Weather – May 2009, Vol. 64, No. 5 115 SSTs SSTs

2007), and 2000). This This 2000). et al., et al., 2004). et al., alone do not explain variability throughout alone do not explain variability throughout although sub-Saharan , rainfall is West in the South with SSTs correlated negatively and Cordery, (Opoku-Ankomah Atlantic with the correlated 1994), and positively SST (Adler the associated increase in soil albedo and increase the associated as a was proposed in temperature, decrease Charney (1975). by cause of Sahelian of self-perpetuat- this mechanism However, 2001) was quickly chal- (Leroux, ing drought resurfaced have lenged and these arguments 2007). (Govaerts and Lattanzio, recently echoes the bipolar structure of higher SSTs at the , and in the South Atlantic in the North which Atlantic SSTs and lower explain reductions in areas has been used to 2001); it is the monsoon (Leroux, by affected in the with observationsconsistent of SSTs North and location and the strength Atlantic anticycloneof the Azores 1991), (Hastenrath, and similar variations in the South Atlantic 2001). Otheranticyclone factors (Leroux, include variations in the African investigated Jet Easterly (AEJ) Jet Easterly Tropical and the (TEJ), 2001; Price (Leroux, ENSO (Ofori-Sarpong and Annor, 2001). At 2001). At and Annor, ENSO (Ofori-Sarpong with associated best ENSO is only strongly rainfall in the Sahel with a non-stationary or no clear association with the Guinea (Ward region Study area and methods Study area agro- the four encompasses study area The 1), of (Figure regions ecological A on Zones although emphasis is placed Agency (GMA) Meteorological and C (Ghana stations falling in classification) with two B covers Zone B. D and one in Zone Zone the dry Ghana, of southeastern strip coastal mean annual rainfalls of between where 740 and 890mm support very little agricul- B is part rainfall anomaly in Zone The ture. of the coast from area of that of a larger Benin which in Ghana to Points Three Cape low The as the . to is referred series of a complex to rainfall is attributed and atmospheric interac- tal (oceanic) coas D in the 1982). Zone tions (Acheampong, north of the country similar experiences (1000mm) but has a single rainfall totals in this potential agricultural The season. wet high rainfall variability is diminished by zone

dry and

et al. et al., ., 1997) and in et al N) and low (high) N) and low o (2000) argue that (2000) argue et al. 1997; Tanu, personal com- Tanu, 1997; 2003) provide information information 2003) provide 1997). The national study of The 1997). et al., et al., et al., 2007) as part south- of a general in Ghana (1951–2000) in Ghana 2003). Diagnostic studies (Ward, 1998; studies (Ward, 2003). Diagnostic Natural and human-induced changes have changes have Natural and human-induced Much of the literature on rainfall in Ghana on rainfall in Ghana Much of the literature (1997) conclude that the tragic consequence consequence that the tragic (1997) conclude in the Sahel on the countries of drought Gu focus. explains and justifies the regional (2003) observe and Adler that a high (low) sea-surfaceSouth (SST) Atlantic temperature rainfall in the with high (low) is associated (south of 8 Guinea Coast may be viewed in the long term (Weldeab in the long term be viewed may et al., shift the of ward in the seasonal migration A (ITCZ). Zone Convergence Inter-Tropical and the nature into deal of research great causes of rainfall variability in the subregion on the Sahel (Mahe has concentrated 2001). Nicholson practical difficulties in obtaining data out- reason, be a contributing side the Sahel may practical Servatwhile on a more note concerning the forcing of rainfall Africa West of the forcing concerning surface Continental conditions global SST. by the persist- in determining a role also play (Zheng and condition of the drought ence with loss of vegetation, The 1998). Eltahir, munication; Gyau-Boakye and Tumbulto, Tumbulto, munication; Gyau-Boakye and rainfall variability analysed in Ho Tanu 2000). and observed the risk of a Tamale, and be higher to season spell during the rainy in the south (Ho), than the north (Tamale). in the intrasea- present are Similar patterns Tamale and sonal rainfall variability of (Adiku Opoku-Ankomah and Cordery (1994) sug- gests that variability in the northern is zone of the country, distinct that of the rest from and influ- of the ITCZ the movement due to Boateng by as noted SSTs, of Atlantic ence (1967). cause of the anom- as the been investigated (Giannini rainfall in the subregion alously low et al., Giannini Rainfall in Ghana variability reportedDeclining rainfall has been through- years the past 50 over Africa West out rainfall in the Sahel. Limited stu dies with- stu Limited rainfall in the Sahel. a similar itself point to in the humid zone exam- for reduction in annual rainfall totals, (Servat d’Ivoire ple in Côte 2000). Tumbulto, and Ghana (Gyau-Boakye or sta- on selected regions has concentrated tions (Adiku

