The 2008 North Atlantic Hurricane Season is rapidly approaching now, with less than 3 months until the season starts on June 1st. This means it is time for the second annual Hurricane Warning Hurricane Season Forecast. Last year's forecast was fairly successful, with a call for above average activity with 11-14 named storms, 6-9 hurricanes, and 3-5 major hurricanes. There were 15 named storms storms in actuality, with only 14 fully tropical storms, 6 hurricanes, and 2 major hurricanes.
El-Niņo Southern Oscillation (ENSO)
The first factor that will be considered for the 2008 hurricane season forecast is ENSO. ENSO is a large scale oscillation of ocean temperatures in the Pacific between warm and cold that has effects on the weather across the globe. In recent months, a strong La Niņa has formed, which is the cold half of the ENSO cycle. The strong La Niņa can be seen on the chart below; it is the area of cold water anomalies stretching from South America to the western equatorial Pacific.
A La Niņa usually results in lower wind shear across the North Atlantic, providing an environment more favorable for tropical cyclone formation and propagation. The question then arises, will the La Niņa continue through the hurricane season? My forecast is for the La Niņa to continue into the hurricane season since it is very strong at this point but, it should start to weaken towards ENSO neutral (a relative lack of ocean temperature anomalies) towards the latter part of the season. The models that are used to predict ENSO (below) are in agreement with this assessment, although it should be noted they are subject to large errors.
Sahelian Rainfall Anomalies
The Sahel of Africa is considered a birth place of tropical cyclones. Every year, dozens of tropical waves depart Africa for the Atlantic from this region, sometimes becoming monstrous Cape Verde hurricanes. This has spawned research into the rainfall patterns of the region, in order to determine if they have any effect on tropical cyclone formation in the Atlantic. Sahelian rainfall has been below average during the last year over the southern Sahel, with the northern Sahel experiencing near normal to even above normal rainfall. In the figure below, I have put a black circle around the coastal region of the Sahel because that is the part of it which I believe is the most relevant to tropical cyclone formation. My theory is as follows. Drier than normal conditions over the coastal Sahel cause easterly waves to become drier and hotter just before they enter the Atlantic. This results in a greater temperature difference between the wave and the water, which can then help the wave become more significant and perhaps provide the extra nudge that will result in it becoming a tropical cyclone down the road. Of course, a dry Sahel is a double edged sword, and it will likely contribute to Saharan Air Layer (SAL) outbreaks. However, since the northern Sahel is near normal to wetter than normal, the more sandy part, maybe that will inhibit SAL outbreaks at the same time. It is up in the air, as always, on how much of an affect the SAL will have on tropical cyclones in the Atlantic this year. This is because the SAL likely depends on other factors than rainfall in the Sahel, such as the strength and position of the Azores high.
Normals and Adjustments Based On Current Factors
There are normally 10 named tropical cyclones in the Atlantic, 6 hurricanes, and 2.5 major hurricanes (according to the National Hurricane Center). The first adjustment for this season that I will make is +2 named storms since in recent years we have been finding tropical cyclones that we wouldn't have normally found in the past, due to their size and sometimes remoteness. The next adjustment will be another +2 named storms due to drier than normal Sahel conditions. I will finally add another +3 for La Niņa. Normal ratios will then be used to determine the amount of hurricanes and major hurricanes.
The Forecast
Named Tropical Cyclones: 16-18
Hurricanes: 8-10
Major Hurricanes: 3-4
Essentially I'm calling for a well above average season, probably more active than last year.
