From the Queensland Dept of transport website (available through...

From the Queensland Dept of transport website (available through a Google search, for King Tides, in Australia):


The term "king" tide has no scientific definition although in popular usage it refers to any high tide well above average height. The popular concept is that the "king tides" are the higher high waters which occur around Christmas time. Equally high tides occur in the winter months but these are at night and are not so apparent as those at Christmas which generally occur during the daylight hours.

Queensland has a relatively large and variable tidal range, for example .
. 7.14 metres at Hay Point 3.85 metres at Thursday Island
. 6.41 metres at Mackay 3.42 metres at Cairns
. 4.78 metres at Karumba 2.71 metres at Brisbane
4.69 metres at Gladstone 2.13 metres on the Sunshine Coast
..
The "king" tides are no more than the very highest tides that occur at each place. They:-
. occur naturally and regularly;
. are predicted in the tide book; but
. are not an every day occurrence.
.
In a lunar month the highest tides occur at the time of the new moon and full moon (when the gravitational forces of sun and moon are in line) these are called "spring" tides and they occur about every 14 days. The highest of the spring tides occur during the summer months of December, January and February and also in the winter months of June, July and August.

Accordingly in any one year there will be two spring tides that are the highest for the year - one during summer and one during winter. These are referred to as "king tides".

The "king tides" occur because of the combined influence of a number astronomical factors which are related to the sun and the moon (and their alignments), and the gravitational attraction they each have on the water surface of the earth.

The earth moves around the sun in an elliptical orbit that takes a little over 365 days to complete. The sun has an influence on the tides. Its gravitational force is greatest when the earth is closest to the sun (perihelion early January each year) and least when the sun is furthest from earth (aphelion early July each year).

The moon has a larger effect in the tides than does the sun.

The moon moves around the earth in its elliptical orbit that takes about 29 days to complete. In a lunar month the highest tides occur at the time of the new moon and full moon (when the gravitational forces of sun and moon are in line) these are called "spring" tides and they occur about every 14 days. Because of the elliptical orbit, the distance between the moon and earth changes. The gravitational force is greatest when the moon is closest to earth (perigee) and least when it is furthest from the earth (apogee) about two weeks after perigee.

The combined effect of the moon's phase and the varying gravitational forces of the sun and moon result in the highest of the spring tides occurring during the summer months of December, January and February and also in the winter months of June, July and August.

The situation at mid summer of 1998/1999 is a good example:-
.
. The Sun
. The earth was at its closest to the sun on 3 January 1999 (Perihelion) and furthest away (Aphelion) on 7 July 1999.
. The sun was at its furthest south of the equator on 22 December 1998.
.
. The Moon
. The moon was at its closest point to the earth (Perigee) on 31 December 1998 [and furthest away (Apogee) on 11 January 1999].
. The full moon occurred on 2 January 1999.
.
. Relative Position of the Sun, Moon and Earth
. At 1300 Hours 2 January 1999
. Moon's Greenwich Hour Angle 44 Deg 17'
Sun's Greenwich Hour Angle 224 Deg 05'
. Moon's Declination N19 Deg 33'
Sun's Declination S22 Deg 58'
.
This means that the sun and moon were on opposite sides of the earth and were almost on a perfectly straight line. In addition both the sun and moon were very nearly at their respective closest points to the earth. These are very favourable conditions for high spring tides i.e. the "king tides".

At the Mooloolaba tidal station, the spring tide occurred with two high tides of equal height on 2 and 3 January 1999.

During normal weather conditions, the height of the "king tides" will be similar from year to year. However in abnormal weather conditions (severe storms or cyclones) the low air pressures and strong winds at these times can elevate the sea level above the expected height.