Temperature and Rain Variability in Southwestern Mediterranean Region, 1980-2022

doi:10.21203/rs.3.rs-4266328/v1

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Temperature and Rain Variability in Southwestern Mediterranean Region, 1980-2022

https://doi.org/10.21203/rs.3.rs-4266328/v1

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Mean temperatures and rainfall evolution have been examined from 1980 to 2022 in Sidi Bel Abbes northwestern Algeria fifty six km far from Alboran Sea in the extreme southwestern Mediterranean Sea. Then, one came to the fact that, contrary to many other scientific results, the rainfall tends to increase even if this augmentation seemed to be slight and difficult to explicate and clarify, when on the other hand temperatures are also clearly augmenting in a really very alarming way collaborating to the global warming.

Nevertheless, this trend was not regular and linear, because temperatures and rainfall evolved abruptly alternating rainy years with drier ones when temperatures are growing almost regularly year after year. However, this saw-toothed evolution of the rains remains less pronounced than that of average temperatures, which have increased significantly over these past four decades.

Climate of our region seems to be a semi-arid Mediterranean Köppen and Geiger "Csa" climate with rainy cold winter and very dry and hot summer, even if winter becomes slightly more humid but summer substantially more swelter.

Temperatures. Precipitations. Mediterranean Sea. Northwestern Algeria. Climate Anomaly

Global warming is a well-observed but perfectly observed phenomenon on our planet which originates from a significant rise in average temperatures for more than a century (Bouaoune and Dahmani-Megrerouche, 2010; Lebid et al., 2013). A majority of scientific authors argue that this increase in temperatures is due to a substantial increase in greenhouse gases in the Earth's atmosphere leading to heat retention which would, in turn, lead to an increase in temperatures especially in our region bordering southern Mediterranean which conclude to a well significant increase in temperatures (Parmesan et Goye, 2003; Somot et al., 2007; Bounoua et al., 2009; Mariotti and Dell'Aquila, 2012; Alexandre-Tudo et al., 2019; Sangam and Savitha, 2019; Klingelhöfer et al., 2020). According to United Nations Fact sheet on Climate Change (2006), Africa seems to be the most vulnerable continent to the impact of climate change (Oluwatosin et al., 2022; Bouabdelli S. et al, 2022), instead of the fact that Africa has contributed very little to the planet's changing climate—between two and three percent of global emissions—but yet it stands out as the continent with the greatest vulnerability. It has been evident that Africa is already suffering from the effects of climate change since the early1970s, and these effects have given rise to a variety of novel and unusual phenomena (Bouabdelli S. et al, 2022; Bedair H. et al., 2023).

Nevertheless, it appears that the corollary of this trend is also due to a decrease in precipitation. These findings were based on the fact that the climate can mainly evolve from a semi-arid cline of the "Csa" type to a sub-tropical climate of the "Bsh" type, according to certain previous studies referring to our region (Peel et al., 2007, Meddi et al., 2010; Zeroual A. et al, 2019). This is how one conducted this study with the aim of corroborating or invalidating this thesis, because although if the temperatures have obviously greatly increased, it would be wiser to examine the evolution of precipitation at the same time in order to know whether the climate will be drier or not.

Could we call this a climatic anomaly since it is the deviation from an average well established climatic state (Merle, 1980)? Estimation of parameters is calculated by comparing the annual precipitations with those calculated over a given reference period (Laala and Djamel, 2016; Zeroual A. et al., 2017).

Our region is situated southwestern of the Mediterranean just towards southeast Spain on the Alboran Sea. It suffers both cold and mild winds from the Northwest which blow from North Atlantic in autumn and winter making them humid and cold, and hot Sirocco winds which blow from the North of the Sahara around mid-spring and throughout the very torrid summer, although the "Chergui" or east wind is very hot sometimes (Somot et al., 2007; Meddi et al.; 2010; Mariotti and Dell'Aquila, 2012; Lebid et al., 2015; BenkhamAllah et al., 2021). Nevertheless, these are physical conditions that have existed since at least five thousand years ago but have not increased temperatures, it is, rather, greenhouse gases that have enforced the very dry Azores anticyclone coming also from North Atlantic which sets in very quickly around mid-spring and sometimes lasts until mid-autumn now, preventing any moisture from reaching our region (Meddi et al., 2010; Lebid et al., 2015, Zeroual et al., 2017).

