Winter season of 1997 - 1998 in Cuba

• Introduction
• Season Analysis
• Conclusions
• References

Introduction

With the arrival of the winter season to the Cuban Archipelago, normally a significant decrease in rainfall occurs, that’s why this stage is called the dry season with only 23% of the total amount of rains in the year.

These rainfall is usually associated to frontal systems affecting the country, which are followed by cold air masses causing a temperature decrease. However, these usual conditions, adequate for a good harvest of the main agricultural product of the country, sugar cane, can be significantly altered by climatic fluctuations such as the El Niño/ Southern Oscillation (ENSO) event.

Under the influence of an ENSO event, the position of the jet stream is radically changed, causing completely anomalous weather patterns not only in North and South America, but also in such far regions as Africa and Antartida. An illustrating example of the great anomalies this event can cause is the winter season of 1982 - 1983 in Cuba, mainly between January and March 1983 (Rubiera, 1984). In these months, very severe weather events took place, with occluded lows on the Gulf of Mexico, which caused frequent heavy rains over a great part of the national territory. These rains exceeded 3 to 5 times the historical records for those months; it was the most humid and rainy season in 50 years, matching the precipitation level of the rainy season.

This paper intends to analyze the 1997 - 1998 winter season in Cuba, which was also under the influence of a strong ENOS event, an important factor on it’s behavior. It must be pointed out that this time, despite the negative effects produced over the national territory, the country was relatively fortunate. In several occasions, cloud bands with severe weather associated, crossed over the Florida Peninsula and the strait of the same name, causing human loses and huge economic damages in the United States (similar or bigger than those caused by Hurricane Andrew on august 22nd 1992). These by no means can be compared to events occurred here.

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Season Analysis 

Taking as a basis the chronology made by Rodriguez et al (1984) and it’s further updating at the Department of Weather Forecasting of the Institute of Meteorology, tables 1 and 2 were built.

 Table 1. Date of entrance and classification (*) of cold fronts that affected Cuba during the 1997 - 1998 winter season.

Number	Date of entrance	Classification
1	October 19th 1997	Weak (TC)
2	November 3rd 1997	Weak
3	November 7th 1997	Weak
4	November 16th 1997	Moderate
5	November 24th 1997	Weak
6	December 1st 1997	Weak
7	December 5th 1997	Weak (TC)
8	December 14th 1997	Moderate (TC)
9	December 14th 1997	Moderate
10	December 27th 1997	Weak
11	December 29th 1997	Moderate (TC)
12	December 30th 1997	Moderate
13	January 9th 1998	Weak
14	January 16th 1998	Weak
15	January 24th 1998	Weak
16	January 27th 1998	Moderate (TC)
17	February 3rd 1998	Moderate (TC)
18	February 3th 1998	Moderate
19	February 6th 1998	Moderate (TC)
20	February 23rd 1998	Weak (TC)
21	March 2nd 1998	Weak (TC)
22	March 9th 1998	Moderate (TC)
23	March 10th 1998	Moderate
24	March 21st 1998	Weak (TC)
25	April 10th 1998	Weak (TC)
26	April 22nd 1998	Weak

* Due to the Island nature and the geographical position of Cuba, cold fronts are classified according to the strength of the Northern winds blowing immediately after the frontal line. They will be:

• Weak, wind speed less than 35 km/h
• Moderate with speed between 36 and 55 km/h
• Strong with winds over 55 km/h.
• (TC) means that the cold front swept all over the country.

As Table 1 is compared to historical records, the following features arise:

The arrival of 26 fronts to the National Territory qualifies this season as active, taking into consideration that the national average is of 19.8 fronts per season with a standard deviation of ± 4.9. Since the 1916 - 1917, there have been 14 seasons with 26 or more fronts. Last time it happened was in 1987 - 1988, also with 26 fronts.

Amongst all fronts arriving, 15 (58%) were classified as weak, and the remaining 11 (48%) classified as moderate. Like in the last winter season (Garcia and Pérez Suárez 1997) no strong cold front affected the country. Historically, 48% of the cold fronts affecting the Country are classified as moderate, 42 % as weak and 10% are strong.

It is known that during its motion eastward, across the National territory, cold fronts weaken, and as an average, only half the number of them reaches the extreme East of the country. This time, 12 fronts did, making 46% of the total, very close to the historical mean.

Table 2 shows other data related to cold fronts climatology in Cuba. An analysis of it reveals further characteristics of this season.

The 1997 - 1998 winter season had a normal beginning, different from that of the preceding year. From November to April the number of fronts arriving was equal or superior to the historical mean, highlighting significantly the seven systems on December as compared to the historical mean of two. This amount of front entries in December had only occurred before in 1975 - 1976, when an equal number entered in January.

The first cold front arrived to the western tip of Cuba on October 19 and displaced east all over the country with little rain activity.

