The Saharan dust clouds that travel thousands of kilometres and have an impact on the climate, weather, environment and health of human beings, as well as of animals, plants and complete ecosystems, marine and land, of the territories under its influence, including Cuba, which is only affected from March to October, also produce a great biogeographical dispersion of pathogens.

The Saharan dust storms incorporate into the atmosphere millions of tons of dust that cross over the Atlantic in different ways, depending on the time of year. These clouds travel thousands of kilometres and give rise to transcontinental and transoceanic events that link very distant points of the planet.

Constituted by paniculated material of different mineral compounds, these storms play a fundamental role in the incorporation into the atmosphere of enormous amounts of bioparticles and other compounds, as well as their transportation, which is why these events contribute to the dispersion at very distant areas of microorganisms and the biogeographical spread of pathogens. Thus a bridge is established between continents through which the microbes contained in a certain geographical area can have an impact on ecosystems located thousands of kilometres leeward of the source regions, with the subsequent effects on health. Then the biogeographical dispersion of pathogens gains a new dimension: these major dust events have an impact on the climate, weather and health of human beings, animals and plants and complete ecosystems, marine and land.

It is possible to link the microorganisms transported by the African dust to some epidemic outbreaks in the Caribbean and that they affect agriculture and cattle raising. The epiphenomena caused by fungi that, for example, damage sugarcane and bananas on a commercial scale, can appear in the Caribbean region several days after the inrush of the dust, which suggests that the spores could have travelled with it. Moreover, pathogen bacteria of rice, beans, fruit and some tree species, as well as bacteria that can infest hogs, foul and cattle, have been identified in the Saharan dust clouds arriving in the Caribbean.

Among the microorganisms present in the transported dust many are sufficiently aggressive pathogens to produce epidemics. It is estimated that 30 percent of the bacteria isolated in the dust are known pathogens.

Made up by a large mixture of biological and mineral dust components, these clouds contain fungi, viruses, bacteria, staphylococci, faecal components, mite, all types of detritus, pollen, mineral elements and compounds that include heavy metals like mercury, as well as persistent organic contaminants, among them insecticides, pesticides and herbicides. Among their mineral particles a relevant role is played by those with a diameter of less than 10 and 2.5 microns and the superfine ones, because of their impact mainly on respiratory and cardiovascular diseases, since given their diameter they are found in the range of breathable particles. It should be taken into account that in the dust clouds that get to Barbados and Miami, from a third to half of the total mass has an aerodynamic diameter of less than 2 or 2.5 microns.

The effects of the dust on the weather and climate are of transcendental importance for the fate of the planet. In the first place, the climate is affected by a radioactive strengthening that can be direct and indirect. The direct takes place through the dispersion and absorption of the incidental solar radiation, which prevents a large part of the solar radiation from reaching the surface of the Earth. It is important to assess that in this process the interaction between the dust and the land infrared radiation also takes place.

The indirect radioactive strengthening is linked to the clouds’ microphysics, since it alters its life span and the production of rain. In both cases, the radioactive strengthening is negative, which is why it contributes to the planet’s cooling. This makes it absolutely necessary that, given the enormous amounts of dust incorporated into the atmosphere every year, it be included in the weather forecast models in their widest spectrum. Moreover, this implies the development of complex models that include not just the presence or amounts of dust, but also its composition, size of its particles, vertical stratification and transformations, among other aspects. Only thus will there be a clearer vision of the planet’s radiation balance, which is equivalent to warming or cooling, and make a real assessment of its impact on climate change.

These dust clouds modulate the cyclogenesis and evolution of tropical cyclones in the North Atlantic when a marked inversion of temperature in the low levels of the atmosphere takes place, increasing the stability, the vertical cutting off of the wind and contributing to the decrease in the temperature of the sea’s surface. In terms of the evolution of the tropical cyclones, it is known that the conditions of the environment outside the cyclone as well as the changes in its internal structure and force condition its future evolution (wind, precipitations). This process of interaction between the dust and the cyclone is complex, but the final effect is the weakening of the organism, which could reach its total destruction, according to what different authors have stated.

