Station-based Analysis of Variability and Change in the Nigerian Hydroclimate

The atmospheric effect of greenhouse gas emissions is posing an increasing threat to the stability of the global climate. Like many developing nations, the western Africa nation of Nigeria faces risks from climate change, with potential effects on the environment upon which Nigerians rely and on broader social constructs, including the national economy. Nigeria's diverse topography, which stretches from dry northern regions of the sub-Sahara to lush southern rainforests along the Gulf of Guinea, accentuates susceptibility to a variety of climate-related hazards, including warming, irregular rainfall patterns, and extreme weather occurrences. Driven by the influence of tropical climates on the global climate system and the importance of climate variability and change specifically within Nigeria, this study of the Nigerian hydroclimate explicitly characterizes historical variability and change through analysis of in-situ daily climate data.
Daily maximum and minimum air temperature and total precipitation data from 1982 through 2011 were obtained from the Nigeria Meteorological Service for 20 locations across the country. Given the limited temporal extent of the data, two popular satellite-derived precipitation products were tested for usability as supplements to the in-situ data. Each of the satellite-derived products depicts rainfall with an unrealistically high frequency and with a temporal trend that is opposite reality. Only in-situ data were analyzed further, beginning with a methodology to define the climatological wet and dry seasons across the country. The critical wet season across Nigeria was found to last between 120 days (north) and 200 days (south), beginning April/May and ending September/October, with wetness migrating from nearer the southern coastline northward through the country during the Northern Hemisphere summer, before retreating south again. As with seasonality, the spatial distribution of precipitation amount and frequency relates to distance northward from the southern coast. Wet season precipitation approaches 2500 mm from an average of more than 115 wet days along the coast, to only about 350 mm and 35 days across far northern Nigeria. Conversely, the dry season produces 300 mm from 30 wet days across the south, and only 80 mm from less than 10 days across the north. The wet season in Nigeria accounts for greater than 90% of annual precipitation and number of wet days. Nigeria experienced a warming and wetting of the climate during the 30-year study period, during both the wet and dry seasons. However, a change in the equitable distribution of precipitation across wet days (i.e., daily intensity) is not greatly evident, as it is for many other regions of the world. Thus, the likely benefit of greater precipitation does not appear to be mitigated by the risks associated with an increase in the frequency of high-intensity rainfall events. But tempering the positive precipitation signal is the likely detrimental effect of warming. Inter-annual variability in the wetness of the critical wet season is evident in the synoptic atmospheric expression of the inter-tropical convergence zone/discontinuity, but also in sea surface temperatures within the Gulf of Guinea. Historically, sea surface temperatures are considerably higher during the wettest wet season years compared to the driest years, possibly indicating a short-distance teleconnection that may offer seasonal predictability.