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3. HYDROGRAPHY


3.1 Introduction
3.2 Wind and runoff
3.3 Characteristic hydrographical features and water masses
3.4 Hydrographic structure in June-July 1982
3.5 Hydrographic structure in November-December 1982
3.6 Hydrographic structure in May 1983
Discussion

3.1 Introduction

The South Equatorial Current (Fig. 3.1.1), situated at about the latitude of 10°S, diverges at Cape Delgado as it impinges on the coast, and feeds the south flowing Mozambique current and the north flowing East African coastal current, (Newell, 1957). The velocity of the East African coastal current varies between 2 and 0.25 m/s, being faster during the Southwest Monsoon season. During the first survey there were no current measurements done with the exception of station 408 on section IV where the current was found to be 1.4 m/s with direction 342°T.

Fig. 3.1.1. The surface currents of the Indian Ocean in the Northern summer.

Most authors consider that the East African coastal current, which flows northwards off the coast of Tanzania throughout the year, leads to northward flow through the Zanzibar channel and still persists during the Northeast monsoon. Harvey (1977), however, analyzed current measurements from H.M.S. Dalrymple (from surveys done between 1951 and 1954) and found that the vector mean residual currents at three of the six positions in the channel had a southward component, see Fig. 3.1.2. He claims that the majority of the observations used were made during or at the end of the Southwest Monsoon, and none was made during a period of predominantly northerly winds.

Fig. 3.1.2. Residual currents in the Zanzibar Channel (Harvey, 1977).

Fig. 3.1.3 shows that the tidal amplitudes on the Tanzanian coast are comparatively larger than most places in the Indian Ocean. The mean neap and spring tidal ranges, for Dar es Salaam, are 1.1 m and 3.3 m, respectively. The ranges for other places on the coast do not differ very much from these values.

Fig. 3.1.3. The spring tide ranges (m) in the Western Indian Ocean.

3.2 Wind and runoff

At the time of writing this report, the meteorological observations for 1982 were not available at the Meteorological Office in Dar es Salaam. Therefore, the data used are those from 1978 and 1979. The monthly wind direction at six land stations along the coast of Tanzania are shown in Table 3.2.1. At Zanzibar station, however, observations for the months of May, November and December were carried out only at 0300, 0600, 1200, 1500 and 1800 hrs.

The Southwest Monsoon season lasts from April to October and the Northeast Monsoon from November to March. In March the winds tend to subside for all stations. This is the transition period, at which time there is reversal from the Northeast to the South-west Monsoon. Another transition period occurs in October. During the Northeast Monsoon, at Tanga the wind blows from the northeast, at Zanzibar from the north, at Dar es Salaam from the east-northeast, and at Mtwara from the north. The wind blows from the south for all stations during the Southwest Monsoon. Records from Zanzibar station show that the frequency of winds from the westerly direction is almost double than those of other stations. This is most probably due to the fact that the station is situated on the western part of the Island, a kind of a leeward side.

On the other hand, wind data from the ship during this survey are as follows:

a) During the first survey the wind was blowing from the south, all along the coast.
b) During the second survey the wind was blowing from the southeast.
c) During the third survey the wind blew from the southeast and the south.
It is evident that the land topography affects the wind observations. Offshore wind observations, therefore, may differ slightly from those on land. The records, also, show a strong diurnal variation. There are pronounced nocturnal offshore winds. However, the winds tend to calm down between 2100 and 1300 hrs except for the months of May and June.

Table 3.2.1. Frequency distribution of wind direction (number of observations, C = calm).

Tanga 1978

Zanzibar 1978

Dar es Salaam 1979

Mtwara 1979

Fig. 3.2.1 shows the monthly freshwater discharge from six major rivers for each particular year. The records are old because of lack of equipment and logistic problems facing the Water authority. The figure is intended to show the seasonal variations in freshwater discharge into the ocean. For example, the figures for the Rufiji river could be misleading because in 1967 the hydroelectric power plant at Kidatu had not been built yet.

