Tides

Tides: caused by the gravitational attraction of the moon,
sun, and earth.
a. Since the moon is much closer to the earth than the sun, it has a greater influence on the tides.
1). This attraction causes a "bulge" of the water at the nearest point of the earth to the moon.
2). The corresponding "bulge" on the other side of the earth is caused by centripetal force, created by the rotation of the earth on its axis.
3). The highest of high tides (or corresponding lowest of low tides) occurs when the sun, moon, and earth are in line with each other, whereas the smallest difference between the high and low tides occur when the sun is at 90 degrees to the line between the moon and earth.

b. Since the earth rotates once on its axis in 24 hours, a given point on the earth will be closest to the moon at a certain time, then 12 hours later, it will be at its farthest from the moon. Both of these produce a bulge, so that there is a high tide at the closest point and a second high tide 12 hours later.

c. But, while the earth is spinning, the moon itself is not stationary. It advances slightly on its own orbit around the earth. The moon orbits earth about once every 29.5 days, which constitutes a lunar month. So, each day it takes a given point on earth about 24 hours and 50 minutes (not 24 hours) to catch up and come to its closest position relative to the moon again. Thus, a full tidal cycle takes 24 hours and 50 minutes, meaning that the difference between the 2 high tides is 12 hours and 25 minutes

d. This fairly simple explanation is complicated by the effect of landmasses at any given point plus the ever-present effect of the earth spinning on its axis.

e. Types of tides:
1). Most familiar type is where there is 2 high tides of equal magnitude and 2 low tides of equal magnitude per day. But the sequence changes daily, as it takes 12 hours and 25 minutes to complete one of these 2 cycles, so that the high tide tomorrow would be 12.5 minutes later than it was today. This type is found on most North and South Atlantic coasts, southern Mediterranean, Red Sea, Persian Gulf, and most of the western coast of Africa and Australia.
2). There is also a single tide once a day, as in the Gulf of Mexico and the extreme North Pacific, and in parts of SE Asia, eastern Australia, and Antarctica.
3). The 3^rd type is also a twice a day sequence, but where the two high tides (or low tides) are of unequal magnitude. This is common in the northern and eastern Indian Ocean, Indonesia, and much of the eastern Pacific. It is also characteristic of the Caribbean Sea and the northern Mediterranean. This is the kind of tidal cycle that we have here off the coast of California.

Movements of water in the oceans are usually beneficial, but may be profoundly adverse in certain circumstances:
a. El Nino current:

1). Off the west coast of South America. It is an extension of the warm water of the equatorial counter-current (flowing eastward at 2-4 degrees N. Lat) which normally dissipates before getting to the coasts of Ecuador and Peru.
2). In infrequent years, when the waters of the western Pacific are warmer than usual, this tongue of warm water carries farther south and diverts the normally cold coastal currents offshore, and
3). Results in large kills of fish that are dependent on the cold waters or a migration of those fish offshore. The shore birds and other animals like sea lions that are dependent on these fish for food starve and perish. Thus, the guano production is diminished, and the fertilizer industry is negatively affected.

b. The Davidson counter-current:

1). Flows northward along the Pacific coast of central America and western Mexico, especially noted along the west coast of Baja California,
2). Transports tropical forms into the cooler temperate waters during the summer months when it is strongest and deposits these forms.
3). These forms may persist for a time until the waters too cold and they then perish.
4). Some of these forms do not perish from the cooler waters but they do not breed in the temperate waters (for example, moray eels are thought to breed only in the warmer waters of southern Baja California or the Gulf of California but not in southern California waters even though they survive for a number of years locally; hence, the presence of large moray eels in San Diego waters).

c. The Gulf Stream, running along the Atlantic coast of Florida, transports tropical species north. carrying these species out of their normal range of distribution.

d. Surface currents, aided by winds, bring many offshore forms adrift to shore.

1). Velella--sailor of the wind.

2). Pelagia--purple-striped jellyfish.