Wednesday, January 24, 2007

Caribbean tides

An interesting URL pertaining to the tides in the Caribbean as well as the Gulf of Mexico.
http://www.du.edu/~jcalvert/geol/tides.htm

Tides in the Gulf of Mexico and the Caribbean Sea


The Caribbean Sea is wider than the Gulf, about 1900 miles from Central America to the Leeward Islands, but narrower, about 550 miles wide, and with a slightly larger area, 750,000 mi2. A tectonic plate occupies the area of the Sea, wedged between the subducting Pacific Plate on the west, and itself subducting beneath the Atlantic plate on the east. Its average depth probably does not exceed that of the Gulf by much, but on its northern side, in a line along the Sierra Maestra of Cuba and between the Caymans and Trinidad, is a subduction trench, the Bartlett Deep, that reaches 22,788 ft. in depth. The structure and history of the Caribbean are also not well known, but both the Gulf and the Caribbean are of great geological interest.
I have compared the tides at Biloxi, Galveston, Tampico and Progreso, Yucatan, which are spaced around the Gulf. If the tidal plots are superimposed in front of a strong light, it is found that low water occurs almost precisely at the same time at these widely-separated points. High water is almost as simultaneous, but there is some variation. It is clear that water is not sloshing from side to side of the Gulf, but the level at the centre is rising and falling with a period of one day. It would be impossible for all this water to enter and leave by the straits of Florida and Yucatan and distribute itself around the Gulf in the time available, so this simple drum-like oscillation is the only possibility.
Furthermore, the period of oscillation must be quite close to 24 hours, so that the diurnal stimulus can resonate to give the observed tidal ranges. The semidiurnal stimulus would not excite a significant amplitude, as is observed. The favoring of diurnal oscillations and the discouragement of semidiurnal oscillations explains the observed tides quite well, at least qualitatively.
Caribbean tides show more variations than Gulf tides, but are small in amplitude, less than 50 cm, and diurnal tides are prominent. In the eastern Caribbean, the tides at St. Croix, Virgin Islands, and at Willemstad, Nederlandse Antillen, are almost entirely diurnal, like Gulf tides. The amplitude of the spring tide is about 40 cm, and of the neap tide, 15 to 20 cm at both locations. High spring tide at St. Croix occurs about 3 hours earlier than at Willemstad, but low tide about 2h 20m later. As the amplitude of the diurnal tide decreases and the small semidiurnal tide increases at neap tides, the time sequence may vary irregularly. The tides are not simultaneous, as in the Gulf.
Further west, the tides at Colón, Panama and Port Royal, Jamaica have a similar pattern, the diurnal tide predominating, but with a significant semidiurnal component that is increasingly important towards neap tides. The phases at these two rather widely-separated ports are almost exactly the same, for both components. Most of the time, there is one high tide a day, with a small advance and recession between the low tide and the next high tide. The neap tide may, however be double, with two high tides about 10 hours apart. The Colón tide is about 2h later than the Port Royal tide.
At Belize City, in the far west of the Caribbean, the tide is mainly semidiurnal, but with a diurnal component that increases toward spring tides, as usual. Typically, there are two high tides with a shallow minimum between them, separated by deeper low tides once a day. The tidal range is small, from little more than 10 cm up to 25 cm. The phase of the semidiurnal tide is 180° out of phase with the semidiurnal component at Colón, so there seems to be some semidiurnal sloshing in this area.
The diurnal resonance does not appear to be as strong in the Caribbean as it is in the Gulf, nor is it as simple. There is good evidence for semidiurnal sloshing from phase relations in the western Caribbean, from north to south. The small tidal ranges show that the tidal oscillations are not part of the Atlantic tides, which have much larger amplitudes.

Monday, January 22, 2007

http://geography.about.com/library/maps/blocean.htm
This site is quite interesting as it shows a breakdown of stats pertaining to each of the oceans.
A little late I know but it is finally up and working. The video in today's class was neat to see those amazing underwater structures built from the lava flows and hot minerals from within the Earth. It is so unbelieveable that all of those living organisms could exist at such dept and temperatures.