4.1. The Variations in the SST and Temperature
The monthly long term mean of the SST from the Niño 3 and Niño 4 boxes are compared with the monthly long term mean temperature from Nigeria so as to establish the possible link between the mean states of the ENSO regions and Nigeria (
Figure 5). The SSTs are in good agreement with the temperature pattern from Nigeria; but better match is seen with the Niño 3 SST than with the Niño 4 SST. This is supported by the stronger correlations of 0.83, 0.54, 0.73, 0.58 and 0.74 between the Niño 3 SST and the respective temperatures from Abuja, Ikeja, Sokoto, Maiduguri and Port-Harcourt when compared with the corresponding values of −0.59, −0.78, 0.31, 0.54 and −0.67 from the Niño 4 box. The correlations indicate that a rising Niño 3 SST will increase the temperature over Nigeria while negative correlation from Niño 4 box suggests that the warming reduces in Nigeria as the SST intensifies in the Niño 4 region; these are attributes of La Niña event. The magnitudes of the Niño 3 SSTs are lower than those recorded within Nigeria since the temperatures were measured over land while the high values in Niño 4 region were estimated over the warm pool. The observed patterns are also in agreement with the Niño 3 and Niño 4 SSTs obtained in the pre-industrial simulation from Kiel Climate Model [
3]. Thus, the agreement with the SST shows that there is good link between the mean climate in Nigeria and that from the ENSO regions.
Furthermore, the results indicate that the mean temperature is generally low in January and August in all the selected areas but the highest temperature is obtained in the North with the peak values diminishing southward, as depicted in
Figure 5. The temperatures in December–February are slightly higher in the central-southern locations compared to the North, while the situation is reversed in August. The temperature intensifies as the Sun crosses the Equator around March and moves northward, leading to the peak intensity in March from the southern regions which shift to April–May in the North with increased magnitudes which have the highest value in Sokoto. The secondary peak in October–November occurred as the Sun returns southward while, the lowest temperature is seen between July and September in southern Nigeria under the moisture bearing cool wind from Atlantic Ocean.
Figure 5.
The comparisons between long-term monthly mean temperature and SST (°C) averaged over 1980–2010 (1983–2010 for Abuja). The color codes are as follows: Niño 3 (blue), Niño 4 (red), Ikeja (purple), Port-Harcourt (brown), Abuja (green), Sokoto (orange) and Maiduguri (black).
Figure 5.
The comparisons between long-term monthly mean temperature and SST (°C) averaged over 1980–2010 (1983–2010 for Abuja). The color codes are as follows: Niño 3 (blue), Niño 4 (red), Ikeja (purple), Port-Harcourt (brown), Abuja (green), Sokoto (orange) and Maiduguri (black).
4.2. Comparisons of the Temperature and SST Anomalies
Investigation using anomalies of the SST and temperature is done so as to be able to compare the climate variations from the study area with ENSO events. The anomalies are obtained by subtracting the mean values, averaged over 1980–2010 (1983–2010 for Abuja) from the monthly records; the results are shown in
Figure 6. According to the US (United States) NOAA definition, an El Niño (La Niña) event occurs when consecutive three months running mean of the Niño 3 SST anomaly, SSTA, is above +0.5 (below −0.5), while some other reports put it at seven months long warming (cooling) of the Niño 3 region [
24]. In this work, the peaks of the temperature anomalies from the different locations are in good agreement with the SSTA from the Niño 3 and Niño 4 regions; however, the matches are better with the Niño 3 SSTA than with the Niño 4 SSTA, which has the peak value around May–June (
Figure 6). The peaks of the temperature anomalies are seen in February–March from the South-Central Nigeria, while it occurred in April in the North. The temperature anomalies are low and negative in August, followed by small positive peaks in October (November) from the North (South) with the exception of Ikeja, which stays negative from June till December. In addition, negative anomalies are seen between November and February in the northern regions.
Figure 6.
The anomalies of the monthly mean temperature averaged over 1980–2010 (1983–2010 for Abuja). Color codes are the same as in
Figure 5.
Figure 6.
The anomalies of the monthly mean temperature averaged over 1980–2010 (1983–2010 for Abuja). Color codes are the same as in
Figure 5.
4.3. Comparison of the Results with ENSO Events
The comparison of the mean temperature and the mean rainfall from the study areas in Nigeria with ENSO events is done to further establish the links between ENSO characteristics and the climate patterns in the country. Several ENSO events were considered in pairs to understand the changes in the temperature (T, °C) and the corresponding Rainfall (R, mm) during El Niño and La Nina episodes (
Figure 7).
Figure 7.
The bar charts of the (a) monthly mean temperature (T, °C) and (b) Rainfall (R, mm/day) during some El Niño and La Niña events. The values covered 1980–2010 (1983–2010 for Abuja). ENT, LNT are El Niño, La Niña Temperatures respectively, while ENR, LNR are the respective El Niño, La Niña Rainfall values. Names of the study areas are shown in short forms on X-axis.
Figure 7.
