Seasonal variation of tides, currents, salinity and temperature along the coast of Mozambique Item Type Journal Contribution Authors Sete, C.I.; Ruby, J.; Dove, V. Publisher Maputo : Instituto Nacional de Hidrografia e Navegacao Download date 03/10/2021 15:57:37 Link to Item http://hdl.handle.net/1834/188 UNESCO (IOC) CENADO ODINAFRICA Centro Nacional de Dados Oceanográficos SEASONAL VARIATION OF TIDES, CURRENTS, SALINITY AND TEMPERATURE ALONG THE COAST OF MOZAMBIQUE By Cândida Sete (*), Jafar Ruby (*) and Verónica Dove (**) (*) Instituto Nacional de Hidrografia e Navegação (INAHINA) (**) Instituto de Investigação Pesqueira (IIP) January, 2002 ACKNOWLEDGEMENTS This report was prepared by: Candida Sete - Physical Oceanographer at the National Institute of Hydrography and Navigation- INAHINA Jafar Ruby - Physical Oceanographer at National Institute of Hydrography and Navigation - INAHINA Verónica Fernando Dove - Physical Oceanographer at Fisheries Research Institute - IIP The team members would like to express their gratitude to: • Eng. Albano Gove – Director of INAHINA • Dr. António Mubango Hoguane Chair of UNESCO Marine Science at Eduardo Mondlane University - UEM • Dr. Rui Silva from the Fisheries Research Institute - IIP • Mr. António Olívio Sitoe from INAHINA – CENADO For their contribution and support for this report. This report was made possible through the support provided by the UNESCO - IOC. 2 Contents Page Acknowledgements 2 1.General socio-economic situation of Mozambique 4 1.1.Geography 4 1.2. Economic driving forces 4 1.3. General 4 1.4. Objective of the study 4 1.5. Methodology of the study 5 2. Physical environment 5 2. 1. Climate 5 2.2. Wind conditions 5 2.3. Precipitation 9 2.4. Rivers 9 3. Sea Level variations in the coast of Mozambique 10 4.Temperature and salinity variation along the Mozambique coast 22 5. Current patterns along Mozambique coast 45 6.Overall conclusions 51 7. References 53 8. Appendix 59 3 1.GENERAL SOCIO-ECONOMIC SITUATION OF MOZAMBIQUE 1.1. Geography Mozambique is situated on the eastern coast of Southern Africa, between 10° 27´ S and 26° 52´ S latitudes and 30° 12´ E and 40° 51´ E longitudes. It is bordered by the Republic of Tanzania in the north, by Malawi, Zambia and Zimbabwe in the west, by South Africa and Swaziland in the south and by the Indian Ocean, which makes up to 2700 km of coastline. 1.2. Economic driving forces Agriculture is the most productive and important sector in the national economy accounting for about 24.6% of gross domestic product (GDP), almost 40% of exports and employing 77.6% of the labour force. Services account for over half of GDP, although the sector employs less than 10% of the working population. Industry contributes with almost the same proportion to the country’s employment as services, but its contribution to GDP is much more modest, of about 8.6% in 1997. 1.3.General Mozambique has the third largest coastline in Africa characterised by wide diversity of habitats including sandy beaches, sand dunes, coral reefs, estuarine systems, bays, mangroves and sea grass beds. Coastal resources such as fisheries, agriculture and forestry contribute significantly to the national income and provide social and economic benefits to about two thirds of the population living in the coastal areas. Different research cruises have been carried out in the Mozambique coastal waters, although they are not undertaken in a regular basis. They are carried out in research and commercial vessels belonging mainly to foreign institutions or companies. The objectives vary from oceanographic research to evaluation and assessment of fisheries along the coast. Because the research cruises were occasional and directed to specific areas depending on their objectives, the existing oceanographic information is little and does not cover satisfactory the coast. 1.4. The Objective of the Study The main objective of the study is to describe as concisely as possible the oceanographic conditions of the Mozambique coastal waters. This could be of value for further hydrodynamic studies of coastal areas with particular emphasis on physical processes that most influence the coastal and marine resources, contributing for its sustainable management. Due to the lack of data, it was not possible to extend this study to analyse the variation of other parameters as suggested in the terms of reference. 4 The possible beneficiaries of this study include, among others, port services, coastal engineering, coastal management, scientific research and students. 1.5. Methodology of the study The approach in the methodology used for this report consisted of: Data collection, Collection of information from existing reports and other documents, Desk study. The data collected will be evaluated with respect to: Geography Seasonal variation of sea level, salinity, temperature and currents. The following steps were undertaken: Data collection from existing reports and studies, Information from existing reports and hydrographic maps with regard to tides, temperature and salinity variations, currents, winds, etc. 2. PHYSICAL ENVIRONMENT 2.1.Climate The climate in the region north of the Zambezi River is under the influence of the equatorial low- pressure zone with the NE monsoon in the warm season. The climate south of the Zambezi River is influenced by subtropical anti-cyclonic zone. North of Sofala along the Zambezi River lay a transitional zone, Intertropical Convergence Zone, ICZ, with high rainfall figures (Saetre and Paula e Silva, 1979). 