For the insurance and reinsurance industries, knowing what's in store over the next 12 months is critical.
Tropical cyclones are the largest and most powerful storms on the planet and can leave destruction in their wake through a combination of torrential rain, huge seas and damaging storm surges.
In 2005 there were 28 named tropical storms in the North Atlantic - the highest number in a 150-year record. One of these, named Katrina, was the most costly hurricane ever to have made landfall in the USA. The overall cost of Katrina was estimated at $81.2 billion and the impact on oil and gas production was hugely significant for global markets. Lloyds estimated its losses to be £2.9 billion as a result of the three biggest Atlantic hurricanes alone.
With the insurance industry facing increasing exposure to weather associated risks, the Met Office has established a seasonal tropical storm forecast for the Atlantic hurricane season offering potentially huge benefits to business and the safety of life.
What exactly is a tropical cyclone and how do we forecast them? Joanne Camp, one of the Met Office's tropical storm specialists, explains:
A 'tropical cyclone' is the generic term for a low-pressure system over tropical or subtropical waters. Once these storms reach a certain intensity they become known as hurricanes, cyclones or typhoons depending on which ocean they form over. Whatever they are known as, they share intense convective activity with thunderstorms, torrential rain and sustained winds of more than 73 miles per hour. Less intense storms with winds over 39 miles per hour become named tropical storms. They form and grow over the tropical oceans, fed by the high ocean temperatures and a vast supply of moisture.
The degree of activity over the whole season varies from year to year and is measured in two ways. The first and most widely-known is to count the number of tropical storms observed over the season. However, the total number of storms tells us little about variations in the intensity and lifetime of storms from one season to the next. A more complete method is by measuring something we call the Accumulated Cyclone Energy (ACE). This is a measure of the intensity and duration of all tropical storms over the season.
Unlike statistical methods employed by many other forecast centres, the Met Office uses the same weather forecast and climate prediction computer model for our tropical storm forecasts. So our forecasts reflect the dynamic interactions happening between the atmosphere and the oceans, rather than rely on an assessment of historical relationships between storm numbers and preceding observed conditions. Multiple forecasts are made using slightly altered starting conditions - we call this an ensemble forecasting technique - which allows us to estimate the range of likely storms.
Dynamical models have a proven record in predicting the number of tropical storms. The change from the 28 named tropical storms in the record-breaking 2005 Atlantic season to the much quieter season in 2006, when only 10 named storms were observed, was very well forecast using this method. As a result of that success we've been making these forecasts since 2007, and the results have been really encouraging. For the 2012 season we will have improved models at our disposal. With finer resolution modelling and improved atmospheric processes being introduced we expect that success to grow.
As well as the routine forecast of the total number of tropical storms and the Accumulated Cyclone Energy (ACE) index issued each May and freely available on the Met Office website, a more detailed analysis is prepared for our customers. We know that the financial and human cost associated with these storms is enormous; in-depth forecasts and a greater awareness of the factors that could affect tropical storm activity are of huge benefit. Throughout the tropical storm season we are in constant touch with our customers. It's important that we work closely with them so they can ask us questions knowing we're on hand with immediate advice.
The largest insured losses occur with tropical storm landfalls and knowing the risk of this happening is naturally of greatest interest to the insurance sector. That's why we are using our expertise in the weather to work towards offering a forecast of landfall in the future. We're also looking further ahead than the next season trialing experimental forecasts for up to five years ahead. This work has been recognised by the industry with the award of the Lloyds Science of Risk Prize in 2010.
Download the PDF Weathering the storm (PDF, 734 kB)