Spatial variation of rainfall in Nigeria during the ‘little dry season’

Atmospheric Research, 22 (1988) 137-147

137

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Spatial Variation of Rainfall in Nigeria During the 'Little Dry Season' J. 'BAYO OMOTOSHO

Department of Applied Geophysics and Meteorology, Federal University of Technology, P.M.B. 704, Akure (Nigeria) (Received March 26, 1987; accepted after revision November 2, 1987)

ABSTRACT Omotosho, J. 'Bayo, 1988. Spatial variation of rainfall in Nigeria during the 'little dry season'. Atmos. Res., 22: 137-147. Rainfall analysis of 33 Nigerian stations for more than 30 years of data has revealed the existence of a latitudinal belt of more pronounced dryness between 6 ° and 8.5°N within a coastal region of a general rainfall minimum during the so-called 'little dry season' of July/August. Temperature, relative humidity and equivalent potential temperature analyses indicate stronger subsidence and inversion in the lower/mid-troposphere over 6 °-8.5 ° N than elsewhere in the coastal region up to 10 ° N. It is suggested that the lower rainfall within 6 ° and 8.5 ° N is due to the stronger subsidence associated with outflows from deep convective systems located to the north of the area. RESUME L'analyse des relev~s pluviom~triques de 33 stations nig~rianes sur plus de 30 anndes r6v~le l'existence d'une ceinture zonale de s~cheresse plus prononc~e situ~e entre 6 ° et 8,5 °N h l'intdrieur d'une rdgion cSti~re oh un minimum pluviom~trique g~n~ral est observ~ pendant la petite saison s~che de juillet-aofit. Les analyses de la tempdrature, de l'humidit~ relative et de la temperature potentielle 6quivalente indiquent une subsidence plus forte, avec inversion, dans la basse troposphere entre 6 ° et 8,5 °N qu'ailleurs dans la rdgion cbti~re jusqu'h 10 ° N. On sugg~re que le d~ficit de pluie entre 6 ° et 8,5°N est dfi h la subsidence plus forte associ~e aux courants venant des syst~mes convectifs profonds situds au nord de la r~gion.

INTRODUCTION

O v e r N i g e r i a , i n d e e d t h e w h o l e o f W e s t A f r i c a , r a i n f a l l is o f t w o r e g i m e s : a bi-modal maximum south of 10°N and a single maximum north of this latit u d e . T h i s d i s t r i b u t i o n is p a r t l y a r e s u l t o f t h e s e a s o n a l o s c i l l a t i o n s o f t h e I n tertropical Discontinuity, ITD. During the northward advance of the ITD, the lower-level warm and moist southwest airstream generally increases in depth t o w a r d t h e E q u a t o r . P a r a d o x i c a l l y , h o w e v e r , t h e r e is a r e l a t i v e m i n i m u m i n

0169-8095/88/$03.50

© 1988 Elsevier Science Publishers B.V.

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the monthly precipitation of each of the stations located south or' 10 =N when the ITD reaches its northernmost limit of 21°N during July/August and the depth of the moist flow over these coastal areas reaches a peak (Hamilton and Archbold, 1945). This relative rainfall minimum is now popularly termed the 'little dry season' although the word dry here does not imply totally rainless conditions as in the period November-February. According to the early classification of Hamilton and Archbold, and later by Adejokun (1966), the weather zone in which this relatively dry region is embedded is zone D (Fig. 1 ) which is a stable region with drizzle or light rain from mostly stratocumulus clouds. Cloud growth is suppressed by a temperature inversion in the 850-750-mb layer due to mid-tropospheric subsidence. However, the single zone C of Hamilton and Archbold has been recently subdivided into two separate zones C1 and C2 (Fig. 2) by Dhonneur (1971). Zone C~ houses the well known squall lines with Ce having predominantly layer clouds and monsoon rainfall. Ireland (1962) was the first to assign values to the northern and eastern limits of the 'little dry season', LDS, of 9°N and 5°E, respectively. Adefolalu ( 1972 ) and Adekoya (1979) later showed that the LDS extends to 12 ° W and 7 ~:E. The area affected by the LDS (approximate 6 ° lat. by 19 ° long. ) is therefore large and significant spatial variations in precipitation may be expected to occur.

This paper presents the rainfall variation over Nigeria in detail. Reasons for the variations are also discussed.

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All the data used for this work were extracted from the climatological records at the Department of Meteorological Services, Oshodi, Lagos. Mean monthly rainfall data for 33 stations in Nigeria were analysed for each month in the year. All stations have rainfall data for over 30 years except Owerri, Ikom and Birni-Kebbi with only 6 - 8 year data. Since stations with shortperiod data are few, they will not adversely affect the results to be presented. Also, the data for each station were used to find the average rainfall for the latitude on or close to which it is located. For example, the averaged rainfall for latitude 6 ° N admitted data for all stations within the band 5.5 ° N and 6.5 ° N. In this way the variations of rainfall with latitude and season are obtained. Surface mean monthly pressure, temperature and relative humidity data were also extracted for the computations of the mean monthly equivalent potential temperatures (0e) and grouped as described above to obtain its latitudinal variation. Taking latitude 8 ° N as reference the departures of 0e at other latitudes from those at 8 °N were then finally computed in order to relate the rainfall

