SOMETCUBA Bulletin |
Volume 4 Number 1 |
January 1998 |
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Amongst all natural disasters, tropical hurricanes are the first cause of human life and economical loses, due mainly to the strong wind and heavy rains, the huge waves and intense sea penetrations. Nevertheless, these systems have also a positive effect, as they bring beneficial rains to zones suffering from long droughts.
During the just ended cyclonic season on the Atlantic basin, meteorologists dedicated to the vigilance and forecasting of tropical hurricanes had a relative ease as compared to the huge job they carried out in the last two years, since there were 19 organisms in 1995 and 13 in 1996. Never before in the current century had there been two so much active consecutive seasons.
By contrast, a great labor was developed by forecasters in charge of these systems in the neighbouring basin of the eastern Pacific, since an intense cyclonic activity took place there in 1997, causing great damages and human loses in the coastal areas of the Mexican Pacific, outstanding amongst these were those caused by hurricane Pauline in the touristic resort of Acapulco.
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The North Atlantic region (included the Golf of Mexico and the Caribbean Sea) classifies as the fourth place in the World in the formation of tropical cyclones with an average of 8.4 per year (for the period from 1886 to 1996), out of these, 5 reach the rank of hurricane.
The 1997 season was little active, since only seven tropical storms developed and only three reached the hurricane stage. Besides, a tropical depression developed (from July 17 to 19). Reduction of cyclonic activity this year was due to the developement of a very strong ENSO (El Niño/Southern Oscilation) event, which imposed strong streams from the west in the upper troposphere, a very unfavourable condition for the genesis and developement of tropical cyclones.
On table 1, a tropical cyclones classification is shown, according to the maximum sustained wind intensities on the surface (WMO 1996).
Table 1.Classification of tropical cyclones. |
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Classification |
Maximum sustained wind speed (averaged over a minute) in km/h |
Tropical Depression Tropical Storm Huricane |
62 63 - 117 118 |
The little activity this year was mainly noticed in the zone of the Gulf of Mexico and the Caribbean Sea. One organism developed in the Gulf, Danny, which was the only one to enter land, while no formation at all took place on the Caribbean Sea nor any organism crossed above it’s waters coming from the east, a completely opposed situation to that faced in the 1996 season. (Ballester et al, 1997). The six remaining organisms formed on the oceanic area at rather high latitudes (Fig. 1).
The most intense hurricane was Erika, with a rank of 3 according to the Saffir/Stimpson scale (see table 2). This organism was the only one generated from a tropical wave proceeding from Africa, which is the main genesis mecanism in the Atlantic.
Table 3 shows the average frequencies of organisns developing into tropical storms per month and the distribution for 1997.
The most significant thing in this table is the developement of four storms in July, despite the small yearly average. Since 1870 this had only occurred in 1966 and 1995. All these storms reached the rank in the first 16 days of the month, making this the most active period of the season. The tropical depression also developed in July. Another interesting characteristic is the lack of activity in August, a somewhat frequent fact in this century before 1930, but taking place only in 1941 and 1961 after that date.
Another significantly anomalous month was September, precisely the one with the greatest frequency on the Atlantic, in which only one cyclon developed this time, this did’t happened since 1976.
A general resume of the tropical cyclones on 1997 is shown in table 4.
June 30th - July 4th
It originated in the afternoon of the 30th in a well defined circulation pattern in the lower levels associated with a frontal low, 385 km south of Cape Hatteras, North Carolina. With a slow motion to the east, it gained some intensity and was classified as the first tropical storm of the season on the afternoon of July 1st. Overnight it bent it’s path northeast, and keeping it’s motion towards the first quadrant, it degraded to tropical depression in the afternoon of the 3rd and in the small hours of the nextt day it lost it’s tropical condition over the Atlantic.
July 11th - 12th
Tropical storm Bill originated in an area of perturbed weather in the high levels of the atmosphere at noon on July 11th some 340 km west of the Bermudas. It’s motion was characterized, during it’s brief existence, by a northeasterly component. Bill intensified quickly reaching the rank of hurricane in the morning of the 12th and in the night of that day it lost it’s tropical condition.
