ODINAFRICA/GLOSS Training Workshop on
Sea-Level Measurement and Interpretation.
Oostende, Belgium, 13-24 November 2006
Regional Oceanography and Climate
East Africa
Shigalla Mahongo
Tanzania Fisheries Research Institute
P.O. Box 9750
Dar es Salaam
TANZANIA
EMAIL: SHIGALLA@YAHOO.CO.UK
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 20oC. Water
temperature is usually
between 20-30oC 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.
Surface Water Temp (oC)
Latitude
Feb
May
Aug
Nov
10°N–0°S
25–26
28–29
21–25
26–27
0°N–10°S
26–28
28
24–26
27
10°N– 20°S
28
25–28
23–24
25–27
20°N– 30°S
25–28
22–25
20–23
22–25
Source: IUCN/UNEP (1985)
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
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 1,0002,000 mm, though local variations are common.
Climate: RAINFALL
THE EFFECT OF ENSO

Warming of seawater (2-8oC 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 1997-98 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
Point des Galets
REUNION
Port Louis
MAURITIUS
Tamatave
MADAGASCAR
Fort Dauphin
MADAGASCAR
Victoria (Mahe)
SEYCHELLES
Mogadishu
SOMALIA
Kismayo
SOMALIA
Inhambane
MOZAMBIQUE
Tulear
MADAGASCAR
Malindi
KENYA
Dar es Salaam
TANZANIA
Moroni
COMOROS
Lindi
TANZANIA
Mkoani (Pemba Is)
TANZANIA
Zanzibar Town
TANZANIA
Mombasa
KENYA
Nosy Be
MADAGASCAR
Mocambique
MOZAMBIQUE
Mocimbo de Praia
MOZAMBIQUE
Beira
MOZAMBIQUE
(m)
0.4
0.5
0.6
0.7
1.2
2.1
2.4
2.6
2.7
3.1
3.2
3.3
3.4
3.6
3.6
3.6
3.6
3.6
3.7
5.7
Oceanography
TIDES
FORM FACTOR:
F = (HK1 + HO1)/( HM2 + HS2)
F = 0.00 - 0.25
F = 0.25 - 1.50
F = 1.50 - 3.00
F > 3.00
Semidiurnal
Mixed, Mainly Semidiurnal
Mixed, Mainly Diurnal
Diurnal
Oceanography: TIDES






















Location
Maputo A
Antonio Enes
Diego Garcia
Nacala B
Pemba B
Kerguelen
Dzaoudzi
Rodrigues
Nozy Be
Zanzibar
St Paul
Dar es Salaam
Lamu B
Mombasa
Gan
Reunion
Crozet
Port Louis C
Male B
Port Victoria B
Praslin
Hanimaadhoo
Country
Form Factor
MOZAMBIQUE
0.04
MOZAMBIQUE
0.07
UK
0.10
MOZAMBIQUE
0.12
MOZAMBIQUE
0.12
FRANCE
0.12
MAYOTTE
0.14
MAURITIUS
0.14
MADAGASCAR
0.15
TANZANA
0.16
FRANCE
0.17
TANZANIA
0.18
KENYA
0.22
KENYA
0.23
MALDIVES
0.30
FRANCE
0.35
FRANCE
0.35
MAURITIUS
0.42
MALDIVES
0.48
SEYCHELLES
0.49
SEYCHELLES
0.58
MALDIVES
0.78
Tide Type
Semidiurnal
Semidiurnal
Semidiurnal
Semidiurnal
Semidiurnal
Semidiurnal
Semidiurnal
Semidiurnal
Semidiurnal
Semidiurnal
Semidiurnal
Semidiurnal
Semidiurnal
Semidiurnal
Mixed, Mainly Semidiurnal
Mixed, Mainly Semidiurnal
Mixed, Mainly Semidiurnal
Mixed, Mainly Semidiurnal
Mixed, Mainly Semidiurnal
Mixed, Mainly Semidiurnal
Mixed, Mainly Semidiurnal
Mixed, Mainly Semidiurnal
Data: UHSLC, F obtained using SLPR2 (DOS)
Oceanography
TIDES
3
Maputo A, January 1974
2
1
0
-1
Semidiurnal
-2
F = 0.04
-3
10
20
30
Oceanography
TIDES
3
2
Antonio Enes, July 1967
1
0
-1
Semidiurnal
-2
F = 0.08
-3
10
20
30
Oceanography
TIDES
3
2
Diego Garcia C, January 2000
1
0
-1
Semidiurnal
-2
F = 0.10
-3
10
20
30
Oceanography
TIDES
3
2
Nacala B, December 1982
1
0
-1
Semidiurnal
-2
F = 0.12
-3
10
20
30
Oceanography
TIDES
3
Pemba B, March 1983
2
1
0
-1
Semidiurnal
-2
F = 0.07
-3
10
20
30
Oceanography
TIDES
3
2
Kerguelen, January 1998
1
0
-1
Semidiurnal
F = 0.12
-2
-3
10
20
30
Oceanography
TIDES
3
2
Dzaoudzi, January 1995
1
0
-1
Semidiurnal
-2
F = 0.14
-3
10
20
30
Oceanography
TIDES
3
2
Rodrigues, January 2003
1
0
-1
Mixed, Mainly Semidiurnal
F = 0.14
-2
-3
10
20
30
Oceanography
TIDES
3
2
Nosy Be, January 1994
1
0
-1
Semidiurnal
-2
F = 0.15
-3
10
20
30
Oceanography
TIDES
3
2
Zanzibar, January 2004
1
0
-1
Semidiurnal
-2
F = 0.16
-3
10
20
30
Oceanography
TIDES
3
2
1
Saint Paul, January 2000
0
Semidiurnal
-1
F = 0.17
-2
-3
10
20
30
Oceanography
TIDES
3
2
Dar es Salaam, January 1990
1
0
-1
Semidiurnal
-2
F = 0.18
-3
10
20
30
Oceanography
TIDES
3
2
Lamu B, January 2003
1
0
-1
-2
Semidiurnal
F = 0.22
-3
10
20
30
Oceanography
TIDES
2
Mombasa, January 1999
0
-2
Semidiurnal
F=0.23
10
20
30
Oceanography
TIDES
3
2
1
Gan, January 2003
0
-1
Mixed, Mainly Semidiurnal
0.30
-2
-3
10
20
30
Oceanography
TIDES
3
2
1
Reunion, January 1985
0
-1
Mixed, Mainly Semidiurnal
-2
F = 0.35
-3
10
20
30
Oceanography
TIDES
3
2
1
Crozet, January 1999
0
Mixed, Mainly Semidiurnal
-1
F = 0.35
-2
-3
10
20
30
Oceanography
TIDES
3
2
1
Port Louis C, January 2003
0
Mixed, Mainly Semidiurnal
-1
F = 0.42
-2
-3
10
20
30
Oceanography
TIDES
3
2
1
Male B January 2003
0
Mixed, Mainly Semidiurnal
-1
F = 0.48
-2
-3
10
20
30
Oceanography
TIDES
3
2
Port Victoria B, Jan 1992
1
0
-1
Mixed, Mainly Semidiurnal
F = 0.49
-2
-3
10
20
30
Oceanography
TIDES
3
2
1
Praslin, Jan 1-31, 1989
0
-1
Mixed, Mainly Semidiurnal
F = 0.58
-2
-3
10
20
30
Oceanography
TIDES
3
2
1
Hanimaadhoo, January 2002
0
-1
Mixed, Mainly Semidiurnal
F = 0.78
-2
-3
10
20
30
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 12o 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 26oS 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 50-65 million
m3/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 20oC 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.