For Peer ReviewTrends in Spatio-Temporal Variability in Annual Rainfall in Ghana, (1951-

2000)

Journal: Weather

Manuscript ID: WEA-07-0100.R1

Wiley - Manuscript type: Research Article

Date Submitted by the Author:

28-Jan-2008

Complete List of Authors: Owusu, Kwadwo; University of Florida, Geography Waylen, Peter; University of Florida, Geography

Keywords:rainfall variability, declining rainfall, Kwahu plateau, humid Ghana, annual rainfall

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Rainfall variability in humid West Africa (south of 8oN) is less studied than that

of the Sahelian zone, responsible for the general consensus of a downward rainfall trend

in West Africa over the last few decades. Such a generalization may not be useful for

modelling and planning purposes. Rainfall mechanisms in the drier Sahel and the humid

Guinea Coast region differ; the former region has a single rainfall peak (in summer) but

the later region has a bi-modal seasonal distribution. It is therefore not necessarily true

that a failure in the rainfall regime, and its subsequent impact on agriculture and

livelihood, in one zone means the same for the other. In fact, it has been observed that a

warmer South Atlantic is associated with more rainfall in the Guinea Coast region and

less rainfall in the Sahel (Gu and Adler 2003). It is also suggested that the El Niño-

Southern Oscillation phenomena (ENSO) may be more strongly associated with Sahelian

rainfall than with that of the Guinea Coast region (Ward et al. 2004). Therefore there is a

need to investigate rainfall trends and variability in the humid areas to better inform

agricultural decision-making. Agricultural productivity has decreased in recent decades

following declining rainfall since the early 1970s, with the rainfall showing signs of

improvement after 2000.

This analysis will contribute to the knowledge of rainfall trends and aid in

agricultural and allied industrial decision-making in this important agro-climatic zone.

The study utilizes annual rainfall totals over 50 years to identify the spatial and temporal

dimensions of rainfall variability in Ghana by comparing the characteristics of the period

1951 to 1970 and 1981 to 2000, hereafter referred to as P1 and P2 respectively. Specific

emphasis is placed on the sub-humid areas of mid-Ghana because of its national and

regional significance to agricultural production and food security.

Rainfall variability in Ghana

Declining rainfall has been reported throughout West Africa over the past 50

years and may be viewed in the long term (Weldeab et al. 2007) as part of general

southward shift in the seasonal migration of the Inter-tropical Convergence Zone (ITCZ).

A great deal of research into the nature and causes of rainfall variability in the sub-region

has concentrated on the Sahel (Mahe et al. 2001). Nicholson et al. (2000) argue that

practical difficulties in obtaining data outside the Sahel may be a contributing reason,

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while on a more practical note Servat et al. (1997) conclude that, the tragic consequence

of drought on the countries in the Sahel explains and justifies the regional focus. Gu and

Adler (2003) observe that a high (low) south Atlantic sea surface temperature (SST) is

associated with high (low) rainfall in the Guinea coast (south of 8oN) and low (high)

rainfall in the Sahel. Limited studies within the humid zone itself point to a similar

reduction in annual rainfall totals, for example in Côte d’Ivoire (Servat et. al., 1997) and

in Ghana (Gyau-Boakye and Tumbulto, 2000).

Much of the literature on rainfall in Ghana has concentrated on selected regions

or stations (for example, Adiku et al. 1997, Tanu, personal communication and Gyau-

Boaakye and Tumbulto 2000). Tanu analyzed rainfall variability in Ho and Tamale, and

observed the risk of a dry spell during the rainy season to be higher in the south, (Ho),

than the north (Tamale). Similar patterns are present in the intra-seasonal rainfall

variability of Accra and Tamale (Adiku et al. 1997). The national study of Opoku-

Ankomah and Cordery (1994) suggests that variability in the northern zone is distinct

from the remainder of the country, due to the movement of the ITCZ and influence of

Atlantic SSTs, as noted by Boateng (1967).

Natural and human induced changes have been investigated as the cause of the

anomalous low rainfall in the sub-region (Giannini et al. 2003). Diagnostic studies (Ward

1998; Giannini et al. 2003) provide information concerning the forcing of West Africa

rainfall by global sea surface temperature (SST). Continental surface conditions also play

a role in determining the persistence of the drought condition (Zheng and Eltahir 1998).

