Ocean Tides
- At any particular time in the deep sea there will be large collection of waves.
In terms of the wave periods, there will be waves ranging from a fraction of a second
to days.
- The long waves are generated by gravitational forces (between the earth, sun and
moon) and are called tides.
- The main tides have periods around 12 hours (semi-diurnal, one high tide and one
low tide every 12 hours) and 24 hours (diurnal, one high and one low tide every
24 hours).
- Different regions have different dominating tides.
- The main tide off South Africa is the semidiurnal principal lunar tide (referred
to as M2).
- The second largest semidiurnal tide is the principal solar tide, S2 (47% as important
as the M2).
- The main diurnal tide is the principal solar tide, K1 (58% of the M2).
- The second largest diurnal tide is the diurnal principal lunar tide O1 (41% of M2).
- Viewed from space, tides appear as “bulges” with a typical height of 0.5 – 1.0m.
- The bulges associated with the semi-diurnal tide tend to remain “under” the moon
as well as directly on the opposite side of the earth. The basis for the bulge under
the moon is the gravitational pull of the moon. The basis for the bulge on the opposite
side is the fact that the presence of the moon causes the earth to wobble.
- However, as the earth rotates around its own axis and the bulges try to follow the
moon, the continents obstruct their propagation and the tides start rotating inside
the ocean basins. This can be in a clockwise or anticlockwise sense.
- In the South Atlantic the M2 tide rotates anti-clockwise.
- The main tides have periods around 12 hours (semi-diurnal, one high tide and one
low tide every 12 hours) and 24 hours (diurnal, one high and one low tide every
24 hours).