et al., Trends in spatio-temporal in Trends The study The

variability annual rainfall in N) is less studied than that of N) is less studied than that of o This analysis will contribute to the know- to will contribute analysis This the Sahelian zone, the data for which are which are for the data the Sahelian zone, of a consensus the general for responsible over Africa West in rainfall trend downward Such a generaliza- decades. the last few modelling and not be useful for tion may Rainfallplanning purposes. mechanisms in the drier Sahel and the humid Guinea has region the former differ; region Coast a single rainfall peak (in summer) but the has a bi-modal seasonal dis- region latter not necessarily tribution. It is therefore in in the rainfall regime true that a failure subsequent impact – and its one zone on – means the and livelihood agriculture In fact, it has been the other. same for observed is that a warmer South Atlantic rainfall in the Guinea with more associated the Sahel and less rainfall in region Coast It 2003). is also suggested and Adler, (Gu that the El Niño-Southern Oscillation phe- strongly be more may nomenon (ENSO) with Sahelian rainfall than with associated (Ward region that of the Guinea Coast and aid in agricul- ledge of rainfall trends tural and allied industrial decision-making in this important zone. agroclimatic 2004). Therefore there is a need to inves- a need to is there Therefore 2004). and variability in the rainfall trends tigate agricultural inform better to humid areas productivity Agricultural decision-makers. decades following in recent has decreased with the early 1970s, declining rainfall since of improvement signs the rainfall showing after 2000. to 50 years over annual rainfall totals utilizes dimensions identify the spatial and temporal of rainfall variability comparing in Ghana by the characteristics of the period 1951 to to referred 2000, hereafter 1970 and 1981 to Specific emphasis as P1 and P2 respectively. of mid- on the subhumid area is placed Ghana because of its national and regional production and agricultural to significance security. food Rainfall variability in humid West Africa Africa West Rainfall variability humid in (south of 8 University of Florida, Gainesville, FL, USA Gainesville, University of Florida, Kwadwo Owusu and Kwadwo Waylen Peter 116

Weather – May 2009, Vol. 64, No. 5 Trends in annual rainfall in Ghana Meteorological classification. Agency Figure 1.Agroecological rainfall zonesofGhanawithselected stationsbasedonGhana means andthedotted linesshow the+/–1standard deviations. Figure rainfall 2.Annual in , fluctuations 1951–2000.Thesolidlinesrepresent the20-year

