Coming to blows.
THE ATLANTIC HURRICANE SEASON HAS come round again and people all over the Caribbean, Central America and south-eastern USA are ready once more to cope with one of nature's most dangerous forces. The 2000 season in the Atlantic basin is predicted to be above-average, with stronger, longer-lasting storms than normal, meaning a heightened chance of coming face-to-face with one of the most destructive of all natural hazards.
The word `hurricane' is the local name for a strong tropical cyclone, an area of intense low atmospheric pressure. It is derived from Hurricane the Caribbean god of evil. Elsewhere, these tropical storms come with different labels, such as `typhoon' in the western Pacific. They have winds of at least 119 kmph, which marks them out from more mundane systems. If the winds reach this speed it is called a hurricane. The fiercest hurricanes call produce winds in excess of 300 kmph.
These spiralling masses of air are spawned over warm tropical waters and rely on a complex combination of processes to grow, mature, and eventually die. Warm ocean water is needed to fuel the tropical cyclone's heat engine, so they only form when the temperature of the top 50 metres or so of the ocean surface is at least 26.5 [degrees] C. The air above has to be marked bra fairly rapid drop in temperature with height, allowing thunderstorms to form initially. It is thunderstorm activity that allows the heat from the ocean waters to be liberated, helping the tropical cyclone to develop. These conditions also need to occur at least 500 km from the equator. This is because the force exerted by the spinning Earth (the Coriolis force) has to be at a certain strength for tropical cyclone formation, and this strength drops off towards the equator.
The Atlantic hurricane season officially started on June 1st and lasts until the end of November, although the Atlantic basin shows a very peaked seasonality with the large majority of hurricane days occurring in the period from August to October. As we near the second anniversary of Hurricane Mitch, which devastated Central America towards the end of October 1998, it is timely to remind ourselves of the awesome power wielded by the king of atmospheric hazards. Mitch chalked up his place in history as the most deadly hurricane to strike the Western Hemisphere in the last two centuries. The death toll of 11,000, with thousands of others missing, is second only in the history of Atlantic hurricanes to the Great Hurricane of 1780, which killed approximately 22,000 people in the eastern Caribbean. More than three million people were either left homeless or severely affected by Mitch. Estimates of the total cost of the damage are around US$5 billion, a terrible burden for a group of the world's poorest countries.
After threatening Jamaica and the Cayman Islands, Mitch moved westward for the assault on Central America. Fierce winds and huge waves up to 13 metres in height savaged the coastline, while inland torrential rain resulted in floods and mudslides which virtually destroyed the entire infrastructure of Honduras and devastated parts of Nicaragua, Guatemala, Belize, and El Salvador. Entire villages and their inhabitants were swept away in the torrents of floodwaters and deep mud that hurtled down the mountainsides. Hundreds of thousands of homes were destroyed. In worst-hit Honduras, the president claimed Mitch had destroyed 50 years of progress.
The prediction that this year will be an above-average hurricane season in the Atlantic basin typically means 11 or more tropical storms, of which seven or more will become hurricanes. The year 2000 is prolonging a run of particularly damaging hurricanes in the area in recent years. The extreme impacts from Hurricanes Marilyn (1995), Opal (1995), Fran (1996), Georges (1998) and Mitch (1998) in the USA and throughout the Caribbean attest to the high level of Atlantic hurricane activity lately. In fact, a record 33 hurricanes hit the Atlantic basin between 1995 to 1999. But fears that this enhanced hurricane activity is a sign of global warming kicking in are premature. Over the long term, scientists have not seen any increase in the intensity or frequency of Atlantic hurricanes during the period of reliable records, which stretches back to the mid1940s. As it happens, the 1991-94 period was the quietest on record in the Atlantic, with fewer than four hurricanes per year.
The link between global warming and hurricanes appears at first sight to be a straightforward one. A planet warmed by increasing amounts of greenhouse gases in the atmosphere may display increased tropical sea surface temperatures (or SSTs) and warmer SSTs could lead to more frequent and intense hurricanes. Indeed, global warming may also mean the areas of ocean surface where the critical 26.5 [degrees] C is exceeded may shift. However, the formation of tropical cyclones depends not only on sea surface temperature, but also on various atmospheric factors. Climatologists feel there is not enough evidence to suggest any major changes in where tropical cyclones form or occur. Little or no change in the number of tropical cyclones is expected as our planet warms. While not much is known about how the average intensity or size of tropical cyclones may change due to global warming, it is thought that wind speeds during peak intensity may increase by five to ten per cent. Some optimists, however, believe this may be an overestimate because of simplifications in the calculations.
Overall, these suggested changes are quite small compared to the observed large natural variability of hurricanes and tropical cyclones. Predicting hurricane frequency is a complex business, but one in which experts are improving all the time. While scientists have not identified any long-term trends, either up or down, they have seen cycles in hurricane frequency that operate on timescales between 25 and 40 years. The period from the 1970s to the early 1990s was a quiet one for major Atlantic hurricanes. At least part of the reason for this was cooler than usual waters in the North Atlantic. The late 1920s through to the 1960s saw more hurricanes thanks to a warmer North Atlantic. It is possible that greater activity since 1995 marks the start of another active period that may last up to 40 years.