The peak outflow from these rivers occur between March and May. During October/November the outflow is at its minimum. Apart from Pangani, there is a well pronounced seasonal variation for all the rivers. The outflow of the Pangani river runs more or less constant throughout the year except for the months of May and June. This uniformity can be attributed to the nature of the drainage area. The drainage area of Pangani gets rain almost throughout the year.

3.3 Characteristic hydrographical features and water masses

Vertical profiles for temperature, salinity and oxygen for section IV June 1982 are presented in Fig. 3.4.4. It shows most of the hydrographical features common to the Tanzanian coastal waters. dt sections were not drawn because dt is largely controlled by temperature. However, where salinity decreases with increasing depth the rate of increase of dt with depth is reduced.

The thermocline lies between 50 and 200 m. There is a shallow salinity maximum at about 150-200 m depth. The minimum zone in the vertical profile of oxygen is found slightly below or at the same depth as the salinity maximum, (c.f. Sætre and Silva, 1979, p. 39).

Fig. 3.3.1 shows the temperature-salinity relationship from the three cruises of this survey for all stations. The main water masses have been identified by using Rochford (1964), Warren et al. (1966) and Wyrtki (1971).

Fig. 3.2.1. Monthly average freshwater runoff from six major rivers (? = data not available).

Fig. 3.3.1. The temperature/salinity relationship for all stations observed during the three surveys.

Surface Water. This is a water mass lying above 100 m. It is formed in the Bay of Bengal and the eastern Indian Ocean area and brought to the west by the South Equatorial Current. The temperature range is 22 to 30°C, whereas its salinity lies below 35.4 ‰. Near to the coast the salinity is kept low by the freshwater runoff.

High Salinity Water. Fig. 3.3.1 also shows a salinity maximum between 18 and 19°C, which corresponds to 150 - 250 m depth. According to Wyrtki (1971), there are two possible sources of this water, namely, Subtropical Surface Water and Arabian Sea Surface Water. These are best separated in the temperature-oxygen diagram (ibid.). This water is formed in the northern Arabian Sea and spreads southwest to the eastern coast of Africa. Rochford (1964) shows that at the Equator it widens and develops a branch that flows southeast. At about 10°S all branches turn to flow to the east. However, the outermost on section IV, during the second and the third cruise, had two maxima. This could be caused by an intrusion of lower salinity water, most probably coming from the eastern side of the Indian Ocean within the South Equatorial Current, (Harvey, 1977).

Indian Ocean Central Water. The layer between 250 and 500 m has a linear T-S relationship. This layer is thought to belong to the Central Water Mass. It was not possible to identify other water masses because the hydrographic sections covered the upper 500 m only. However, literature shows there are deeper water masses. For example, the Antarctic Intermediat, the Indian Ocean Deep and the Red Sea water.

3.4 Hydrographic structure in June-July 1982

The vertical distribution of temperature, salinity and oxygen in the eight hydrographic sections are presented in Figs. 3.4.1-3.4.8. The surface temperatures was 26-27°C. As already mentioned in section 3.3, a salinity maximum and an oxygen minimum occupy more or less the same depth, about 200 m. Following Wyrtki (1971) the mixed layer depth, D, is defined as tS - tD 1°C, where tS is the temperature at the surface and tD the temperature at D, but where there was a shallow thermocline less than 10 m in depth the drop in temperature across this thermocline was ignored in determining the depth of the mixed layer.

Section I (Fig. 3.4.1). No calculations were made for the geostrophic velocity but visual inspection indicates that there was a northward current. The average surface temperature was 26.5°C. The depth of the mixed layer was 85 m. The salinity profile shows that around Moa Bay there is low salinity water being produced, probably from the runoff.

