Observed trends in diurnal temperature range over Nigeria

DIKEVictor Nnmi,LIN Zhohui,WANGYuxi n NNAMCHIHycinth

aInternational Center for Climate and Environment Sciences,Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing,China;

bCollege of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing,China;

cEnergy,Climate,and Environment Science Group,Imo State Polytechnic Umuagwo,Ohaji,Nigeria;

dGEOMARHelmholtz Centre for Ocean Research Kiel,Kiel,Germany

ABSTRACT The long-term trend of diurnal temperature range(DTR)over Nigeria was examined using daily station-based datasets for the period 1971-2013.The results show that the regionally averaged DTRhas decreased signi f i cantly(-0.34°Cper decade)over the Nigerian Sahel(north of 10°N),but there has been a slight increasing trend(0.01°Cper decade)over the Nigerian Guinea Coast.The annual decreasing trend of DTRin the Nigerian Sahel is mainly attributable to the signi f i cant increasing trend in daily minimum temperature(T min,0.51°Cper decade),which far outstrips the rate of increase in the daily maximum(T max,0.17°C per decade).In contrast,the comparable trends in T min(0.19°Cper decade)and T max(0.20°Cper decade)may explain the non-signi f i cant trend of the DTR averaged over the Guinea Coast region.It is observed that the DTR has decreased more in boreal summer(June-July-August)than in boreal winter(December-January-February)for the regions.Furthermore,it is found that the signi f i cant DTR declining trend over the Nigerian Sahel is closely associated with an increasing trend of annual and summer precipitation in the region,but the increasing DTRtrend in the Nigerian Guinea Coast region can be attributed to the decreasing trend of cloud cover over the region.

KEYWORDS Diurnal temperature range;maximum/minimum temperature;trend;Nigeria

1.Introduction

There are growing concerns on the decreasing trend of diurnal temperature range(DTR)over many land areas since 1950,which ismainly resulting from alarger increase in minimum air temperature(Tmin)relative to that of maximum air temperature(Tmax),especially in drier regions(IPCC 2007;Zhou et al.2009).The DTRis de f i ned as the difference between daily Tmaxand Tmin,it embodies the asymmetric variation in the diurnal changes of these surfaceair temperatures,and it isconsidered asan indicator of climate changes because of its sensitivity to radiative energy balance(Karl,Karoly,and Arblaster 2004).Longterm trends in DTRhave been investigated for different locations(Ding,Wang,and Lu 2018;Wang et al.2014),and it is found that DTRhas decreased over many land areas(Ding,Wang,and Lu 2018;You et al.2016).Further results suggest that thisdeclinemay beattributableto theincreasing effectsof precipitation and cloud cover(Dai,Trenberth,and Karl 1999),as precipitation can reduce the DTR by dampening Tmaxthrough daytime evaporative cooling effects,and this effect could be larger in dry regions than in humid regions.On theother hand,cloudscan also affect the DTRby reducing incident shortwave solar radiation to the Earth's surface during the day,which also dampens Tmaxand increases Tminthrough itsability to intercept outgoing longwave radiation(Dai,Trenberth,and Karl 1999;Zhou et al.2009;Shen et al.2014).However,the long-term trend of precipitation and cloud amount varies regionally,and thiswill lead to diverse DTRtrendsin different regions due to the combined impact of these factors(Zhou et al.2009).For instance,Shen,Liu,and Lu(2017)observed a decrease in DTRin some climate regionsbut an increase in some others,and thisunderscoresthe need to evaluate DTR trends in different climatic regions,and further to investigate the associated in f l uencing factors(Yang and Ren 2017).

As an important index of climate change,the DTR provides more information on climate change than the mean temperature(Karl,Karoly,and Arblaster 2004;Shen,Liu,and Lu 2017).Although the increasing trend of temperature extremes over Nigeria has been investigated by many previous studies(e.g.Abdulsalami2015;Abatan et al.2016),the DTR trend has not received considerable attention in Nigeria.As suggested in Dai,Trenberth,and Karl(1999),the precipitation effect on DTR could be larger in dry regions than in humid regions.Thus,it is interesting to examine the trends of DTRin the dry and humid zones of Nigeria,i.e.the Nigerian Sahel and Nigerian Guinea Coast,using station observations,and further to reveal the possible regional difference in DTR trends within the country,and the underlying factors that may be responsible for the regional differences.

