Figure 1 shows the topography (m) of West Africa, the Atlantic Ocean and some African continents. The rectangle indicates the study area of the northern region of Senegal. On the figure, we can see that our study area is between 15°N-17°N and 17°W-13°W. Our study area is bordered to the west by the Atlantic Ocean, this justifies the study of Sea Level Pressure (SLP). To understand the different levels of sea level pressure used in the literature Fig. 2 is shown.
Figure 2 shows the average global surface pressure. The study area, which is located in the north part of West Africa, is surrounded by two anticyclones, between 15°N- 60°N and 90°W- 05°W (1010hPa) and 40°S-10°S and 30°W-0°E (1020hPa). Puebla and Nieto (2009) explained that the changes in precipitation trends considering the regression between the Pressure at Sea Level (SLP) defined between the area from 20°N-80°N and 60°W-30°E over the Euro-Atlantic area and precipitation. Barros et al. (2008) studied sea level pressure between 20°-45°S and 70°-45°W. Busuioc et al. (2001) chose the area between 40°W-40°E and 40°–70°N for their research. Kim et al. (2004) have chosen SLP as their field of study at (15°N–70°N, 60°E–150°W). The importance of this figure allows us to show that our study area, northern Senegal, is surrounded by two high sea level pressures, namely the Azores High and the St. Helena High. Since our study focuses on the influence of these two high pressure areas, this figure allows us to locate the coordinates of these high-pressure areas in relation to our zone.
As the study is based on the influence of sea level pressure on interannual variability. It is interesting to see how surface pressure varies in different months.
Figure 3 shows the monthly average of Sea Level Pressure. It is observed that SLP, varying between 1006–1016 hPa over 20°S and 20°N and 1018–1022 hPa beyond 20°N and below 20°S. The southerly high pressure of 1022hPa or greater approached 20°S and sea level longitude (located between the continents of Africa and America) from June to October. This period corresponds to the rain belt defined by Nicholson and Grist (2003). While the northern one approaches the 20°N zone and the sea longitude (located between the two continents Africa and America) from June to July. The high-pressure system in the south becomes lower than 1022 hPa from August to October. This figure of the mean monthly pressure indicates the relationship of the Sea Level Pressure with the seasonal cycle of precipitation in our study area. Crespo et al (2019) show that the pressure adjustment mechanism is the main driver of the convergence of meridional surface winds in the eastern tropical Atlantic.
The deficit of rainfall was associated with an increase in the subsidence of the air and the surplus with a more intense monsoon circulation. It is necessary to look at the wind characteristics at 700 hPa and 200 hPa. The wind at these two pressure levels plays an important role on the interannual variability of rainfall. They correspond to the position of the African Easterly Jet (AEJ) and the Tropical Easterly Jet (TEJ). In Fig. 3, it can be seen that the SLP to the north and the south accompanies the rainfall belt. It is necessary to represent the Sea Level Pressure in JJAS.
Figure 4a shows the 700 hPa average wind (JJAS) and GPCP precipitation (JJAS) while Fig. 4b shows the divergence of the wind (JJAS) at 700 hPa (vectors) and the Sea Level Pressure (JJAS). The mean wind (JJAS) at 200 hPa (vectors) and precipitation from GPCP (JJAS), the wind divergence (JJAS) at 200 hPa (vectors) and the Sea Level Pressure (JJAS) are shown in Figs. 4c and 4d, respectively. Maximum precipitation accompanies the mean east wind of the northern hemisphere, which corresponds to the African Easterly Jet (JEA) (700 hPa) and the Tropical Easterly Jet (JET) (200 hPa). The high pressure located between 25°N-40°N and 50°W-20°W is accompanied by the wind subsidence at 700 hPa and 200 hPa. Monerie and al. (2012; 2013) have shown that the forecast rainfall deficits towards the west are associated with an increase in air subsidence and the surplus with a more intense monsoon circulation and a reinforcement of the Africa Easterly Jet and anomalies in the zonal circulation between the Indian and Atlantic oceans, favouring the subsidence of the air and the transfer of humidity outside the region. The wind convergence is observed at 200 hPa over (15°N-17°N; 17°W-12°W). Janicot and Fontaine (1993) established a link between decreases in the altitude divergence in the upper troposphere above West Africa linked to a strengthening of the Walker-type atmospheric cell above the Atlantic, and the occurrence of droughts affecting the whole of West Africa.
Figures 4e and 4f show the rotational wind at 700 hPa and at 200 hPa. It can be seen that the sea surface pressure (greater than or equal to 1015 hPa) is located between two zones (10°S-40°S and 40°W-0°E, and 15°N-40°N and 50°W-25°W). It is found that the rotational wind at 700 hPa is from north east, i.e., of negative sign. This weakness is synonymous with a minimum of vortex. Sultan (2011) has shown that a maximum of relative vortex marks the increase in cyclonic circulation linked to the continental thermal depression. The figure illustrates the ITCZ defined as the zone into which the converging fluxes from the subtropical anticyclones move: The Saint Helena high (southern hemisphere), and the Azores and Saharo-Lybian (northern hemisphere) high. The ITCZ corresponds to the low-pressure area between the subtropical high-pressure belts (Leroux, 2001). We found that there is probably a relationship between Sea Level Pressure and precipitation. So, we are going to try to study statically the relationship between these two variables.
