Monthly rainfall data from meteorological stations in Nigeria are analyzed from 1951 to 1992, in relation to some Tropical climate systems: Tropical South Atlantic (TSA) sea surface temperature index, North Atlantic Ocean (NAO) atmospheric index, Tropical North Atlantic (TNA) sea surface temperature index, Central Indian Precipitation (CIP) index and Outgoing Longwave Radiation Anomaly (OLRA). The analysis also includes August break (Monsoon break intensity (MBI) ) and annual rainfall anomaly index (RAI). The analyses show that the rainfall anomalies although sometimes intense do not have predictable patterns. The teleconnection between CIP and total rainfall in Nigeria suggests that the rainfall patterns in Nigeria is likely to be modulated by the Tropical Easterly Jet (TEJ) connecting rainfall pattern in Central India to that in Nigeria. The August break is observed to be highly variable and does not show a clear pattern of variability. Its variablility may be connected with multiple forcings from ocean and mesoscale circulations.
Nigeria has witnessed series of episodes of climate extremes which have continued to pose serious threats to lives and properties. Floods and drought conditions across the country have had far reaching and devastating effect on her economy and food security. Nigeria depends largely on the vagaries of weather, especially rainfall for its rain fed agriculture. The monsoon over West Africa is a seasonal prevailing wind blowing from the South Atlantic Ocean. Monsoon systems affect more than one-third of world’s population in numerous and varied ways, from agricultural irrigation to catastrophic disasters on an enormous spatial scale, such as flood or drought (Maher and Hu, 2006). The activity of this wind system has also been closely linked to Hydro power plants, infrastructural development, flood control dams, desert encroachment and visibility. Studies have linked the climate of Nigeria to various global systems such as; Intertropical Convergence Zone (ITCZ) (Chineke et al.,2010, Akinsanola and Ogunjobi, 2014), El Nino Southern Oscillation (Okeke et al.,2006, Olaniran, 2002) and West African Monsoon (Adeyemi et al.,2001, Chineke et al.,2010,). Other vagaries include Sea Surface Temperature (SST) which affects rainfall patterns in the Sahel (Thorncroft and Hodges, 2001). How these global and regional climate drivers interact and affect climate change is still not known. The ITCZ is the zone of contact between the Tropical Continental (CT) air mass and the Tropical Maritime air mass (MT) and sweeps across West Africa once every year. This region of contact between the two air masses at the surface is therefore a zone of moisture discontinuity and is also known as the Zone of Intertropical Discontinuity (ITD). The ITCZ is both a zonal and global phenomenon (Clement et al., 2009). It is responsible for the rainy season in the tropics. Understanding, interpreting and exploring the dynamics of weather and climate extremes is still an active area of research today (Nnamchi et al., 2015).
1.2 Climate of Nigeria.
Nigeria (Latitude 4o-13oN and Longitude 2o-14oE) is located in the tropics and borders the Gulf of Guinea between Benin on the West and Cameroon on the East. It has an area of 923,765 square km. The country’s land mass extends from the Gulf of Guinea in the South to its boundary with Niger and Chad republics in the North. (Thorncroft et al., 2001).
Nigeria has the following climate zones:
(i) The Tropical monsoon climate which is found in the southern part of the country. This climate is influenced by the monsoons originating from the South Atlantic Ocean, which is brought into the country by the (Maritime Tropical) air mass. The Tropical monsoon climate has a very small temperature range. For example Warri town in the southern part of Nigeria, records a maximum of 28oC for its hottest month while its lowest temperature is 26oC in its coldest month (Houghton et al.,1995). The southern part of Nigeria experiences heavy and abundant rainfall. The annual rainfall recorded in this region is very high, usually above the 2,000 mm rainfall totals expected for tropical rainforest climates worldwide. Over 4,000 mm of rainfall is recorded in the coastal region of Nigeria around the Niger delta area. The southern region experiences two rainfall peaks, usually in the months of June and September.
(ii) Tropical Savannah Climate is extensive in area and covers most of Western Nigeria to Central Nigeria beginning from the Tropical rainforest climate boundary in Southern Nigeria to the Central part of Nigeria, where, it exerts enormous influence on the region. The tropical savannah climate exhibits a well-marked rainy season with a single peak known as the summer maximum due to its location from the coast (Olaniran 2002).
(iii) The Sahel Climate or Tropical dry Climate is the predominant climate type in the far northern part of Nigeria. Annual rainfall totals are lower compared to the southern and north central part of Nigeria. Rainy season in the northern part of Nigeria lasts for only three to four months (June-September). The rest of the year is hot and dry with temperatures climbing as high as 40oC (Adeyemi et al., 2001).