In this paper,using a daily dataset from 16 meteorological stations for the period 1971-2013,we analyze the DTR trends in the aforementioned two climate regions of Nigeria.Furthermore,we make an attempt to investigate the possible factors responsible for the regional differences in DTR trends over Nigeria.The study area,data,and methods are described in section 2.The results,along with some discussion,comprise section 3,followed by concluding remarks in section 4.

2.Study area,data,and methods

2.1.Study area and data

Nigeria islocated in the West African region between the latitudes of 4°N and 14°N and the longitudes of 2°Eand 15°Eand has a total area of approximately 925796 km2(Figure 1(a)).The climate is dominated by West African monsoon circulation;the prevailing winds transport moisture from the Gulf of Guinea in the low levelsof the atmosphere.There are two major climatic regions in Nigeria:the Nigerian Sahel and Guinea Coast.These regions are characterized by variant climatic patterns(Nnamchiand Li2011).In the Guinea Coast region,temperatures rarely exceed 32°C,but the humidity is very high and the nights are warm;whereas,in the Nigerian Sahel,midday temperatures rise above 36°Cin summer,but with relatively cool nights,with the minimum temperaturedropping aslow as19°Cduring the dry season.In thisstudy,the DTRtrend wasinvestigated for the abovementioned two regions of Nigeria.The Nigerian Sahel is delineated as(10°-15°N,3.0°-15°W),while the Guinea Coast is delineated as(4°-10°N,2.5°-15°W)(Figure 1(a)).

Thedataused in theanalysiscomprise dailyrainfall,Tmin,and Tmax,spanning 43 years (January 1971 to December 2013).These data were acquired from 16 synoptic weather stations of the Nigerian Meteorological Agency,and the datasets have been employed for many studies(e.g.Abatan et al.2016).As shown in Figure 1(a),there are four stations(Gusau,Maiduguri,Bauchi,and Gombe)in the Nigerian Sahel region,and the other 12 stations are located in the Guinea Coast region.Monthly meansof total cloud cover(TCC)from ERA-Interim are also used in the analysis These data have a spatial resolution of 0.25°and cover the region(89.5°S-89.5°N,0°-359.3°E)(Dee et al.2011).

2.2.Methods

The temperature data were subjected to quality control and homogeneity assessment before being used in this study.DTR index was selected from the core indices recommended by CCl/CLIVAR/JCOMM ETCCDI(http://etccdi.paci f i cclimate.org)(Peterson 2005;Karl,Nicholls,and Ghazi 1999),which is de f i ned as the difference between Tmaxand Tmin(Peterson 2005).The inherent trends were computed using the Theil-Sen estimator(Theil 1950;Sen 1968)and tested for statistical signi f icance based on the Mann-Kendall rank test(Mann 1945;Kendall 1975).A trend was considered to be statistically signi f i cant at p＜0.05 and p<0.1.This method is widely accepted for its robustness in examining trends in climate data(e.g.Shen et al.2014;Abatan et al.2016).The correlation coefficients were tested for signi f i cance based on the Student's t-test.

3.Results and discussion

3.1.Spatial difference in DTRtrends over Nigeria

In thissection,wepresent thespatialdifferencein the DTR trends over different climate regions in Nigeria.Figure 1(b-f)show theannualand seasonal-averaged DTRtrend at 16 meteorological stations over Nigeria during 1971-2013.It is found that the annual-averaged DTRhas decreased signi f icantly in the Nigerian Sahel and increased marginally over the Guinea Coast region.A closer look at the trends at the stations suggests that,in the Nigerian Sahel,the DTRhasdecreased signi f ciantly at threeout of four stationsover theregion.Thus,the DTR has decreased signi f icantly at a rate of-0.84°C,-0.32°C and-0.15°Cper decade at Gombe,Bauchiand Maiduguri respectively,all of which are statistically signi f ciant at the 95%con f idence level.Meanwhile,thedecreasing trend at Gusau(-0.15°C)isnot statistically signi f icant.Relatively,in the Guinea Coast region,the DTRhasdecreased at six out of twelve stations.The declines at Ikeja(-0.19°C)and Lokoja(-0.17°C)are statistically signi f ciant at the 95%con f idence level,while the declines at Ibadan(-0.03°C),Ijebu(-0.06°C),Iseyin(-0.02°C)and Enugu(-0.02°C)are not.However,the DTRhas increased at Awka and Asaba,at a rate of 0.26°Cand 0.31°Cper decade respectively.

