The Oceans

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

Sea Water

Ocean Circulation

Surface circulation is driven by the winds.

Deep circulation is the result of dense, cold, salty water sinking, rising tomerge with the surface circulation, then returning to the polar regions to sinkagain.

Tides

There are Two Tidal Bulges

• Moon's Gravity Pulls Oceans - Near-side Bulge is Easy to Understand
• Moon and Earth actually orbit around the Earth-Moon Center of Mass (about 1500 km beneath the surface of the Earth)
• Motion of Earth Around Center of Mass Creates a Bulge on the Far Side of the Earth

Both Moon and Sun Cause Tides

• Spring Tides
  • New or Full Moon
  • Sun and Moon Pulling in Parallel Directions
  • Lunar and Solar Tides Add Up
  • Unusually Large Tidal Range
• Neap Tides
  • First or Last Quarter
  • Sun and Moon Pulling At Right Angles
  • Lunar and Solar Tides Partially Cancel
  • Unusually Small Tidal Range

Tides Affect Earth's Rotation

Tides in the oceans cannot move freely because of the continents. They actually flow ina complex manner as shown below.  Most often they circulate around nodes, verymuch the way a wave rotates around a cup as you oscillate it (left). In narrow, parallel-sidedbodies of water the tides move as waves, for example in the South Atlantic. The map belowshows the position of high tide crests at three-hour intervals.

• Rotation and Friction Causes Tides to Lead Moon
• Bulge Pulls Moon, Throws into Larger Orbit
• Friction Slows Earth
  • Precambrian (900 m.y.): Year = 500 Days, Day = 18 Hr., Month = 23.4 Days
  • Cambrian (500 m.y.): Year = 400 Days, Day = 22 Hr.

Tidal Phenomena

• Range 0-50 Ft
• Tidal Bore
• Tidal Falls or Rapids
• Tidal Channels & Deltas

Tides and History

The Boston Tea Party, December 16, 1773

• Near Earth's perihelion: strong solar tides
• Near lunar perigee: strong lunar tides
• Full moon on December 13: spring tide
• Overall effect: unusually large tidal range

Ideally, the tea should have been thrown overboard just after high tide, so the ebbing tide could carry it out to sea. In reality, because the colonists had only a few hours to stage their protest, they could not plan for the tides. The Boston Tea Party fell not just at low tide, but an exceptionally low tide at that. The incoming tide simply washed the tea ashore, and work parties with shovels cleaned it off the waterfront and threw it back into the harbor next day (pollution was not a concern for most people in those days!).

The Battle of Tarawa, November 20, 1943

• Near Earth's perihelion: strong solar tides
• Near lunar apogee: weak lunar tides
• Near last-quarter moon: neap tide
• Overall effect: unusually small tidal range

The effect of tides on the Boston Tea Party is a humorous footnote to history. The effect of tides on the battle for Tarawa during World War II was anything but. Tarawa was one of the easternmost Pacific islands held by the Japanese, and the assault was the first amphibious assault in the Pacific during World War II. Sketchy tide data for the island suggested a tidal range of about seven feet. However, there were puzzling rumors of periods when the tides on Tarawa almost ceased, a condition local mariners called a A

Tarawa is a type example of an atoll, a flat-topped submarine mountain capped by coral. Most atolls, like Tarawa, have a wide shallow lagoon ringed by low coral islands. The only island of consequence at Tarawa was one with an airfield. The plan was for Allied ships to stand offshore in deep water and send landing craft into the lagoon. The landing craft would go as far in as possible and discharge troops.

The invasion was set for November 20, 1943 when tide conditions were expected to befavorable. At low tide in the early morning, the bombardment would begin. As the tide roseand water levels in the lagoon reached 1.5 meters (five feet), landing craft would headashore and by noon, at high tide, heavier craft could come ashore bringing tanks andsupplies.

This isn't the sort of thing you can call off and reschedule if things go wrong, asthey did. Once an attack is under way, the enemy knows your intentions. Any delay merelygives the enemy time to reinforce or escape.

Unfortunately, the rumors of almost-tideless periods at Tarawa were true. November 20 was near last-quarter moon, resulting in a neap tide. Military planners knew about the risks of neap tide but did not realize the moon was unusually far from earth as well, weakening its tidal effects even more. Also, the Earth was only seven weeks from perihelion, meaning solar tides were unusually strong as well. The top diagram shows what planners expected. The bottom one shows what actually happened.

Landing craft hit bottom hundreds of meters offshore and the Marines had to wade ashoreunder heavy fire. Once ashore, they had to fight without assistance, because supply shipscould not come in. For 48 hours, the tidal range was only 60 centimeters (two feet), andit was four days before the tidal range increased to normal. 1027 Marines werekilled and 2292 wounded in the battle.

Deep Ocean Phenomena

Turbidity Flows

A powerful earthquake off Newfoundland in 1929 caused a submarine landslide on the edge of the continental shelf. Submarine cables in the slump area broke immediately but cables downslope broke up to several hours later. Apparently a dense current of suspended sediment traveled several hundred kilometers across the sea floor.

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Created 21 May 1997, Last Update 3 November 1999