|This map shows hurricane tracks over New Zealand in the early part of the 20th century. Note that the North Island is more likely to be hit than the South Island. Also note that storms are more prevalent during certain decades and seasons. Hurricanes affecting New Zealand are not very strong, as judged by the Saffir-Simpson hurricane scale (see below). Only on two occasions did hurricanes of category one (970 mbar) reach NZ (bold print). As the world climate changes, more and heavier storms can be expected.|
Hurricanes do not occur in a narrow band around the equator because
here the coriolis forces are zero (they work up and down). Thus
winds cannot focus into an area of low pressure and spin around. Hurricanes
do not occur further than 35 degrees from the equator because there the
seas are too cold to power them. As hurricanes move from the warm waters
nearest the equator outward to the cooler regions, coriolis forces
increase gradually, forcing them to spin faster and tighter, thereby increasing
their destructive power. Cyclones thus become most destructive shortly
before the end of their paths. For an explanation about how coriolis forces
or geostrophic forces work, read circulation/deflection.
World map showing primary and secondary storm tracks and average atmospheric pressure.
Courtesy Encyclopedia Britannica.
|How do tropical cyclones form?
As winds transfer heat from the warm areas on the planet to the cooler ones, they swirl around while they are deflected by Coriolis forces, caused by the rotation of the Earth. In the Northern Hemisphere, moving objects are deflected to the right, resulting in low pressure areas (cyclones) cycling anticlockwise. In the Southern Hemisphere, this is the reverse.
a stationary tropical cyclone, winds arriving from all directions, cause
waves to radiate out in all directions. But once the system moves, the
pattern changes as shown in this diagram for a Southern Hemisphere cyclone.
The most powerful winds now arise from where the centre came from, sending
large waves out ahead of the storm. The wind also pushes the water ahead
of it, causing the water level to rise (storm surge). The size of this
storm surge and its waves, depends largely on how the storm's centre has
been moving, for the rotating winds around it are capable of cancelling
each other's waves.
Note that storm sized winds (but not of hurricane force) blowing for a long time from one direction, are capable of developing sea states equally destructive as hurricanes.
storm surge has two components: the pressure difference between high and
low pressure areas and the water level swept up by winds. The diagram attempts
to quantify the barometric effect. In the left half, the ocean is flat
and the atmospheric pressure between a high and a low is represented as
if the atmosphere extended further out in space. The atmosphere's pressure
is almost equal to that of 10 m of water. It is expressed in bar, where
one bar is the average atmospheric pressure on Earth. Due to the centrifugal
force of a rotating Earth, the atmospheric pressure is less at the equator
than at the poles. In New Zealand, it is about 1015 millibar.
The effect of a high pressure area is that of pushing the sea level down. A hurricane of category one (970 mbar) is thus theoretically capable of causing a 45 cm storm surge (1015-970). In practice, winds are the overriding factor (see below).
When a hurricane moves ever faster in one direction, very high waves can form underneath, arriving without warning. It is thought that this may have happened in the case of Cyclone Heta that destroyed much of the island state of Niue  but how does this work? The drawing shown here has three panels. The top panel shows the waves radiating out from a stationary cyclone and the next two panels of what happens when it 'chases' waves in one direction.
With a stationary cyclone, the winds radiate out in all directions at equal strengths. It causes waves to build up toward the periphery of the cyclone and then to gradually diminish as they radiate out. As they move further away, their heights diminish but their wave lengths increase, causing them to run ever faster. Such waves can cover thousands of kilometres but they are always preceded by small waves before the bigger ones arrive.
When a hurricane moves, its leading winds become stronger while its trailing winds become weaker. The stronger forward winds build up higher waves from the ones that would otherwise have escaped. However, as their wave lengths increase, they run away ever faster. Thus when a hurricane accelerates to keep up with them, these wave become monstrous without there being smaller waves to warn of their arrival.
 Callaghan David, Jeff Callaghan, Peter Nielsen and
Tom Baldock (2006): GENERATION OF EXTREME WAVE CONDITIONS FROM AN ACCELERATING
TROPICAL CYCLONE, ICCE 2006 Abstract Nr 1496
Cyclone Heta: Indepth article on this web site about Cyclone Heta and the damage it caused.
|What are a storm's consequences?
Because both winds and waves increase rapidly with wind speed, scientists Saffir and Simpson have devised a hurricane scale in categories, each category being twice as destructive as the previous. The severity of a hurricane can be related to its barometric pressure, but a large degree of variation remains.
Note that the maximum wind speed increases in a gradual fashion but the storm surge increases more rapidly. Maximum wave heights of a possible fully developed sea are not shown. In the table, the damage caused by hurricane winds is shown but not that of hurricane seas, which can inflict far more damage, although only to coastal settlements. Also the torrential rains from a weakening hurricane, can cause more damage than that caused by its winds.
|The threat of hurricanes to coastal dwellings does not come from wind alone. In this picture a typical situation is shown of many a coastal settlement, which are only 2m above spring high tide level. A storm may bring a one metre surge with 6 metre waves, allowing waves to nip over coastal walls and revetments, but a class three hurricane arriving with a 3m surge and 12m waves, will flood these settlements during high tide, causing major damage. The storm surge lifts the water high above the beach, allowing higher waves to ride much further inland than usual.|
Large storms always cause damage to beaches and dunes but these can repair themselves slowly after the event (See dunes & beaches). The high waves stir the sand deep down and up-root marine organisms living there. Both the sand and the organisms are transported towards the beach, causing wash-ups, sometimes of disastrous proportions.
Storms leave their unmistakable signature in the underwater environment,
both beneficial and detrimental. Overall, one sees more species diversity
in sheltered areas than in exposed areas. Here are some of the important
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