A Neap tide in costal Louisiana produces 2 high and 2 Low tides very quickly in a 24 hr period, yet all other days we have one high and one low. Why does this happen?

Question

I’m familiar with neap tides. It’s my understanding that they occur when the sun, moon, and earth form right angles and the moon phase is at 1/4 and 3/4. Yet this occurrence happens on a waning or waxing gibbous every time. The range is very small and changes quickly. I can’t wrap my head around the science that produces a completely different tidal sequence twice a month, then what you would see play out on any other day

In: Earth Science

Answer 1

Part of your premise is incorrect- there are two tide cycles per day all through the month on just over a 12 hour cycle. If you look up tide charts in your local area you should see this – each day has am and pm high/low tides, with a few exceptions when the am tide is near noon, pushing the next tide cycle into the next day

With a neap tide the gravity from the moon is pulling the water at right angles to the direction it’s being pulled by the gravity of the sun, so you get smaller tides.

With spring tides the moon is aligned with the sun so they’re both pulling the water in the same direction, causing larger tides.

Answer 2

There are a bunch of things going on in the Gulf, but biggest factor is probably that any water entering/leaving the gulf has to come from through a fairly small opening of Yucatan – Cuba – Florida. The result is that it can be very hard for the water to get in/out of the Gulf. When the tides are strong (and there is a lot of water that needs to be moved) the end result is basically that the earth spins more quickly than the water can react. When the tides a weak the constriction is less of a problem.

Answer 3

While the sun and moon both energise the tides, the result in different locations has alot to do with the way the water slops around when constrained by the coastline and the contours of the sea floor. Often there’s a resonance involved, where a particular stretch of water is shaped to favour going backwards and forwards at a particular rate.

Experiment with a shallow tray of water, lifting one end rythmically up and down to make waves. You’ll find that you can make it slop around much more if you match your rythm to the time it takes the wave to go up and back the length of the tray. That’s the kind of thing that can explain many different weird tide patterns around the world.

The Gulf of Mexico is also relatively shallow in places with limited connexions to the Atlantic. The tide varies from diurnal to semi-dirunal along its coastline and, in the transistion areas, there are unusual patterns.