The loss of vegetation, associated increase in soil albedo and decreasing temperature, was

proposed as a cause of Sahelian drought by Charney (1975). However, this mechanism of

self-perpetuating drought (Leroux 2001) was quickly challenged and these arguments

have resurfaced recently (Govaerts and Lattanzio 2007). SSTs alone do not explain

variability throughout West Africa, although sub-Saharan rainfall is negatively correlated

with SSTs in the South Atlantic (Opoku-Ankomah and Cordery 1994), and positively

correlated with the Gulf of Guinea (Adler et al. 2000). This echoes the dipolar structure

of higher SSTs in the South Atlantic and at the Equator, and lower SSTs in the North

Atlantic which has been used to explain reductions in areas affected by the monsoon

(Leroux 2001), and is consistent with observations of SSTs in the North Atlantic and the

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strength and location of the Azores anticyclone (Hastenrath 1991), and similar variations

in the South Atlantic anticyclone (Leroux 2001). Other factors investigated include

variations in the African Easterly Jet (AEJ) and the Tropical Easterly Jet (TEJ), (Leroux

2001, Price et al. 2007), and ENSO (Ofori-Sarpong and Annor 2001). At best ENSO is

only strongly associated with rainfall in the Sahel with a non-stationary or no clear

association with the Guinea Coast region (Ward et al. 2004)

Study area and methodsThe study area encompasses the four agro-ecological regions of Ghana (Fig.1),

although emphasis is placed on zones A and C (Ghana Meteorological Agency (GMA)

classification) with two stations falling in Zone D and one in Zone B. Zone B covers the

dry coastal strip of southeastern Ghana, where mean annual rainfalls of between740 and

890 mm supports very little agriculture. The rainfall anomaly in Zone B is part of a larger

area from the coast of Cape Three Points in Ghana to Benin and is referred to as the

‘Dahomey gap’. The low rainfall is attributed to a complex series of coastal (oceanic) and

atmospheric interactions (Acheampong 1982). Zone D in the north of the country

experiences similar rainfall totals (1000 mm) but has a single wet season. The agricultural

potential in this zone is diminished by high rainfall variability and poor soils. The most

productive zones of the country in terms of food and cash crop production are zones A

and C, both of which are extremely dependant on traditional rain-fed agriculture (Adiku

et. al. 2007).

Monthly rainfall totals, provided by the GMA, covering the period 1951 to 2000,

are divided into two 20-year periods 1951-1970 and 1981-2000 (P1 and P2). This

division is based on observations of a period of mostly above normal rainfall in P1 and

mostly below normal rainfall in P2, while the 1970s shows a more transitional pattern

(Fig. 2), This is also consistent with the identification of (multi-decadal) trends or

oscillations in West African rainfall (Lare and Nicholson 1994 and Ward et al. 2004).

Three classes (or terciles) of above normal, normal and below normal rainfall are defined

by the 33rd and 66th percentiles of annual totals estimated over 1951-2000. This typology

corresponds with the GMA forecast (www.meteo.gov.gh/forecast_farmers.html) and is

appropriate for agricultural applications. In addition to using long term records at 15

stations from GMA which form the core of the analysis, we also utilize less complete, but

Deleted: corresponding to a shift in climate experienced in many regions around the world during the mid 1970s which has been widely reported (forexample, Waylen et al. 2000; Chavez et. al. 2003).

Deleted: delineation

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spatially more numerous, data from the Global Historic Climate Network (GHCN)

covering Ghana, Benin, Burkina Faso, Côte d’Ivoire and Togo to demonstrate the spatial

extent of the declining rainfall pattern.

Results and discussionMean annual rainfall totals within all four agro-ecological zones experienced a

decline from P1 to P2, except Kete-Krachi, in zone C (Table 1). The reductions at six of

the 15 stations were significant at the 0.01 level and an additional four at the 0.05 level.

Stations experiencing significant declines are located towards the southwestern forest and

the coastal zones A and D (Fig 1). No consistent pattern of changes in the standard

deviation (SD) emerges, which might be expected given the sample sizes. Although none

of the changes in the SD of the 14 declining stations is significant, 11 stations

experienced a decline while four saw an increase in the SD in P2. Figure 3 plots the

combined changes in the mean and SD from P1 to P2. As expected, most of the stations

display declining SD when means decrease in P2. A few stations like Konongo, Agogo

and Effiduasi have declining means with little change in the SD. Both combinations

(declining mean and possibly stable standard deviation) suggest that P2 will display a

higher frequency of rainfall in the lower tercile.

