Tuesday, October 4

The sting in the tail of a tropical cyclone


POWERFUL STORM: Tropical Cyclone Yasi causes severe damage in Townsville, Australia.

IT was early in my teaching career, in the late 1960s, in teaching meteorology to senior students at a Nautical College in the UK when one from the British Virgin Islands, in the Caribbean, asked, “Sir, you have explained temperate cyclones (low pressure systems) but what is the difference between tropical cyclones, typhoons and hurricanes?” “An astute question,” I quietly said to myself, for he had clearly experienced hurricanes in the Caribbean.

Perhaps my explanation now would have been more detailed. Much later in life I have experienced all three in different degrees. During the UK Great Storm of Oct 15-16, 1987 – the worst storm to hit Southern England for 284 years – there were hurricane force winds of 190km per hour (kph). At 4am, I was awoken by the rattling of open windows nearly snapping off their hinges. I was in the eye of a hurricane, which had originated in the Gulf of Mexico, drifted across the North Atlantic Ocean with its tail end hitting my house!

In the eye of this cyclone, the air was bone dry. That night, 15 million trees were uprooted and 19 lives lost in Southern England. The trees were destroyed through a very wet autumn and the fact that they had retained their leaves later than usual, offered resistance to the wind. A third of the tiles were ripped off my roof and smashed to pieces. The total insurance claims in the UK amounted to 1.5 billion pounds. Three years later, the Burn’s Day storm on Jan 25-26 in Scotland was classified as a Category 3 hurricane costing insurance companies 3.37 billion pounds. Both storms were stings in the tail of tropically generated hurricanes migrating several thousand kilometres from their origin.

In 1996, in Sabah, the tail end of a typhoon that had already swept across the Philippines with devastating effect made landfall on my hotel. To see tablecloths and crockery fly off tables, dead leaves rush in and staff furiously fighting the elements to batten down the shutters sent shudders down my spine. All in, the so-called, ‘Land below the wind’!

Later in 2006, whilst in Mauritius, again the tail end of an Indian Ocean tropical cyclone hit the beach resort where I was staying. To feel the wind strength and to see huge storm waves breaking over the shore coral reef and churning up the inshore waters drove me to my room on the ground floor albeit about five metres above the high tide mark. Fortunately the storm surge stopped as it lapped around the base of my verandah leaving its debris of discarded plastic water bottles and nylon fishing nets abandoned by carefree oceangoing vessels – all their junk was on my doorstep.

The generic terms hurricanes and typhoons are not easily distinguished for they are both forms of tropical cyclones with almost identical degrees of gravity. Whilst the most dangerous of natural hazards to man, they are part of the atmosphere’s circulation in the transfer of energy from the equator to the poles. A tropical cyclone is a low pressure system over tropical or subtropical waters at 26.5 degrees Centigrade and above, with winds circulating clockwise in the southern hemisphere and anticlockwise north of the equator. Up to 10km high and 480 to 650km wide, a cyclone can travel as fast as 65kph.

Early in its development, it is a weak tropical depression but when wind speeds exceed 65kph, it is a tropical storm. When wind speeds reach over 120kph, it is termed a typhoon in the North West Pacific and a hurricane in the Atlantic. Interestingly in the Indian Ocean, the term tropical cyclone is used. So it is merely a matter of nomenclature of regionally specific names depending where one lives in the tropics between 5 and 30 degrees latitude. (See Table of the Saffir-Simpson Scale of Hurricane Intensity and Storm Damage devised in 1970.)

The Borneo Post has kept us all well-informed of the paths and damage caused by Typhoon Talas in Western Japan with wind speeds of 108kph, a deluge of rain of 305 millimetres in 12 hours and a death toll of nearly 100 people. At the same time, Tropical Storm Lee, in the wake of Hurricane Irene, with wind speeds of up to 120kph was sweeping across Louisiana, in the US. The super typhoon Nanmadal in late August caused 20 deaths and the evacuation of 61,000 people in Luzon in the Philippines, gusts there reaching 230kph. Hurricane Irene in the US caused 50 deaths and probably in excess of US$10 billion in damages.

As I write this, Grade 4 Hurricane Katia is sweeping from the Caribbean across the Atlantic on a northeasterly path, following the direction of the warm ocean current of the North Atlantic Drift, its tail may hit the northern parts of Wales, Ireland, and Scotland with fears there of storm surges in coastal areas coinciding with high tides. On average, a tropical cyclone travels at between 480 and 650km per day and the equivalent of 46,600km before it decays.

Why are tropical cyclones given names? Over the Philippines and Japan these are not personal names but descriptive adjectives dependent on the Asian country that first identified them. In the US, female and male names in alphabetical order are given in rotation to hurricanes, thus Irene was followed by Lee and now by Katia.

What of the other effects, as air is sucked in from high pressure areas to the low pressure centre of the tropical cyclone, to create high velocity winds? High seas driven by the force of the winds can cause problems for container ships at sea. A storm surge may cause extensive flooding and ultimately extensive damage in river estuaries as experienced in New Orleans and this week in South Pakistan. Torrential rain – for a tropical cyclone can evaporate as much as 2.3 billion tones of moisture from the sea each day – has the capacity to trigger landslides and extensive flooding inland.

Hitting densely populated and low-lying areas a few metres above sea level river surges can cause an inordinate loss of life unless imminent warnings to evacuate are in place. Much meteorological debate occurs worldwide on the effect of global warming upon the increasing frequency of tropical cyclones. Climatic models intimate that the intensity in these could occur in the future.

I just wonder what with the recently declared return in 2011 of the La Nina effect, are more tropical cyclones yet to be generated this year and in 2012? Who knows? Meteorologists and atmospheric physicists worldwide must provide adequate warnings as to when, where and how such phenomena can suddenly affect us. Even in Sarawak, with changing climatic patterns, we are not exempt from the sting in the tail of a typhoon which perchance could migrate even in our direction.

For further information go to: The Australian Bureau of Meteorology (www.bom.gov.au), Atlantic Oceanographic and Meteorological Laboratory (www.aoml.noaa.gov), and Malaysian Meteorological Department (www.met.gov.my).