Figure 1.(a)Distribution of weather stations in Nigeria used in this study,with the horizontal black line demarcating the Nigerian Sahel(above)and the Guinea Coast(below).(b-f)Annual and seasonal trends of DTRacross Nigeria.Upward-pointing triangles indicate positive Theil-Sen slopes,and downward-pointing triangles negative slopes.The back(gray)and red(brown)trianglesare statistically signi f i cant at the 95%and 90%con f i dence levels respectively.

However,the DTRtrends vary seasonally.Figure 1(c-f)show the DTRtrend in boreal winter(December-January-February,DJF),spring(March-April-May,MAM),summer(June-July-August,JJA),and autumn (September-October-November,SON)for each station(see Table 1 for the ratesof change per decade).It isfound that the magnitude of the DTRdecline averaged over the Nigerian Sahel region islarger during SON(-0.47°C)and JJA(-0.37°C)than in the DJF(-0.33°C)and MAM(-0.25°C)per decade,and thissituation isrelatively thesameat nearly allstationsover the region.At Maiduguri,the annual DTRhas decreased signi f i cantlyasaresult of thelarge DTRdecreaseduring JJA(-0.3°C)and SON(-0.15°C),which are statistically signi f icant at the 95%con f i dence level.This is similar to the observed situation at Bauchi,where the signi f i cant annual downward trend of DTRis as a result of larger decreasing trends during SON(-0.05°C)and JJA(-0.34°C).At Gombe(Gusau)the decline is also signi f i cant,at a rate of-0.6°C(-0.17°C)per decadein JJA,and-1°C(-0.3°C)perdecadein SON.Although,there are indicationsof declinesduring DJF and MAM at the stations,the magnitude of the trends in SONand JJAisstronger.Indeed,thestationsin the Nigerian Sahel have decreasing trends at annual and seasonal time scales.

Table 1.Seasonal and annual trends of DTR(°Cper decade)over Nigeria.The superscripts‘a’and‘b’indicate trends that are statistically signi f i cant at the 95%and 90%con f i dence level respectively.

In the Nigerian Guinea Coast,the regional-averaged DTR has decreased during summer(JJA)at about-0.005°C per decade,while there has been no noticeable change during SON.The region has also experienced increasing trends during DJF(0.06°C)and MAM(0.03°C)per decade.These differing seasonal trends explain the marginal increasing annual trend over the region.Moreover,the DTR has decreased at Benin at a rate of-0.02°C,at Calabar at a rate of-0.03°C,at Ibadan at a rate of-0.004°C,at Ikom at a rate of-0.003°C,and at Iseyin at a rate of-0.05°Cper decade during JJA,but there is no noticeable change at Enugu.However,Bida and Ijebu have recorded a marginal increase at rates of 0.03°C and 0.04°C respectively per decade,while Asaba and Awka have recorded statistically signi f i cant increasing trends in all seasons,at 0.31°C and 0.26°C per decade respectively(Figure 1(b)and Table 1).During SON,the DTRhasdecreased at a rate of-0.04°Cat Benin,while it has decreased at Bida,Enugu,Ijebu,Ikeja and Lokoja at rates of-0.07°C,-0.07°C,-0.05°C,-0.2°Cand-0.3°C per decade respectively.In Figure 1(c),DJFis characterized by marginal decreasing trends at the stations;the trends are decreasing at rates of-0.07°C,-0.1°C,-0.12°C,-0.15°C and-0.02°C per decade at Enugu,Ibadan,Ijebu,Ikeja and Lokoja respectively,while the increasing trends seen at Benin,Bida,Calabar,Ikom and Iseyin are non-signi f i cant.The DTR hasalso decreased during MAM at some stations Lokoja(-0.2°C)and Ikeja (-0.16°C)recorded statistically signi f i cant decreasing trends,while the decreasing trends at Benin(-0.05°C)and Ijebu(-0.06°C)are not statistically signi f i cant.The other stations show nonsigni f i cant increasing trends,except Asaba and Awka with signi f i cant increasing trends,as illustrated in Figure 1(d).The above results indicate inhomogeneity in the seasonal DTRtrends at the stations in the region,which characterizes the annual trend of DTR in the Nigerian Guinea Coast.

