Oceanography

3.1.3.Tidal phenomenon

A tide is the alternative rise (high) and fall(low) of sea level due to the gravitational forces exerted on the earth by the moon and the sun. More commonly, These will be two highs and two lows in a day in most areas. There are known as semi-diurnal tides. But in some areas there will be one high and one low in a day. These are known as diurnal-tides.

At a given place, the tide generally has two cycles per lunar day(24 h 50min). A time lag exists between the moment when the moon crosses the meridian(in the north or in the south) and the high water. This time lag, which has a fixed value for each place, is called HWF&Ch(high water full and change). The solar tide has two cycles per day(24 h). High water due to this solar tide is at the same time each day.

The gravitational attraction of the sun and the gravitational attraction of the moon always affect two sides of the earth at the same time. On one side, the waters of the ocean are drawn away from the earth; on the other, the earth is drawn away from the water. The gravitational attraction of the moon and the sun are added together. Thus, the combined gravitational attraction of the sun and the moon produces the highest tide called the spring tide. This can occur both at full moon and at new moon. However, this does not happen when the sun and the moon are exactly in the same direction(new moon), or in the opposite direction(full moon). It occurs generally later(mostly one or two days). The time lag is called the age of the tide. The tide are least when the gravitational attraction of the sun is counter acting that of the moon, or when the sun and the moon are at right angles in relation to the earth. Thus, the weakest tide, called neap tide occurs one week later i.e., 7 days after the occurrence of spring tide(first and third quarter of the moon). Due to the moon’s influence only, the tide each day would be exactly 50 min and 28s later than the preceding day. Because of the sun’s contribution, this phase lag is not constant. It is less just before and after spring tide (about 35 min) and more around neap tide, one week later(about 80 min).

The moon and the sun are rarely on the equator. They may have a (northerly or southerly) declination, which may be about 23.5° for the sun and 28.5° for the moon. The effect of this is that the tide generating force, H-H₀ does not describe two equal sinusoids, while the earth makes a complete revolution and the maxima will be different. This difference is called the daily inequality and the tidal curve may have a shape like that shown in.

When the declination of a celestial body is considerable, the resulting tide will then not be so strong as the tide which is generated by the same celestial body when it is on the equator. For the sun the latter is the case in the equinoxes(21^st march and 22^nd September). In those periods extra high tides may occur. These are called as equinoctial spring tides.