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Hurricanes

IN SHORT - Hurricanes are dramatic weather events which occur over the warm waters zones of the oceans, in both Earth's hemispheres. As they are called 'hurricanes' in the Atlantic Ocean region, they are termed 'typhoon' in the western part of the Pacific Ocean. All such events may reach 60 mi (100 km) wide as gale winds and heavy winds are seen up to 120 mi

(200 km) from the hurricane's center. The weather services categorize the various steps leading to a hurricane, as the hurricanes themselves range over a 5-degree scale. Natural pattern cycles seem to exist about those weather events, as they may too be affected by the global warming question

Hurricanes

Hurricane Ophelia on Sept. 14, 2005. picture Naval Research Laboratory, Marine Meteorology Division

Hurricanes are well known to USA southeastern coasts dwellers. Hurricanes are appearing in summer and fall in the tropical zone of the North Atlantic Ocean. Warm waters there are having moist air rising and transforming into clouds. Pressure differences are then adding winds leading to loose clusters of showers and thunderstorms. Weather and persisting water heat make such features turn into hurricanes, those vast zone of spiralling clouds and winds. Clouds and winds are spiralling inwards, toward the "eye" of the hurricane. Winds and storms are most violent just around this center, in the "eye wall" as the eye proper has just calm winds and clear skies

Hurricanes may be about 60 mi (100 km) wide as gale winds and heavy rains are seen up to 120 mi (200 km) from the hurricane center. The eye varies between 10 mi (16 km) and 30 mi (48 km). The stronger the hurricane, the smaller the eye. Just in the hurricane's eye, the air, coming from up to the lower stratosphere, is sinking down, as in the ring of strong thunderstorms around it, it's strongly hurled upwards. Hurricanes may last from days to about one month. As better monitoring, prediction, and preparedness plans are lowering the direct effects of hurricanes, indirect damages are still important, under the form of landslides and floods which may cause thousands deaths, or due to an increase in coastal populations and properties values. Hurricane Andrew e.g. caused $ 26.5 billion in damages in Miami, Fla. metropolitan area

Once pushed westwards by trade winds, hurricanes are naturally taking a polewards path. This leads them directly on USA southeastern coasts. From there, due to their water heat-engine fading, their move North, remaining dangerous, and eventually turn into usual North Atlantic cyclones

Other Names, Categories

In the western part of the Pacific Ocean such weather events are known like "typhoons" as elsewhere they are termed "tropical cyclones". "Tropical cyclones" on the other hand is the generic name for any of such meteorological events; an hurricane is a tropical cyclone. All such events are working in the same way

Once the process started, a gradation of weather may occur before turning into a real tropical cyclone. An organized circle of winds and thunderstorms around a center is called a "tropical depression". It's the first stage. If winds there eventually reach 39 mph (63 km/h) or more, the storm turns into a "tropical storm". At last, if the winds reach 74 mph (119 km/h) the tropical storm turns into a tropical cyclone, that is a hurricane -or a typhoon. Names are attributed to these events starting at tropical storms. The official Atlantic hurricane season runs from June 1 through November 30. 6 to 7.5 hurricanes are seen there along this period each year, along with about 9-10 tropical storms. 1.5 to 3.8 hurricanes may be qualified major ones as, generally, 5 hurricanes are reaching the U.S. coastline on a three-year timespan. On the other hand, a below-normal hurricane season in the Eastern and Central Pacific has see six to eight hurricanes only, with two to four of them becoming major storms

The Saffir-Simpson hurricane scale comprises 5 degrees with winds ranging from 74-95 to over 155 mph. Hurricanes in USA are affecting the Atlantic and Gulf coastal zones, as inland areas may be affected by correlative weather such as high winds, tornadoes or floods. Japan is a good target too for local hurricanes -called "typhoons"- with an average of more than a dozen each year!

An Increasing Trend in the Atlantic Ocean During the Next 20-30 Years

A trend has been seen since 1995 in the Atlantic Ocean temperatures and circulation patterns. This trend is bringing more numerous and much stronger hurricanes. Hurricanes in the Atlantic Ocean are affected by a cycle lasting for decades -it's called the "thermohaline circulation"- and which is alternating mild and stronger periods. This cycle has been observed since about the year 1900 and we entered a period of strong activity since about 10 years as this period is expected to last during the next 20-30 years. On the other hand the El Niño and El Niña phenomenons are interacting with this hurricanes pattern as are currents in the Atlantic Ocean too. When El Nino is absent, storms do not move North. They stay at lower latitudes as currents may bring the hurricanes further West. La Nina, on the other hand, is affecting too the hurricane season as when the event occurs on the Pacific Ocean, hurricanes are stronger in the Atlantic Ocean due to the changes La Nina brings to the wind patterns in the upper and lower levels of the atmosphere. Concurrently, the increase of storms in the Atlantic comes with a decrease of them in the Pacific Ocean. The modified winds do not act like a shear to hurricanes anymore in the Atlantic, as such an effect is increased in the Pacific. The main contributor to the Atlantic Ocean hurricanes seems well to be a weak El Niño and strong Western African monsoons, with the temperature of the surface waters of less importance. Such an increasing trend is expected to cause more damages as more people are now living in the affected areas than there were in the last period of strengthened activity of the 1930s-1960s

The dust blown off the coasts of West Africa, from the Sahara, recently were studied lowering the sea surface temperatures of the North Atlantic Ocean some years, likely impacting the cycle of the hurricanes there. When sea surface is colder, less hurricanes are triggered

Another view if that global warming is affecting too the conditions in the West African Atlantic where the very first symptoms of storms and hurricanes are appearing. Such a modification might bring to stronger such weather events. Another view still is that the increasing trend since 1995 might be due to a large scale oceanic current system called the "Great Ocean Conveyor" which is moving warm water from the Pacific Ocean into the Atlantic one through the Indian Ocean. It's working on a 15-20 year cycle. When the conveyor is active, the Atlantic Ocean climate is warmer and wetter as the Pacific Ocean one is colder and dryer (that is it endures an El Niño). And the reverse (El Niña in the Pacific Ocean) when the conveyor is inactive. The conveyor have entered an active phase since about 1995. A warmer Atlantic Ocean usually means an increased hurricanes rate