In some occasions cold fronts arriving to Cuba at this time become stationary, and after loosing the thermal contrast, they remain as frontal throughs, which can produce noticeable accumulates of rain. None of this happened this time and the month’s total rainfall remained below the mean (BVC 1997a).

Four fronts arrived during the month of November, However, the month was a warm one, due mainly to the high minimum temperatures registered (BVC 1997a). It was caused by the flux of warm air from the south that persisted for several days.

Table 2. Amount of cold fronts that have affected Cuba by months since the 1916 - 1917 season until 1996 - 1997 and behavior during the last two seasons.

Seas.	Sep	Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Jun	Total
1916 1917	7	108	207	261	289	273	240	158	108	4	1605
Ave	0.09	1.33	2.56	3.22	3.57	3.37	2.96	1.95	0.72	0.05	--
1996 1997	0	0	2	2	3	2	0	1	1	0	11
1997 1998	0	1	4	7	4	4	4	2	0	0	26

The existence of a subtropical jet stream over the South West of the Gulf of Mexico and it’s interaction with the cold fronts caused high values of precipitation in the Western and Central regions of Cuba.

The first frontal system of the month of November (second of the season) oscillated over the Central Provinces producing rainfalls of over 100 mm/24 h at different sites.

The front that arrived on November 16 (fourth of the season) also caused significant rains on days 15 and 16. Another rainy event occurred with front number 5, when in state of dissipation produced accumulates of 138.5 mm/24 h at General Carrillo, province of Villa Clara and 150 mm/24 h at Mapos, province of Sancti Spiritus on days 25 and 26 respectively.

Despite the 7 systems arriving during December, mean temperatures for the month were high, mostly the minimum means, which were the highest since 1986 (BVC 1997c). However on day 14, fronts 8 and 9 arrived. The latest was a secondary front followed by a polar air mass causing low temperatures of interest on days 16, 17 and 19.

Also due to the arrival of fronts 11 and 12 the last two days of the month showed low temperatures of interest. Particularly on the early hours of the 31st in the plains of Havana - Matanzas with 7.0° Celsius at Batabanó and Bainoa, 7.2 at Colón and 7.4 at Indio Hatuey and Tapaste.

Frontal systems caused abundant precipitation this month, making it one of the rainiest Decembers since 1941 (BVC 1997c).

The trend towards high mean temperatures continued in the first month of 1998, with positive anomalies of over 1 ° Celsius registered all over the country (BVC 1998a).

Once more frontal systems arriving caused numerous precipitation, though the most noticeable were those associated to the last front of the month of December. This one stationed over the Eastern Region on days 2, 3 and 4 of January causing precipitation of 400 mm/24 h at the province of Holguín on day 3.

Four cold fronts showed up also on February, but like in the previous month, minimum temperatures kept presenting positive anomalies in the Western and Central regions. (BVC 1998b).

The first front of the month (seventeenth of the season) came preceded by a squall line that caused an accumulated of 128.6 mm/24 h at Casablanca station on day 2. This is a record for this month at the station.

The winds from the North region increased with the arrival of a secondary front (number 18), this situation caused coastal flooding at some low zones of the north coast in the West part of the country, including the Havana Mole in the early hours of day 4.

The next front came accompanied by a line of severe local storms and followed by a mass of continental air, which provoked a significant decrease of temperatures with the most significant value of 5.6 ° Celsius at Bainoa.

Once more four fronts arrived in March, but unlike in the previous months, the mean temperatures were slightly lower than normal, though the minimums kept high (BVC 1998c). The month was rainy, mostly in the eastern provinces due mainly to frontal systems.

Cold front 21st came preceded by prefrontal storms and followed by a cold dry air mass that caused minimum temperatures of interest in the early hours of day 4 with the most significant value of 7.7 ° Celsius at Bainoa, 8.4 at Jovellanos and 9.2 at Tapaste.

The last two fronts of the season arrived on April. The second of them (26th of the season) brought along a mass of cold air that caused temperatures below 10 ° Celsius in the West of the country.

A significant feature of the season under analysis is the frequent occurrence of winds from the South over the West part of the country. These winds were called "Sures" by Rodríguez and Ballester (1985) and are produced mainly by the influence of extratropical cyclones moving over the Gulf of Mexico or adjoining states of North America. The high pressures related to the anticyclone of the Azores contribute to the increase of the barometric gradient.

This winter season presented a completely abnormal behavior with regard to this meteorological event, which, among other things caused high temperatures and damage to fruit and tobacco plantations. 18 "Sures" were registered in this season. This is a new record for the amount of cases of this event in a single season and widely exceeded the previous record of 13 for the season of 1935 - 1936, according to the chronology of Ballester (1997) which starts in the season of 1918 - 1919.

In Table 3. The behavior of "Sures" for the just ended winter season and historical values for the period 1918 - 1997 is presented. Here it can be seen that the historical mean of 5.5 was widely superated as well as the monthly frequencies from November to May.

Table 3. Historical monthly mean frequency of "Sures" and behavior for the 1997 - 1998 winter season.

