23 July 2009
There was an intriguing paper in Science this month about tropical cyclone predictions.
Every spring, there is considerable media attention to the forecasts from Colorado State and NOAA, on how many hurricanes we can expect. Every spring, I remind my viewers that these forecasts have little skill. Nonetheless, you cannot learn to forecast something unless you try and trying brings new knowledge and insight. So have at it, but tell the public that there is low confidence in the forecast.
The connection between El Nino, and La Nina, and Atlantic hurricanes has long been known, and the current ENSO phase is an important ingredient in these seasonal forecasts of tropical cyclone activity. Specifically, an El Nino tends to increase wind shear over the Atlantic and reduce the number of hurricanes, while La Nina does pretty much the opposite. An El Nino seems to be brewing right now and may hold down the number of storms this year.
Some El Nino years have turned out to rather active though, and three scientists at Georgia Tech. (H.-M. Kim, P. J. Webster, J. A. Curry, Science 325, 77 (2009) may have discovered one reason!
CENTRAL PACIFIC WARMING (CPW)
El Nino is a warming of waters in the Eastern Pacific. Eastern Pacific Warming (EPW). La Nina is a cooling of waters in the Eastern Pacific (EPC). There also seems to be a cycle of warming in the sea surface temps. of water in the Central Pacific. This (CPW) warming has a strong connection to hurricanes too. Not only that, but it has a strong connection to storms that affect the Gulf and East coast of the U.S, Mexico and Canada! Apparently even stronger a connection than El Nino.
Greg Holland from NCAR wrote a perspectives piece on the paper by Webster-Kim and Curry and speculates that perhaps some El Nino events in the past have been misidentified even.
Wait, there’s more!
Forecasting the development of an El Nino, or La Nina ahead of time is very difficult. Especially if we are between cycles. There is a forecast wall, in the early spring, that makes it nearly impossible. So, insurance companies and residents cannot know whether an El Nino or La Nina is coming for hurricane season. These CPW events are, according to Kim et.al. MUCH more forecastable!
So they may have found a new method of forecasting tropical cyclone numbers, that is more accurate, and easier to forecast than in the past!
You do that, and you get published in what is perhaps the most distinguished peer reviewed journal, of them all!
ps: If you want to really understand how El Nino’s begin and end, then Taichiro Sakagami has made it easy. His videos are highly recommended.