mm 1000 1250 1500 1750 2000 2250 2500 750 9016 9018 902000 1990 1980 1970 1960 1950 Year declining rainfallpattern. to ofthe demonstrate thespatialextent Faso,Benin, Burkina Côte d’Ivoire and Network (GHCN) covering Ghana, numerous, datafrom theGlobalHistoric also utilize lesscomplete, butspatiallymore GMA whichform thecore oftheanalysis, we using long-term records at15stationsfrom for agricultural applications. additionto In forecast_farmers.html) andisappropriate with theGMA forecast (www.meteo.gov.gh/ over 1951–2000. correspondsThis typology 66th percentiles ofannualtotals estimated normal rainfallare definedby the33rd and normalandbelow-ciles) ofabove-normal, 1994; Ward West African rainfall(Lare andNicholson, trends oroscillationsin of (multi-decadal) This isalsoconsistent withtheidentification show amore transitionalpattern (Figure 2). below normalrainfallinP2,whilethe1970s ly above normalrainfallinP1andmostly ofaperiodmost- based onobservations and 1981–2000(P1P2). This divisionis divided into two 20-year periods1951–1970 GMA, covering theperiod1951to 2000,are fed agriculture (Adiku are depen extremely dent ontraditionalrain- areproduction Zones AandC,bothofwhich interms offood andcash-crop the country and poorsoils. The most productive zones of case ofNavrongo highlightsthedualrainfall the AkosomboDaminearly 1960s. The Volta, which wascreated by thefillingof explainedby itsproximity topartially the in P2. anomalymay be The Kete-Krachi with adecliningmeanandincreased SD both highermeanandSD, andatNavrongo whichhas terns atKete-Krachi, are observed ofrainfallinthelowerfrequency tercile. SD) suggest thatP2willdisplay ahigher tions (decliningmeanandpossiblystable with littlechangeintheSD. Bothcombina- Agogo andEffiduasihave decliningmeans decrease inP2.Afew stations likeKonongo, stations display decliningSDwhenmeans SD from P1to P2.As expected, mostofthe the combined changesinthemeanand an increase intheSDP2.Figure 3 plots stations experienced adeclinewhile4saw the 14decliningstationsissignificant, 11 Although noneofthechangesinSD givenmight beexpected thesamplesizes. standard deviation(SD) emerges, which No consistent pattern ofchangesinthe and thecoastal Zones AandD(Figure 1). are located towards thesouthwestern Stations experiencingsignificant declines level and anadditionalfour atthe0.05level. the 15stationswere significant atthe0.01 Zone C (Table 1). The atsixof reductions from in P1to P2,except atKete-Krachi agroecological zones experienced adecline annualrainfalltotals withinallfourMean Results anddiscussion Monthly rainfalltotals,Monthly provided by the Notable deviationsfrom pat- thetwo et al., 2004). Three classes(orter- et al., 2007). regions. The annual rainfall totals between Table 1 P1 and P2 in Zone A dropped from around Difference in the mean, standard deviation and coefficient of variation of annual rainfall 1800mm to about 1600mm, Zones B and D

between 1951–1970 (P1) and 1981–2000 (P2). saw a reduction from around 1200mm to Trends in annual rainfall Ghana Station Zone Mean (mm) (P1–P2) SD (mm) (P1–P2) CV (%) (P1–P2) about 1000mm while Zone C also experi- enced reductions from 1400mm to 1200mm. Agogo A 174.5* –8.8 –3.1 These reductions are similar to those report- Effiduasi A 307.8** –11.1 –4.3 ed for the Sahel within the same periods. A 175.9* 81.6 3.7 Figure 4 also indicates similar reductions in neighbouring West African countries. Ho A 188.1** 20.2 –0.8 The differences in the absolute and per- Konongo A 317.1** 14.0 –2.0 centage mean annual rainfall totals are Kumasi A 291.6** 97.4 3.2 shown in Figure 5(a) and 5(b) respectively. P2 has seen a reduction as high as 300mm A 136.9* 106.9 6.4

(20%) in the forest regions of Zone A to as Weather – May 2009, Vol. 64, No. 5 Takoradi A 335.3** 72.9 –0.6 little as 100mm (10%) for Zones B and D. Accra B 260.5** 40.7 –3.6 The results of F- and t-tests shown in Table 1 and Figure 4 indicate that stations in Zones C 169.8* 105.1 6.1 A and B, as well as one in Zone C, have seen Kete-Krachi C –39.7 –147.2* –10.3 a significant reduction in the mean annual Kintampo C 150.9 52.0 1.7 rainfall totals from P1 to P2. In general the reductions seen in Zones C and D, that occu- C 115.1 25.5 0.4 py the northern half of Ghana, are minimal, D 93.3 –33.4 –4.7 ranging from 150.9mm in Kintampo to an increase of 39.7mm in Kete-Krachi. Tamale D 77.3 51.3 3.1 Results of the tercile frequency analysis * Significant at 0.05 level. shown in Table 2 are similar to those of the ** Significant at 0.01 level. parametric tests in Table 1. Almost all the sta- tions in Zones A and B showed signs of being 600 800 1000 1200 1400 1600 1800 significantly wetter (more years reporting 400 400 totals in the upper tercile than expected, and fewer years in the lower tercile) in P1 Kete-Krachi than in P2. Indications of drier conditions 350 350 (significantly fewer upper-tercile and more Takoradi lower-tercile occurrences) are exhibited in Agogo P2. Some stations like Wenchi and Tamale 300 300 (Zones C and D) towards the north and east Kumasi of the study area indicate wetter conditions in P1 but no significant drying in P2. 250 250 The distribution of significant decreases Accra Ho in rainfall appears to bisect GMA Zone Tamale Konongo A. A physical basis for such a subdivi- 200 200 Berekum sion may be offered by a consideration of Nsawam