Seas	Sep	Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Total
1918 1919	0.04	0.05	0.08	0.27	0.65	1.22	1.72	1.04	0.32	5.4
1997 1998	0	0	1	4	2	5	3	2	1	18

There had been no "Sures" on November since 1988, furthermore, in this occasion, it was qualified as strong (wind speed over 55 km/h) which happened before only in 1963, another year when a strong ENSO event was present, according to the chronology of Cárdenas (1996).

On December, a record was established with four cases, which influenced in the high temperatures of that month, despite the arrival of 7 cold fronts. "Sures" registered on December, as well as those from January, April and May were classified as moderate (wind speed between 36 and 55 km/h).

On February, the record from 1935 of 5 "Sures" was equaled. And the damage potential, (defined by Ballester, 1997), superated (17.7 x10³ knots² ) the existing maximum from February 1940 ( 16.6x10³ knots² ), when an ENSO event was also taking place. Tree of these five "Sures" were classified as strong, they occurred on day 2, from day 15 to 17 and on day 22 with a gust of 90 km/h at Casablanca, Havana City.

The first of the February cases lasted for 6 hours with wind speeds between 60 and 70 km/h and a maximum gust of 114 km/h at Casablanca station. It was produced by the displacement of an extratropical low over the Center-Eastern portions of the Gulf of Mexico.

The "Sur" that affected us on days 15, 16 and 17 reached it’s maximum strength on day 16, when for approximately two hours, the wind remained between 70 and 80 km/h. It had maximum gusts of 115 km/h on day 15 and 116 km/h on day 16 at Casablanca. It was due to a combination of a complex system of low pressures covering great part of the South of the United States, the Gulf of Mexico and the Caribbean, blocked by an intense anticyclone with it’s center moving slowly over the North East of the United States and the adjacent Atlantic.

For the first time, a maximum gust of over 100 km/h was registered on February, this is also the first time that two "Sures" reach maximum gusts of over 100 km/h, and besides, on consecutive days. For these reasons, this has been the most anomalous month of February with regard to this event.

Two of the three "Sures" of March were qualified as strong. The first of them was on days 8 and 9 with gusts of 96 km/h while the other occurred from the 18th to the 20th of the month.

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Conclusions

The winter season of 1997 - 1998 was an active one with 26 fronts reaching Cuba. It also highlighted by the abundant precipitation and the high temperatures, characteristics common to those seasons on which the ENSO event has been present and the high frequency of Sures

Very significant was the arrival of 7 fronts on December, which had only happened before in 1975.

The lowest temperatures of the season were those associated to fronts 12, 19 and 21.

A new record was settled for the number of "Sures" with 18, with the month of February been the most significant by the intensity of the winds. This month of February has been, besides, the most severe with regard to "Sures" since 1919.

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References 

• Ballester, M. (1997): Los "sures" en la mitad occidental de Cuba. Cronología actualizada, potencial de daño y tendencias. Boletín de la Sociedad Meteorológica de Cuba. Vol. 3 No 2.
• BVC (1997a): Boletín de Vigilancia del Clima, Centro Nacional del Clima. Vol. 8 No. 10.
• BVC (1997b): Boletín de Vigilancia del Clima, Centro Nacional del Clima. Vol. 8 No. 11.
• BVC (1997c): Boletín de Vigilancia del Clima, Centro Nacional del Clima. Vol. 8 No. 12.
• BVC (1998a): Boletín de Vigilancia del Clima, Centro Nacional del Clima. Vol. 9 No. 1.
• BVC (1998b): Boletín de Vigilancia del Clima, Centro Nacional del Clima. Vol. 9 No. 2.
• BVC (1998c): Boletín de Vigilancia del Clima, Centro Nacional del Clima. Vol. 9 No. 3.
• BVC (1998d): Boletín de Vigilancia del Clima, Centro Nacional del Clima. Vol. 9 No. 4.
• Cárdenas, P. (1996): Cronología ENOS por índice IE [inédito]. Centro Nacional del Clima, Instituto de Meteorología, C. Habana, Cuba.
• García, O. y R. Pérez Suárez (1997): Temporada invernal de 1996-1997 en Cuba. BSMC Vol. 3 No.2.
• Rodríguez, M., C. González y J. Valdés Quiñones (1984): Cronología de los frentes fríos que han afectado a Cuba desde la temporada de 1916-1917 hasta la temporada de 1982-1983. Reporte de investigación No. 4. Instituto de Meteorología.
• Rodríguez, M. y M. Ballester (1985): Cronología de los sures que han afectado a la mitad occidental de Cuba desde la temporada 1916-1917 hasta la de 1977-1978. Reporte de investigación No. 10. Instituto de Meteorología.
• Rubiera, J.M. (1984): Enero - Marzo de 1983: Un análisis del período invernal más severo registrado en Cuba. Memorias del I Seminario Taller de Desastres Naturales, UNDRO, La Habana pp. 13-19.

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