Water to Shore

In 1776, Ben Franklin left Philadelphia by ship for France. By carefully measuring water temperatures along the way, he was able to map the Gulf Stream.

The motion of waves just moves a column or molecule of water up and down. Wouldn’t that mean that some water never hits a shore?

All water — even that in the center of the ocean — does eventually strike a shore. In fact, water goes a lot farther than the nearest shore. It’s possible to think of all the water on Earth as being connected. Water molecules go from ocean to air, to land, to lakes, to rivers, to ice sheets and finally back to the oceans. Water goes everywhere . . .

In the ocean alone, water is carried by ocean currents, for example, the Gulf Stream flowing up the Atlantic coast, then curving down to the British Isles. These sorts of currents are driven by winds which stir and mix water. What’s more, as water cools, it sinks. And, over the course of thousands of years, water that was on the top of the ocean moves around to the bottom — to create a giant conveyor-belt of ocean water.

And it’s true, the motion of the waves just moves a column of water up and down. But as waves comes near a shore, in shallow water, they “feel the bottom,” of the ocean. At that point, waves become steeper and develop some forward motion. Eventually, waves might become steep enough to break as surf, and run up on the beach.

This question was posed to a few scientists, and these are some replies:

Bill Patzert, JPL Oceanographer: The question, ” … if all the water in the ocean including the center part eventually hits a shore?” The simple answer, “An unqualified YES!” One way the think of this is that every 3000 years all the oceans eventually evaporate and fall back into the oceans and on the continents … the global recycling system. Yea, I know, that’s not what you meant. But, that’s the big picture … the hydrological cycle that controls climate. Also, water cools and sinks at high latitudes, and rises in upwelling zones like the equator and along coasts. This is another pathway. I think of water in the oceans, air, lakes, rivers and in aquifers as all connected. Ocean to air to land to lakes to rivers to ice sheets and back to the oceans. Around the surface of the oceans, sinking to the bottom, returning to the surface. Water goes everywhere! The ocean currents are driven by winds which stir and mix; create waves (which actually have a net forward movement, “Stokes drift”); have ocean-wide patterns of great currents; and, as mentioned above, exchange heat and water with the atmosphere. Drop a tennis shoe in the ocean … what happens? Read this really interesting article: http://community.seattletimes.nwsource.com/archive/?date=20030307&slug=flotsam07m  . I know that some ocean debris (let’s call them “water particles”) are adrift for decades or even centuries, but with all the islands and coastlines in the various oceans, eventually surface floating water or debris hits a shore or the ocean bottom. Enough?

  1. A particle of water in a wave in the open ocean does move only up and down. It is just the path of successive up and down movements, at the surface, that we see as waves. As waves get near the shore, in shallow water, they “feel the bottom” and become steeper, and develop some forward motion. Eventually, they may become steep enough to break as surf, and run up on the beach with only forward motion.
  2. And, the travel of water in the ocean does depend on ocean currents. A fine example is the Gulf Stream, which is a wide river of ocean water flowing from the Caribbean, up our Atlantic coast, then curving around to the British isles, giving them a warmer climate and rain and fog. This flow was mapped with floating bottles by none other than Benjamin Franklin in the 18th century and helped sailing ships travel to England faster. The water diffuses and returns by counter currents, much of it at lower depths since it is now cool and therefore heavier. So you can see that ocean circulation can be a complex problem, and it may be years before that particle makes landfall, if ever!

Dork Sahagian: All this depends on the time and space scales in which you are interested. For reasonable size scales (say, a few meters on a side), there is a characteristic time during which the ocean mixes. This is on the order of a thousand years. This means that water that was on the top moves around to the bottom, water in the Pacific reaches the Atlantic, etc. Chance are, most parcels of water will interact with a shoreline during that time. Still, there could be some spots in the bottom of trenches, etc. that take longer to mix with the rest of the ocean. The details of deep-sea circulation are not so well understood, but the basic thermohaline circulation (the so-called “Conveyor-belt”) was recognized years ago, and is described in every elementary textbook.

So yes, even water in the middle of the ocean eventually hits a shore, but it is carried by slow currents (Gulf Stream being most famous) rather than waves (they just move particles in vertically oriented circles).

I hope this helps.

Steven Webster: Jorge, it’s rather like the statement that some of the molecules of oxygen we breathe today were probably breathed by Jesus Christ in his day (or Benjamin Franklin, or pick your historical luminary). Molecules in the atmosphere and ocean, including water molecules, get pretty well spread around, given enough time. That’s why there is DDT in the water in the ocean in Antarctica even though it has never been applied within 6,000 miles of there. The molecular motions caused by waves are a very local phenomenon, but these same molecules are also affected by local, regional and ocean-wide currents. Some of these are driven by tides, and some by winds (which are often seasonal in their occurrence). And some are driven by the major oceanic circulation. Molecules sinking at high latitudes in the Northern Hemisphere may come back to the surface 1000 years later in Antarctica! And the diatoms carried in these waters may spring back to life after 1000 years of quiescence.

This web site indicates the average residence time for a molecule of water in the ocean is about 2000 years: http://www-naweb.iaea.org/napc/ih/documents/userupdate/description/Precip1.html

The global water cycle: http://www.globalchange.umich.edu/gctext/Inquiries/Inquiries_by_Unit/Unit_10a.htm

In other words, I think it is safe to assume that every molecule of ocean water is, given enough time, going to lap against some shore. It will also evaporate into the atmosphere and fall somewhere as rain, then pass through a plant and get transpired back to the atmosphere again, fall as rain, flow down a watercourse to the ocean, pass through the cooling system of a passing ship, be taken into the ballast water of the ship, get transported thousands of miles to a distant port, be discharged there and taken into the cooling system of a coastal power plant and discharged again, and taken into a nearby desalination plant and distributed to the home of a family named Smith where it will be used to make ice to cool a gin and tonic.

By a route we all recognize it will either be expired to the atmosphere by Mrs. Smith, or will find its way to a wastewater treatment plant and, thence, to the ocean again. Or, in dry California, to groundwater recharge and to an irrigation well in the Salinas Valley, through a growing head of lettuce, back to the atmosphere, and away we go again. Given enough time, your listener may share a molecule of his/her tissue water (we’re all about 85% water) with any number of the rest of us now and for millennia to come. And lettuce heads, too.

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