Dynamical Concept (Theory) of Tides

Emphasizes the effect of basin shape and basin depth and Coriolis effect on the astronomically created progressive wave tide

I. review of equilibrium concept of the tides

One crest (high tide) of the progressive “tidal wave” is created by the gravitational attraction of the moon.. The earth rotates from west to east underneath this bulge.

II. the dynamic concept of the tides

A. tide wave is a shallow water wave

The wave-length of the progressive wave is half the circumference of the earth. From the equator to a distance of almost 2/3 of the way to the poles, this means that 1/20 of the wavelength is a much larger number than the depth of the ocean basin. Therefore, the tidal wave acts like a shallow water wave and is altered by friction with the bottom

B. the tidal wave is altered by the continental land masses

The tidal wave is changed by refraction, reflection, and diffraction as it encounters the continents. The wave is slowed, so that it lags behind the moon’s overhead location. The degree to which the wave is changed is different in each oceanic basin and sub-basin.

C. formation of standing waves in separate basins

Within each basin and sub-basin, a standing wave (a rhythmic back and forth oscillation in sea level within a sub-basin) is created by earlier tides and this further alters the tide. While the time and level of the tides are still predictable, the calculation is more complex and requires a series of onsite tide level observations.

D. resultant dynamic tide pattern

The tide in the Southern Ocean is least constrained by land masses, and is dominated by the progressive wave component of the tides.

The tide in the Atlantic, Pacific, and Indian Oceans is more constrained, and the standing wave component of the tides is strengthened. In each, a series of standing waves are created, each oscillating around a node, (called an amphidromic point) where sea level does not change. Because of Coriolis effect, the oscillation receives a rotational deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. However, in a manner that may seem counter-intuitive, this causes the standing wave to rotate around the amphidromic point in a counter-clockwise direction in the Northern Hemisphere and a clockwise direction in the Southern Hemisphere.

The range of tidal sea level change from high tide to low tide is generally greater the further the location is from an amphidromic point.

Marginal Seas

Mediterranean Sea

A. basin is a remnant of the Tethys Sea, (600 –4 Mybp) which was an arm of the Panthalassa (of which the Indo-Pacific is a remnant) and also connected to the newly forming Atlantic Ocean (180 Mybp)

2. basin has been through episodes as a salt lake and a dry salt desert, followed by re-flooding by a oceanwater river entering from the Atlantic

C. high salinity

1. little river inflow (Nile, Danube, Don, Rhone, Po Rivers)
2. high evaporation rates, especially in the eastern area

D. circulation pattern

1. sinking surface water in eastern area produces a bottom current which flows westward and eventually out into the Atlantic Ocean as the Mediterranean Intermediate Water (MIW).

2. Surface water enters from the Atlantic through the Strait of Gibralter to replace the outwardly flowing deep water.

Black Sea

1. precipitation and inflow of freshwater exceed evaporation

2. isolated from the Mediterranean by a 20 m deep sill in the Bosporus Strait

3. this results in a stratification of lower salinity (16 %_o ) surface water

and higher salinity (22.3%_o) deep water at 180 m

4. no dissolved oxygen in deep water

Baltic Sea

A. currently is estuarine-like, with lower salinity surface water and higher salinity deep water. The deep water is increasingly hypoxic or anoxic during the summer

B. 8000 ybp it was a freshwater lake

C. 9500 ybp it was an arm of the ocean (Yoldia Sea)

D. 10000 ybp it was a freshwater lake

Caribbean Sea

1. isolated from the Atlantic by the chain of islands called the Antilles. However, the sill in the Anegada Passage is 2300 m deep, so not completely isolated.

2. The Caribbean current flows from east to west across the Sea and into the Gulf of Mexico

3. upwelling occurs along the northern coast of South America

Gulf of Mexico

1. connected to Caribbean at Yucatan Straits by 1900 m sill

connected to Atlantic at Strait of Florida by 1000 m sill

2. central moves clockwise, peripheral water moves counter-clockwise

Bering Sea

1. somewhat isolated from the Pacific on the east by the Aleutian Island chain
2. shallow continental shelf in northeast, deep basin in southwest
3. surface flow is into Sea from Pacific and then through Bering Strait into Arctic Ocean
4. currently an extremely valuable fishing area

Gulf of California

1. basin formed by rift
2. tremendous silt load brought in by Colorado River in past
3. removal of freshwater for irrigation has reduced freshwater inflow and silt input
4. seasonal upwelling along east (winter) and west (summer) coasts of the Gulf
5. southern part stratifies during summer
6. hydrothermal vents in bottom covered by layer of silt, through which hot water rises

Red Sea

1. basin formed by rift since 180 Mybp, and mostly since 20 Mybp
2. no river input and very high evaporation rate
3. circulation at mouth (Bab el Mandeb) to Indian Ocean is like that at mouth of Mediterranean Sea.
4. Hydrothermal vents create hot brine pools
5. Suez Canal at north end has led to transfer of some Red Sea fishes into the Mediterranean

Arabian Sea

1. circulation is dominated by monsoon wind circulation
1. wind blows from northeast from Nov.-Mar.
2. wind blows from southwest from Mar.-Nov.

2. creates upwelling off Somalia during southwest monsoon

Bay of Bengal

1. strong freshwater inflow from Ganges and Brahmaputra Rivers
2. southwest monsoon create upwelling along the east India coast