CHANGES IN NEAR-SURFACE WIND SPEED IN THE SOUTH OF BENIN FROM 1961 TO 2016

Houngninou B. Etienne 1 , Allé C. S. Ulrich 2 , Guédjé K. François 1 and Kougbéagbédè Hilaire 1 . 1. Département de Physique, Faculté des Sciences et Techniques, Université d’Abomey-Calavi. 2. Chaire Internationale en Physique Mathématique et Applications (CIPMA Chaire UNESCO), Faculté des Sciences et Techniques, Université d’Abomey-Calavi. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History


…………………………………………………………………………………………………….... Introduction:-
In the context of the current climate change, recent studies focused on the trends of temperatures and precipitations instead of changes in near-surface wind speed (Shenbin et al., 2006;Pryor et al., 2009). This fact has particularly been true in West Africa, partly because of the importance of precipitations for the economy and food safety in that region (Barbier et al. 2009;Sultan et al., 2012). However, changes in near-surface wind speed could have some important socio-economic and environmental consequences (Fallot, 2008;Guo et al., 2011). Zhou et al. (2006) have demonstrated that the decrease of the wind speed has caused the reduction in wind energy supply in the delta of Pearl River in China. The decrease of the near-surface wind speed would be the first factor responsible for the decrease of atmospheric evaporative demand on the Thibetan plateau (Shenbin et al., 2011) and in Australia (Roderick et al., 2007). Therefore, understanding changes in wind speed can help us understand climate changes better and its environmental, ecological and socio-economic, and impacts (Guo et al., 2011). Several studies have analyzed the trends of changes in near-surface wind speeds over these last 50 years in several regions of the world mainly in Europe (Brazdil et al., 2009;Pirazzoli et Tomasin, 2003;Smits et al., 2005), Asia (Guo et al., 2011;Zhang et al., 2007), America (Pryor et al., 2009;Tuller, 2004), Australia (McVicar et al., 2008, Roderick et al., 2007, Antartic (Aristidi et al., 2005;Turner et al., 2005) and Alaska (Lynch et al., 2004). For West Africa, less attention has been given to long period changes of the near-surface wind speed. Studies on near-surface wind speed mainly focused on lithometeors issues (Kalu, 1979;McTainsh, 1980;Chamard and Courel, 1988;Ozer, 1997). This study aims to document changes in near-surface wind speed in South of Benin from the anemometric data of the synoptic station in Cotonou (Benin).

ISSN: 2320-5407
Int. J. Adv. Res. 5 (11), 1223-1232 1224 Data and methods:-Study Area and Data used Benin is located in West Africa in the tropical area between 6˚30´ and 12˚30´ North latitude and 1˚ and 3˚40´ East longitude (Fig 1). Two masses of air, the monsoon and the harmattan, traverse Benin. Each becomes dominant depending on periods of the year (Gbaguidi et al., 2011). The monsoon is higher during the rainy seasons (March-November). The harmattan is higher in the dry season (November-February). The average daily speed is roughly 4 ms -1 (Allé et al., 2013). Benin has a less hilly terrain particularly in the South with a vegetation destroyed by the anthropic activities (Gbaguidi et al., 2011).
The anemometric data used are the maximum daily near-surface wind speed, which were measured at 10 m above the ground. They derive from the synoptic station in Cotonou (06˚21´N; 02˚23´E), which is installed in the South of Benin on the Atlantic coast since 1952. However, it is only from 1961 that the frequent recordings of the wind speed started. That is the reason why the period from 1961 to 2016 has been chosen to undertake this study. It is considered as the possible longest period of recordings of the near-surface wind speed in Benin. These data show fewer gaps from 1961 to 2016 (less than 10%). The gaps have been considered as missing data. Next, Pettitt test (Pettitt, 1979) has been implemented in a recurrent way in order to identify a change in probability's law of the mean annual wind speed (breakage). Finally, the use of Mann-Whitney´s test (Mann and Whitney, 1947) allowed us to compare the different sub-periods.

Changing in wind speed on a monthly scale:-
The analysis of the change in wind speed has been refined at monthly scale. In this respect, a linear regression has been done every month between the mean monthly wind speed and the years for each sub-period. The slopes of the linear regressions were used to verify the persistence at the monthly scale of the decennial trends.
Change in the wind category occurrence:-Some wind speed categories have been made (Table 1) based on the Beaufort scale (Hirsch et al., 2016). For each sub-period, the occurrence of a category used corresponds to the ratio between the number of this category and the total number of observations made over the sub-period. The comparison of the occurrences between the sub-periods informs us on the changes in category occurrences. Results:-

Average features of the maximum daily near-surface wind speed in the South of Benin
The analysis of the distribution of the maximum daily near-surface wind speed shows that 90% of these winds are located between the interval ]4 ms -1 , 12 ms -1 ] (Fig 2), with an mean annual of 7.8 ± 1.2 ms -1 . They are higher from March to October with a landing at 8.5ms -1 (Fig 3).  These results also show that the sub-periods P1 and P2 are not significantly different in terms of wind speeds (pvalue > 0.05). However, the comparative analysis of the sub-periods trends P1 and P2 clearly shows that these two sub-periods show reverse trends (Fig 5). In fact, the sub-period P1 is characterized by a progressive decrease of the winds (slope = -0.15 ms -1 a -1 ) that continued during the sub-period P2, but with a lower intensity (slope = -0.03 ms -1 a -1 ). At the reverse, the sub-period P3 is characterized by a neat increase of the wind speeds with a slope inversely closer to that of P1 (slope = 0.11 ms -1 a -1 ).     Years 1961-1977 2009-2016 1978-2008 1228 Change in the monthly scale of the wind speed: -Fig 6 sums up the linear regression slopes between the mean monthly of the maximum daily near-surface wind speed and the years. After analyzing this Fig, it appears that all the months have been affected by the decrease of the winds (negative slopes) during the sub-period P1 and P2 in various proportions. The months between May and August are the months that have recorded the highest decreases (slope from -0.18 to -0.21 ms -1 a -1 ) during the subperiod P1. During the sub-period P2, this decrease of the winds has considerably slowed down on overall months as shown the quasi-null slopes observed. The sub-period P3 is characterized by an important increase of the winds over January to July except for February, whereas the decreasing trend is still persistent over the second mid of the year (August to December). The increase of wind speeds observed is observed between P2 and P3 on the yearly scale is due to the intensification of winds over the first mid half of the year, except for February. shows that the occurrence of winds of types "Fresh breeze" to "Near gale" has decreased during the sub-period P2 compared to the sub-periods P1 and P3. In fact, winds of types "Fresh breeze ", "Strong breeze" and "Near gale" have respectively decreased from 34 to 9%, from 17 to 3% and from 5 to 1% between P1 and P2. During the subperiod P3, these winds have made a revival and increased to 45%, 22% and 5% exceeding their historical level of P1 (1961)(1962)(1963)(1964)(1965)(1966)(1967)(1968)(1969)(1970)(1971)(1972)(1973)(1974)(1975)(1976)(1977). In the opposite, the frequency of the winds of types "Gentle breeze" and "Moderate breeze" has increased during the sub-period P2 compared to sub-periods P1 and P3. Their occurrences were respectively 28% and 57% during sub-period P2 against 5% and 37% during P1 and 4% and 21% during P3. The overlapping of these variations with decennial trends of the daily wind speeds shows that the decrease of the winds between P1 and P2 and their revival between P2 and P3 are respectively due to the decrease and the increase in the occurrence of strong winds (Fresh breeze, Strong breeze and Near gale).