EFFECT OF ALANTIC SEA SURFACE TEMPERATURE ON SEASONAL RAINFALL OVER SOME COASTAL CITIES IN NIGERIA (FROM 1991-2000.)

EFFECT OF ALANTIC SEA SURFACE TEMPERATURE ON SEASONAL RAINFALL OVER SOME COASTAL CITIES IN NIGERIA (FROM 1991-2000.)

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ABSTRACT

This study examines the effect of Atlantic Sea Surface Temperature on rainfall in some selected coastal stations in Nigeria for a period of 10 years (1991 – 2000). Correlation between rainfall and Atlantic SST of  the coastal stations and to determine the trend of Atlantic SST and rainfall of the coastal stations over the 10 years period (1991 – 2000) were determined. Sea Surface Temperature and monthly rainfall data were used for this study. The Atlantic SST covers 10 years spanning between 1991 – 2000 was made available by the European Centre for Medium Weather Forecasting (ECMWF) and the rainfall data for Ikeja and Calabar  used for this study were collected from Nigerian Meteorological Agency (NIMET) and Ondo (Okitipupa) State was made available By Climate Research Unit (CRU). Statistical analysis which include mean, standard deviation, trend analysis and correlation analysis were carried out. The rainfall and SST data were standardized to reduce problems inherent in the analysis of rainfall: the highly diverse means and variances and the randomness of the convective process reflected in individual station. The result showed that Atlantic SST has a little role to play in determining the rainfall of the coastal states. The trend of Atlantic SST was found to be increasing while the rainfall trend in Ikeja and Okitipupa was decreasing and that of Calabar was increasing. The correlation coefficient (R2) of Atlantic SST against Ikeja is 0.119 while that of Atlantic SST against Okitipupa is 0.018 and that of Atlantic SST against Calabar is 0.014. This result which shows that there is no significant correlation between the Atlantic SST and the coastal stations and that Atlantic SST has little or no effect on Rainfall in these coastal stations.

CHAPTER ONE

 INTRODUCTION

BACKGROUND TO STUDY

Sea surface temperature (SST) is the temperature of the sea surface representing the condition in the surface mixed layer underlying the ocean skin. It is a geophysical variable of importance to marine meteorological services, the world climate research programme and also other large and small-scale research programmes. Its distribution on local, regional, hemispheric or global scales are of interest to scientists for the study of variety of processes in the sea, coastal areas and in the atmosphere. These ranges from local air-sea interaction and their relationship to local weather which include rainfall, which is the considered in this study. Processes in the ocean often have thermal expression at the sea surface e.g. Mesoscale eddies and upwelling. The importance of SST cannot be overemphasized in the field of meteorology and water resources, it has diverse uses ranging from forecasting of weather such as development of showers, thunderstorm, fog and rainfall amount [1, 2, and 3].SST may also determine areas of fish catchment by fishing fleet.

The heat content of the world’s oceans is estimated to have increased by 14.2x1022 J during the period of 1961 to 2003 [4]. This is no surprise as there has been a unanimous agreement to curb the rate of global warming, which is also evident on sea surface temperature. Some evidence of global warming in Nigeria has been observed using sea surface temperature (SST) for the period 1989-2006 in East Mole Station, about 2 kilometers from the coast [5]

The surface water off the Nigerian coast is basically warm with temperature generally greater than 24oC. Sea surface temperature show double peaked cycles which match quantitatively the cycle of solar heights. Between October and May, south Atlantic sea surface temperatures range from 23.39oC-27.55oC while during the peak of the rainy season of June-September; he range is between 22.29o-25.47oC. [6]  showed  that seasonal variation  of  SST  of the  Atlantic  ocean,  2km off  the  coast at Lagos(east mole station),peak in April (30oC) and November (29oC), and the minimum in August (26oC), corresponding to the observed monthly and seasonal rainfall distribution in Lagos. The lowest SST in August may be related to the extension of the cold tongue of the St Helena High in the South Atlantic to the Gulf of Guinea, overriding the warm Guinea current.

STATEMENT OF PROBLEM

The few studies of the relationship between SSTs and the atmospheric parameters in the Atlantic have been concerned with the influence of SSTs on rainfall and have produced evidence both direct forcing by SSTs and of complex changes of atmospheric parameters which force both SSTs and rainfall [7]; [8]; [9]. A phenomenon called ENSO also referred to as El Nino (an exclusive warming of the upper ocean in the tropical eastern pacific lasting three or more season) influences SST and has a drastic effect on coastal rainfall. An ENSO episode is primarily evidenced through the appearance of SST anomalies and this was evident in the South Atlantic sea surface temperature, it was discovered that this had a profound effect in creating drier than normal conditions with below normal rains, the conditions with below normal rainfall, especially in strong El Nino years like 1983/1984, 1987/1988, 1991/1992, 1992/1993, 1997/1998. The most recent El Niño event began in the spring months of 1997.

 Instrumentation placed on the analysis of Buoys in the Pacific Ocean after the 1982-1983 El Niño beganrecording abnormally high temperatures   off the coast of Peru. Over the next couple of months, this strength of these anomalies grew. The anomalies grew so large by October 1997 that this El Niño had already become the strongest in the 50+ years of accurate data gathering.

Also, La Niña is associated with cooler than normal water temperatures in the Equatorial Pacific Ocean, unlike El Niño which is associated with warmer than normal water. There are other factors that contribute to coastal rainfall apart from SST. In the coastal area thunderstorm are of greater importance to rainfall [10].

RESEARCH OBJECTIVE

This study seeks to investigate the relationship between SST and rainfall in some of the coastal stations of Nigeria, also to find the correlation between rainfall and SST’s of south Atlantic and to develop statistical model to estimate monthly/seasonal rainfall over the selected station using January, February and December SST of east mole and south Atlantic ocean.

SCOPE OF THE STUDY

Climate of Study areas

Nigeria is a maritime state with a coastline of approximately 853 km and lies between  latitude 4°10' to 140'N and longitude 2 °45' to 8 5'E. The Nigerian coastline stretches from the western border with the Republic of Benin to the eastern border with the Cameroon Republic. SST was taken from eastmole in Victoria Island which is a coastal station in  Nigeria and located at longitudes 3027E and latitude 6025N with an altitude of 2.5m [11]. Also south Atlantic SST was taken at a location (0-20°South, 30°West-10°East), Nigeria as a maritime state, is signatory to the Law of the Sea Convention. Her sovereignty extends beyond her internal waters to her territorial sea of 30 nautical miles. In 1978, Nigeria established an Exclusive Economic Zone which is an area beyond and adjacent to the territorial sea extending 200 nautical miles from the baseline. The surface area of the continental shelf is 46,300 km2 while the EEZ covers an area of 210,900 km2 (World Resources 1990), within which Nigeria exercises sovereign rights for the purpose of exploring, exploiting, conserving, and managing the natural resources. Much of Nigeria's population and economic activities are located along the coast with over 20% of the population inhabiting coastal areas. The Nigerian coastal zone and its resources have vast

implications for the economy.


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