Presentation on theme: "ODINAFRICA/GLOSS Training Workshop on Sea-Level Measurement and Interpretation. Oostende, Belgium, 13-24 November 2006 Shigalla Mahongo Tanzania Fisheries."— Presentation transcript:
ODINAFRICA/GLOSS Training Workshop on Sea-Level Measurement and Interpretation. Oostende, Belgium, November 2006 Shigalla Mahongo Tanzania Fisheries Research Institute P.O. Box 9750 Dar es Salaam TANZANIA Regional Oceanography and Climate East Africa
THE EAST AFRICAN REGION We define the East African Region to include: Mainland countries of: Somalia, Kenya, Tanzania & Mozambique Islands of: Seychelles, Comoros, Madagascar, Mauritius & Reunion
THE EAST AFRICAN REGION A large part of the East African coastal plain is low-lying, very variable in width and relatively unindented. The extensive coastal plains in Madagascar are associated with major rivers and are the result of deposition of soil from the upland plateau. The oceanic islands have only a few streams, most of which are seasonal.
THE EAST AFRICAN REGION The island states are generally volcanic in origin Characterized by very narrow coastal plains. These coastal plains are almost absent in some areas, for example in Seychelles.
THE EAST AFRICAN REGION There are several large rivers along the Eastern African mainland shores, with largest rivers running through Mozambique.
CONTINENTAL SHELF Along much of the continental margin of eastern Africa, the continental shelf varies markedly throughout the region. It is generally very narrow and the shores dip down steeply to the ocean floor. The fringing reef is a dominant feature along most of the edge of the shelf, usually at 45 m depth.
LENGTH OF COASTLINE, CONTINENTAL SHELF AREA AND COASTAL POPULATION Sources: Gaudian et al., 2002; COI, 2004
SURFACE TEMPERATURE Surface air temperature rarely falls below 20 o C. Water temperature is usually between o C with small annual variations. Away from the Equator, there is a clear distinction between the cold winter and hot summer seasons. The highlands have a marked temperature differential, which determine the local humidity. LatitudeFebMayAugNov 10°N–0°S25–2628–2921–2526–27 0°N–10°S26–282824– °N– 20°S2825–2823–2425–27 20°N– 30°S25–2822–2520–2322–25 Source: IUCN/UNEP (1985) Surface Water Temp ( o C)
Climate The Monsoon Wind System The climate of this region is subject to alternating and distinctive seasons, the southern and northern monsoons, which have a marked effect on air and water temperature, winds and rainfall. A monsoon is defined by a seasonal change in wind direction between January and July During the months of November-March (southern hemisphere summer), the prevailing trade wind is the NE monsoon. South of the Equator, this wind becomes more north-westerly due to the effect of the earth’s rotation. The NE monsoon wind is usually steady and light, about 5 m/sec. The wind (and season) is usually associated with short rainy periods and the true rainy season begins after the NE monsoon, from March-June.
Climate The Monsoon Wind System From June-September (winter months), the region experiences a complete reversal in wind direction, a feature unique to the Indian Ocean, and the SE monsoon wind prevails. This is a stronger wind, reaching an average velocity of 9 m/sec, especially in the southern parts of the region. In northern parts of the region, this wind veers to the west and in some places it may be referred to as a south-westerly wind. Winds are a particularly important feature in the region, driving the water circulation and affecting wave action, local climate, biological processes and human activities.
Climate: Monsoon Winds
Climate Other Wind Systems Two other wind types that occur in the Region are cyclones and daily onshore and offshore breezes. The shore breezes are generated by the temperature difference between land and sea, which causes these generally light winds to blow off-shore during the night and onshore during the day. Onshore winds are noticeably stronger during the flooding springtide. Cyclones are far less understood, but they occur more frequently in the Comoros, Madagascar and the Mascarene islands. Cyclones are more common during the months of January to March and have devastating effects, especially when they coincide with high spring tides.
Climate RAINFALL Most of the areas near the Equator have two distinct rainfall seasons centered around March-May and Sept-Nov. The seasons are related to the double passage of the low pressure ITCZ, which follow the seasonal movement of the sun. Away from the Equator, most of the rain is concentrated within the summer seasons. The peaks in the northern and southern hemisphere are centred around June-Aug & Dec-Feb, respectively. The seasonal rainfall patterns are significantly modified in some areas by meso-scale systems, e.g. the land/sea breeze effects. Other factors include the intensity, location and orientation of the monsoonal wind systems, tropical cyclones, sub-tropical anticyclones, jet streams, extra-tropical weather systems and easterly/westerly waves.
