A study of climatic variability in Nigeria based on the onset, retreat, and length of the rainy season

Abstract

The long-term variability of rainfall conditions in Nigeria in terms of the onset, retreat, and length of the rainy season has been analysed, using pentad data for the period 1919-1985. Data were grouped into four areas, arranged in a south-north transect; the Coastal, Guinea-Savanna, Midland and Sahelian Zones. The series for retreat of rainfall showed evidence for quasi-triennial and quasi-6-year oscillations, while that for rainy season length displayed quasi-biennial and quasi-triennial oscillations. No consistent spectral peaks emerged for changes in the date of onset of the rainy season. There is spatial coherence in variation in the date of the retreat of rainfall over the whole country, whilst for the date of onset of the season spatial coherence is limited to southern Nigeria (Coastal and Guinea-Savanna Zones). Northern Nigeria (Midland and Sahelian Zones) and southern Nigeria (Coastal and Guinea-Savanna) emerge as distinct areas in terms of spatial coherence in the variation of the length of the rainy season. There is also evidence for a secular change in the date of the retreat of rainfall for the whole country during the period 1939-1985, and in the date of onset of rainfall for southern Nigeria for 1968-1985.

Onset, retreat and length of the rainy season over Cameroon

“…These zones are similar to those defined using other criteria (Olaniran, 1989(Olaniran, , 1983b). Thus we can consider:…”

Characteristics of precipitable water over Peninsular Malaysia from satellite and in situ data

“…It is also likely that such inconsistency may result from the interference of topography (Schulz et al 2009), a general feature of the study area, with the prevailing circulation aloft. The bimodal oscillations of WST and W exhibit phase semblance with the findings of Wong et al (2009), for precipitation in Peninsular Malaysia and those of Olaniran and Sumner (1989) and Adeyemi (2009), using radiosonde data taken over the coastal region of West Africa. The NW, WW, and SW experience two rainy seasons, which are fairly distinguishable during the inter-monsoonal periods of March to April and October to November, when substantial precipitation, linked with the reversal of the wind directions as well as the influence of local topography in the regions ensues.…”

“…For instance, Peixoto and Oort (1996), who studied the climatology of relative humidity in the atmosphere based on global radiosonde data, have observed these differences. Also, Balogun and Adedokun (1986) and Olaniran and Sumner (1989) have confirmed such differences over West Africa. Four seasons: NEM, pre-monsoon, SWM, and early NEM have been identified for Peninsular Malaysia (Varikoden et al 2011).…”

“…The primary and secondary minima of both WST and W respectively coincide with the retreat of the SWM and NEM, while their maxima counterparts coincide with the onset of NEM and SWM. The dip in precipitation during these periods may be due to several factors such as coastal upwelling and circulation aloft, which becomes divergent and subsident as a result of the frequent occurrence of inversions and isothermals in the upper atmosphere along the coast (Olaniran and Sumner 1989;Willoughby et al 2002). WSM is observed to be higher than WSL in all the climatic regions.…”

Evidence of climate change based on rainfall records in Nigeria

“…According to Sumner (1988), continued changes in annual precipitation receipt beyond decadal periods may indicate pronounced regional climate change. Olaniran and Sumner (1989) studied climatic variability in Nigeria based on the onset, retreat, and length of the rainy season. The present study attempts to investigate evidence of climate change based on rainfall seasonality and the replicability of rainfall regimes in Nigeria.…”