To investigate the relative effects of Tminand Tmaxon the variation in DTRin Nigeria,we present a comparison of their annual and seasonal time series.Standardized annual-averaged DTRanomalies during 1971-2013 over the Nigerian Saheland Guinea Coast regionsare shown in Figure 2,along with the time series of the annual mean Tmaxand Tmin.The long-term linear trend of DTRis also shown in the f i gure.Figure 2(a)illustrates that,in the Nigerian Sahel,the annual DTR has decreased signi f icantly.The rate of decline is-0.34°C per decade,and this decline is most obvious during 2000-2013.In this period,the DTR magnitude is smaller,the reason for which is that the magnitude of the increasing trend in Tminishigher when compared with theincreasing trend in Tmax;speci f i cally,Tminincreased at a rate of 0.51°C per decade during 1971-2013,which by far exceeds the 0.17°Cper decade increase in Tmaxin the Nigerian Sahel.The seasonal trends of DTRalso suggest there has been a more signi f i cant decrease over the Nigerian Sahel.This decreasing trend ismost notable in JJA-0.37°Cand SON-0.47°C,ascompared with DJF-0.33°Cand MAM-0.25°C per decade(Table 2).During this period,Tminincreased rapidly compared with Tmaxat the stations.For instance,at Bauchi,Tminincreased signi f i cantly in all seasons,while the Tmaxincrease was less signi f i cant.More speci f i cally,Tminincreased during DJF,MAM,JJAand SONwith magnitudes of 0.5°C,0.4°C,0.5°C and 0.6°C per decade respectively,while Tmaxincreased at 0.2°C,0.2°C,0.15°C and 0.14°C per decade during DJF,MAM,JJA and SON respectively.Thisispractically the same situation asat the stations where the DTR has decreased signi f i cantly.Nonetheless,this analysis shows that the signi f i cant decreasing DTRin the Nigerian Sahel is demonstrative of the fact that the rate of increase in Tminhas exceeded the rate of increase in Tmax(Figure 2(b)).

Figure 2.Normalized time series of annual-averaged DTR,T max and T min over(a)the Nigerian Sahel and(b)Guinea Coast.The dotted lines are trend lines.

Table 2.Seasonal trends of DTR,T max,T min(°Cper decade),precipitation(PRE)(mm per decade)and TCC(0-1)averaged over two regions in Nigeria.The superscripts‘a’and‘b’indicate trends that are statistically signi f i cant at the 95%and 90%con f i dence level respectively.

Figure 2(b)shows the interannual variation and trends of the DTRaveraged over the Guinea Coast.It is evident that the DTRhas increased marginally,at a rate of 0.01°C per decade in the region.This is connected to the signi f icantlyincreasing Tmin(0.19°C)and Tmax(0.20°C)per decade.Obviously,the Tmaxnarrowly increased by as much as the Tmin,and this perhaps explains the marginal increase in DTRin the region.However,there are some differences among stations(Table 1).For instance,the DTR has decreased signi f i cantly at Ikeja.On closer inspection,at this station,the variation of seasonal trends in Tminand Tmaxsuggests that Tminhas increased at rates of 0.43°C,0.38°C,0.33°Cand 0.36°Cper decade in DJF,MAM,JJAand SONrespectively,while Tmaxhasincreased at ratesof 0.3°C,0.20°C,0.14°Cand 0.20°Cper decade.Meanwhile,at Awka,where the DTRhas increased,the rate of increase in Tmaxhas outstripped the rate of increase in Tminin all seasons.The disparity in the increase in Tminand Tmaxat the stations in the Nigerian Guinea Coast region may have contributed to the non-signi f i cant increasing annual DTRtrend for the region.

