La Nina, tropical cyclones and extra tropical transition

 

source: http://www.beady.com/roundtheworld/Trip_pages/Australasia/QLD.HTM

 

“Is there going to be a tropical cyclone when I am camping at Coromandel this Xmas ?”

NO, there will not be a tropical cyclone. However, there maybe a extra-tropical cyclone, a low pressure that has moved out of the tropics and transitioned into a mid-latitude low. More about that later.

“But seriously, will there be more tropical cyclones in the Pacific and are they headed our way ?

YES! We are currently experiencing a strong La Nina circulation, which is associated with more tropical cyclones in the area west of the dateline. NIWA estimate that there will be 9-12 cyclones in the Southwest Pacific between November and April. See the NIWA update.

 

“When was the last La Nina summer? “

Early 2006 was an La Nina period.  This is the summer that an extra tropical cyclone ripped through Bay of Island Sailing week, cancelling a couple of days racing.

 

“What is this extra tropical transition?”

A tropical cyclone is different from the usual cyclones we see in New Zealand. It has no associated frontal systems. As it moves south it encounters colder sea surface temperatures, and strong westerly winds aloft. On satellite imagery it changes from being the classic circular tropical cyclone to an asymmetric cyclone we are used to seeing.

 

Classic tropical cyclone imagery.

source http://www.jeffsweather.com/archives/2007/06/tropical_cyclon_2.html

Jones (2003) developed the climatology shown below of the number of tropical cyclone in each of the areas, and the number the transition.

Monthly total number of tropical cyclones in each basin (1970-1999), with shaded areas representing the proportion that become extratropical cyclones, source (Jones, Harr et al. 2003)

 

SO the answer, seems like there is a greater than normal number of cyclones about, and this means a greater than normal chance of one transitioning . The same thing can be said about the chance of  such a storm when we head round the North Island.

 

Want some Xmas reading ??? Download the paper ….

Jones, S., P. Harr, et al. (2003). "The Extratropical Transition of Tropical Cyclones: Forecast Challenges, Current Understanding, and Future Directions." Weather and Forecasting 18(6): 1052-1092.