Standard Deviation (mm) Effiduasi topography and proximity to the ocean. Figure 6 shows that the stations that have 150 150 experienced significant reductions in mean Navrongo annual rainfall totals lie on the windward side of the Kwahu with regard to 100 100 600 800 1000 1200 1400 1600 1800 the rain-bearing southwesterly monsoon winds, while the northeastern stations, on Mean Annual Precipitation (mm) the leeward, display much smaller changes. The plateau (identified in Figure 6) has an average elevation of 460m, and stretches Nsawam Konongo Kumasi Kintampo from in the southeast to Wenchi Effiduasi Ho in the west. Gorshkov and Makarieva (2006) Berekum Navrongo note that in West Africa (north of 10°N) Ejura Tamale mean annual precipitation declines in an Kete-Krachi Takoradi exponential manner with distance inland. Agogo Wenchi Drawing a line between and Adieko Accra Period 1 (Côte D’Ivoire), parallel to the coast of southwestern Ghana and approximately Figure 3. Mean and standard deviation of annual rainfall totals between P1 and P2. orthogonal to the southwest monsoons (Figure 6), perpendicular distances are mechanism in West Africa. Its proximity to with the declining rainfall pattern reported measured to 22 stations falling in the rec- the Sahel gives rise to the single rainfall dis- for the Sahel. tangle defined (Figure 7). A similar analy- tribution, a reduced mean and increased SD. The general observation is of declining sis of the relationship between distance The greater frequency of rainfall totals in the mean annual rainfall being concentrated between the Axim-Adieko line and the lower tercile () in P2 is consistent towards the southwest forest and coastal combined GMA and GHCN data reveals a 117 118

Weather – May 2009, Vol. 64, No. 5 Trends in annual rainfall in Ghana on thewindward side oftheplateau. oftheprogressionstations are part noted ever Figure 7 seemsto indicate that these River Valley andisless easilydefined;how- Figure by the 6) thedivide isbisected Tano very strong very relationship (r absolute andpercentages are apparent inthesouthwest region ofGhana. Figure 5.(a)Absolutedifferences (mm)inmeanannualrainfall (P1-P2)and(b)expressed aspercentage ofmeanrainfall inP1. area ofSunyani and Wenchi (8 the inland. In break withstationsfurther southwest marked oftheplateau andavery Figure 4.Regional meanannualrainfall in1951–1970(P1)and1981–2000 (P2)Central West Africa. 008060402005 03 01 -10 0 10 20 30 40 50 0 200 400 600 800 1000 2 =0.885)to the mm o N, 2 o W in and the dry northeast tradewinds, itisno northeast and thedry enced by themoistsouthwest monsoon in Ghana,and West Africa ingeneral, influ- winds. to thedry With rainfall distribution Guinea andSudanSavanna andexposed sideismainlycoveredthe northeastern by oftheHarmattanwinds, while the impact ested andshieldedby thetopography from The southernslopesoftheplateau are for- totals. onmeanannualrainfall significant impact to orientationcanhave northwest sucha coincidence thataplateau withasoutheast the notionofdownward trends in annual inthispapersupport The observations Conclusion % Greater reductions inboth Table 2 The number of years in different rainfall terciles for 1951–1970 (P1) and 1981–2000 (P2) and the statistical significance of these changes.