In the areas that remain under its influence, these dust clouds impose their conditions; by influencing the atmosphere’s parameters they modulate the rain regimen. Recent studies carried out in Havana confirm the effect of these dust clouds on the storm areas, in which they can stimulate a great electric apparatus, the marked reduction of precipitations thus contributing to drought. This effect must be taken into account in the regions where the precipitations produced by the summer storms have a significant weight on the total of rain.

This subject’s increasing importance is due, in the first place, to the development of highly specialised satellites that have firstly enabled a global, regional and local vision of the occurrence of the storms and the transport of dust, as well as a rapprochement to the wide sphere of transcendental effects that this desert dust has on the environment, weather, climate and health of human beings, animals and plants, including the marine and land ecosystems vital for the planet, to the point of reaching on occasions the category of disasters.

For a country like Cuba, located in the Caribbean Sea, leeward of the Sahara – the principal source of the planet’s dust – and frequently hit by tropical cyclones, the development of researches geared at clarifying the effects of the Saharan dust clouds on different scales of temporal space becomes absolutely necessary, as well as characterising their behaviour, measuring their concentrations, chemical composition and classifying their biological components. On the Caribbean island, where summer rains are closely linked to electrical storms and determine to a great extent the country’s vital water resources, during the spring and summer the effects of the Saharan dust clouds are frequent, depositing enormous amounts of dust, with a heterogeneous charge of mineral particles, biological compounds, many of them pathogens and some sufficiently aggressive to produce epidemics, and animal and plant diseases.

The present article features the presence of the Saharan dust clouds over the national territory from 1998 to 2009, on different scales of temporal space (country, provinces, days, months and years), according to elements like the presence or not of the dust, percentage and amount of days with dust, consecutive days with the presence of dust and the value of the aerosol index (AI).

Materials and methods

This analysis works in different scales of temporal space that include days, weeks, months and years; essentially the national territory (the whole) and the provinces (the parts), according to the administrative political division in force at the moment of drawing up the project’s databases.

The basic tools were the images of different types of satellites, sensors and spectral channels. The satellites are: Earth Probe, AURA, GOES and METEOSAT. The images are used with different levels of priority, according to the area under study and the specific concerning phenomenon; however, the article is fundamentally based on the TOMS Earth Probe, OMI AURA and GOES – METEOSAT Split Windows images. The processing of these images is manual and is carried out by placing a map of Cuba with the administrative political division over the image to be analysed, to determine the presence or not of the dust over the area under study, which can be ratified through the value of the aerosol index, determined with the TaiSat Software. The entire processing of data is carried out through techniques of the multivariate statistics.

Saharan dust in Cuba from 1998 to 2009

The first Saharan dust clouds get to the island mainly in March, though they can do so in April. The maximum percentages of days are especially registered in July and June, though they have also been registered in May and August. The last clouds of the season are generally observed over the national territory in September and October.

The maximum value of the Aerosol Index reported is four, in July.

When the percentage curve of days with dust from 1998 to 2009 is analysed, one can observe that the years 1998 and 1999 remain close to 70 percent, a data that considerably increases in 2000, up to 97 percent of days with dust, to then experience a very sudden drop to 54 percent in 2001.

From here on the transport of dust rapidly intensifies, continuing between 2002 and 2004 with all the days with dust during several months.

Following this there is a continuous drop in the arrival of the dust, with the minimum value in 2006: 32 percent. Then a discrete increase begins, which continues until 2009 in 81 percent of the days. In Fig. 1 one can see the cyclical behaviour in terms of the arrival of the dust.

Though the period under study is relatively short and does not allow for a rigorous characterisation, the transport of dust seems to be represented by cycles of approximately five years, which could establish a cyclical behaviour, in the occurrence of the storms as well as in the transport of dust. This fact appears reflected in the graph of percentage of days with dust per year and is of great importance due to the repercussion it can have on the behaviour of certain weather and climate patterns in the region, as well as the transport of pathogens and other compounds endangering health, like the persistent organic contaminants and heavy metals.

The Aerosol Index has its highest registry in the years corresponding to the period of maximum arrival of the dust: 2002 and 2003, when an AI of four is reported. Another notable value (3.5) is reported in 1999, 2000 and 2004.

