides. Tides are actually long-period waves that roll around the planet as the ocean is ‘pulled’ back and forth as the moon and the sun interact with the Earth in their monthly and yearly orbits.
The next thing you need to know is that the moon follows an elliptical path around the Earth in its monthly orbit, and the Earth follows an elliptical path in its yearly orbit around the sun. This means that, at times, the moon and the sun are closer to Earth. At other times, they are farther away. What happens when the moon and the sun are close to the Earth? You guessed it: the gravitational pull they exert is stronger, resulting in slightly higher tides.
While both the moon and the sun influence tides, the moon plays a much larger role because it is so close to the Earth. Its gravitational pull is about twice as strong as that of the sun. Now consider these two cases:
The Effect of a Full or New Moon
During full or new moons—which occur when the Earth, sun, and moon are nearly in alignment—average tidal ranges are slightly larger. This occurs twice each month. The moon appears new (dark) when it is directly between the Earth and the sun. The moon appears full when the Earth is between the moon and the sun. In both cases, the gravitational pull of the sun is ‘added’ to the gravitational pull of the moon on Earth, causing the oceans to bulge a bit more than usual. This means that high tides are a little higher and low tides are a little lower than average. These are called ‘spring tides.’
The Effect of Perigee
Once every 28 days, the moon reaches a ‘perigee,’ its closest point of approach to the Earth. This is the point at which the gravitational pull of the moon is strongest. During these periods, there will be a slight increase in the average range of tides.
What happens when a full or new moon coincides with perigee?
Full or New Moon + Perigee
About three or four times a year, the new or full moon coincides closely in time with the perigee of the moon—the point when the moon is closest to the planet. These occurrences are often called ‘perigean spring tides.’
The difference between ‘perigean spring tide’ and normal tidal ranges for all areas of the coast is small. In most cases, the difference is only a couple of inches above normal spring tides. The largest difference occurs in certain areas of the Alaska coast where the range of the tide may be increased by around six inches. But considering that these areas have an average tidal range of more than 30 feet, the increase is but a small percentage of the whole (less than a two percent increase).
Perigean Spring Tides And Coastal Flooding
Major coastal flooding doesn’t always occur whenever there is a perigean spring tide, however perigean spring tides may often cause minor coastal flooding in some very low-lying areas. Major coastal flooding typically occurs in response to strong onshore winds and barometric pressure changes from a coastal storm. If a storm strikes during a perigean spring tide, flooding could be significantly worse than it otherwise would have been. In some instances, perigean spring tides have coincided with a shift in offshore ocean circulation patterns and large scale shifts in wind that have resulted in unexpected coastal flooding. It is expected that occurrences of minor “nuisance” flooding at the times of perigean spring tides will increase even more as sea level rises relative to the land.
NOAA’s tide and tidal current predictions take into account astronomical considerations due to the position of the moon and the sun.
Compiled by Paul Johnson
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