2.2.Wind conditions The winds in the northern part of Mozambique are influenced by the monsoon system with NE winds during the southern summer and SW winds during the southern winter. Central and Southern Mozambique are dominated by the SE trade winds. The bathymetric conditions of Mozambique Channel are shown in figure 1.1. The Figure 1.2 shows some meteorological stations along the coast, that could be taken as representative for the situation at sea. This figure shows appreciable agreement with wind direction at sea while the force tended to be higher in the sea (Saetre e Silva, 1982). 5 According to Tinley (1971) northern Mozambique is affected by the extension of the East African monsoon system with winds blowing North to Northeast during the southern summer and South to Southwest during the southern winter. Central and southern Mozambique is affected by the Southeast Trade Wind System and receives easterly prevailing winds trough out the year. The figure 1.3 shows, on a monthly basis, 10 years average distributions of wind frequencies and forces per directions for the selected meteorological station (Saetre and Silva, 1982). Figure 1.1. Bathymetric map of the Mozambique Channel. 6 Figure 1.2. Location of the main rivers and the meteorological and runoff gauge stations. The numbers are the total runoff in the indicated subareas. 7 Figure 1.3. Mean monthly distribution of wind direction (%) and force (m/s) at four selected coastal Mozambique stations for mean year 1968- 1977 (Saetre and Silva, 1982). 8 2.3. Precipitation The average annual precipitation is about 1200 mm. The rainfall is mainly restricted to the warm season November to April. According to the classification of Köppen, the northern Mozambique (Cabo Delgado, Niassa, Nampula and Zambezia Provinces) and coastal region have a tropical rain savannah climate, whereas the upland areas of the interior have a humid temperate climate. Ocean currents, particularly the Mozambique warm current, may influence the rainfall. 2.4. Rivers The rivers shown in Figure 1.3 drain about 141 km3 of water rich in nutrients into the coastal waters. About 80% of this water enters the ocean from Sofala Bank, central Mozambique. Zambezi River, the largest river in eastern Africa, alone, contributes with 67% of the total river discharge in the whole country (Seatre and Jorge da Silva, 1982). Table 2.1. Seasonal characteristics of Mozambican rivers Flood season Flood peak River Period % of total Period % of total runoff runoff Lurio Meluli Ligonha Moluocue Melela Jan-April 67-82 Feb-Mar 37-52 Nipiode Licungo Incomati Umbeluzi Maputo Dec-Mar 60-64 February 17 Zambezi(1960- Jan-Apr 56 Feb-Mar 32 1974) Pungoe Dec-April 74-84 Feb-Mar 42 Buzi Save Dec-April 99 Feb-mar 52 Limpopo January-April 81 Feb-Mar 53 9 3. SEA LEVEL VARIATION IN THE COAST OF MOZAMBIQUE Figure 3. Tidal station network in the coast of Mozambique. 10 Four sea level observation stations are presently operating in permanent basis in the Mozambique coast, and have provided an important contribution supplying data for operational and research applications. Apart from the actual observation network, tidal data have been obtained for different locations from the past and have been used to generate different products and inputs to different studies. Two of the tidal stations (Inhambane and Pemba) are also part of the GLOSS network. For the purpose of this study, the main tidal characteristics along the coast of Mozambique will be summarised and the seasonal variation of tides will be presented in this report. Tides in the coast of Mozambique, are of semi-diurnal type (two low waters and two high waters a day) with a daily inequality of about 10-20 centimeters in Maputo, Inhambane, Chinde, Quelimane and Angoche, to about 30-40 centimetres in Beira, Pebane, Ilha de Moçambique, Nacala, Pemba and Mocímboa da Praia. 3.1.Tidal Amplitudes The tidal range varies quite considerably between extreme neap tides and extreme spring tides. Tidal amplitudes vary over the year and along the coast. a) Maputo Port region. The tidal range varies from less than 30 centimetres and about 380 centimetres (reference to chart datum) during extreme spring tides. Tidal amplitudes vary over the year between 80 centimetres during neap tides and about 300 centimetres during spring tides (see figure below). Seasonal variation of tides at Maputo harbour (1974 data) 450 400 350 300 250 200 150 100 50 Low and High waters (cm) 0 1 37 73 109 145 181 217 253 289 325 361 397 433 469 505 541 577 613 649 685 721 Time (day x 2) Figure 3.1 a) The variation of tides at Maputo harbour.