(iv) Alpine Climate or highland climate or mountain climate, are found on highlands regions in Nigeria. Highlands with the alpine climate in Nigeria, are well over 1,520 metres above sea level. Due to their location in the tropics, their elevation is high enough to reach the temperate climate line in the tropics thereby giving the highlands, mountains and the plateau regions standing above this height, a cool mountain climate.
Temperatures throughout Nigeria are generally high; diurnal variations are most pronounced than seasonal ones. Highest temperatures occur during the dry season; rains moderate afternoon highs during the wet season. Average highs and lows for Lagos are 31oC and 23oC in January and 28oC and 23oC in June (Akinsanola and Ogunjobi, 2014). In the north, temperatures can reach as high as 44oC before the onset of the rains or drop as low as 6oC during intrusion of cool air.
Nigeria has four observed seasons which are:
A long rainy season which starts in March and lasts to the end of July, with a peak period in June over most parts of southern Nigeria.
A short dry season is in August and lasts for 3-4 weeks. This is due to the ITCZ moving to the north during the summer solstice.
A short rainy season follows the brief dry period in August and lasts from early September to mid-October as the ITCZ moves south again, with a peak period at the end of September. The rains are not usually as heavy as those in the long rainy season.
A long dry season starts from late October and lasts to early March with peak dry conditions between early December and late February. Vegetation growth is generally hampered, grasses dry and leaves fall from deciduous trees due to reduced moisture.
Many global and continental systems are linked to the climate of Nigeria. They include Tropical Easterly Jet (TEJ) (Olaniran, 2010), El Nino Southern Oscillation (Okeke et al.,2006), North Tropical Atlantic Sea Surface Temperature (Cook et al.,1998).
1.3 Tropical Easterly Jet (TEJ)
The TEJ is part of the Indian summer monsoon system and it extends from India over Africa in the Northern Hemisphere summer months, generally at a height of around 12-15 km (Thorn croft et al, 2001). The west-east axis of the TEJ is located between 4-10oN. On the southern side of the axis, conditions are conducive to the ascent of air and consequently rainfall occurrence whilst the northern side is marked by subsidence. (Yanjun et al., 2008). This can produce wet conditions from the middle belt to the south and dry conditions over the extreme north.
1.4 Tropical North Atlantic Index (TNA)
The Tropical North Atlantic Index is Anomaly of the average of the monthly Sea Surface Temperature SST from 5.5N-23.5N and 15W-57.5W. The TNA is also part of the major source of moisture for the monsoon system of Nigeria.(Farmer and Wigley,1985).
According to Palmer (1986), warming of the tropical Atlantic ocean reduces the meridional gradient of sea surface temperature (SST) south of the ITZC and this results in a weakening of the Hadley meridional circulation (i.e. the pattern of circulation of the atmosphere over the tropics). The weakened circulation reduces the intensity of the southwest monsoon flow into West and Central Africa and consequently rainfall over southern Nigeria. (Olaniran 2002)
1.5 El Nino Southern Oscillations and Other Teleconnections
Teleconnection studies are on-going to establish linkage between Nigerian rainfall and continental climate system (Nnamchi et al.,2015). El Nino is the term used to describe the excessive warming of the upper ocean in the tropical eastern Pacific lasting up to a year or even more. The cooling phase of El Nino is called La Nina. Changes in the El Nino events are related to changes in the Pacific Circulation. These changes have been reported to have a drastic impact on the ITCZ status and this could be a global effect reaching Nigeria (Clement et al.,2009).
1.6 Outgoing Long Wave Radiation.
From most literature studied so far, it is obvious that not much work has been done with regards to the effect of outgoing long wave radiation (OLR) on rainfall pattern in Nigeria. The OLR have long been observed by satellites as proxies in detecting deep convection over the tropics. (Sukumaran and Stordal, 2013). The OLR might provide physical insight into the study of rainfall variability in Nigeria (Okeke et al., 2006).
1.7 What is Rainfall Anomaly?
Rainfall anomaly is the deviation of the annual rainfall from the normal or expected mean annual rainfall of a place (Olaniran, 2002). The deviation gives rise to departures from the mean annual rainfall either above it or below the mark. The departures could be deficit or surplus depending on the mean. The departures are referred to as anomalies. A persistent departure from the normal e.g. above mean rainfall index of the same sign constitutes a climatic fluctuation. (Farmer and Wigley, 1985).These climatic fluctuations are indices of climate change impacts.
1.8 Purpose of Study
The purpose of this work is to
- To measure the intensity of the monsoon break and to determine the association of the monsoon break with the rainfall anomalies and the climate indices.
- investigate the relationship between rainfall anomaly (including monsoon breaks) and other climate systems purported to drive the rainfall pattern in Nigeria.