Notable deviations from the two patterns are observed at Kete-Krachi, which has

both higher mean and SD, and at Navrongo with a declining mean and increased SD in

P2. The Kete-Krachi anomaly may be partially explained by its proximity to the Volta,

which was created by the filling of the Akosombo dam in the early 1960s. The case of

Navrongo highlights the dual rainfall mechanism in West Africa. Its proximity to the

Sahel gives rise to the single rainfall distribution, a reduced mean and increased SD. The

greater frequency of rainfall totals in the lower tercile (droughts) in P2 is consistent with

the declining rainfall pattern reported for the Sahel.

The general observation is of declining mean annual rainfall being concentrated

towards the south-west forest and coastal regions. The annual rainfall totals between P1

and P2 in zone A dropped from around 1800mm to about 1600mm, zones B and D saw a

reduction from around 1200mm to about 1000mm while zone C also experienced

reductions from 1400mm to 1200mm. These reductions are similar to those reported for

Deleted: The hypergeometric distribution and standard F- and t-tests are employed to identify changes in rainfall occurrences in each tercile, and means and variances of annual totals, under the null hypothesis of no significant changes between P1 and P2. The hypergeometric distribution arises when a random selection is made (without replacement) among objects of two distinct types. In its general form the hypergeometric distribution is describe by three parameters: N, the population; k, the number of success in the population; and n, the sample size. ¶¶

p(x) =

−−

n

N

xn

kN

x

k

¶

¶In this application N corresponds to the total number of complete annual records at a station between 1951 and 2000. The total number years falling in above normal or below normal terciles in this record – roughly a third, constitutes k, and the number of years of record available in P1 (P2) determines n. Significance levels,α of 0.05 and 0.01 are used throughout.

Deleted: declining

Deleted: A n

Deleted: observed

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the Sahel within the same periods. Figure 4 also indicates similar reductions in

neighbouring West African countries.

The differences in the absolute and percentage mean annual rainfall totals are

shown in Figs. (5a) and (5b) respectively. P2 has seen a reduction as high as 300mm

(20%) in the forest regions of zone A to as little as 100mm (10%) for zones B and D. The

results of F and t-tests shown in Table 1 and Fig. 4 indicates that stations in zones A and

B, as well as one in zone C, have seen a significant reduction in the mean annual rainfall

totals from P1 to P2. In general the reductions seen in zones C and D, that occupy the

northern half of Ghana, are minimal, ranging from 150.9mm in Kintampo to an increase

of 39.7mm in Kete-Krachi.

Results of the tercile frequency analysis shown in table 2, are similar to those of

the parametric tests in Table 1. Almost all the stations in Zones A and B showed signs of

being significantly wetter (more years reporting totals in the upper tercile than expected,

and fewer years in the lower tercile) in P1 than in P2. Indications of drier conditions

(significantly fewer upper tercile and more lower tercile occurrences) are exhibited in P2.

Some stations like Wenchi and Tamale (zones C and D) towards the north and east of the

study area indicate wetter conditions in P1 but no significant drying in P2

The distribution of significant decreases in rainfall appears to bisect GMA zone

A. A physical basis for such a sub-division may be offered by a consideration of

topography and proximity to the ocean. Figure 6 shows that the stations that have

experienced significant reductions in mean annual rainfall totals lie on the windward side

of the Kwahu Plateau with regard to the rain-bearing south-westerly monsoon winds,

while the north-eastern stations, on the leeward, display much smaller changes. The

plateau (identified in Fig. 6) has an average elevation of 460m, and stretches from

Koforidua in the south-east to Wenchi in the west. Gorshkov and Makarieva (2006) note

that in West Africa (north of 10°N) mean annual precipitation declines in an exponential

manner with distance inland. Drawing a line between Axim and Adieko (Côte D’Ivoire),

parallel to the coast of south-western Ghana and approximately orthogonal to the south-

west monsoons (Fig. 6), perpendicular distances are measured to 22 stations falling in the

rectangle defined (Fig. 7). A similar analysis of the relationship between distance

between the Axim-Akiedo line and the combined GMA and GHCN data reveals a very

Deleted: low

Deleted: application of the hypergeometric tests to tercile occurrences

Deleted: (

Deleted: )

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strong relationship (r2 = 0.885) to the south-west of the Plateau and a very marked break

with stations further inland. In the area of Sunyani and Wenchi (8oN, 2oW in Fig. 6) the

divide is dissected by the Tano river valley and is less easily defined, however Fig. 7

seems to indicate that these stations are part of the progression noted on the windward

side of the plateau.