The study area is located in North West Algeria in the quite middle of the region called Oranie because its capital and largest city is Oran on the Mediterranean shore (Fig. 1). It is a region which is bordered to the north for more than 500 km by the Mediterranean Sea or more precisely by the eastern part of the Alboran Sea which constitutes the westernmost part of the Mediterranean Sea. It is the least salty part of the Mediterranean Sea because it mainly receives its waters from the North Atlantic through the Strait of Gibraltar. Waters which bathe the coast of Oranie are then less salty than those of other more eastern regions and they are stirred by the Algerian marine current which licks the coasts to go up towards the Balearic Islands in the middle of the Algerian-Provençal basin of the western Mediterranean.

The urban region of Sidi Bel Abbes, subject of our study, is formed by a quadrilateral which more precisely includes south of the Tell Atlas massifs of the Tessala Mountains and the western part of the Beni Chougran and the north of both the Dhaias and Saida Mountains.

It is thus a small plain of almost 50 km on each side with an average altitude of almost 450 to 500 m above sea level. It extends squarely from the southwest to the northeast. The southwest part of the plain is higher than the northeast part by more than 300 m which direct the main flow streams and Wadis.

The plain, at the foot of the Seba Chioukh mountains in the extreme southwest, has a height of nearly 640 m and extends gently towards the north west to descend approximately 300 m towards the north-eastern tip, discharging the waters of the Mekerra Wadi into the Sig dam overlooking the great Macta depression which flows into the Mediterranean Sea 50 km to the north in the greatest Algerian industrial area of Arzew bay.

Then, Sidi Bel Abbes, a city of nearly 300,000 inhabitants is located at 55 km as the crow flies south of the Mediterranean Sea north western Algeria just towards the Spaniard region of Andalucía. It is a relative modern city which extends over more than 90 km² in the middle of a depression of nearly 470 m altitude. It is surrounded by hills whose altitudes vary between 700 m west, nearly 1000 m north, 900 m east and more than 1300 m south (Fig. 2).

It is subject to various influences depending on the season, as in autumn and winter, humid winds from the Iceland depression blow in from the north-west or west bringing with them humidity, freshness, even freezing temperatures, while from the middle of the spring until early autumn it is rather the Azores anticyclone which moves eastwards and permanently affects our region with dry and hot southerly or easterly winds (Lebid et al., 2015; BenkhamAllah et al., 2021).

Most wet months are those of winter followed by those of Autumn while springs ones are dryer when the summer is almost totally arid. Then, region of Sidi Bel Abbes belongs to the "Csa" type climate according to the Köppen and Geiger classification, making it a region with a typical semi-arid Mediterranean climate with mild winters and hot summers, although for some years it was exceptionally classified in the sub-humid (Köttek et al., 2006).

The Sidi Bel Abbes region is thus characterized by a climate of the classic Mediterranean type of the semi-arid bioclimatic stage with a continental tendency with cold or even freezing and humid winters and hot or even scorching and particularly dry summers with short intermediate seasons.

The predominant winds are the west to northwest winds which bring humidity from the nearby North Atlantic and which arise, mainly, from the Icelandic depression which begins to activate at the very beginning of autumn. and which is sweeping across Europe and North Africa, after the Azores anticyclone has moved towards a more southwesterly sector.