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cessation (October) of rainfall. minima to atmospheric behaviour. Due to lack of upper air data over the zone of interest, investigation of the atmospheric behaviour during the period is based only on surface data. SPATIAL VARIATION OF RAINFALL The typical rainfall distribution patterns during the onset, peak and cessation of the rains are depicted in Fig. 3 (April, J u n e - A u g u s t and October, respectively). Fig. 3a shows a general decrease of rainfall as one moves northward. By July, a definite minimum in rainfall can be seen centred along latitude 7 °8 ° N between the orography-influenced maximum over the Jos Plateau to the north and the coastal maximum to the south. This situation persists until October (Fig. 3e) when the rainfall pattern turns back to the early season situation. During the onset and cessation periods, isohyets generally run east-west across the entire country. The latitudinal variation of zonal mean rainfall tbr each month shown in Fig. 4 portrays a linear variation up to June and again from October. The linearity breaks down during July to September. This detail is missing in the analysis by Omotosho ( 1985 ) because much fewer data were used in that study.

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Nonetheless, the present work is in agreement with the previous analysis for all months except July to September and will give reliable rainfall estimates at any latitude upto June and from October to December. This will be particularly useful for agricultural planning and water resources management, as the probable onset a n d / o r cessation of any agriculturally adequate rainfall amount can be estimated (Omotosho, 1985). SURFACE EQUIVALENT POTENTIAL TEMPERATURE ANALYSIS

Equivalent potential temperature (0e) is a most useful parameter for studying the thermodynamic behaviour of the tropical atmosphere and has been employed extensively for this purpose (e.g., Obasi, 1964; Garstang et al., 1967; Adefolalu, 1972 ). The deviations of mean monthly surface 0e at other latitudes from the values at 8°N (Fig. 5) show that the coastal areas have higher values of 0~ than all other latitudes during the beginning (Jam-May) and at the cessation of the rains. From June to October, however, the zonal belt around latitude 8°N has higher 0~ than the surrounding latitudes. However, while the distribution of surface temperature (Fig. 6a) shows higher temperatures, the relative humidity in Fig. 6b depicts a relatively drier zone even though both are non-conservative parameters. It is interesting to note that the zone is located at the northern part of the 'little dry season' area. If the lower relative humidity in the zone is assumed to

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146

deep convective systems in C 1 with their strong upper level outflow will thus be associated with intense subsidence and warming at some distance away from their location. As zone C2 is immediately adjacent to these propagating systems and the coastal regions to the south are already under a cool and stable airmass of the south Atlantic anticyclone, it is suggested here that the higher temperatures and lower relative humidity in the zonal band of 6 °-8.5 : N must be due mainly to the subsidence warming, arising from the deep convective outflow to the north of the area, this subsidence air being mixed down to the surface. Thus the inversion (Fig. 1 ) will be strengthened and become more pronounced with consequently stronger mid-tropospheric subsidence and cloud-growth suppression in the area. Precipitation will therefore increase away from the zone as instability increases southwards (as well as northwards) in response to the weaker subsidence which, to the south, is now due only to the southern anticyclone influence. However, the upper air data necessary to investigate this statement are presently lacking and the findings must therefore be tentative. CONCLUSIONS

The existence of a drier zonal belt between latitudes 6 ° to 8.5: N during the 'little dry season' of southern Nigeria has been revealed. It is suggested that although the zone is within the area of general mid-tropospheric subsidence which occurs during July and August, the suppression of cloud growth and rainfall is more pronounced over this latitudinal belt due to the additional subsidence resulting from outflow from deep convection in the adjacent weather zone C~ to the north. If in the future upper air data become available over the area, it will be necessary to investigate the divergence/convergence and vorticity distributions in order to unravel clearly the reasons for the observed rainfall minimum in the area.

REFERENCES Adefolalu, D.O., 1972. On the mean equivalent potential temperature of the tropical atmosphere and the "Little Dry Season" of West Africa. Niger. Q. Meteorol. Mag., 2 ( 1 ): 15-40. Adejokun, J.A., 1966. The three-dimensional structure of the Inter-Tropical Discontinuity over Nigeria. Niger. Meteorol. Serv. Tech. Note, 29. Adekoya, J.O., 1979. Little Dry Season in West Africa. M.Sc. Thesis, Dept. of Meteorology, Florida State University, U.S.A. Dhonneur, G., 1971. General Circulation and Types of Weather over Western and Central Africa. Annex-IV, GARP-GATE 23 Design, p. 22. Garstang, M., LaSeur, N.E. and Aspliden, C., 1967. Equivalent potential temperature as a measure of the tropical atmosphere. U.S. Army Rep., 67-10: 38-43.

147 Hamilton, R.A. and Archbold, J.W., 1945. Meteorology of Nigeria and adjacent territory. Q. J.R. Meteorol. Soc., 72: 231-265. Ireland, A.W., 1962. The little dry season of southern Nigeria. Niger. Meteorol. Serv. Tech. Note, 24. Obasi, G.O.P., 1964. Thermodynamic and dynamic transformation over Ikeja/Lagos. Niger. Meteorol. Serv. Tech. Note, 33. Omotosho, J. 'Bayo, 1985. The separate contributions of line squalls thunderstorms and the monsoon to the total rainfall in Nigeria. J. Climatol., 5: 543-552. Reed, R.J., Norquist, D.C. and Recker, E.E., 1977. The structure and properties of African wave disturbances as observed during phase III of GATE. Mon. Weather Rev., 105: 317-333.