Climate RAINFALL The ITCZ is not a zone of rainfall. It is a zone of instability within which a number of factors can lead to a triggering of the rainfall mechanism.
Climate: RAINFALL THE EFFECT OF TROPICAL CYCLONES Tropical cyclone tracks for the 2000 season, resulting in the Mozambique floods
Climate RAINFALL Global scale systems like the El Niño/Southern Oscillation (ENSO) also play an important role. Most of the region experiences annual rainfall of between 1,000- 2,000 mm, though local variations are common.
Climate: RAINFALL THE EFFECT OF ENSO Warming of seawater (2-8 o C above normal) is experienced at intervals of 3-10 years in the western coast of South America, refers to as El-Niño (Spanish for Little Boy - Peruvian anchovy fishermen of the 1800s). La-Niña (Spanish for Little Girl) is a cold event that typically (but not always) occurs after an El-Niño event. El Niño and La Niña are opposite phases of the El-Niño Southern Oscillation (ENSO) cycle, which describes fluctuations in the strength of oceanographic and atmospheric circulation patterns across the Pacific. The effects of El Niño are felt worldwide. Severe floods in occurred in southern Kenya during the El-Niño, and dumped massive quantities of sediments along the coast. In Tanzania, there were prolonged high levels of freshwater in the upper reaches of the Rufiji Delta. Coral reefs of the Western Indian Ocean were also affected (bleaching).
Oceanography TIDES The tides over the entire mainland coast of East Africa, the western coast of Madagascar and the small islands of the Aldabra group are predominantly semidiurnal, with spring tidal ranges of 2-4m. The range is over 5m on large estuaries along the Mozambican coast. The tides of Seychelles, Mauritius and Reunion are mixed but mainly diurnal except at Rodriguez (semidiurnal). The spring tidal ranges are also smaller, about 1m or less. Tidal currents can be extremely strong, particularly along breaks in the reef or narrow passages.
Oceanography: TIDES MEAN SPRING TIDAL RANGES (m) Point des Galets REUNION0.4 Port LouisMAURITIUS0.5 Tamatave MADAGASCAR0.6 Fort Dauphin MADAGASCAR0.7 Victoria (Mahe) SEYCHELLES1.2 Mogadishu SOMALIA2.1 Kismayo SOMALIA2.4 Inhambane MOZAMBIQUE2.6 Tulear MADAGASCAR2.7 Malindi KENYA 3.1 Dar es Salaam TANZANIA3.2 Moroni COMOROS3.3 Lindi TANZANIA3.4 Mkoani (Pemba Is) TANZANIA3.6 Zanzibar Town TANZANIA3.6 Mombasa KENYA3.6 Nosy Be MADAGASCAR3.6 Mocambique MOZAMBIQUE3.6 Mocimbo de Praia MOZAMBIQUE3.7 Beira MOZAMBIQUE5.7
Oceanography TIDES FORM FACTOR: F = (H K1 + H O1 )/( H M2 + H S2 ) F = Semidiurnal F = Mixed, Mainly Semidiurnal F = Mixed, Mainly Diurnal F > 3.00 Diurnal
Oceanography: TIDES LocationCountry Form Factor Tide Type Maputo AMOZAMBIQUE0.04Semidiurnal Antonio EnesMOZAMBIQUE0.07Semidiurnal Diego GarciaUK0.10Semidiurnal Nacala BMOZAMBIQUE0.12Semidiurnal Pemba BMOZAMBIQUE0.12Semidiurnal KerguelenFRANCE0.12Semidiurnal DzaoudziMAYOTTE0.14Semidiurnal RodriguesMAURITIUS0.14Semidiurnal Nozy Be MADAGASCAR0.15Semidiurnal ZanzibarTANZANA0.16Semidiurnal St PaulFRANCE0.17Semidiurnal Dar es Salaam TANZANIA0.18Semidiurnal Lamu BKENYA0.22Semidiurnal MombasaKENYA0.23Semidiurnal GanMALDIVES0.30Mixed, Mainly Semidiurnal ReunionFRANCE0.35Mixed, Mainly Semidiurnal CrozetFRANCE0.35 Mixed, Mainly Semidiurnal Port Louis CMAURITIUS0.42 Mixed, Mainly Semidiurnal Male BMALDIVES0.48Mixed, Mainly Semidiurnal Port Victoria BSEYCHELLES0.49 Mixed, Mainly Semidiurnal PraslinSEYCHELLES0.58 Mixed, Mainly Semidiurnal HanimaadhooMALDIVES0.78 Mixed, Mainly Semidiurnal Data: UHSLC, F obtained using SLPR2 (DOS)SLPR2
