You are here:

Oceanography/Need to know how far the moon drags ocean water? East-west and north-south.


Hello, I'm an author and really need some tide related info for authenticity of script. I can't find this info anywhere, and hope you might advise.

When the moon sweeps 180 degrees across the sky, over an ocean. I realize the water is lifted by up to a meter, but how far and fast does the bulk of the seawater get dragged east to west by the moon's gravity during this 12 hour period?

Also, is there similar drag of seawater from say -50 degree Latitude up toward the Equator.

It's impossible to find out about this online, but logic suggests the water is dragged a lot , but need a rough idea how far.

Any help or advice will be appreciated, Thanks Rodney

global M2 tide
global M2 tide  
Tides can be pretty complicated so I'll just try to give you some basic information. First of all, the main tidal influence is the moon, by about a factor of 2 compared to the sun, so imagining the tidal height to be "in phase" with the moon is a tempting generalization. However, the influence of various other tidal components as well as the coastlines and bathymetry (ocean depth) complicate things considerably.

I've attached a an image showing so-called co-tidal (time) and co-range (amplitude) values for the world's ocean's tides. For example, note the blue ellipse off the west coast of North America. Notice how there are white lines that converge to a point in this ellipse, sort of like spokes on a wheel. These white lines are the co-tidal lines representing the time of maximum tide amplitude (range) for that part of the ocean. There are 12 lines representing the progression of the maximum at 2 hour intervals (= 24 hours in a day). The center of the ellipse is where the amplitude is zero (its called an 'amphidrome'). You can see other amphidromes in other parts of the oceans as well as areas of large tidal ranges. In addition to this overall tidal variation, the sea level and currents driven by the tides can vary quite a bit locally (you've probably heard of the tides in the Bay of Fundy in Canada).

As far a "dragging" water east-to west, it does't really work that way (I'm interpreting dragging as meaning the movement of water horizontally). The moon and sun are really lifting the water surface vertically as the Earth rotates around. Horizontal currents due to the tides are caused by water piling up unevenly due to coasts and bathymetry. This water then sloshes back and forth in synch with the tidal cycle. These are the flood and ebb tides you've heard of.

Tidal predictions are based on fitting data (sea surface height) to a bunch of tidal components, each representing a particular frequency (very messy, really). The image I've attached shows the progression of the tides (vertical lifting) about as well as any other general representation.

There is really very little north-south transport of water due to the tides, at least over large distances. Transporting water from 50 degrees north (or south) is totally unrealistic.

Hope this helps!


All Answers

Answers by Expert:

Ask Experts


randy patton


Physical oceanography, surface and internal wave characteristics, ocean currents, fluid mechanics, geophysical fluid dynamics, ocean optics, coastal dynamics, modeling and simulation, data analysis, El Nino and related large scale dynamics Not an expert in marine biology (some in bioluminescence) or chemical oceanography


26 years as professional scientist for research company working mostly on Navy and other government contracts. Projects included modeling, simulations and data analysis related to Non-acoustic Anti-submarine Warfare (NAASW). Other projects included remote sensing of ocean features, statistical analysis of ship tracks, ocean optics instrumentation development, synthetic aperture radar (SAR) and sonar (SAS).

Journal of Physical Oceanography, 1984, "A Numerical Model for Low-Frequency Equatorial Dynamics" (with M. Cane)

MS Physical Oceanography, MIT, 1981 BS Applied Math, UC Berkeley, 1976

Past/Present Clients
Currently an Expert for All Experts in Advanced Math

©2017 All rights reserved.