3.2.Possible reason for the regional difference in the DTRtrend over Nigeria

It has been found that the DTRshows a strong inverse correlation with rainfall amount and cloud cover(Zhou et al.2009;Wang et al.2014;You et al.2016).To investigate the possible reason for the regional difference in the DTR trend over the Nigerian Saheland Guinea Coast regions,we examinetheeffectsof precipitation and cloud cover on the DTRtrendsin these two climatic regionsof Nigeria.

Figures 3(a,b)show the normalized time series of the regional-averaged DTR and precipitation for SON and JJA over the Nigerian Sahel and Guinea Coast regions,wherein the straight lines indicate the long-term linear trend.There is an indication that,when the DTRis low,precipitation is relatively high and,conversely,when precipitation is low,the DTR is high.To further strengthen this point,we also examine the correlation between the interannual variation of DTR and rainfall,the results of which suggest the annual-averaged rainfall is negatively correlated with the annual-averaged DTRin the Nigerian Sahel,with a correlation coefficient of r=-0.37,and in the Nigerian Guinea Coast region with r=-0.25.This indicates that the correlation between DTR and rainfall over the Nigerian Guinea Coast is relatively weak,while the correlation over the Nigerian Sahel is signi f i cant at the 95%con f i dence level.

Furthermore,in the Nigerian Sahel,the time series of the seasonal-averaged DTR and precipitation are well related,with correlation coefficientsof-0.53,-0.49 and-0.36 during JJA,SON and MAM respectively;although,a non-signi f i cant positive correlation(r=0.18)can be found during DJF.Meanwhile,in the Nigerian Guinea Coast region,the correlation coefficient is 0.002 for JJA and -0.07 for SON,inferring a weak relationship between DTRand precipitation during boreal summer in the region.Conversely,there are signi f i cant negative correlations of r=-0.54 and r=-0.42 for DJFand MAM respectively.The large difference in the correlation coefficient during boreal summer and winter perhaps explains the non-signi f i cant negative correlation found between the annual-averaged DTRand precipitation in the region.This is consistent with the f i ndings of Dai,Trenberth,and Karl(1999),who suggested the effect of precipitation on DTRcould be larger in dry regionsthan in humid regions.

The trends of seasonal and annual mean rainfall averaged over the Nigerian Sahel suggest that rainfall has increased signi f i cantly in JJA,at a rate of 10 mm per decade during 1971-2013,and 5.4 mm per decade in SON(Figure 3(a)and Table 2).Similarly,the trend implies that it has increased at a rate of 4.2 mm per decade annually over the region.Furthermore,Lin and Dike(2018)suggested that there are more incidence of heavy rainfall in the region.Thisshowsthat a signi f i cant decline in DTR over the Nigerian Sahel is well connected with the signi f i cant increasing rainfall over the region,and thiscan be explained by the damping effect of precipitation on Tmaxthrough evaporative cooling(Zhou et al.2009;Shen et al.2014).Relatively,the rate of rainfall has also increased over the Nigerian Guinea Coast,at a rate of 7 mm per decade during JJA and 5.8 mm per decade during SON(Figure 3(b)),while it has increased annually at a rate of 4 mm per decade in the region.Meanwhile,the annual-averaged DTRtrend is found to be increasing marginally,at 0.01°C per decade,with a maximum increasing trend of DTR found in DJF.This suggests that the increasing DTR trend found in the Nigerian Guinea Coast region cannot on the whole be explained by the increasing trend of rainfall in the region.

Figure 3.Normalized time series of seasonal(JJAand SON)DTRand precipitation anomalies during 1971-2013(thick and dotted linesshow the DTRand precipitation anomaliesrespectively,while the straight lines are trend lines)over(a)the Nigerian Sahel and(b)the Guinea Coast.

Figure 4.Normalized time series of annual DTRanomalies during 1971-2013 and ERA-Interim TCCanomalies during 1979-2013(thick and dotted lines show the DTRand TCCanomalies respectively,while the straight lines are trend lines)over(a)the Nigerian Sahel,(b)the Guinea Coast,and(c)for Awka(MAM).