July 13th - 16th
Originated on the morning of July 13 in an area of low pressures some 450 km southsouthwest of Cape Hatteras. The same day in the afternoon it was classified as a tropical storm while it remained almost stationary. In the morning of day 14, it started a slow displacement northnortheast and during the night turned between eastnortheast and east. degrading to tropical depression in the early hours of the 15th. It kept the same direction and intensity until it dissipated in the afternoon of the 20th.
July 16th - 26th
It formed on the afternoon of July 16th in an area of low pressures on the northwest of the Gulf of Mexico. It kept almost stationary until early next day, when it started moving northeast as it was gaining intensity and organization qualifiying as tropical storm in the morning of the 17th. Danny kept intensifiyng and reached the rank of hurricane in the first hours of the 18th, passing north of New Orleans and stationing over the outlet of the Mississippi. With a very slow motion approximately between northeast and eastnortheast, it kept close to the coast of the states of Mississippi and Alabama until the afternoon of the 19th, when it entered on land near the border between Alabama and Florida, quickly loosing intensity, degrading and dissipating in the afternoon of the 20th.
After dissipating, Danny moved slowly through the southeast of the United States, keeping it’s circulating cloud pattern. As it exited to the sea by the coasts of North Carolina it was again cataloged as a tropical storm on the 24th. Afterwards, it kept moving with a component towards the first quadrant close to the northeastern coast of the United States and became extratropical in the morning of the 26th.
September 3rd - 15th
It originated within a strong tropical wave proceeding from Africa in the early hours of the 3rd, some distance east of the island of Trinidad, in the southern group of the lesser Antilles. With a quick motion westnorthwest it reached the rank of tropical storm in the afternoon of that day. Since the first hours of the afternoon it turned it’s path almost northnorthwest to turn again westnorthwest it’s fast motion until the afternoon of the 4th. After showing some disorganization on it’s cloud pattern in the mornimg the next day, caused by it’s motion in an environment with a high wind shear, it reorganized in the afternoon and started moving slowly in the same direction, increasing it’s intensity to turn into a hurricane in the morning of the 5th. The intensification process continued and it’s motion turned to the northeast until it became almost stationary north of the Lesser Antilles. Early on the 7th, it started a slow motion to the north and later to the northnortheast with greater speed, reaching it’s maximum intensity (200 km/h) in the afternoom of the 8th, which remained for almost 24 hours.
It kept the northnortheast path gradually diminishing it’s intensity until the afternoon of the 10th when it bent it’s trajectory to the northeast and eastnortheast degrading to tropical storm in the evening of the 11th. It kept this direction until the afternoon of the 12th when it started moving between the eastsoutheast and southeast. In the morning of the 14 it reassumed it’s direction towards the first quadrant, passing over the Azores one day later. Erika started loosing it’s tropical characteristics since the 15th.
October 7th - 8th
It formed in an area of low pressures 880 km eastsoutheast of the Bermudas in the morning of October 7th. With a generally eastward motion it reached the rank of tropical storm in the morning of the 8th, It bent it’s path to the eastnortheast and during the night it lost it’s tropical characteristics.
October 16th - 17th
It developed in an area of low pressures over the Atlantic northeast of the northern group of the Lesser Antilles in the morning of October 16th. During it’s brief life Grace kept a trajectory towards the first quadrant moving between east and eastnortheast. In the morning of the next day it degraded to tropical depression and quickly became extratropical.
- Ballester, M. , O. García y J. Rubiera (1997): Temporada ciclónica de 1996 en el Atlántico Norte. Dpto. de Pronósticos, INSMET-CITMA. Aseguradora del Turismo, La Isla S.A., 20 pp.
- OMM (1996): WMO/TD No. 494 Tropical Cyclone Programme, Report No. TCP-30. Ginebra, Suiza.
- Simpson, R. H. y H. Riehl (1981): The hurricane and its Impact. Lousiana State Univ. Press, Ba Ton Ronge (ISBN )-8071-0688-7) 398 pp.
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