AEF forecast for overall Atlantic tropical cyclone activity for the 2006 Hurricane Season

Issued April 2006

AEF is pleased to offer a forecast of overall tropical cyclone activity for the Atlantic basin. This forecast is based on the same Index technology used to produce the regional AEF hurricane risk forecasts. The forecast is based on projections of the Atlantic SST anomaly and the El-Niño/Southern Oscillation (ENSO) climate indices for the upcoming hurricane season.

Recent research has shown that the Atlantic Sea-Surface Temperature (SST) is directly related to tropical cyclone activity. The Atlantic SST forecast for the 2006 season continues to call for warm conditions in the tropical Atlantic. Warm conditions in the Atlantic have persisted for the past 11 years. Atlantic tropical cyclone activity has been above normal in 9 out of the past 11 years.

The The ENSO forecast for 2006 calls for neutral conditions during the spring/early summer and remaining this way during the hurricane season.

Forecasts for tropical cyclone activity in the Atlantic basin

*Average represents the mean number of storms from 1950-2004.

Brief Description of Climate Indices

Atlantic SST anomaly: The Atlantic SST anomaly represents the deviation of the sea-surface temperature (SST) across the Atlantic basin from the long-term average. Tropical cyclone development is favored when the Atlantic SST is warm and is reduced when the Atlantic SST is cool. Research has identified a pronounced multi-decadal variability in the Atlantic SST Index meaning that there are extended periods (~15-20 years) of warm and cool phases occurring in the Atlantic.

NAO: The North Atlantic Oscillation Index is based on the normalized surface pressure difference between Reykjavik, Iceland and Gibraltar in the Azores. The pressure difference between these two points is a measure of the westerly winds across the Atlantic basin. Scientists view the NAO as a measure of the strength and location of the high pressure system over the central Atlantic Ocean, commonly referred to as the Bermuda High. Our research has shown that the Bermuda High is important in defining the most prominent tropical cyclone pathways in the Atlantic.

ENSO : The El Niño /Southern Oscillation (ENSO) represents a significant change in sea surface temperature over the eastern and central equatorial Pacific that occurs at irregular intervals, generally ranging between two and seven years. These changes in sea surface temperature result in changes in large-scale atmospheric convection that affect wind patterns around the globe. During El Niño events (ENSO warm phase), an increase in tropospheric vertical shear (the difference between the upper-level and near-surface winds) inhibits tropical cyclone genesis and intensification. During La Niña events (ENSO cold phase), a reduction in tropospheric wind shear enhances tropical cyclone activity. Our ENSO measure is based on the Niño 3.4 region in the central and eastern Pacific.