Dr Steve Smith examines why this year's so far benign hurricane season is not living up to expectations.

To many observers, the 2006 hurricane season has so far been rather underwhelming. In reality, to date the season has simply been more average than the very high activity seen in the previous two seasons. The activity in the Atlantic in 2006 has been driven primarily by two major factors, sea surface temperatures and wind shear (changes of wind speed with height in the atmosphere).

While the waters of the Atlantic have been warm in 2006, they have not been substantially warmer than the 1950-2000 average. Typically, the difference between 2006 temperatures and the average have been approximately +/- 1˚C. This means there has not been the same massive amounts of energy in the ocean to fuel hurricanes that we saw in 2005, where sea surface temperatures reached over 2˚C above average in some areas. High levels of wind shear, meanwhile, have been prevalent so far in 2006. This has meant that storms either struggle to form or are quickly disrupted once they do form.

Going forward, it appears that an El Niño has begun in the Pacific. El Niño is strongly correlated with reduced hurricane activity in the Atlantic as the changes in atmospheric circulation associated with the phenomenon increase Atlantic wind shear. While this is not currently suppressing hurricane activity in the Atlantic, we do expect it to reduce activity towards the end of the season.

2006 has so far seen a number of storms forming in the Pacific basin. It certainly appears, at first blush, that the Pacific is more active this year. In reality, however, the Pacific has had a roughly average season so far, given that it typically sees more than twice the number of storms in any given year than the Atlantic.

On average, we expect 27 named storms in the Pacific, compared to ten on average in the Atlantic. As of 28 September, the Pacific had seen 14 named storms and only nine in the Atlantic. Given these statistics and the fact we are roughly halfway through the tropical cyclone season in these basins, both the Pacific and the Atlantic basins certainly seem on track to have approximately “average” years of activity.