The industry enjoyed a benign hurricane season in 2006, but the same cannot be expected in 2007 warns Dr Steve Smith. Warmer sea temperatures and natural weather patterns are likely to cause an above average storm season this year.
The Earth’s climate is a massively complex system, which defies accurate prediction on almost any timescale. However, there are signals that can be exploited to determine broad trends in the climate.
Hurricane season outlook
Various forecast teams make predictions of the number of storms expected during a hurricane season. But predictions of the number of storms should be ignored because the actual numbers can be misleading. However, the underlying climatic conditions that drive the numerical forecasts are valid and can usefully inform our thinking.
There are two major factors that influence the overall activity during a hurricane season – sea surface temperatures (SSTs) and the El Niño Southern Oscillation (ENSO).
Warm waters provide fuel for hurricanes – the warmer the water, the more fuel a hurricane can draw upon to strengthen and grow. Indeed, there is a critical threshold (26.5°C) of SST, below which hurricanes cannot form. However, as has been shown during the 2006 season, warm waters are a necessary but not sufficient condition for hurricanes to form.
In 2006 the SSTs in the Atlantic were roughly 1°C warmer than average and yet the 2006 season was a very average hurricane season. While warm waters are very important to hurricane development, they appear to have a smaller impact on what actually starts a hurricane. Nevertheless, Atlantic SSTs are a major indicator for hurricane activity since once the seed of a storm is present in the Atlantic, the sea temperatures determine to a significant extent whether that seed will develop or will fade away.
“While the season is not expected to have activity approaching that seen in 2005, an active season along the lines of 1995 and 2004 is a distinct possibility
Currently, Atlantic SSTs are warmer than they were in 2006 but not quite as warm as 2005. Further, the extent of water that can support a hurricane (greater than 26.5°C) is about the same as 2006 but not as great as 2005. However, in the Caribbean it is clear that current SSTs in some areas are the same as, if not higher than, 2005. Turning to the Gulf of Mexico, current SSTs are broadly similar to 2006 and are lower than 2005. If this trend continues, the SSTs in 2007 are likely to be warmer in general than 2006 but not as warm as they were in 2005 when Hurricanes Katrina, Rita and Wilma made landfall. The Caribbean Sea, however, is a concern since SSTs there are at least as warm as they were in 2005 in some areas.
The second major influence on hurricane activity is the phase of the El Niño. When ENSO is in the warm phase (waters of the Equatorial Pacific are warmer than average – El Niño), hurricane activity is suppressed. When ENSO is in the cool phase (the Equatorial Pacific is cooler than average – La Niña), hurricane activity is enhanced. The widely accepted explanation for the influence of ENSO on hurricane activity is simple: during El Niño, wind shear across the Atlantic Basin is increased; during La Niña, windshear is decreased. The presence of wind shear is detrimental to hurricane development.
The 2006 season saw the development of El Niño through the middle of the season. Due to this, the 2006 season was all but shut down by September. However, the mechanism by which this El Niño was thought to work was unusual – rather than increasing wind shear, this El Niño increased the stability of the Atlantic atmosphere.
Hurricanes need an unstable lower atmosphere to enable the thunderstorms in the hurricane’s core to intensify. The El Niño in 2006 made the atmosphere more stable, an unforeseen and previously unmeasured El Niño phenomena.
The 2006 El Niño faded to neutral fairly quickly after the end of 2006. The neutral ENSO phase continues to persist in the Pacific, however there are many indicators showing that a move to the cool La Niña phase is becoming more probable in the upcoming months. Model forecasts of ENSO for the next six to eight months clearly show a La Niña developing through the summer.
Given these conditions, it is likely that hurricane activity in the Atlantic Basin will be above average. It is also likely that hurricanes will form in the Caribbean and near the Bahamas ie closer to land. This would increase the likelihood of landfalls, due to the impact of the high wind shear forecast over the Equator. While the season is not expected to have activity approaching that seen in 2005, an active season along the lines of 1995 and 2004 is a distinct possibility.
Dr Steve Smith is vice president of Carvill America.
A warming planet - Climate change
In early February of this year, the International Panel on Climate Change (IPCC) began the process of completing its fourth assessment Report (4AR) on the scientific basis of climate change. In the six years since the last assessment, the science surrounding climate change has improved markedly and has effectively removed any doubt concerning the veracity of climate change. The 4AR science section has concluded that there is very high confidence (90% probability) that the earth is warming due to human activity. The likely change in average sea surface temperature expected over the next 100 years is determined to be in the 1 to 6 degrees Celsius range and the likely rise in sea level is determined to be in the 0.2m to 0.5m range. In terms of catastrophic weather events (ie events of significance to the insurance community), the 4AR science section concluded that:
â€¢ It was very likely (>90% probability) that warm spells/heatwaves would increase in frequency;
â€¢ It was very likely that the frequency of heavy precipitation events would increase; and
â€¢ It was likely (>66% probability) that intense tropical cyclone activity would increase.
The picture painted by the science of global climate change is a stark one for the insurance market.
The likelihood is that many catastrophic events will increase in frequency and/or severity, ranging from the obvious, such as tropical cyclones and flood events, to the not so obvious, such as wildfires increasing due to drought and the climate change induced activity of insects.
In a very real sense, however, the impact of climate change may already be with us. One recent theory based on climate change argues that the recent increases in Atlantic SSTs, and the related increases in hurricane activity, are related not to a natural climatic cycle in the Atlantic but are in fact a response to climate change.