Autumn, winter and to a lesser extent spring are quite cool and humid seasons. The low height of the Tessala mountains to the northwest of Sidi Bel Abbes is not sufficient to attenuate the strength of these Atlantic winds, while the real screen remains the Sierra Nevada in the south of Spain in Andalusia which literally bursts the clouds coming from the North Atlantic on this mountain range, hence the great majestic rivers of Andalusia. These clouds do not have time to rehumidify over the short distance of 140 km from the Alboran channel of the sea which separates south Spain from Oranie, hence the relative weakness of the precipitation which reaches the latter. This is how central and especially eastern region of Mediterranean Algeria receives more precipitation on average than our western region. Then, central Algeria known as Titteri and Kabylie receive nearly 700 mm on average while northern Constantine or the Aures mountains which peak at 2300 m receive precipitation which can reach 1000 mm on average per year because the westerly winds which tumbling from Spain have plenty of time to recharge by crossing nearly 600 to 1000 km of fairly warm sea where evaporation remains very strong even at certain times of winter compared to land which cools more quickly.

But, however, generally towards the end of July and the beginning of August, certain particularly violent stormy episodes can affect the region. These storms are generally from east but sometimes come up from the south, hence the fact that they carry with them red dust from the Sahara. Some years these storms even reach northern Europe, even the British Isles or even Scandinavia with characteristic red dust rains. Although these rains are beneficial because they bring a certain welcome humidity during these heatwave periods and they are also basic which helps neutralize the acidity of some soils, dust remains particularly disturbing because it penetrates everywhere due to its very low granularity and when it gets wet it forms a gluey and very sticky layer.

Episodes of intense precipitation in short durations of the "Cévenol" type begin at the very end of summer or the beginning of autumn and floods are the annual lot of practically the entire Mediterranean area of North Africa and our Oranie region is not spared. On the contrary, soils which are particularly stiffened by a burning summer sun become veritable slabs and rains, which are unable to penetrate these soils, rush down slopes in veritable torrents in a very short time. These soils absorb almost nothing at all and all the water surges quickly and floods the lower parts of the region carrying huge amounts of debris from the higher parts.

Then, climate of Sidi Bel Abbes is, like that of its region of Oranie, a semi-arid climate dependent on the climatic conditions which reign over most of the southern part of the Mediterranean with cold, rainy winters and hot summers. and dry, influenced by both continental and marine effects. The climate of this region is characterized by a strong contrast between summer and winter, especially for precipitation and the hydrological cycle on the surface. Complex climatic mechanisms operate there and involve the atmosphere, the North Atlantic Ocean, the Mediterranean Sea, Alpine glaciers, continental vegetation and aerosols of desert or anthropogenic origin (Laurent Li, 2003).

Data used are based on those recorded at the station located at the aerodrome one kilometer south-east of the city and qualified as a professional station within the World Meteorological Organization (WMO) under the identification number international 60520. This station is located at 35°10'26" N latitude and 00°35'36" W longitude and is 481 m above sea level.

Data have been gathered from the Algerian National Meteorological Office, both got temperatures amounts and rainfall quantities, and this from the year 1980 until the year 2022 in order to establish a cause to effect relationship to see, or not, these parameters evolution and variations related to time (ANMO, 2023). The result one would like to achieve is to know whether an increase in average temperatures would lead to an increase or rather a decrease in rainfall in our region over a period of more than forty years.

Fourier analysis using the FFT (Fast Fourier Transform) is a method that takes into consideration each block of four consecutive points to smooth the curves. The more this number of points increases, the less the smoothing is pronounced and the variations are more apparent.

One will report, firstly, the average temperatures recorded in the town of Sidi Bel Abbes, forty two years long, then one will draw a curve which will represent the evolution of these temperatures during this period to deduce the general tendency.

Secondly, one will operate in the same way with the precipitation reported during this same period of forty years, then after having examined the global propensity of this rainfall one will make a comparison between these two inclinations to see if correlation exists or not or if there exists a relationship which would be directly or inversely proportional between these two trends.

Our paper examined the evolution over time of average temperatures and then of precipitation. Corresponding curves relating the different recorded temperatures have been traced and confined in (Fig. 3) and (Fig. 4) for temperatures. Average temperatures have been reported in order to have a more detailed and more explicit view, and to see, after if there is a relation between them and rain. A calculation of the linear regression line then a second degree polynomial interpolation which remains largely explicit, have been conducted in order to obtain a more mathematical view of the phenomena encountered.

