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WEATHERWISE Tropical CYCLONE season BY JOHN LAW, METEOROLOGIST T he tropical cyclone season in the southwest Pacific draws to an end as we approach winter. The season is normally defined as the period from November to April the following year. However, the weather doesn't follow a rigid calendar and tropical cyclones have been known to form in the month of June. During the 2012 – 2013 season, six tropical cyclones formed nearby, four originating in the southwest Pacific and two in the Coral Sea.The monitoring and forecasting of tropical systems is looked after by a Regional Specialized Meteorological Centre or a Tropical Cyclone Warning Centre around the globe depending on the location of the cyclone. TCWC Wellington, based at MetService, looks after the area that extends from 160°E to 1200W and between 25°S and 40°S. Although it is very rare for any tropical cyclones to form in the Wellington area of responsibility, fully fledged tropical cyclones do arrive from the Brisbane or Nadi areas and they may retain their cyclone status until 30°S. Sometimes an ex-tropical cyclone will approach New Zealand and severe weather watches and warnings need to be issued. Even if land areas are not affected, warnings are issued for vessels over the open sea. By ensuring there is one official voice for tropical cyclones, national meteorological agencies work together to provide clear forecasts and warnings and avoid the issue of potentially conflicting information. This is especially important near the boundaries of different areas of responsibility. The forecasting team at Wellington work directly with the forecasters in Brisbane and Nadi to ensure the best forecasts and warnings are made and all backup arrangements are in place. Our neighbouring TCWCs are Brisbane, run by the Australian bureau of Meteorology and RSMC Nadi, by the Fiji Meteorological Service. Of the seven named tropical cyclones this season, four were named by Fiji (Evan, Freda, Gary and Haley) and three named by Brisbane (Sandra, Tim and Zane). A fourth tropical cyclone, Oswald, was named by TCWC Brisbane over the Gulf of Carpentaria but the cyclone did not make it to the Coral Sea and dissipated over Queensland. The remnants of tropical cyclone Oswald moved southwards along the Queensland coast and bringing with it some heavy rain and strong winds. The cyclones are classified by a category system numbering Cat 1 to Cat 5 depending on the strength of the winds near the centre of the system. WHAT ARE THE DIFFERENT CATEGORIES? Cyclone Category Average Typical Central Wind Speed Strongest gusts pressure (km/h) (km/h) (hPa) 1 Gale 63-88 > than 125 < than 985 2 Storm 89-117 125-164 985-970 3 Hurricane 118-159 165-224 970-955 4 Major Hurricane 160-199 225-279 955-930 5 Major Hurricane < than 200 < than 279 > than 930 Track maps 2013 This map shows the path of the tropical systems through the season. The name marks the start of the track and the path shows the trace of the system even after it has been downgraded but still distinguishable. RSMC locations and areas of responsibility THE CYCLONES Name Category Active between Deepest Central pressure (hPa) Peak average wind+ (knots) Peak average wind+ (km/h) TC Evan 4 12-19 Dec 2012 943 100 185 TC Freda 4 29 Dec to 1 Jan 2013 940 100 185 TC Gary 3 21-27 Jan 2013 965 80 148 TC Haley 1 10-11 Feb 2013 990 40 74 TC Sandra 4 8-15 Mar 2013 930 100 185 TC Tim 2 14-17 Mar 2013 984 50 93 TC Zane 2 30 Apr-1 May 2013 983 60 111 + Gusts may be 50 percent, or above, higher than the average wind. 14 ■ NZ AQUACULTURE ■ JULY/AUGUST 2013