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 cells of Zone 12.

Location and extent:

Zone 12 runs parallel 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 zone

(Zone 11) on the South American Coast• Clustering rainfall data for the 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

29.25

26.75

22.25

20.25

17.25

13.75

05.25

03.75

03.25

01.25

05.25

03.75

02.25

00.25

Ann_Avg

36.15

36.61

36.41

 36.15

35.86

35.38

 34.2

34.7

32.43

32.7

33.25

30.54

30.2

31.62

Explanation

• Annual average salinity values tend to increase from south to north within the zone (30.2 to 36.77 units).

• Intra-class: Within certain classes, annual average salinity increases with latitude in certain classes, especially above Latitude 5.25. Below this parallel, the changes in the value do not follow a recognisable trend.

Table 2: Salinity data (Zone 11)

Latitude19.25

16.25

12.25

10.75

9.25

4.25

3.75

2.25

0.75

0.25

Annual average salinity (units?)

35.84

35.29

34.59

33.33

31.87

32.66

 27.11

27.45

28.93

26.02

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 Zone 12.

Table 3: Precipitattion data (zone 12)

Latitude

29.75

26.75

24.25

22.75

14.75

8.75

6.75

4.25

3.75

0.75

Annual total

252.2

286.7

43

44.1

514.6

3853

3030

2097

2221

1968

Average Maximum

 51.9

61.8

10.7

16.9

213.5

1023

732.6

564

368.5

351

Explanation

• Generally, rainfall heavier south than north of the zone.

• The lowest figures seem to be associated with the middle parts of the zone while highest figures are associated with the south below Lat. 9. Medium figures are associated with the extreme north and mid latitudes of the zone.

• 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 zone 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