During the beginning of the period one notice that the evolution of temperatures is not stable, it is as if the trend hesitates to assert itself. Let's thus notice that temperatures oscillate greatly between decreasing and increasing trends. It is only during the middle of the period that these temperatures begin to stabilize and increase almost regularly. Towards the end of the period examined, temperature changes follow exactly the quadratic curve which tends to flatten out and the increase in temperature becomes less pronounced. One have the impression that it is a thermodynamic physical system which, after a certain transitional start-up period, has arrived at a stabilization plateau in which average temperatures would practically no longer increase and would follow a fairly regular plateau.

One could imagine that it could be that by following this trend, one reaches a maximum increase in average temperatures and the Earth would therefore be able to dissipate adequately the temperature received more efficiently than it did there over thirty or forty years ago. It appears, therefore, that a temperature close to 17°C seems to be an asymptotic temperature and that the average temperature in our region will probably not become higher than this value around 2036 according to our evaluation. It is obvious that this reasoning no longer applies if one only consider the linear regression line which would tend to increase to infinity, which would be totally utopian. Would this mean that the Earth will not warm more than that? The future will perhaps tell us if in the meantime humanity does not adopt more environmentally friendly behavior by trying as much as possible to make the supreme effort to reduce, just a little, its high greenhouse effect gas emissions.

Then, the first impression is that average temperatures have greatly increased from the beginning of the period to its end. One can see, at first glance, that there are certain time parts with different slopes of evolution, it is not a straight line, certain slopes are steeper than others and there is even a practically horizontal level, this which means that the evolutions were not linear, although one did not know exactly why, since there are several factors affecting this parameter due to the geographical location of our region.

One started with an average temperature of 14.25°C to end at 18.75°C, a phenomenal increase of more than 04.50°C in more than forty years. Nevertheless, this evolution was not made in a linear or rectilinear way but rather in the form of periods in which the increases succeeded the decreases but with an overall tendency towards the increase. However, one can divide this evolution into three main parts in which one have for the first part a wave or ramp evolution going from 14.25°C to nearly 17°C in 2000, i.e. more than twenty years with a slope of 0.14°C of irregular increase per year, to settle on a plateau and this until practically 2020, i.e. also about twenty years with a less pronounced slope of 0.038°C per year to end up finding a strong tendency to the increase over the last two years with a slope of 0.5°C per year.

This demonstrates the strong irregularity of the evolutions of the average temperatures with a clear tendency towards the heighten, which augurs of a true greenhouse effect leading inevitably towards a worsening of the global warming.

Nevertheless, it has been realized that when examining this figure, the trend is towards a reduction in the amount in the increase in temperatures and that the latter tend to want to reach a plateau or a horizontal asymptote which would describe a maximum potential temperatures that our atmospheric system can reach. This is a hypothesis that remains posed, because we do not know what the reaction of this atmosphere would be to a continued increase in average temperatures and will this increase continue when it reaches its maximum saturation with greenhouse gases .

Using the Fast Fourier transform with four smoothing points we can see that the trend is not regular, and that although hesitant at the beginning, then stable in the middle, the average temperatures tend to increase dramatically towards the end of the period, and especially from these last three years which have been particularly trying with strong heatwaves periods.

For precipitations data have been confined in (Fig. 5) and then processed as the same manner as temperatures.

From the examination of (Fig. 5), one can see at first sight that the evolution of precipitation is also absolutely irregular since there are quite pronounced ups and downs throughout the considered period. Here too, one could divide our overall time interval into periods that one would call rainy and others that seemed to be drier, and this over several years. The evolution, too, is not very regular although in the long run it has an upward trend like average temperatures. For this purpose, one can also divide this period into three periods, one of which therefore begins in 1980 and ends in 2008 with a remarkable average rainfall of around 325 mm to experience good growth between 2009 until around 2019 with an average of about 455 mm to suffer an unfortunate episode of drought in the last three years with a little over 200 mm as an average since 2020.