Between 1998 and 2009, 2002 was the year with the highest percentage of days with dust per months, since a great deal of the spring and summer registered the total of those days, in April, May, June and July, which indicates that during those months the national territory was affected on a daily basis by Saharan dust clouds.

The Aerosol Index maintained very high values during almost the entire season, with the highest amount registered (four) in July and 3.5 in April, June and August, followed in 2004 with all the days with dust during May, June and July, as well as 2003 with 100 percent of the days with dust in June and July. The lowest levels of the period correspond to 2006 (32% of the days in July and August) and 2005 (35% in July). Starting 2006 one observes a tendency toward an increase in the levels of dust, with 48 percent in August 2007, 43 percent in June 2008 and 84 percent in July 2009. In the years corresponding to the decade of the 1990s and in 2000 one observes the following levels: 74 percent, 71 percent, 97 percent in 1998, 1999 and 2000, respectively.

Indicators linked to the effects of the dust clouds

To be able to understand the behaviour of the dust clouds, systematise their applications in different economic, social, scientific spheres and be able to assess the interaction between the country and the provinces, and between the latter themselves, it is necessary to define a group of indicators that allow us to assess, in a very synthetic and homogeneous way, the presence and potential effects linked to the characteristics of the daily, monthly and seasonal (accumulative) characteristics of the transport of the dust on processes like electrical storms, drought, ecosystems like the coral reefs, internal water pools and the health sphere, where it can be bronchial asthma and acute respiratory infections in human beings, or diarrhoea in the cattle. These clouds can arrive as major dust events that cover thousands of square kilometres and affect the entire national territory (see Fig. 2).

Fig.2. Image IR GOES-Meteosat 16 Jul. 2009 0600z shows an extensive dust cloud over Cuba’s Eastern region.

– Dust season: It is defined as the arrival of the dust to the national territory, which takes place between March and October, directly linked to the Saharan dust clouds and that does not necessarily imply the establishment of the Saharan Air Layer (SAL), as it is called. This transport is linked generally to the trade winds, but can take place through the waves of the East and other meteorological phenomena.

The dust present in a lesser degree in the Cuban territory, in November and the winter months, is considered as coming from other sources, mainly from the west, and is studied independently.

– Monthly Activity (MA): It is directly defined as the amount of dust days corresponding to the month, expressed in percentages. It has five categories: very weak (0 – 19), weak (20 – 39), moderate (40 – 59), strong (60 – 89) and intense (90 – 100).

– Dust Season Intensity (DSI): It is the average percentage of dust days during the months of the dust season. This indicator can be national and provincial , and has four categories: weak (0 – 14), moderate (15 – 29), strong (30- 54) and very strong (55 – 100). This definition allows us to make a quick and precise assessment of the annual behaviour of the dust, over the national territory as well as for each province, individually, as well as their comparison.

– Index of Affectation (IA): It is defined as the reason for the provincial DSI (pDSI) and the national DSI (nDSI).

Dust season intensity from 1998 to 2009

The analysis of the potential intensity of the season leads us to a different way of thinking. We don’t see here a month or a week with a great deal of dust, with its direct effect on a specific process, as can be the case of bronchial asthma; but rather the total effect of a dust season, with its consequences on systems like the coral reefs, land aquifers, the flowering of marine toxic algae, forests, human beings and animals.

Analysing the seasons based on these principles reveals the seasonal behaviour of the Saharan dust clouds on the national territory, as a whole – as appears in Fig. 3 – and in its integral components, the provinces (see Fig. 4). The maps of dust obtained based on the EP-TOMS and OMI Aura images shows us different scenarios, which is explained as follows.

Behaviour of nDSI

The analysis of the period under study shows 1998 with high levels of intensity, which reach up to 50 percent and decrease the following year to an accumulated 45 percent. That decrease continues and in 2000 is reaches the figure of 40 percent, with a sudden fall in 2001, when it reached 20 percent.