Figure 5 shows the correlation between SLP and JJAS precipitation between 5°S-20°N and 20°W-0°W. There is a negative correlation over 15°N-17°N and 17°W-13°W. This figure also indicates that, in the study area, SLP is related to precipitation with the opposite sign (i.e. higher values of sea surface pressure in the north are accompanied by a decrease in precipitation). Janicot and Fontiane (1993) showed that a deficit season in the Sahel occurs when there is a “dipolar” structure of surface pressure (higher values in the North Atlantic, lower values in the South Atlantic). Monerie and Roucou (2015) have shown that climate change in the West African monsoon region in autumn is due to increased pressure with abnormally high-pressure circulation over Europe.
The two levels of sea pressure were identified in Fig. 4 (10°S-40°S and 40°W-0°E, and 15°N-40°N and 50°W-25°W). The PCs of these SLP are shown in Figs. 6, 7, 8, and 9.
Figure 6 shows the first three PCs of the SLP data between 15°N-40°N and 50°W-25°W during summer (JAS) from 1979 to 2003. PC1 is in a negative phase for the years 1979–1984, 90–94, 1996–1997, and 2002. These components account for 62.2% of the total variance. The PC2 of SLP data was generally in its positive phase for the years 1981,1983, 1986-87, 1991-94, 1997 and 2002 while the PC3 of SLP data was in its negative phase between the years 1979-81, 1984, 1987-88, 1990, 1993, 1995-98 and 2000 and in its positive phase between 1982-83, 1985-86, 1989, 1991-92, 1994, 1999 and 2001–2003. The years 1982–1983 corresponded to years of drought in our study area, Surraud (1954) showed that any strengthening or displacement towards the south of the Azores anticyclone causes a southward shift of the ITCZ. Zhang et Hook, 2014 showed that an early (late) demise is associated with an anomalously strong (weak) North Atlantic subtropical high, which extends over the Mediterranean and the Sahara throughout the demise period. PC2 and PC3 respectively represent approximately 24.2% and 6.5% of the total variance. The first three corresponding EOFs are shown in Fig. 7.
Figure 6 shows the temporal component while Fig. 7 shows the spatial component. PC1 is negative over the entire area representing 62.2%. PC2 is negative between 28°N-40°N and positive between 15°N-28°N showing a north-south dipole. PC3 is positive between 50°W-25°W and 15°N-24°N. Our study area is located in this zone. According to Fig. 6, PC3 was in its positive 1982–1983; this period corresponds to a period of drought in our study zone. The SLP zone between 15°N-40°N and 50°W-25°W allows us to see its influence on our study area located from 15°N onwards. We have seen in Fig. 2, that the North High is on 15°N-40°N and 50°W-25°W.
Figure 8 shows the first three PCs of Sea Level Pressure (SLP) data between 40°S-10°S and 40°W-0°E during summer (JAS) from 1979 to 2003. PC1 is in a positive phase for the years 1979, 1988, 1990–1992, 1994–1995, 1995 and 2000–2001. It can be noted that the positive phase 2000–2001 corresponds to the return to normal rainfall over this region. These components represent 69.4% of the total variance. The PC2 of the SLP data was generally in its positive phase for the years 1981, 1983–1986, 1988, 1991–1992, 1995–1996, 1999 and 2001–2003, while the PC3 of the SLP data was in its positive phase between the years 1980–1983, 1991–1995, 1997 and 1999–2003 and in its negative phase between 1979, 1984–1990, 1996 and 1998. PC2 and PC3 account for about 12.5% and 8.9% of the total variance, respectively. The first three corresponding EOFs are shown in Fig. 9.
Figure 9 shows the temporal component while the Fig. 10 shows the spatial component. The PC1 is positive or null over the whole area representing 69.4%. The PC2 is negative between 26°S-40°S. PC3 is positive between 30°S-10°S and 40°W-0°E. We will now see the wavelet of the two SLP and precipitation in our study area. Figure 10 shows the average spectrum of wavelets over the period 1979 to 2003 of the SLP data between 15°N-40°N and 50°W-25°W during July August and September 1979–2003. There are peaks in the 4-year band in 1996–1999; oscillations of more than eight years in 1983–1990 and indicate intervals of high variance over the entire study period.
The average wavelet spectrum over the period 1979 to 2003 of the SLP data between 40°S-10°S and 40°W-0°E during July August and September 1979–2003 is shown in Fig. 11. A 4-year variability is observed over the period 1989–1993. The figure shows an interval of high variance between 1979–1996 and low variance between 1997–2003. Chiang and Vimont (2004) suggested that the Pacific and Atlantic modes are analogous, governed by physics intrinsic to the ITCZ/cold tongue complex. This explains the mode of variability of more than 4 years.
Figure 12 shows the average wavelet spectrum over the period 1979 to 2003 of the GPCP precipitation data between 15°N -17°N and 17°W-13°W during July August and September for 1979–2003. There are fluctuations of 1 to 2 years from 1994–1995 and peaks of 8 years from 1984–1991. Interval of high variance (1984–2003) and low variance (1980–1984) are also noted in the figure. We can conclude that the eight-years oscillations observed in 1984–1991 on the Precipitation data are in phase with the same duration’s oscillations with the SLP data of the northern anticyclones. Fontaine et al. (1998) showed that a warm phase of Enoa type tends to move Walker cell eastward, which causes an abnormal upward transport over the Pacific and an abnormal subsident transport over the peri-Atlantic space. The variability of more than 8 years is due to the EL NINO phenomenon which is in phase with the North and South high (Chiang and Vimont, 2004; Fontaine 1998). Chang et al. (2000) showed that there is a significant remote influence of the Pacific ENSO on the variability of the tropical Atlantic.