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WEATHERWISE WHAT IS IN STORE FOR THE COMING TROPICAL CYCLONE SEASON? BY DANIEL CORBETT, METEOROLOGIST A s we slowly head into summer in the South Pacific we start to think about warmer weather and holidays, but also it is the time when the tropics come to life. The warming waters across the region, combined with ideal environmental conditions, help bring the cyclones to life. In the South Pacific basin we call these tropical systems cyclones, but they go by different names in other parts of the world. Typhoon is a term used mostly in the northwest Pacific region, whereas hurricane is the preferred term in the northeast Pacific and the Atlantic basin, when maximum winds sustained over one minute, or average over 10 minutes, exceed 63 knots. In the Southern Hemisphere, especially in the Australia-South Pacific area, the tendency is to use tropical cyclone as a generic term whether the maximum winds are gale, storm or hurricane in strength. Hurricane (as a term), tends to be used more informally in this part of the world. Tropical cyclones (hurricanes/typhoons), acquire energy through a process called latent heat release. This happens when water evaporates from the ocean and rises to a level where the moist air becomes saturated and condensation takes place and is released as latent heat. Tropical cyclones are fuelled by a different heat mechanism compared to other cyclonic windstorms, such as nor���easters, European windstorms, and polar lows. The characteristic that separates tropical cyclones from other cyclonic systems, is that at any height in the atmosphere, the centre of a tropical cyclone will be warmer than its surroundings; a phenomenon called ���warm core��� storm systems. In addition, the winds in a cold core system will be stronger with increasing height, but this is not the case in warm core (tropical cyclones) systems. MetService operates a Tropical Cyclone Warning Centre. This is one of six TCWC���s around the globe, along with six Regional Specialized Meteorological Centres, all monitoring tropical activity in their respective areas. Whenever a tropical cyclone forms in the area south of 25 south from 160 east to 120 west, or moves from the Nadi area of responsibility into this area. Tropical cyclones that form in the Brisbane area of responsibility do move towards the Wellington area but they almost always pass through the Nadi area first. The coordinate Need a break from the water? ? Subscribe to RV Travel Lifestyle Magazine, sent to your home, plus receive the digital version for only $50.00 for both.(Save $23.40 rrp) SUBSCRIBE FROM ONLY $50! Digital subscriptions can be read on the device of your choice. Ph: 06 306 6030 Email: subs@rvmagazine.co.nz www.rvmagazine.co.nz 56 Professional Skipper January/February 2013 25 south 160 east represents the triple point where all three tropical cyclone warning areas of responsibility coincide. Wellington monitors all tropical cyclones in its area until they are re-classified to a depression, formerly cyclone..(name). This normally happens by the time the time the cyclone has reached 30 south. Even after the cyclone has been reclassified, Wellington may still continue to issue warnings on the system if required. There are many factors that can affect the number of tropical systems that develop, where they develop, and then where they track. The position of the South Pacific Convergence Zone can play a big part in the development of these systems. This zone of converging winds and moisture may be further south than normal in La Nina years, and further north in El Nino years. Another helpful ingredient in the formation of tropical cyclones over a season, is how the active phases of the Madden Julian Oscillation time out with respect to various developing convective systems. An easy way to imagine it, would be to think of the SPCZ as a line of showers and thunderstorms that bubbles away with the heating of the day. If you add in an active phase of MJO, it is like having a simmering pot on the stove where you then suddenly turn up the heat to high, allowing it to come up to the boil. One other final key ingredient for the sustained life of the systems is favourable winds in the path of the storm. Too much wind in the wrong direction (shear), can tear the storm apart and knock it dead in its tracks. This upcoming season with a neutral to very weak El Nino set up across the Pacific, we can expect a normal to perhaps slightly above normal season overall. A normal season would feature 10 tropical cyclones across the Southwest Pacific. This season, at least two of the cyclones could reach category 3 (Safir Simpson scale 1-5), with mean wind speeds of at least 118km/h. Most of the cyclones over a typical season tend to frequent tropical waters, but usually at least one ex-tropical cyclone does come close to New Zealand and its surrounding waters.