Identifying the reasons behind these hurricane `cycles' is an area of active research. Meanwhile, a number of characteristics of the atmosphere and oceans outside the Atlantic have been identified as being important for the formation of Atlantic hurricanes. One important influence, recognised since at least the 1930s, has its origin in North Africa. A series of disturbances in the atmosphere, known as `African easterly waves', move towards the west in the trade winds that blow across the Atlantic Ocean. These easterly waves often serve as the `seedling' circulations for a large proportion of tropical cyclones over the North Atlantic. In fact, nearly 85 per cent of major hurricanes in the Atlantic originate from easterly waves.
The number of easterly waves generated over North Africa varies from year to year, but on average they total about 60. However, the number formed bears no relationship to how much tropical cyclone activity occurs over the Atlantic each year. The exact nature of the relationship between African easterly waves and Atlantic hurricane formation remains a puzzle for atmospheric scientists.
Another notable influence on Atlantic hurricanes lies to the west, in the Pacific Ocean. During some years, sea surface temperatures in the central and eastern tropical Pacific are warmer than normal (the phenomenon known as El Nino) and this tends to inhibit the formation and intensity of tropical cyclones in the Atlantic. Conversely, cooler-than-average SSTs in the central and eastern tropical Pacific, known as La Nina, enhance cyclone activity in the tropical Atlantic. During last year's hurricane season, La Nina was marked in the Pacific, contributing to the high frequency of Atlantic hurricanes. This year, the La Nina phase has been fading out, but its influence is still likely to mean more Atlantic storms than usual. Just how El Nino and La Nina events in the Pacific affect hurricanes in the Atlantic is complex, but the influences extend to effects on Atlantic pressure systems, winds and sea surface temperatures.
Our ability to predict and forecast hurricanes is important because it enables authorities to issue warnings and take evasive action. Attempts over the years to deal with the hurricane problem at its source, by weakening or destroying tropical cyclones as they form, have largely been a failure. Several ideas have been tried, but none with much success.
One approach was to spread some kind of liquid over the ocean surface to prevent evaporation from occurring. Since a tropical cyclone needs huge amounts of evaporation from the ocean to maintain its intensity, cutting off this source of energy should reduce the storm's strength. However, no-one has found a substance that is able to stay together in the rough seas of a tropical cyclone.
An alternative approach involved silver iodide dropped from an aeroplane into the cyclone clouds. Silver iodide encourages raindrop formation, and has also been used to `seed' clouds in desert areas to make them rain. By seeding hurricane clouds, it was hoped that some of the energy that would normally go towards the strong inner core winds, the `eyewall', would be diverted to make rain. Hence the eyewall would be weakened, rendering the hurricane less intense. After 20 years of research the idea was given up as a failure.
Most hurricane researchers tend to agree that since we can't control, alter or destroy hurricanes, we should focus on reducing society's vulnerability through being prepared. We know that coastal areas are most vulnerable to these storms, because tropical cyclones die out over land due to the loss of a moisture source. After only a few hours, a hurricane over land begins to weaken. This is because the storm lacks the moisture and heat sources that the ocean provides. This depletion of moisture and heat reduces the tropical cyclone's ability to produce thunderstorms near the storm centre. Without this convection, the storm rapidly fills and dies out.
Preparing for the hurricane hazard means making sure that houses and other buildings can withstand the force of a hurricane. Laws to enforce appropriate building codes are in place in the US states bordering the Atlantic and Gulf of Mexico, but enforcing such rules is not so easy in the poorer countries of the Caribbean and Central America. However, even in the USA the risk from hurricanes is rising. With population and development increasing along coastal areas, greater numbers of people are vulnerable to the hurricane threat. Economic figures for damage in the USA show a steady increase throughout the 20th century, largely because more and more people are choosing to live in hazardous areas. Damage from Hurricane Andrew (1992) alone was more than US$25 billion in southern Florida and Louisiana, and would have been higher had it hit Miami directly.
The good news in the USA is that the number of people injured or killed by hurricanes has been steadily falling over the last 100 years, largely because of improvements in forecasting and emergency preparations. Our scientific understanding of the atmosphere and our ability to forecast hurricane landfall pathways have increased dramatically over the past two decades thanks to better data from weather satellites and hurricane surveillance planes, coupled with improvements in the computer models used to forecast the movement of hurricanes.
All US states in hurricane pathways have designated evacuation routes and the National Weather Service broadcasts forecasts and warnings 24 hours a day on special weather radios. The most recent models can even sound an alarm, waking people up who are asleep when a storm is about to strike.
It is unlikely that we shall ever be able to control the incredible forces of the hurricane hazard, so we can only concentrate on improving our ability to deal with its potentially catastrophic effects.
What's in a name?
Tropical cyclones with maximum sustained surface winds of less than 63 kmph are called `tropical depressions'. If the wind in a tropical depression exceeds this speed it is upgraded to a `tropical storm' and assigned a personal name. Giving a storm or hurricane a name is not just for the amusement for forecasters. Specific names provide ease of communication when forecasters issue forecasts and warnings to the general public. Individual storms often last a week or longer and more than one can be occurring in the same area at the same time, so names help reduce the confusion about which storm is being described. Personalising these deadly natural hazards also encourages people to pay more attention to the warnings.
Names used for hurricanes that have a severe impact on lives or economies, and are remembered by generations because, of the devastation they caused, are often `retired' by the WMO for at least ten years. This is to facilitate historic references, legal actions, and insurance claims, and to avoid public confusion with another storm of the same name. It will be some time before Mitch, Andrew and Floyd will be allowed back on to the official list of names for Atlantic basin hurricanes.
This year, scientists christened the first hurricane of the 2000 Atlantic season `Alberto'. As Geographical went to press, Hurricane Debby became the first hurricane this season to strike the Caribbean.