The southern slopes of the Plateau are forested and shielded by the topography

from the impact of the Harmattan winds, while the northeastern side is mainly occupied

Guinea and Sudan Savanna. With rainfall distribution in Ghana, and West Africa in

general, influenced by the moist Southwest Monsoon and the dry Northeast Trade Winds,

it is no coincidence that a plateau with a southeast to northwest orientation can have such

a significant impact on mean annual rainfall totals.

ConclusionThe observations in this paper support the notion of downward trends in annual

precipitation that have been highlighted in the Sahel and parts of the humid regions of the

Guinea Coast. Moreover, while displaying declining rainfall, they provide further

confirmation that such trends may well vary considerably between regions. Such changes

are more frequently larger and statistically significant in the southwest than those of

either mid- or northern Ghana.

The Kwahu plateau appears to constitute an important divide in the extent of these

changes. Mean annual precipitation has dropped by 20% in the forest region to the south-

west, twice the proportion experienced in the savanna zones. Such changes are likely to

have significant impacts in the areas of rainfed agriculture which is widely practiced in

Ghana. Large scale rainfall deficits like the one observed here have the potential to

destroy plant cover, reduce evapo-transpiration, increase surface albedo and other aspects

of water and energy balance (Lare and Nicholson 1994) and set in motion a long period

of below normal rainfall. The decline in precipitation with elevation is unusual in the

presence of onshore winds (Barros and Lettenmaier 1994), but given the relatively low

topography, it is consistent with Gorshkov and Makarieva’s (2006) hypothesis of the

absence of forest, and thus a “biotic pump”, in the interior of West Africa.

Deleted: h

Deleted: In long-run, the general regional decline in precipitation may potentially be attributed to global precession of the perihelion from the boreal summer, 10,000 yrs BP to its present position in the boreal winter (Weldeab et al., 2007). This will have reduced summer heating over the continent, thereby influencing the northernmost extension of the ITCZ and associated monsoonal flows.

Deleted: s

Deleted: as much as

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The identification of the Kwahu plateau as an important divide in the changing

rainfall pattern in Ghana may pose a problem for planning and modelling based on the

GMA agro-ecological classification. Although the higher annual rainfall totals in the

south-west supports the differentiation of zones A and C, the influence of the plateau on

rainfall in the southern parts of the country makes extending zone A to the interior less

realistic.

Acknowledgement

The authors wish to thank the Ghana Meteorological Agency for the generous provision of rainfall data.

References

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Adedoyin, J. A. (1989) Global-scale sea-surface temperature anomalies and rainfall characteristics in northern Nigeria. International Journal of Climatology, 9, pp. 133-144

Adiku, S. G. K., Mawunya, F.D., Jones, J. W. and Yangyouru, M. (2007 shown as 1997 in the text ) Can ENSO Help in Agricultural Decision-Making in Ghana? In: Sivakumar, M. V. K. and Hansen, K. (Eds.) Climate Prediction and Agriculture. Springer, Berlin Heidelberg

Adler, R.F., Huffman, G. J., Bolvin, D. T., Curtis S. and Nelkin, E. J. ( 2000) Tropical rainfall distribution using TRMM combined with other satellite and rain gauge information. Journal of Applied Meteorology, 39, pp. 2007-2023

Barros, A. P. and Lettenmaier, D.P. (1994) Dynamic modeling of orographically induced precipitation. Reviews of Geophysics, 32, (3): pp. 265-284

Boateng, E. A. (1967) A Geography of Ghana. Cambridge University Press: Cambridge.

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Charney, J. G. (1975) Dynamics of deserts and drought in the Sahel. Quarterly Journal of Royal Meteorological Society, 101, pp. 193-202

Chavez, F. P., Ryan, J., Lluch-Cota, S. E. and Niquen, M. C. (2003) From Anchovies to Sardines and back: Multidecadal change in the Pacific Ocean. Science, 299, pp. 217-222 [delete if editing is agreed]Ghana Meteorological Agency. 2008. http://www.meteo.gov.gh/seasonal_forecast.html09/01/2008

Giannini, A., Saravanan, R. and Chang, P. (2003) Ocean forcing of Sahel rainfall on interannual to interdecadal time scale. Science, 302, pp. 1027-1030

Gorshkov, V. G. and Makarieva, A. M. (2006) Biotic pump of atmospheric moisture, its links to global atmospheric circulation and implications for conservation of the terrestrial water cycle. Russian Academy of Sciences, Petersburg Nuclear Physics Institute, preprint 2655: pp. 47

Govaerts, Y. and Lattanzio, A. (2007) Surface albedo response to Sahel precipitation changes. Eos, Transactions American Geophysical Union, 88, pp. 25-26

Gu G andAdler R. F. (2003) Seasonal rainfall variability within the West African Monsoon system. CLIVAR Exchanges, 8, pp. 11-15

Gyau-Boakye, P. and Tumbulto, J. W. (2000) The Volta lake and declining rainfall and streamflows in the Volta river basin. Environment, Development and Sustainability 2, pp. 1-10

Hastenrath, S. (1991) Climate Dynamics of the Tropics. Updated Edition from Climate and Circulation of the Tropics. Kluwer Academic Publishers: Dordrecht.