Oceanography TIDES Semidiurnal F = 0.04
Oceanography TIDES Semidiurnal F = 0.08
Oceanography TIDES Semidiurnal F = 0.10
Oceanography TIDES Semidiurnal F = 0.12
Oceanography TIDES Semidiurnal F = 0.07
Oceanography TIDES Semidiurnal F = 0.12
Oceanography TIDES Semidiurnal F = 0.14
Oceanography TIDES Mixed, Mainly Semidiurnal F = 0.14
Oceanography TIDES Mixed, Mainly Semidiurnal F = 0.35
Oceanography TIDES Mixed, Mainly Semidiurnal F = 0.35
Oceanography TIDES Mixed, Mainly Semidiurnal F = 0.42
Oceanography TIDES Mixed, Mainly Semidiurnal F = 0.48
Oceanography TIDES Mixed, Mainly Semidiurnal F = 0.49
Oceanography TIDES Mixed, Mainly Semidiurnal F = 0.58
Oceanography TIDES Mixed, Mainly Semidiurnal F = 0.78
Oceanography CURRENTS In the Indian Ocean, the steady South Equatorial Current (SEC) flows from the eastern part to northern Madagascar and Africa. The SEC is created by the southeast trade winds, which push the water to the west. The main stream of this current flows about 12 o south of the equator. Since the Indian Ocean is closed off to the north, the wind and ocean current systems are restricted to a small area and are forced southwards. When the SEC reaches the western side of the Indian Ocean it splits into two main currents: one flowing south, comprising the Madagascar Current and the perennial Mozambique Current, and the other flowing north, called the East African Coastal Current (EACC).
Monsoon Winds & Major Ocean Currents
Oceanography CURRENTS South of Madagascar, the East Madagascar current and the Mozambique current join at about 26 o S to form the southward flowing Agulhas Current. The Agulhas Current is dominant along the eastern coast of South Africa, rarely dropping below 21°C. The EACC flows as far as southern Somalia, after which its direction depends on the season. During the SE monsoon, the flow of the EACC is increased to 4 knots and its range extends further north, becoming the Somali Current, and later joins the Indian Monsoon Current.
Oceanography CURRENTS During the NE monsoon, the northerly flow of the EACC is reduced to less than 1 knot in places and its course is diverted eastwards toward Australia, becoming the Equatorial Counter Current. The region can therefore be divided into three hydrographic zones (El Sayed, 1989): (a) Somali upwelling zone: north-east of Somali coast (b)Monsoon current zone: Tanzania, Kenya, Seychelles (c) Agulhas and Mozambique current zone: Comoros, Madagascar, Mauritius and Mozambique. In this zone, current flow patterns are subject to seasonal, cyclonic influence in December-April.
Oceanography UP/DOWNWELLING Through much of the region the water depth is between 3,000–4,000 m, though a few areas of depths greater than 5,000 m occur. The deep sea-sea waters are rich in nutrients, that continually trickle down from the surface water layers. The fast flowing Somali Current transports about million m 3 /sec of water penetrating deep into the oceanic water mass and causing an upwelling about its west flank along the north-west Somali coast. This upwelling induces high productivity off the Somali coast as the turbulent activity brings nutrient-rich cold sub-surface waters with temperatures below 20 o C to the surface.
Oceanography UP/DOWNWELLING Most other parts of the region experience down-welling and the surface waters are generally nutrient-poor resulting in low biological productivity. The EACC feeds the continental shelves of Kenya and Tanzania with nutrient-poor mid-ocean water.