Section II (Fig. 3.4.2). In this section the depth of the mixed layer was about 95 m. The section failed to show any discernible direction for the geostrophic current. The average surface temperature was 26.48°C. There was a tongue of low salinity water near the coast on the mainland side.

Section III (Fig. 3.4.3). This was the shallowest section, not deeper than 40 m. Its average temperature was 26.41°C. The surface salinity decreased seaward. The water appeared to be well mixed because of the combination of strong tidal currents and the wind in the Zanzibar channel.

Section IV (Fig. 3.4.4). There were indications of a weak northward current. The surface salinity decreased towards the coast. The oxygen minimum of 2.5 ml/l was situated about 50 m below the salinity maximum. The average mixed layer depth was about 60 m. The average surface temperature was 26.93°C.

Section V (Fig. 3.4.5). The average surface temperature was 27.12°C. The baroclinic structure suggested a northward current. The depth of the mixed layer was about 92 m.

Section VI (Fig. 3.4.6). The effect of freshwater discharge from the Rufiji river is shown clearly by the vertical isolines with salinity increasing seaward. The outermost stations indicate a northward current. The average surface temperature was 27.12°C and the mixed layer was about 91 m deep.

Fig. 3.4.1. Temperature, salinity, density and oxygen content at Section I during the first survey.

Fig. 3.4.2. Temperature, salinity, density and oxygen content at Section II during the first survey.

Fig. 3.4.3. Temperature, salinity, density and oxygen content at Section III during the first survey.

Fig. 3.4.4. Temperature, salinity, density and oxygen content at Section IV during the first survey.

Fig. 3.4.5. Temperature, salinity, density and oxygen content at Section V during the first survey.

Fig. 3.4.6. Temperature, salinity, density and oxygen content at Section VI during the first survey.

Fig. 3.4.7. Temperature, salinity, density and oxygen content at Section VII during the first survey.

Fig. 3.4.8. Temperature, salinity, density and oxygen content at Section VIII during the first survey.

Sections VII and VIII (Figs. 3.4.7 and 3.4.8) showed a weak cyclonic eddy. The average surface temperature was 26.52°C, while the mixed layer depth was found to be 88 m.

In general the surface temperature decreased from north to about 6°30'S along section IV and rose again. The average temperature for the cruise was 26.64°C. The average depth of the mixed layer was 84 m, being shallowest along section IV, where it was 40 m.

3.5 Hydrographic structure in November-December 1982

A few more stations were added during this cruise. The surface temperatures were 1-2°C higher and the thermocline was not as distinct as in the previous cruise. The salinity in the upper layer was also higher (0.1 - 0.2 ‰). The oxygen content was similar for the sections I - VI as observed in June - July but for sections VII and VIII the oxygen content was up to 2 ml/l higher.

Section I (Fig. 3.5.1). The baroclinic structure was weak. There were no indications of any current except in the Pemba channel where there were slight indications of a northward current. The average surface temperature and the depth of the mixed layer were 28.22°C and 28 m, respectively.

Section II (Fig. 3.5.2). This section was extended by adding two more stations seaward. The average surface temperature was 27.84°C and the mixed layer depth was 28 m.

Section III (Fig. 3.5.3). Both the temperature and the salinity profiles show an intrusion of a warmer and fresher water from the mainland side. The average surface temperature was 28.52°C.

Section IV (Fig. 3.5.4). The temperature distribution indicates a strong current shear, especially below 200 m. Near the coast the current had a northward direction. The average surface temperature was 27.70°C and depth of the mixed layer was 47 m. At station 568 two salinity maxima were observed at 50 and 200 m, respectively.

Fig. 3.5.1. Temperature, salinity, density and oxygen content at Section I during the second survey.

Fig. 3.5.2. Temperature, salinity, density and oxygen content at Section II during the second survey.

Fig. 3.5.3. Temperature, salinity, density and oxygen content at Section III during the second survey.

Fig. 3.5.4. Temperature, salinity, density and oxygen content at Section IV during the second survey.

