MEXICO:The frequency of tropical storms has increased over the last decade, writes Brendan McWilliams
Tropical revolving storms, to give them their full resounding scientific title, occur on nearly all the semi-tropical oceans of the world. When they occur on the Indian Ocean, they are known as cyclones; on the China Seas and on the north Pacific they are called typhoons, and when they occur in the north Atlantic or Caribbean region, they are hurricanes.
Hurricanes thrive on heat and moisture. They invariably form over the ocean about five degrees north of the equator and are unviable unless the sea surface temperature in the region is 26 degrees or more.
Once formed in the Atlantic, they usually move westwards for a time towards the Caribbean or the Gulf of Mexico before veering gradually northwards.
If they hit land at any stage, they are deprived of their essential moisture and rapidly decline in strength. For ease of reference, storms of sufficient intensity are given a name. Like other weather phenomena, they are born, intensify, mature and then decay, the various stages of this cycle being mainly defined by the strength of the maximum winds blowing around the centre of low pressure.
To be dangerous enough to earn a name, a storm must have sustained average winds blowing around it in excess of 40mph; but only a proportion of named storms develop into full-blown hurricanes, which, by definition, generate winds of at least 75mph whirling anticlockwise around the eye.
The number of hurricanes, and their intensity, varies from year to year. The year 2005 was exceptional and broke all records when 28 named storms developed, enough to beat the previous record of 21 in 1933 by a substantial margin; 15 of the 28, including the infamous Katrina, matured to full hurricanes, also beating the previous record of 12 full hurricanes in 1969.
It was the first year to have three hurricanes of category 5, the highest number on the Saffir- Simpson scale, and it was also the first year during which four "major" hurricanes, defined as category 3 or higher, affected mainland US.
Last year, by contrast, was comparatively quiet, but the dearth can be easily explained in retrospect. Rather unexpectedly, an El Niño episode - a periodic warming of the surface waters of the equatorial Pacific - began to develop in September, and it is well known that a secondary effect of an El Niño is to suppress hurricane activity in the Atlantic.
This year, too, has been very quiet so far. Before Dean's appearance on August 13th, only three storms deserving of being named appeared.
Dean, however, seems set at the time of writing to graduate to full category 5. Despite the relative quiet, the US National Oceanic and Atmospheric Administration (NOAA), continues to predict more hurricane activity than usual during the remainder of this season. The annual number of Atlantic tropical storms has been found to vary more or less systematically over a period of several decades, responding to what the experts call a "multi-decadal signal".
The average number severe enough to merit names was eight or nine a season during the second half of the 20th century, but increased to 14 for the decade 1996-2005. This recent flurry of hurricane activity was preceded by the relatively quiet period from 1970 to 1994, but the 1950s and 1960s were very active hurricane decades, as at present.
A greater number of storms is likely, for example, when the temperature of the sea water is above average and when the overlying atmosphere is characterised by a low vertical wind shear - when the average wind strength does not increase with height as much as one might normally expect.
The occurrence of these ideal conditions is believed to be related to long-term changes in the Atlantic circulation system which incorporates the Gulf Stream; they have been with us since 1995, and were interfered with in 2006 only by El Niño.
From all this, NOAA has concluded that global warming is unlikely to have had anything to do with the peak in hurricane activity in recent years, since it is part of a well-known cycle and would have happened anyway.
Until comparatively recently, few have disagreed, but this consensus has tended to break down in the past year or two.
A seminal paper in August 2005 by Kerry Emanuel of the Massachusetts Institute of Technology suggests that tropical storms worldwide have, in fact, become more severe during the last decade or so, generating stronger winds, producing more rainfall and exhibiting a longer lifetime than used to be the case.
According to Emanuel, who is widely respected in the meteorological world, this change in the power of hurricanes is "unprecedented and probably reflects the effect of global warming".
Other studies have suggested that when the effects of the multi- decadal signal are eliminated, the frequency of tropical storms, on average, is also on the increase.
These issues are still the subject of much debate, but it is clear that we cannot rule out, even at this early stage, some possible influence of global warming on the power and frequency of north Atlantic hurricanes.