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WEATHERWISE A QUIET TROPICAL SEASON ENDS WITH A FINAL SURGE BY DANIEL CORBETT, METEOROLOGIST T he La Nina Summer that was a bit of a hit or miss at times in New Zealand, also had an effect on the cyclone season across the South Pacific. The cyclone season, which runs from November to April, was quieter than normal with only a handful of storms developing. The upper level winds during much of the recent La Nina summer were not favourable for cyclone development, instead, some of the potential areas of tropical moisture during the season were just entrained into the typical weather systems moving across New Zealand during the summer. The extra boost that helped generate the few tropical cyclones of the season, came from active phases in the Madden Julian Oscillation. In simple terms, MJO is a surge of energy in the upper levels of the atmosphere that circulates the globe every 60-90 days. It enhances the upper atmosphere to bring about an upsurge in thunderstorms. The MJO active phases moved across the Coral Sea during early February and well into late March. Cyclone Jasmine in February was the most powerful cyclone to develop during the season. It developed in ideal conditions with light winds and very warm water in the Coral Sea. This helped the storm to quickly strengthen to category four on the Safir Simpson scale (category five on the South Pacific scale) with maximum sustained winds of 115 knots (213km/hr). The storm tracked southeast just to the north of New Caledonia before tracking into cooler waters to the north of New Zealand. Once in the cooler waters (below 26˚C) the storm became extra tropical and fizzled away as an area of wind and rain to the east of New Zealand. In the third week of March a tropical low slipped down to just north of New Zealand and wrapped itself around an upper low for several days. This area of cloud and moisture descended down across the North Island. Parts of Northland received 200-300mm of rain from this low. The rainfall was very heavy at times with rain rates getting up to around 30mm/ hr at times. One location picked up 345mm during the heavy rain event. The upper level winds in the sub-tropical jet also buckled and developed an upper level low to the west of Northland. This clockwise flow of air continued to feed in tropical moisture, FULL TIME MARINE ENGINEERS WANTED! In this role you will be responsible for the For this role you will need to have: WWW.FULLERS.CO.NZ VIP.S87 56 Professional Skipper July/August 2012 (09) 367 9112. If you wish to apply now, please submit http://careers.fullers.co.nz under Mean Sea Level Pressure April 3, 2012 as well as preventing the initial low and associated rain band from clearing away to the east. It lingered across parts of Northland for over 24 hours. The final surge from the tropics came during late March. This final surge was far more devasting for Fiji. The continuous rain for days on the monsoonal trough brought over 500mm of rain to parts of Fiji. A tropical low sat close to Fiji for a couple of days and then Cyclone Daphne developed to the southwest of the island. The continuous rain for days on the monsoonal trough brought over 500mm of rain to parts of Fiji. Daphne passed south of Fiji but it, along with the first tropical low, began to move south out of the tropics at the beginning of April. The first tropical low moved down to New Zealand at the start of April but Daphne was hot on its trail. An upper level low to the northwest of the North Island allowed the tropical low and its moisture to slowly spread in across the North Island. Daphne was moving at a good clip and kept trucking southeast away from New Zealand, but it did help to elongate and thin the band of moisture from the first tropical low. This in turn made the rain across the North Island patchier but in a sustained southeast flow. The rain certainly accumulated across the Gisborne and Hawkes Bay region. By the morning of the April 4, 179mm had fallen in the Gisborne ranges in Te Puia, and Gisborne picked up 90mm. There were some places that received over 300mm of rain during the period. The approaching low squeezed up against a high over the South Island bringing widespread gales across much of the North Island, especially around exposed coasts. The unsettled spell was replaced with a building anticyclone just in time for Easter. The tropics then began to quieten down again for another year.