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AMPHIDROMIC TIDES

Q. Explain, what is amphidromic point and amphidromic system.
In a bucket of water if water is swirled it tends to pile up against bucket walls. A form of sine wave is seen along bucket wall. A floating particle in the water moves up & down in a cyclic way, which is typical of a simple harmonic motion. Oscillations in the basin appear to be oscillating about a central pivot position (node). A wedge with taper towards the node appears to spin in the basin.

This rotary tidal motion about fixed node is termed as amphidromic system. The node, at around centre of basin has steady water level & is called amphidromic point. Technically, an amphidromic point, also called a tidal node, is a geographical location which has zero tidal amplitude for one harmonic constituent of the tide. The tidal range (the peak-to-peak amplitude, or height difference between high tide and low tide) for that harmonic constituent increases with distance from this point.

Q. What is the origin of the word amphidromic?
The term amphidromic point is derived from the Greek words amphi (around) and dromos, meaning running, referring to the rotary tides running around them.

Q. What are co-tidal and co-range lines?
The Prediction is easily available for standard or secondary port at specific locations. One may want to find out height of tide, range etc for a position remote from the port. The predictions at the remote place may be found with the help of co-tidal charts. Co-tidal charts are accordingly prepared for certain areas where traffic around amphidromic point is dense & the deep drafted vessels may wish to calculate the tidal height to find the under keel clearance. Co-tidal charts show the co-tidal lines which appear to radiate from amphidromic point & join the points of equal ‘Mean High Water Interval (the mean time interval between the mer-pass of moon over Greenwich meridian & the time of next HW at the relevant place. Chart also shows Co-range lines which are the lines joining equal Mean Spring Range. (MSR = MHWS~MLWS).

Q. If¨represents a standard port where the predictions on the date in question are as follows:
0230          5.6m  HW
0845          2.2m  LW
Find the height of tide & the time of occurrence of HW at the ship’s position.
Standard Port is on the co-tidal line 02h.
Ship is on the co-tidal line, 00h 00m. i.e. the HW would occur 02h before the Port. The time of HW at ship’s position will be 0230 –02 = 00h 30m.
The MSRs are 3m & 2m respectively. The ratio of MSRs are 3:2. This means the HW and range at the ship’s position will be two thirds or 0.67 of HW at Std Port. (= 5.6 x 0.67 = 3.75m)
The intermediate time & height of tide is found in the usual manner from the relevant curves provided.   

Q. What is westward intensification? 
The currents making up the western side of the global gyro are much more intense than the currents on the eastern side. Thus, if we consider Atlantic Ocean, the currents on US coast are much stronger than the currents on Portuguese Coast. The currents off of the east coast of the continents are more intense than currents off of the west coast of the continents. This phenomenon is known as westward intensification.

Q. What is the Coriolis force theory for above?
The Coriolis Effect is a turn table effect and is caused due different latitudes of the Earth having different easterly rotational speeds, and the path taken by an object is apparently deflected as it moves between areas of different rotation speeds. The greater the change in rotation speed, the stronger the Coriolis force. The speed of rotation at equator is 902.5 M/h and at any latitude it varies as cosine of latitude. Therefore, speed of Earth’s rotation changes more quickly with latitude near the poles than at the equator. Also, the Coriolis force is strongest near the poles and weakest at the equator.
The high latitude surface currents of the gyros experience a strong Coriolis force at high latitudes. The east moving currents get deflected rather quickly due strong Coriolis force. This causes the current on eastern side of oceans spread out over a wide area as they move towards the equator. Near the equator, the west moving currents experience a weaker Coriolis force, so their deflection does not happen until the water mass reach the western side of the ocean. These western currents therefore are compelled to move through a much narrower area. If the same amount of water is there in circulation, the currents on the western shores of ocean are much stronger.

Q. Is there any other theory?
It is based on Ekman transport theory.  It was put forward in 1902 by Vagn Walfrid Ekman. Ekman Transport is a component of wind-driven ocean current. Ekman transport occurs when ocean surface waters are influenced by the friction force acting on them forming the wind drift current. However, due to the influence of the Coriolis effect, the ocean water moves at a 90° angle from the direction of the surface wind.The direction of transport is dependent on the hemisphere: in the northern hemisphere, transport occurs at 90° clockwise from wind direction, while in the southern hemisphere it occurs at 90° anticlockwise. This also causes the accumulation of water mass to the right or left respectively. Thus, the equatorial current shifts a large mass of water to northwards. Thus, there is a water hill whose peak is more to west rather than in centre of ocean. As the water slides down under gradient that is created a clockwise pattern is set up. The peak being more to the west a narrow band is available for the water to go up ad a much wider band is available for the SME amount of water to come down. This causes the currents on the western side to be much stronger than the one on the eastern shore.  

Q. Why the heap of water due sun’s or moon’s gravitational pull is also seen on the opposite side also?
The differential acceleration on unit mass of water in relation to the unit mass of solid earth is towards sun /moon. Also, the differential acceleration on unit mass of water in relation to the unit mass of solid earth on the opposite side is towards the space. This is because the unit mass of earth is considered at the centre of earth.

Q. What is the cause of the spring tides?
In every 29.531 days (Lunar Month), twice the Moon & Sun are in line. Once in Conjunction & once in Opposition, in both the situations Spring Tide is created by adding of the Tide Raising Forces. Thus at New Moon & Full Moon, HW is higher than normal HWs & LW is lower than normal LWs.

Q. What is the cause of the neap tides?
Twice every Lunation, i.e. every 14.75 days, the Moon & Sun subtend 90°- angle at Earth, or the moon is at 90° elongation. The Moon is then said to be in Quadrature. Sun’s HW & LW tend to oppose Moon’s HW & LW respectively. Thus the net result is HW, lower than normal HWs & LW, higher than normal LWs. These tides are called Neap Tides.

Q. What is priming of tides?
This happens when moon is in the first or third quarter. The heap which should form under the moon, gets deflected towards the Sun or towards the direction opposite to Sun. Assuming that, momentarily, the positions of heap of water, Sun & Moon are stationary, the observer who is rotating along with Earth, will find Sun, heap & Moon in that order, on his meridian. Thus Sun’s effect is to cause time of HW to preceed the time of Moon’s transit. This phenomenon is called Priming.

Q. What is lagging of tides?

This happens when moon is in second or the fourth quarter. Sun will cause the time of HW to follow the time of Moon’s transit. The HW occurs after Moon’s mer-pass.

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