2012 Hurricane Season: Too Soon to Say ~

Traditionally, December is the month when storm-watchers start to make their forecasts (or, more accurately, their guesses) about the next year’s Atlantic hurricane season. Understandably, the top forecasters like to hedge their bets so far before the fact. On December 7th, for instance, Tropical Storm Risk (TSR) predicted a 2012 season slightly more active than the past 60 years’ average, but slightly less active than the past ten years. Wrote TSR’s Professor Mark Saunders and Dr. Adam Lea (both from University College of London), “Based on current and projected climate signals, Atlantic basin and US landfalling tropical cyclone activity are forecast to be about 15% above the 1950-2011 long-term norm but 15% below the recent 2002-2011 10-year norm.” (See “ Extended Range Forecast for Atlantic Hurricane Activity in 2012”) But the whole notion of a “norm” for something as chaotic and catastrophic as a major hurricane — to say nothing of a whole season’s worth of storms — requires a bit of a stretch. That may be why this year one of the top names in long-range forecasting has decided not to make a forecast in December — at least not with the customary level of detail. Colorado State University meteorologists Phil Klotzbach and Bill Gray released their early outlook on December 7 (“ Extended Range Forecast of Atlantic Seasonal Hurricane Activity and Landfall Strike Probability for 2012,” by Philip J. Klotzbach and William M. Gray). But the pair wrote, “We are discontinuing our early December quantitative hurricane forecast for the next year and giving a more qualitative discussion of the factors which will determine next year’s Atlantic basin hurricane activity. Our early December Atlantic basin seasonal hurricane forecasts of the last 20 years have not shown real-time forecast skill even though the hindcast studies on which they were based had considerable skill.” “Skill” is a term weather simulation experts use for a close fit between their projections and the observed real-world events — or, at least, for a better-than-wild-guess fit. To demonstrate “skill,” the Colorado state team has, in effect, run predictions using its model for years that have already gone by, and then seen if this “hindcast” prediction was accurate. But researchers are forced to admit that they don’t fully understand the cause-and-effect relationships in long-range weather patterns, and that their models are too crude to be reliable. Write Klotzback and Gray, “This is the nature of the seasonal or climate forecast problem where one is dealing with a very complicated atmospheric-oceanic system that is highly non-linear. There is a maze of changing physical linkages between the many variables. These linkages can undergo unknown changes from weekly to decadal time scales. It is impossible to understand how all these processes interact with each other. No one can completely understand the full complexity of the atmosphere-ocean system.” This year, the Colorado State team is basing its assessment on two key weather drivers: The Atlantic thermohaline circulation (THC), and the possibility of a Pacific Ocean El Nino event. A strong THC would tend to increase storm activity, while a strong El Nino would tend to suppress storm activity. But the two factors — a strong THC and an active El Nino — are assumed to be independent, and the odds of either one occurring are uncertain. So at this stage of the game, Klotzbach and Gray give 15 percent odds of a very strong THC with no El Nino, which would lead to an active hurricane season; 45 percent odds of a continued moderately strong THC and no El Nino; a 30 percent chance of a continued moderately strong THC plus an El Nino; and 10 percent odds of a weaker THC plus the El Nino. The bottom line (if you can call it that): 15 percent odds: 14-17 named storms, 9-11 hurricanes, 4-5 major hurricanes 45 percent odds: 12-15 named storms, 7-9 hurricanes, 3-4 major hurricanes 30 percent odds: 8-11 named storms, 3-5 hurricanes, 1-2 major hurricanes 10 percent odds: 5-7 named storms, 2-3 hurricanes, 0-1 major hurricanes Tropical Storm Risk’s forecasters, for what it’s worth, are basing their assessment on a different set of factors. Write Saunders and Lea, “TSR’s two predictors are the forecast July-September trade wind speed over the Caribbean and tropical North Atlantic, and the forecast August-September 2012 sea surface temperatures in the tropical North Atlantic. The former influences cyclonic vorticity (the spinning up of storms) in the main hurricane track region, while the latter provides heat and moisture to power incipient storms in the main track region. At present TSR anticipates both predictors to have a small enhancing effect on activity.” Actually, sea surface temperatures and the thermohaline circulation are related factors; any attempt to predict hurricane activity, in fact, has to take into account water temperatures in the storm region. But the truth acknowledged by both forecaster teams is that this far in advance, too little is known about next summer’s conditions to say what the 2012 storm season will bring.