One of the important questions for the upcoming hurricane season is whether El Niño will develop for the 2013 hurricane season. Table 1 displays years since 1950 with similar September-October MEI (Multivariate ENSO Index or Multivariate ENSO Indicator) values to 2012 (from -0.25 to 0.25 standard deviations of normal). Also displayed are the following year’s August-September MEI values. Of the twelve years with similar MEI values to late 2012, five (42%) experienced El Niño conditions (defined as an August-September MEI greater than 0.5 standard deviations above normal) the following year. In addition, we have now gone since 2009 without an El Niño. Typically, warm ENSO (El Niño–Southern Oscillation) events occur about 3-7 years.
THC circulation becomes unusually strong in 2013 and no El Niño event occurs (resulting in a seasonal average net tropical cyclone (NTC) activity of ~ 180) – 20% chance.
THC continues in the above-average condition it has been in since 1995 and no El Niño develops (NTC ~ 140) – 40% chance.
THC continues in above-average condition it has been in since 1995 with the development of a significant El Niño (NTC ~ 75) – 35% chance.
THC becomes weaker and there is the development of a significant El Niño (NTC ~ 40) – 5% chance.
180 NTC – 14-17 named storms, 9-11 hurricanes, 4-5 major hurricanes
140 NTC – 12-15 named storms, 7-9 hurricanes, 3-4 major hurricanes
75 NTC – 8-11 named storms, 3-5 hurricanes, 1-2 major hurricanes
40 NTC – 5-7 named storms, 2-3 hurricanes, 0-1 major hurricanes
Tropical Atlantic SSTs have warmed considerably since the spring of 2012, and currently stand at well above-average levels. Warm anomalies persist across the tropical Atlantic and far North Atlantic, indicative of an active phase of the AMO and a strong phase of the THC.
One big question is how the anomalies in the tropical Atlantic will change over the next several months. During the winter of 2011/2012, the North Atlantic Oscillation (NAO) became strongly positive, which triggered the development of strong trades that cooled the tropical Atlantic considerably. Since that time, the NAO has consistently been negative, implying weak trades and leading to a significant warming of the tropical Atlantic.
Two of the major influences that need to be monitored during the winter of 2012/2013 are the state of ENSO and the strength of the AMO (THC). As mentioned in our discussion, we believe that we remain in an active era for Atlantic basin tropical cyclones, and consequently, if El Niño does not develop, an active 2013 season is likely. However, given our current statistical analysis, it appears that there is a moderate chance (30-50%) that El Niño will develop for the 2013 Atlantic basin hurricane season. By early April of next year, we should have a better idea of the likelihood of ENSO developing. Both dynamical and statistical ENSO forecast models show significantly improved skill for an August-October forecast by early spring.
The following calculations assume that we remain in a strong phase of the THC (positive phase of the AMO) for the 2013 Atlantic hurricane season.
The following names will be used for named storms that form in the North Atlantic in 2013 :
Number of storms forecast : 9
Number of hurricanes forecast : 6
Number of storm day forecast : 64
Number of hurricane days forecast : 20
Category 3 or higher hurricane forecast : 90%
Storm strikes along the U.S. Coast : 3
A significant focus of our recent research involves efforts to develop forecasts of the probability of hurricane landfall along the U.S. coastline and in the Caribbean. While we are not issuing a quantitative forecast in this early outlook, we can still provide interested readers with the climatological probabilities of landfall for various portions of the United States coastline.
Table 2 lists climatological strike probabilities for the 2013 hurricane season for different TC categories for the entire U.S. coastline, the Gulf Coast and the East Coast including the Florida peninsula. We also issue probabilities for various islands and landmasses in the Caribbean and in Central America.
Table 2: Climatological probability (expressed in percent) of one or more landfalling tropical storms (TS), category 1-2 hurricanes (HUR), category 3-4-5 hurricanes, total hurricanes and named storms along the entire U.S. coastline, along the Gulf Coast (Regions 1-4), and along the Florida Peninsula and the East Coast (Regions 5-11). Probabilities of a tropical storm, hurricane and major hurricane tracking into the Caribbean are also provided.
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