P 1 (1951–1970) P 2 (1981–2000) (1951–2000) Trends in annual rainfall Ghana Station Zone Above Normal Below Total Above Normal Below Total Total Agogo A 8* 3 4 15 5 6 9 20 41 Effiduasi A 18** 6 1** 17 3* 6 10* 19 47 Ejura A 6 4 4 14 5 6 7 18 41 Ho A 12** 4* 4 20 5 7 8 20 46 Konongo A 14** 5 1** 20 1** 8 11** 20 50 Kumasi A 12** 5 3* 20 3* 6 11** 20 50

Nsawam A 9 7 4 20 4 8 7 19 49 Weather – May 2009, Vol. 64, No. 5 Takoradi A 11** 5 3* 19 2** 9 9* 20 48 Accra B 8 9 2** 19 3 4 10** 17 46 Berekum C 8 5 4 17 4 6 8 18 44 Kete-Krachi C 6 8 6 20 5 6 9 20 48 Kintampo C 10* 5 4* 19 3 4 7 14 42 Wenchi C 9 5 6 20 6 7 7 20 50 Navrongo D 8 8 4 20 5 6 9 20 50 Tamale D 9 3* 8 20 5 7 8 20 50 * Significant at 0.05 level. ** Significant at 0.01 level.

precipitation that have been highlighted in the Sahel and parts of the humid regions of 11 the Guinea Coast. Moreover, while display- ing declining rainfall, they provide further confirmation that such trends may well vary considerably between regions. Such chang- es are more frequently larger and statisti- 10 cally significant in the southwest than those 700 of either mid- or northern Ghana. The Kwahu Plateau appears to consti- tute an important divide in the extent of 600 9 these changes. Mean annual precipitation has dropped by 20% in the forest region to the southwest, twice the proportion experi- 500 enced in the zones. Such changes are likely to have significant impacts in the 8 areas of rain-fed agriculture which is widely 400 practised in Ghana. Large-scale rainfall defi- cits like the one observed here have the potential to destroy plant cover, reduce Elevation (m) 7 300 evapotranspiration, increase surface albedo and affect other aspects of water and energy balance (Lare and Nicholson, 1994) and set in motion a long period of below-normal 200 rainfall. The decline in precipitation with ele- 6 vation is unusual in the presence of onshore winds (Barros and Lettenmaier, 1994), but 100 given the relatively low topography, it is consistent with Gorshkov and Makarieva’s 5 (2006) hypothesis of the absence of forest, and thus a biotic pump, in the interior of West Africa. The identification of the Kwahu Plateau as -3 -2 -1 0 1 an important divide in the changing rainfall Figure 6. Changes in mean annual rainfall (mm) P1–P2 based on data from GHCN. The dotted rec- pattern in Ghana may pose a problem for tangle indicates the area in which mean annual rainfalls are portrayed in Figure 7 with respect to the planning and modelling based on the GMA increased change southwest of the Kwahu Plateau. agro-ecological classification. Although the 119 120

Weather – May 2009, Vol. 64, No. 5 Trends in annual rainfall in Ghana identified relative totheKwahu Plateau. Figure 7.Changesinmeanannualrainfall withdistance from thesouthwest coast. Stationsare sion ofrainfalldata. orological for Agency thegenerous provi- The authorswishto thanktheGhanaMete- Acknowledgement ing Zone Ato theinterior lessrealistic. makesextend- ofthecountry southern parts the influence oftheplateau onrainfallinthe thedifferentiation ofZonessupports AandC, higher annualrainfalltotals inthesouthwest References Mean Annual Precipitation (mm) Physical Geography causes. along thecoast of Ghana-its nature and Acheampong PK. 32 induced precipitation. modelingoforographicallyDynamic Barros AP, Lettenmaier DP. J. App. Meteor. satellite andraingaugeinformation. bution using TRMM combined withother EJ. S, Nelkin Adler RF, HuffmanGJ, DT, Bolvin Curtis Berlin, Heidelberg. HansenK(eds).Springer: Sivakumar MVK, in in Ghana? agricultural decision-making Yangyouru M. Adiku Mawunya SGK, FD, JonesJW, 1000 1500 2000 Climate Prediction andAgriculture. : 265–284. Geografiska Annaler. A, Series 0 0 0 0 0 0 700 600 500 400 300 200 100 0 0 0 0 0 0 0 700 600 500 400 300 200 100 0 2000. Tropical rainfalldistri- 39 1997.Can ENSOhelpin : 2007–2023. 1982. Rainfall anomaly 1982.Rainfall

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