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Apart from the periodic astronomical effects, there are some non-periodic effects on coastal tides that are difficult to predict far in advance. One's first reaction tends to be that the topology of the coastline, harbor or estuary must play a non-periodic role.
Topology
The topology of harbors obviously plays a role in the arrival times of the highs and lows of the tides, but this is mainly catered for in the harmonic constituent parameters as a phase shift (i.e., a time delay). As shown in part 2, the amplitude of the tide as a function of time sums as follows:
A = m1Cos(2Πt/T1 - Φ1) + m2Cos(2Πt/T2 - Φ2) + ... + mnCos(2Πt/Tn - Φn)
The constants Φn determine the phase shift of each harmonic constituent. However, topology plays a role in conjunction with meteorological effects. Non-periodic variations in tidal heights are mainly caused by strong or prolonged winds and by unusually high or low barometric pressure.
Strong Wind
The effect of wind on sea level, and therefore on tidal heights and times, is very variable and depends on the topography of the area in question. A strong wind blowing straight onshore will pile up the water and cause high waters to be higher than predicted, while winds blowing off the land will have the reverse effect.
Storm Surges
Winds blowing along a coast tend to set up long waves, which slowly travel along the shore, raising sea level where the crest of the wave appears. These waves are known as storm surges. When the trough coincides with the tidal cycle for a location, it is called a negative storm surge.
Barometric Pressure
A difference of 1 millibar from the mean barometric pressure can cause a difference in height of 1 centimeter. A low barometer will tend to raise the sea level and a high barometer will tend to depress it. This is however a complex process that depends on the mean barometric pressure over a considerable area.
Changes due to barometric pressure seldom exceed 30 centimeters but, when the mean sea level is raised or lowered by strong winds or by storm surges, this change will be greatly enhanced.
Summarized from: Land Information New Zealand (LINZ)
The fundamental force behind the tides, i.e., tidal gravity, is fully discussed in the eBook Relativity 4 Engineers.
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Re: Tidal Prediction Part 3
