Expert explains the quiet hurricane season of '06
Read or listen: Why so few hurricanes in 2006? Gallery: Great hurricanes of 2005. Gallery: Eyewitness photos from inside a hurricane
Chris Landsea says an unexpected El Nino probably contributed.
Salazar: Why was 2006 been so much quieter, in terms of hurricane activity?
Landsea: In 2006, we had nine storms, five of which were hurricanes. Only two were major hurricanes, a major hurricane being a category 2, 3, 4, or 5 on the Saffir–Simpson scale.
In 2005, we had all of the ingredients for an extremely busy season. We had that very warm water, and a minimum of wind shear and also a very unstable atmosphere that allowed a lot of thunderstorm activity.
In 2006, in contrast, while we still had above–normal water temperatures, we saw more ordinary amounts of windshear and a more suppressed atmosphere for thunderstorms. So in contrast to 2005, we had a third the number of hurricanes in 2006.
Salazar: Did the El Nino that formed in mid–2006 have anything to do with the quieter hurricane season?
Landsea: We do know that when there’s a moderate, or strong El Nino event in the Pacific, it changes global weather patterns. And one favorable aspect of it is that it cuts down the number of Atlantic hurricanes.
We did see, unexpectedly, the development of an El Nino in 2006. And it probably had a role in reducing the hurricane activity.
Salazar: Hurricane season forecasts can be a bit off sometimes. What’s the “x” factor that can hamper predictions?
Landsea: Well, 2006 certainly was a busted forecast for NOAA. We do two seasonal predictions, one in late May and one in early August.
And both of those predictions were predicting a very busy hurricane season. The late May 2006 prediction said that 8, 9, or 10 hurricanes, and the early August prediction lowered that slightly to 7, 8, or 9. And instead of it being significantly above average, 2006 was pretty near normal. So we did not anticipate the El Nino to form and be a factor in 2006.
And there are other factors that do contribute to a season, one of which is on a longer timescale. It’s called the Atlantic Multidecadal Oscillation, where you get changes in the ocean temperatures and the atmosphere that tend to cause 20 to 40 years of relatively busy, or relatively quiet conditions. Not every year is busy. But what we’ve seen since 1995 is that every year, except an El Nino, has been quite a bit above normal.
And that’s what we were anticipating, that the longer–term swing was going to contribute to a busy year. We didn’t anticipate that an El Nino would be dampening the hurricane season.
Instead, we had an unexpected El Nino form, and it contributed toward a quieter season.
Salazar: I’ve heard scientists describe the Atlantic Multidecadal Oscillation as “natural ups and downs.” How big an effect does this natural cycle have on hurricanes compared to global warming?
Landsea: There certainly is quite a bit of scientific controversy about what’s been contributing toward the busy Atlantic hurricane seasons in recent years. There’s quite a bit of evidence that what we’re seeing is the natural cycle, but there likely is also a contribution due to global warming.
Hurricanes are a heat engine, and it does make intuitive sense that if you provide more energy in the input through very warm, moist, tropical air, that all other factors being equal, you should get a stronger hurricane.
But the question in my mind is, what is the sensitivity of hurricanes to changes in the ocean temperature and the tropical atmosphere? And from what we can tell, that sensitivity is fairly small. We think that about a 2 percent per degree Fahrenheit increase in the winds may occur due to global warming. Given that we’ve had less than a degree Fahrenheit of warming in the last 100 years in the tropical Atlantic and the Caribbean, we’re likely looking at a fairly small contribution of global warming to what’s going on.
The Atlantic Multidecadal Oscillations seem to contribute to a change, where you have about a doubling in the number of major hurricanes when you’re in the busy period. So, while it’s still very controversial, in my estimation most of what we’re seeing is just a natural phenomenon.
Salazar: Thank you, Dr. Landsea.
Thank you for this interview. I was disappointed that there was no mention of the busy hurricane season on both coasts of the Pacific. We know that the west coast of Mexico, and a good deal of Asia’s east coast contended with several large storms this year, and it gets irritating when otherwise intelligent people point to the Gulf of Mexico as some sort of litmus test for climate change. It is as short-sighted as using last year’s active Gulf season, on its own, as a confirmation of global warming. We need to consider a variety of data, glaciers, Greenland pump, ocean temperatures, to name a few, all of which point to some clear conclusions regarding the by-products of human population growth and industrial activity. It was also puzzling that Dr. Landsea mentions the “favorable aspect” of El Nino…I think that he’d have a hard time convincing Californians that there’s anything favorable about the El Nino. It’s easy to appreciate good luck, but harder to acknowledge that our good luck is someone else’s bad luck; when climate-related disaster misses someone, it hits someone else. The questions to ask, therefore, should consider the numbers of events on a global scale, and determinations made regarding the severity and number of events that are climate related. Only in that way will we see how much of the bad luck is due to events that we may be able to attenuate through changes of policy and administration. Of course it’s complicated and politicized, but it is too large and enduring a problem to be confined to the Gulf of Mexico.