The Atlantic Basin hurricane season officially starts next week and the US National Oceanic and Atmospheric Administration’s Climate Prediction Center is already predicting a rough ride.
NOAA’s annual preseason forecast was issued last week and calls for 12 to 18 named storms, six to 10 hurricanes and three to six major hurricanes with winds of at least 111 miles per hour. The official National Hurricane Center seasonal averages are 11 named storms, six hurricanes and two major hurricanes.
The official Atlantic Basin hurricane season runs from 1 June through 30 November.
In terms of probability, NOAA’s 2011 Atlantic Hurricane Season Outlook stated there is a 65 per cent chance this year of an above-normal season with regard to tropical cyclone activity, a 25 per cent chance of a near-normal season and only a 10 per cent chance of a below-normal season.
“This outlook reflects an expected set of conditions that is conducive to above-normal Atlantic hurricane activity,” NOAA stated.
Three climate factors are expected to support the above-normal tropical cyclone activity:
The continuation of multi-decadal period of higher activity in the Atlantic Basin that began in 1995;
A continuation of above-average sea surface temperatures in the areas of the tropical Atlantic Ocean and Caribbean Sea where most hurricanes develop;
El Niño/La Niña-Southern Oscillation neutral conditions most likely, with lingering La Niña impacts into the summer.
In addition, several of the dynamic computer models are forecasting an above-normal season in terms of numbers and strength of tropical cyclones. Other forecasters, such as Phil Klotzbach and William Gray of Colorado State University and the UK-based Tropical Storm Risk, are also predicting a busy hurricane season.
Climatologists have identified a cyclical pattern in the Atlantic Basin of periods of higher than normal tropical cyclone activity followed by periods of reduced activity. These cycles usually last two decades or more; the current period of increased tropical cyclone activity began in 1995.
Key climate indicators of the tropical multi-decadal signal in the area of the Atlantic Basin where most tropical cyclones develop include warmer-than-average sea surface temperatures, reduced vertical wind shear, weaker easterly trade winds, below-average sea-level pressure and a configuration of the African easterly jet that is more conducive to hurricane development from tropical waves moving off the African coast.
“Many of these atmospheric features typically become evident during late April and May, as the atmosphere across the tropical Atlantic and Africa begins to transition into its summertime monsoon state,” the outlook stated. “Several of these conditions are now present and are expected to persist through the hurricane season because of their link to the tropical multi-decadal signal.”
One variable that could have a big impact on the hurricane season is the El Niño/La Niña-Southern Oscillation, better known as ENSO. El Niño is an anomalous warming of the equatoral Pacific Ocean that is known to create higher wind shear in the Atlantic Basin, hindering tropical cyclone development and strengthening. Conversely, La Niña and ENSO neutral conditions support conditions of less wind shear in the Atlantic Basin. A La Niña currently exists but is in the process of transitioning into an ENSO neutral condition.
However, predictions made in the spring of what ENSO conditions will exist in the period between August and October – the peak of the Atlantic Basin hurricane season – have considerable uncertainty. Most models are forecasting ENSO-neutral conditions, but a few models predict the continuation of weak La Niña conditions or the development of weak El Niño conditions, the NOAA outlook pointed out.
“This spread in the model forecasts, combined with the limited predictive skill exhibited by all such models at this time of the year, is a main reason why we are presently indicating only a 65 per cent chance of an above-normal season,” NOAA stated. “If El Niño does not develop, the probability of an above-normal Atlantic hurricane season will be even higher and the actual seasonal activity will more likely be toward the upper end of our predicted ranges.”