WHAT'S AHEAD – WEATHERWISE
CYCLONE SEASON AN EARLY START TO A TYPICAL SUMMER
BY BOB MCDAVITT
Sea surface temperatures in the equatorial parts of the Pacific Ocean were much cooler than normal last summer, and that triggered an extreme La Nina episode. They relaxed in autumn and winter but have shown some minor cooling during spring, so we now have a weak to moderate La Nina.
Global models are picking this La Nina episode to continue into our coming summer. Although there are few indicators as to how intense this event may become, at this stage it is not showing signs of becoming as intense as last summer's extreme La Nina.
In late October, and on behalf of collaborating organisations around the Pacific, NIWA and MetService issued a tropical cyclone outlook covering this cyclone season. In this weak to moderate La Nina, current trends in atmosphere and ocean show an affinity with four previous cyclone seasons from a database that dates back to 1969.
These analogue seasons are 1976-77, 1989-90, 2000-01 and 2008-09. They indicate that the 2011 and 2012 South Pacific cyclone season may be below average, with maybe five to eight named cyclones as compared with the average of nine. For New Zealand, activity from ex-tropical cyclones is expected to be about normal to below normal. Activity is expected to be below normal to west of the dateline, except near
North Queensland, where near to slightly above normal activity may occur.
Higher than normal activity is likely east of the international dateline near the Society Islands and the Austral Islands during the second half of the season.
More information on this outlook is available from http:// www.niwa.co.nz/node/103022
Mariners from other parts of the world need to know that the category scale used to describe cyclone intensity in the South Pacific is not the same as the Saffir-Simpson scale used in parts of the world such as the United States of America. The table gives a comparison:
Beaufort
(*=extended, as used in China and Taiwan)
8 (gale)
9 (severe gale) 10 (storm)
10 minute sustained wind in knots
34-40 41-47 48-55
11 (violent storm) 56-63 12 (hurricane) 13* 14* 15* 16* 17*
64-72 73-85 86-89 90-99
100-106 107-120 >120
Weather features in the South Pacific displayed on a Google Earth map
South Pacific Cyclone Category
1 2 3 4 5
1 2
3 4 5
This time around, La Nina is only a background tropical influence to our weather patterns; a minor player. Other factors are at work as well, allowing for more variety in our likely weather patterns.
Among these weather patterns, two have been identified as being "on the menu": • the anticyclones that typically cross New Zealand during summer are expected to arrive earlier than normal this year. This should bring long periods of dry, warm weather to the north and west of the North Island, and
• troughs and fronts in the Tasman Sea have been deflected away from the west of the South Island over the past few months. This pattern is expected to change during November, so normal rainfall should return to our hydro-storage lakes.
This is my last article for Professional Skipper. I shall be retiring from my position as MetService Weather Ambassador in early 2012. My replacement is MetService media and communication meteorologist Daniel Corbett.
Dan is an extremely experienced broadcast meteorologist and until recently was one of the main weather presenters on the BBC. Along with his previous experience forecasting in "Tornado Alley" in the United States, he brings a wealth of media and communications experience to our team.
56 Professional Skipper January/February 2012
USA Saffir- Simpson Category
Tropical Storm
VIP.S85