In order to investigate the possible reason for the increasing DTR trend in the Nigerian Guinea Coast region,we further analyze the annual time series of averaged TCC and DTR over the two climatic regions in Nigeria,as shown in Figure 4.It can be seen from Figures 4(a,b)that TCC has decreased in the two regions.Given that cloud is inversely correlated with DTR,as more cloud can dampen the Tmaxand increase the Tmin(Dai,Del-Genio,and Fung 1997;Dai,Trenberth,and Karl 1999;Zhou et al.2009),the decreasing trend of cloud cover in the Nigerian Guinea Coast region might be responsible for the increasing DTR trend(Table 2).To strengthen this point that DTR and TCC are connected in the region,we also calculate the correlation coefficient between annual DTRand TCC in these two regions.It is found that annual DTRand TCCare negatively correlated with correlation coefficient r=-0.45 in the Nigerian Guinea Coast and r=-0.15 in the Nigerian Sahel as shown in Figure 4(a-b).This indicates that the correlation between DTR and TCC over the Nigerian Guinea Coast is statistically signi f i cant,while that over the Nigerian Sahel is relatively weak.This suggests that the effect of TCC on DTR is more pronounced in the Guinea Coast than in the Nigerian Sahel,which is an indication that the decreasing cloud cover in the Nigerian Guinea Coast region has an important effect on the increase in Tmax.To show that the effect of TCC is signi f i cant in the Nigerian Guinea Coast region,we present in Figure 4(c)the connection between the DTR and TCC in Awka,where the DTR increasing trend is relatively larger during the MAM season.With correlation coefficient of r=-0.29 when the time series are detrended,it indicates that DTRand TCC are inversely correlated during the MAM season.This suggests that increasing DTR is associated with decreasing cloud cover at the station as shown in Figure 4(c).

The above results further explain the regional difference in DTR in these two regions of Nigeria,which indicates that increasing precipitation in the Nigerian Sahel is well connected to decreasing DTRin the region while decreasing cloud cover is related with the marginal increase in DTR over the Nigerian Guinea Coast region.

4.Conclusions

In this study,the trends of DTR in Nigeria during 1971-2013 are examined based on observed station data.The inherent trends of DTR,Tmin,and Tmax,as well as the possible in f l uence of precipitation and cloud cover,are also investigated.In the Nigerian Sahel,the annual DTRtrend shows a signi f i cant decline over the region at the 95% con f i dence level.Furthermore,the seasonal trends show that the DTR has decreased signi f i cantly during JJA and SON,more so than during DJFand MAM.The decreasing trendsare associated with a faster warming of Tmin,which outstrips that of Tmax,during 1971-2013 in the Nigerian Sahel.Relatively,the annual trends in DTR have increased marginally in the Nigerian Guinea Coast region,and the seasonal trendsover the region suggest a marginal increase,especially during DJF and MAM.The slight increasing annual DTRtrend is an indication that Tminand Tmaxhave increased symmetrically over the region.

Meanwhile,there is an inverse relationship between precipitation and DTRover Nigeria;thesigni f i cant decreasing DTRtrend is well associated with a signi f i cant increasing rainfall trend,and thisisevident in the Nigerian Sahel.The increase in precipitation over the region hastended to dampen the increase in Tmax,while Tminhas increased rapidly.However,the impact of the precipitation trend on the DTR trend is not explicit in the Nigerian Guinea coast region,where a slight increase in the annual DTRis found with the increasing rainfall trend.Further analysis reveals that a decreasing trend of TCCexists in both the Nigerian Sahel and Guinea Coast region,and there also exists an inverse relationship between DTR and cloud cover over Nigeria,especially in the Nigerian Guinea Coast region.The decreasing trend of TCC is associated with the marginally increasing trend of DTR in the Nigerian Guinea coast region,while the effect of cloud cover on DTRover the Nigerian Sahel is relatively weak.This demonstrates that precipitation and cloud cover largely determine the patterns of DTRvariability in Nigeria.

Disclosure statement

No potential con f l ict of interest was reported by the authors.

Funding

This research was jointly supported by the Strategic Priority Research Program of the Chinese Academy of Sciences(CAS)[grant number XDA19030403],the National Natural Science Foundation of China[grant number 41575095],and the CAS‘Belt and Road Initiatives'Program on International Cooperation[grant number 134111KYSB20160010].Victor Nnamdi DIKE acknowledges support from the CAS-TWAS(The World Academy of Sciences)President Fellowship.Hyacinth NNAMCHI is supported by the International Federation for Science(W/4849).