TREND IN SALINITY OF THE WATERS OF THE CENTRAL- WEST-NORTH AFRICAN OCEANIC I ZONE (ZONE 12). (A PILOT STUDY) S. Duadze

INTRODUCTION Objective: This study is an attempt to ascertain whether there is any observable trend in the salinity data recorded in the LOICZ database for the Oceanic I Zone of Zone 12. Location and extent: Cell 12 runs parallelly to the coastline of North-West- Central Africa, spanning from Morocco in the extreme north to Congo in the extreme south. It covers 30  (3180 km) north-south and 55  (6115 km) east-west.

Methodology Clustering salinity data into 5 classes Assessment of any trend between and within the classes Running a similar clustering for the opposite cell (Zone 11) on the South American Coast Clustering rainfall data for the cell (Zone 12) Assessing any observable trend within the rainfall data Assessing the drainage network/basins on the adjoining land masses. Relating rainfall distribution as well as drainage network density and relative size of drainage areas to the observed trend in the salinity.

Results and discussion The salinity data is presented in Tables 1, 2 and 3.

Table 1: Salinity data (Zone 12) Latitude Ann_Avg

Explanation Annual average salinity values tend to increase from south to north within the cell (30.2 to units). Intra-class: Within certain classes, annual average salinity creases with latitude in certain classes, especially above Latitude Below this parallel, the changes in the value do not follow a recognisable trend.

Table 2: Salinity data (Zone 11) Latitude Annual average salinity (units?)

Explanation The trends are almost similar to those of Zone 12. Annual average salinity appears to increase with latitudes (i.e. northwards within the oceanic zone of the zone). Below latitude 3.75 intra-class trend does not seem to give a clear trend. Intra- class trends same as those of Zonw 12.

Table 3: Precipitattion data (Cell 12) Latitude Annual total Average Maximum

Explanation Generally, rainfall heavier south than north of the zone. The lowest figures seem to be associated with the middle parts of the cell while highest figures are associated with the south below Lat. 9. Medium figures are associated with the extreme north and mid latitudes of the cell. Rainfall distribution does not seem to clearly explain the trend in salinity. However, it is not unlikely that the high values record in the deep south of the cell could play a role in the lower salinity there.

Drainage network/basins Drainage network within the zone are denser in the southern latitudes that in the north. Furthermore, all the large rivers and their associated basins are confined more to the south than the north. All these rivers basins, notably the Congo, Niger, Volta, Mano, Sene- Gambia basins drain into the sea. The relatively heavy rainfalls in the south which fall in the terrestrial cells and drain into the ocean (oceanic I) The freshwater possibly does dilute the sea water rendering it less saline The comparatively low density of drainage network in the north with their associated small basins coupled with the relatively low rainfall could not have diluted the sea there than in the south.

Conclusion LOICZ Clustering tool provides the capability for regional surveys. Salinity appears to increase from south to north within the Central- West-North African Oceanic I waters. Discharge from river basins probably underlies the trend observed