Lare, A. R. and Nicholson, S. E. (1994) Contrasting conditions of surface water balance in wet years and dry years as a possible land surface-atmosphere feedback mechanism in the West Africa Sahel. Journal of Climate 7, pp. 653-668

Leroux, M. (2001) The Meteorology and Climate of Tropical Africa. Praxis Publishing: Chichester.

Mahe, G., L’Hote, Y., Olivry, J. C. and Wotling, G. (2001) Trends and discontinuities in regional rainfall of West and Central Africa: 1951-1989. Hydrological Science Journal,46, pp. 211-226

Nicholson, S. E., Some, B. and Kone, B. (2000) An Analysis of recent rainfall conditions in West Africa, including the rainy seasons of the 1997 El Niño and the 1998 La Niña years. Journal of Climate, 13, pp. 2628-2640

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Ofori-Sarpong, E. and Annor, B. (2001) Ranfall over Accra, 1901-90. Weather, 56, pp. 55-62

Opoku-Ankomah, Y. and Cordrey, I. (1994) Atlantic sea surface temperatures and rainfall variability in Ghana. Journal of Climate, 7, pp. 551-558

Price, C. Yair, Y. and Asfur, M. (2007) East African lightning as a precursor of Atlantic hurricane activity. Geophys. Res. Lett., 34, L09805, doi:10.1029/2006GL028884

Servat. E., Paturel, J. E., Lubes, H., Kouame, B, Ouadraogo, M. and Masson, J. M. (1997) Climate variability along the Gulf of Guinea, Part I: detailed analysis of the phenomenon in Côte d’Ivoire. Journal of Hydrology, 191, pp. 1-15

Ward, M. N. (1998) Diagnosis and short-lead time prediction of summer rainfall in tropical North Africa at interannual and multi-decadal timescales. Journal of Climate, 11, pp. 3167–3191

Ward, M. N., Cook, K., Diedhiou, A., Fontaine, B. Giannini, A., Kamga, A., Lamb, P. J., Mohamed, B., Nassor, A. and Thomcroft, C. (2004) Seasonal-to-Decadal Predictability and Prediction of West African Climate. CLIVAR Exchanges, 9, pp. 14-20

Waylen, P. R., Compagnucci, R. and Caffera, M. (2000) Inter-annual and inter-decadal variability in stream flow from the Argentine Andes.’ Physical Geography, 21, pp. 452-465 [delete of editing is agreed]

Weldeab, S., Lea, D. W., Schneider, R. R. and Andersen, N. (2007) 155,000 years of West African monsoon and ocean thermal evolution. Science, 316, pp. 1303 - 1307

Zheng, X. and Eltahir, E. A. B. (1998) The Role of Vegetation in the Dynamics of West African Monsoons. Journal of Climate, 11, pp. 2078-2096

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Fig. 2 Annual rainfall fluctuations in Kumasi, 1951-2000. The solid lines represent the 20-year means and the dotted lines show the +/- 1 standard deviations

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For Peer ReviewMean and standard deviation of annual rainfall totals between P1 and P2 30x30mm (500 x 500 DPI)

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Fig. 4 Regional mean annual rainfall in 1951-1970 (P1) and 1981-2000 (P2) in Central West Africa

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Fig. 5 (a) Absolute differences (mm) in mean annual rainfall (P1-P2) and (b) expressed as percentage of mean rainfall in P1. Greater reductions in both absolute and percentages

are apparent in the south-west region of Ghana.

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Fig. 6 Changes in mean annual rainfall (mm) P1-P2 based on data from GHCN. The dotted rectangle indicates the area in which mean annual rainfalls are portrayed in figure 7 with

respect to the increased change south-west of the Kwahu Plateau 30x32mm (500 x 500 DPI)

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Changes in mean annual rainfall with distance from the southwest coast. Stations are identified relative to the Kwahu plateau

31x25mm (500 x 500 DPI)

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Ago-ecological zones of Ghana with selected rainfall stations based on Ghana Meteorological Agency classification

25x35mm (500 x 500 DPI)

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