Nevertheless, by examining the linear curve of the rainfall averages, it can be noted a slight tendency to increase in these precipitations starting from an average of 325 mm to that of 375 mm, i.e. a net gain of nearly 50 mm per year, and this despite the remarkable last three years. In a first and strictly linear approach, one would think that although the climate is frankly tending to become warmer, there is also a significant increase in the average precipitation.

The average precipitation is 355 mm over the period extending from 1980 to 2022. Examination of Fig. 6 shows us that the variations in this precipitation compared to the average calculated over the period concerned are absolutely not regular. There is an alternation between "rainy" years and "dry" years compared to this average. However, there are periods, more or less long, in which trends assert themselves more or less strongly and for more or less time.

Between 1980 and 1986 there is a perfect alternation, then a succession of three dry years, followed by a wetter year to give way to a series of five particularly dry years to return to an almost alternation followed by around ten fairly dry years until 2009, which was a particularly wet year which would bring in its wake nearly a decade of particularly wet years. However, from 2019 one was witnessing a significantly severe drying with precipitation which is dangerously around the 200 mm isohyet which corresponds to the limit between a semi-arid zone and an arid zone. What is also remarkable is the fact that the number of "wet" years is equal to the number of "dry" years, always compared to the average, i.e. 21 years. However, between 1980 and 2001, the first half of the period, there were 9 wet years and 12 dry years, while during the second half of the period considered the wet years were 12. order of 12 while the dry ones are of the order of 9, i.e. a remarkable symmetry. Likewise, and surprisingly, the pivotal year of 2001 was a year that was just average. However, what remains to be deplored is that the last, most recent period is almost the driest.

The average precipitation between 1980 and 2000 is 322 mm, or 10% lower than the average, while that of the second half of the period concerned, i.e. from 2001 to 2022, is around 386 mm, or nearly 9% higher than the average for the entire period.

However, the curve of the quadratic variations clearly shows that there is an inflection in the average precipitation and that the climate tends to become warmer and less humid. Nevertheless, one think that this relative increase in precipitation would be due to the relative proximity of the ocean masses to the North Atlantic Ocean and to the fact of the increase in temperatures which induced an increase in the rate of evaporation, hence increased humidity and during the meeting of these cloudy masses with the foothills of Tessala which dominate the city of Sidi Bel Abbes to the north there is cooling and precipitation on the plain of the Mekerra and the first foothills of the Tellian Atlas during the relaxation of the cloudy masses. One note, therefore, that it is somehow difficult to affirm whether global warming is automatically accompanied by a decrease in precipitation, because in our case, although average temperatures have undeniably increased in recent years as shown in (Fig. 7), precipitation has also increased, making this assertion rather awkward.

Average temperatures and Pluviometry reported from 1980 to 2022 in Sidi Bel Abbes north-western Algeria close to the shore of the southwestern Mediterranean showed a true frank augmentation for temperatures when it is of a reduced but real increase for rainfall. Then, in a linear point of view and contrary to many other scientific results, the rainfall tends to increase, although this increase is small and difficult to explicate and clarify.

Nevertheless, this trend was not regular and linear, because quadratic study showed that rainfall evolved as temperatures which have increased significantly over these past four decades and after having growing significantly it has felt again to values close to the beginning ones. Also, the trend is towards a reduction in the amount of the increase in temperatures and that the latter tend to want to reach a plateau or a horizontal asymptote which would describe a maximum potential temperatures that our atmospheric system can reach. Then, the main conclusion is that climate of our region remained a semi-arid Mediterranean "Csa" climate with somehow rainy cold autumn and winter but extremely dry and hot summer.

Funding No funding was received for conducting this study.

Author's contribution. All authors have participated effectively in the elaboration of this article.

Conflict of interest. All the authors declare that they have no conflict of interest.

Ethical Approval. Not applicable .

Consent to Participate. Not applicable.

Consent for Publication. Not applicable.

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No competing interests reported.

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Temperature and Rain Variability in Southwestern Mediterranean Region, 1980-2022

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