The 2009 hurricane season starts on June 1st, less than 2 months from now, which means it's time to make my yearly hurricane season forecast. This is the 3rd Hurricane Warning hurricane season forecast. Below are the previous two forecasts compared to what actually happened, and the original text of the forecasts can be found here and here.
2007 forecast: 11-14 named storms, 6-9 hurricanes, and 3-5 major hurricanes
2007 actual: 15 named storms, 6 hurricanes, and 2 major hurricanes
2008 forecast: 16-18 named storms, 8-10 hurricanes, and 3-4 major hurricanes
2008 actual: 16 named storms, 8 hurricanes, and 5 major hurricanes
El-Niņo Southern Oscillation (ENSO)
The first factor that will be considered in the forecast is the El-Niņo Southern Oscillation (ENSO), the periodic warming and cooling of the equatorial eastern Pacific, with the warm phase being called El-Niņo and the cool phase being called La-Niņa. We have been in a La-Niņa for the past 2 years, but that is finally changing. As can be seen in the sea surface temperature loop below, the equatorial eastern Pacific is beginning to warm.
Computer models suggest that warming will continue throughout the year.
Considering current observations and the models, it is probable that an El-Niņo will be developing during the hurricane season. However, ENSO will be roughly neutral most of the season, and even when El-Niņo forms it won't have an immediate effect on tropical cyclone activity in the North Atlantic since there tends to be a lag between the ENSO state and its effect on the atmosphere. Thus, I expect an ENSO neutral season. Statistically, ENSO neutral seasons are even more active than La-Niņa seasons, with on average 12 storms, as compared to an average of 11 storms during La-Niņa seasons and 9 storms during El-Niņo seasons.
Atlantic Multidecadal Oscillation (AMO)
The second factor being considered in the forecast is the Atlantic Multidecadal Oscillation (AMO). The AMO is essentially a measure of the sea surface temperature of the Atlantic, from the equator to Greenland. Tropical cyclones are powered by warm waters, so when the AMO is positive (indicating warmer waters than normal), there tends to be more tropical cyclones. On the flip side, when the AMO is negative (indicating cooler waters than normal) there tends to be less tropical cyclones. The AMO generally stays in one phase (positive or negative) for 20-40 years, for reasons unknown at this point. We have been in a positive phase of the AMO since about 1995, which correlates with the beginning of the current active era.
Things are different this year though, the AMO is currently at its lowest levels since 1996 and is significantly negative as of March. It is unusual for the AMO to stray from the long term trend as much as this year. One possible cause behind this year's anomalous AMO is the historic solar minimum we're in, which can be read about here. During a solar minimum less energy reaches the Earth, which results in cooler temperatures in the atmosphere and oceans. It appears that we will be in solar minimum for awhile longer, which will help keep the AMO lower than normal through this season.
Additionally, Mt. Redoubt in Alaska has erupted several times, to altitudes as high as 65,000 feet (see the satellite image below). This has pumped significant amounts of sulfur dioxide into the stratosphere, which has preceded to disperse around the planet. This sort of eruption has been known to cause lower levels of sunlight across the planet in the past, so Redoubt's eruption may contribute to a lower AMO in coming months.
Normals and Adjustments
Since 1965 there have been an average of 11 named tropical cyclones per year in the North Atlantic basin. I will add +1 for the neutral ENSO and another +1 for a neutral AMO (as opposed to +3 in a positive AMO year). Climatological ratios will then be used to determine the number of hurricanes and major hurricanes. This year I will not have a range for the forecast, just a single number in each category. Since 1965, my forecasting scheme was within 1 named storm of being accurate 38% of the time, within 2 named storms of being accurate 68% of the time, and within 3 named storms of being accurate 77% of the time. I like those odds, but it can't be ignored that 23% of the time things went totally wrong! Some seasons just don't follow climatology.
The Forecast
Named tropical cyclones: 13
Hurricanes: 8
Major Hurricanes: 3
This would be a slightly above average season if the forecast verified, not much different than most of the years this decade.