The years 2002, 2003 and 2004 established a record of potential intensity. Headed by 2002, which presents the highest values, those years registered levels of more than 55 percent. The year 2005 represents a sudden drop in potential intensity of up to 20 percent, which marks a decrease in the presence of the dust in the region, which can take into account a marked influence in the intense cyclonic activity in that season. During 2006 the pDSI rises as compared to the previous year; however, together with 2007, 2008 and 2009, a basin of affectation due to dust is maintained that exceeds an nDSI of 30 percent. Fig.3 (a, b and c), the nDSI maps for 1998, 2002 and 2009 reveal.

Fig. 3 (a,b,c). Shows the nDSI maps for 1998, 2002 and 2009, respectively.

Behaviour of pDSI

The comparative analysis of the provinces for the period under study shows different levels of the pDSI throughout the national territory, for each year, and the changes in the years under study, for each province (see Fig. 4 a, b and c). Moreover, it shows the SAL maps for 1998, 2002 and 2009.

Low levels of pDSI appear in the 1990s, characterised by the light colours with the colour green on an upper border (see Fig. 4a).

eanwhile, 2000 shows higher pDSI levels, with the highest value in Guantánamo (34.8%); 2001 accumulates the lowest levels of intensity over the entire territory (2% and 2.1% in Granma and Cienfuegos, respectively). On the contrary, 2002 takes the darker colours, according to the maximum values of intensity, given that this is the year with the highest pDSI of the period under study (63% in Guantánamo). The years 2003 and 2004 have weak to moderate degrees of intensity, with 2003 with the highest values in Guantánamo and Pinar del Río (72% and 57%). In 2004, the highest figure of the season corresponds to the province of Guantánamo (52.8%). The year 2005, considered as having a moderate monthly activity in some months, has a low pDSI that corresponds to the colours white and blue, which recall 2001, though it does not reach such low levels as this year and is evidently far from of the previous years. This is why it can be considered as the start of a period of declining potential intensity.

The years 2006, 2007, 2008 and 2009 are in a period of declining pDSI, which is seen in the light colours of the annual maps (see Fig. 4c). During those years the maximum value of pDSI remained in Villa Clara (12.25%), shared with Sancti Spíritus in 2008.

a (1998. Total potential intensity of dust season.) b (2002. Total potential intensity of dust season.) c (2009. Total potential intensity of dust season.)

Fig.4 a, b, c. Shows the pDSI maps for 1998, 2002 and 2009, respectively

Index of affectation

The Index of Affectation (IA) offers a quantitative assessment of the relationship between the country and the province, between the whole and the parts, and allows for establishing a comparison between the levels of affectation of the different provinces.

The analysis of the matrix of the affectation index (Table 1) for the period under study shows us that the highest value corresponds to the province of Guantánamo in 2002, with 0.83, and the lowest was Villa Clara with 0.52, followed by Guantánamo with 0.50. This indicator allows us to situate each one of the provinces according to the total accumulative effect of the dust and to compare them between each other to assess where there can be a greater incidence with the national territory of the effects of the season.

Conclusions

• The Saharan dust clouds only affect the national territory from March to October. The definition of the «dust season in Cuba» is based on this criterion.

• The analysis of the distribution curve of maximum percentages of days with dust for the period under study, from 1998 to 2009, shows a cyclical behaviour, approximately represented by five-year cycles. This criterion can be of great interest, for the study of the occurrence of dust storms as well as the transport of clouds, their arrival to Cuba and the impact on the Caribbean.

• The maximum amount of days with dust for the national territory is mainly registered in summer, during the months of June or July.

• The comparative analyses of the pDSI maps shows a marked annual variability in the behaviour of this indicator, which also establishes that the total effects of each one of seasons are different.

• The comparative analysis of the pDSI maps for a specific season shows a great variability in the behaviour of this indicator among the provinces, while the comparison of the maps for the period under study also indicates a marked variability for each one of the provinces, which implies that the total intensity of a dust season is manifested in a differentiated manner in each one of the provinces. This is also variable in the weather and is due to a great extent to the particularities of the transport of the dust.

• The Index of Affectation establishes the relationship of the seasonal behaviour between the provinces and the nation, in addition to allowing for the quantitative comparison between the different provinces, which makes it a basic working tool. (2013)