Atu and tropical cyclones
Tropical cyclones are storm systems which form around a low pressure centre and produce strong winds and downpours of rain. As the term 'cyclone' indicates, they rotate anticlockwise in the Northern hemisphere and clockwise in the Southern Hemisphere. They come from tropical zones and depending on their force and location, they are also known as tropical depressions, tropical storms, hurricanes, typhoons or simply cyclones.
Tropical cyclones are meteorological phenomena which produce extremely strong winds and huge erratic and shifty swell. Tornados, torrential rain and huge swell near to coastal areas are further consequences of tropical cyclones.
They develop over extended surface areas of warm waters and lose force when they hit land, or, as in the case of Atu, when they move across waters at lower temperatures than where they originated.
These systems work with what is known as a warm nucleus, where large amounts of heat from vapourisation is expelled which rises causing considerable water vapour condensation. The heat is distributed vertically around the centre of the storm, so at any height (except near the surface where the water temperature determines the air temperature) the centre of the cyclone is always warmer than the area around it.
The main parts of a cyclone are the eye, the wall of the eye and the rainy sides. The eye is normally cloud-free, warm and increasing in temperature. However, seas can be extremely violent. The eye is circular in shape and can be anything from 3 to 370 kms in diameter.
The eye wall is the area of stormy activity bordering the centre of the cyclone.
It is usually characterised by symmetrical dense cloud (Central Dense Overcast, CDO), which makes the eye perfectly circular. Winds reach higher speeds in this area, the clouds are higher and the precipitation is more intense. When the eye wall weakens so does the tropical cyclone and pressure in the centre increases.
The rainy bands are areas of precipitation and storms which cyclonically rotate towards the centre. The gusts of wind are strong and the most significant precipitations usually occur in areas interspersed with other areas of relative calm.
Atu, the Vanuatu cyclone
Atu, named after the Vanuatu archipelago where it formed a week ago, is 400 miles Northeast of Auckland and has a radius of some 300 miles. It is currently weakening and will soon become a tropical storm. The deep tropical convection and the associated storms have lost force over the past 24 hours, so it has been deduced that the phenomenon has entered a transition. Despite this reclassification the depression created will still produce strong sustained winds of 35 to 45 knots with gusts of 55, accompanied by rough seas and ten metre waves as it moves from South to Southeast.
To get an idea of the importance of this big depression we can see that the model for European meteorological forecasting has calculated pressure of 964 millibars at the eye, whilst the American model has put the figure at 984 millibars.
The winds forming are also increased by an important isobaric gradient of 20 to 35 millibars according to each model along the 300 miles from the outside to the centre of Atu.