I was listening to a 30 second science-bite about hurricanes from David Shukman on the BBC News website yesterday. David was presenting a ‘idiots guide’ to just how hurricanes are formed, and for once the explanation didn’t involve that ‘fast moving ribbon of air high in the atmosphere that the experts call the jet stream’. The basic premise of his explanation was that when a thunderstorm forms over the ocean in the tropics, if derives its energy from the warm seas, and if the temperature of that sea was 26°C or higher, and you added a little bit of rotation, then before you know it you would have yourself a tropical cyclone. Of course this was a thinly disguised bit of propaganda to implant in the viewer’s mind the undisputed fact that: higher sea surface temperatures mean more hurricanes, and of course it must logically follow that’s why they’ve been so many this year.
That got me to thinking about El Niño and La Niña events and how they might influence tropical cyclone development in the eastern Pacific. If you apply the same ‘Shukman’ type logic, the extra warmth of the Pacific during El Niño years, should be apparent in the accumulated cyclone energy. So I have produced a couple of graphs that plot ACE for the eastern Pacific from 1950 for both total ACE and ACE anomalies (fig 2). The pale pink and blue stripes in the lower graph are the ENSO events during that time.
I could do further statistical analysis on the results, but I think I wouldn’t find a strong correlation between SST and the ACE index. Active years do occur in El Niño years that’s true, but they also seem to occur just in La Niña or neutral years. I personally think that the BBC should be explaining that although the warmth of the oceans is a key part in tropical cyclone development, any perceived increase in their number and ferocity is a little more complicated, rather than this glib thirty-second news item with its fancy graphics implies.
I did find an analysis concerning El Niño and intense tropical cyclones in the Journal Nature but was unable to download it without getting a second mortgage on the house. I did find this online debate about the conclusion that the report came up with which might be of interest.