Fig. 3.5.5. Temperature, salinity, density and oxygen content at Section V during the second survey.

Fig. 3.5.6. Temperature, salinity, density and oxygen content at Section VI during the second survey.

Fig. 3.5.7. Temperature, salinity, density and oxygen content at Section VII during the second survey.

Fig. 3.5.8. Temperature, salinity, density and oxygen content at Section VIII during the second survey.

Section V (Fig. 3.5.5) shows a wedge-shape salinity distribution. The average surface temperature was 28.21°C and the depth of the mixed layer was the same as in section IV. The core of maximum salinity was observed to be shallower than during the previous cruise.

However, the zone of maximum salinity in section VI (Fig. 3.5.6) was deeper than during the previous cruise. There was an indication of a northward current. The average surface temperature was 28.14°C and the mixed layer depth was 39 m.

Sections VII and VIII (Fig. 3.5.7 and 3.5.8). The baroclinic structure was weak, and the southward transport was small. The depth of the mixed layer was 49 m, and the average surface temperature was 27.78°C.

The average surface temperature for the whole cruise was 28.05°C and the mixed layer depth was 40 m. The depth of the thermocline was shallower than during the previous cruise. The oxygen content for the water column 5 - 500 m was above 2.4 ml/l.

3.6 Hydrographic structure in May 1983

In addition to the usual eight sections many other surface samples were taken for salinity and temperature. The surface temperature varied between 27.5 and 28.5°C. The surface salinity distribution (Fig. 3.6.1) demonstrated the influence of the freshwater outflow, especially off the mouths of major rivers e.g. Rufiji, Ruvu and Pangani.

Section I (Fig. 3.6.2). Inside the Pemba channel the baroclinic structure did not show any sign for current. The outer stations, however, showed a northward current for depths below 200 m, while the surface water was going south. The average temperature was 28.07°C and the depth of the mixed layer was 66 m.

Section IV (Fig. 3.6.3) shows two distinct cores of maximum salinity for station 84, one at about 130 m and another at 200 m.

Fig. 3.6.1. Surface salinity distribution during the third survey.

Fig. 3.6.2. Temperature, salinity, density and oxygen content at Section I during the third survey.

Fig. 3.6.3. Temperature, salinity, density and oxygen content at Section IV during the third survey.

Discussion

(...)

(...) found that the annual range was greatest in the areas furthest from the equator (4.6°C) and least for the area between 5°30' and 7°5. The depth of the upper mixed layer varied from 20 m (March and November) to 100 m (June/July). This reflects the seasonal variation of the wind speed and direction (Table 3.2.1).

In November the surface salinities were at maximum. They are at minimum in May following the peak freshwater outflow (Fig. 3.2.1). Bryceson (1977), however, found that the salinity started to decrease before the onset of the rains and attributed this to the advection of lower salinity water from the south February onwards. The combination of high salinity and (relatively) low temperature during the second cruise suggest a high rate of evaporation, especially on section IV.

The freshwater from the river discharge is very much restricted to the inshore waters. This is due to the fact that wind blowing from the south (in the southern hemisphere), parallel to a coast on its left, causes an Ekman transport to the left. Consequently, water piles at the coast.

The oxygen content observed during this survey cannot be said to be a limiting factor on fish distribution. The oxygen minimum increased during the November-December cruise. This means either there is a seasonal variation in the oxygen minimum zone or being the time for the monsoon reversal another water mass is fed into the area, as suggested earlier by Bryceson (1977) and Harvey (1977).

The current pattern disturbances observed around Latham Island during the third cruise are eddies which cause a localized upwelling. This idea is supported by the existence of a shallow thermocline and indications of cyclonic eddies along section IV. Similar conditions apply to the area north of Pemba Island. The geostrophic current was observed to flow north. This does not necessarily mean the current is always northbound unless some cruises are scheduled during the Northeast Monsoon season and prove so.


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