The bar charts of the (a) monthly mean temperature (T, °C) and (b) Rainfall (R, mm/day) during some El Niño and La Niña events. The values covered 1980–2010 (1983–2010 for Abuja). ENT, LNT are El Niño, La Niña Temperatures respectively, while ENR, LNR are the respective El Niño, La Niña Rainfall values. Names of the study areas are shown in short forms on X-axis.
The warm events that started in 1982, 1986, 1997, 2002 and 2009 together with the La Niña episodes from 1984, 1988, 1995, 1999 and 2007 were considered along with the years following the beginning of the events. Under temperature, the figures show that an El Niño occurrence leads to high temperature in the year following the event while the temperature is reduced in the year after a La Niña condition (
Figure 7). Further investigation indicates that an El Niño phase will lead to high temperature in the following spring and summer months after the event while the spring and summer temperatures are reduced after a La Niña condition (not shown). Therefore, the strong El Niño condition of 1997/1998 might explain the rise in the overall mean temperature of 1998, which is also higher in all the study locations than the values recorded in 1997, during which the warm event was at the peak (
Figure 7). Conversely, the 1988/1989 strong La Niña event is followed by reduction in the mean temperature in 1989 over Nigeria compared to 1988. Considering the rainfall, the intensities of the mean rainfall are generally lower in an El Niño years than the magnitudes in the year after a strong event while the rainfall becomes more during a La Niña occurrence but the strength reduces after the cold event.
The overall results indicate that the mean temperature over the entire country is higher (lower) after an El Niño (a La Niña) year compared to the magnitude during a strong El Niño (La Niña) event, while the rainfall is stronger during a La Niña case than in the El Niño period. The impacts on the temperature and rainfall rise with the strength of both events. However, there are some exceptions to the above conclusions that might be due to missing or insufficient data in some years from the study locations. The rainfall patterns also suggest that the peak period, December–February, of an El Niño (La Niña) condition is associated with an overall decrease (increase) in the mean precipitation over Nigeria (not shown). In an El Niño year, the reduction in the mean precipitation is strongest in the northern Nigeria when compared to the South. The low magnitude of the rainfall over the country is due to the ITCZ position which shifts more slightly to the South (beyond the mean southern extreme) at the time of an El Niño. Investigation with some ENSO events suggests that the rainfall pattern in El Niño years show good agreement with the southward shift in the ITCZ location—as indicated by the region with the peak rainfall (pink-purple-blue color) (
Figure 8a–c for 1982/1983, 1986/1987, and 2002/2003, respectively). However, less drought is expected as the intensity of the rainfall rises over the country in a La Niña year due to the northward spread of the rainfall along with the regions of the strongest precipitation which represent the ITCZ location (
Figure 8d–f for 1984/1985, 1995/1996, and 2007/2008, respectively). Further study using the rainfall patterns over the Tropical Pacific suggests that the above changes in the ITCZ position and the rainfall in Nigeria is caused by southward (northward) movement of the ITCZ location over the Tropical Pacific during an El Niño (La Niña) episode (
Figure 8g–i for El Niño;
Figure 8j–l for La Niña).
Figure 8.
El Niño Episodes and La Niña Episodes: The rainfall patterns during El Niño episodes of (
a) 1982/1983, (
b) 1986/1987, and (
c) 2002/2003; and La Niña cases of (
d) 1984/1985, (
e) 1995/1996, and (
f) 2007/2008; and (
g–
l) the respective rainfall patterns over the Tropical Pacific (both sets of maps are on the same scale). (source:
www.esrl.noaa.gov/psd/cgi-bin/data/composites/printpage.pl).
Figure 8.
El Niño Episodes and La Niña Episodes: The rainfall patterns during El Niño episodes of (
a) 1982/1983, (
b) 1986/1987, and (
c) 2002/2003; and La Niña cases of (
d) 1984/1985, (
e) 1995/1996, and (
f) 2007/2008; and (
g–
l) the respective rainfall patterns over the Tropical Pacific (both sets of maps are on the same scale). (source:
www.esrl.noaa.gov/psd/cgi-bin/data/composites/printpage.pl).
In addition, the comparison between the El Niño events (
Figure 8g–i) and the La Niña cases (
Figure 8j–l) show southward spread of the ITCZ location and the associated rainfall during an El Niño episodes when compared with the La Niña years during which the ITCZ width becomes thin and stays in the far North. Hence, the increase in the rainfall above the background mean during a La Niña year is strongest in the southern Nigeria and reduces northward. The highest values are observed in Port-Harcourt followed by Ikeja, Lagos (South) with slight increase in Abuja (central) while almost none is seen in the North. The rainfall patterns also have broader bases and maintained moderate levels in all locations in Nigeria during La Niña events (e.g., 1988 and 1999) compared to El Niño periods (1997) (not shown), thus suggesting that the rain starts early during La Niña year and spreads over several months of the year, especially in the South. Though the least rainfall is recorded in December–January across Nigeria, the small increase in the precipitation within the southern-central regions at the time of La Niña, supports a slight northward shift of the mean ITCZ position during the period when compared to neutral condition. The overall results suggest that the magnitude of the average rainfall in Nigeria is low (high) during an El Niño (a La Niña) event (
Figure 7).