A summary of oceanographic knowledge of the Agulhas and Somali Current LMEs

The oceanographic conditions in the Agulhas and Somali Current LMEs are distinct. In the southern part of the west Indian Ocean, the Agulhas Current system dominates. This current is one of the largest western boundary currents in the world and is fed from a range of complex sources. By contrast, the Somali Current in the northern part of the west Indian Ocean, is a shallow current that has the unusual characteristic of seasonally reversing direction. In addition, there are currents that carry water past the islands of the south west Indian Ocean and which form part of the typical wind-driven subtropical gyre.

Knowledge of the oceanography of these different systems, including their physical, chemical, biological and geological features, is highly variable. Parts of the Agulhas Current system have been comprehensively studied, but knowledge about other parts – the Mozambique Channel and the seas around Madagascar, for instance – is scant. And, while some features of the Somali Current have been relatively well observed in the past, no studies are taking place inshore because of security concerns in Somalia. In addition, few dedicated oceanographic studies have been undertaken in the vicinity of most of the islands of the south west Indian Ocean. The influence of all these currents on the circulation over the adjacent shelves, the local chemistry, biology and sediment movement has been inadequately investigated.

This summary of what is known about the oceanography of the west Indian Ocean is strongly skewed towards physical oceanography. Considerably less is known about the chemical oceanography, biological oceanography and marine geology of the region.

 

Agulhas Current system

The Agulhas Current system may be considered to consist of three components: a source region, the northern Agulhas Current proper and the southern Agulhas Current. The sources of the Agulhas Current are the subtropical gyre of the south Indian Ocean, the region east of Madagascar and the Mozambique Channel. Of these, the subtropical gyre is the main contributor by volume. The flow through the Mozambique Channel consists largely of intense, anti-cyclonic eddies that are formed in the narrows of the channel and that move poleward from there. These eddies may be 200km in diameter and extend all the way to the sea floor. Their influence on adjacent shelf regions is not known.

 Between the narrows of the channel and the Comoros Islands, the flow seems to be in the form of an anti-cyclonic gyre, but this conclusion is based on very few observations and the flow may in fact be quite variable. The currents on the eastern side of the channel, outside the direct influence of the Mozambique eddies, are unknown. There are some observations that suggest a net southward movement, but other measurements indicate a northward movement. Remote sensing data have shown the presence of weak eddies in this region that draw water off the shelf. Whether this is an important process from a chemical and biological point of view is not yet known. In fact, there are no published observations of currents on this shelf region whatsoever.

The flow along the east coast of Madagascar is equally poorly known. A fast and intense current, the East Madagascar Current carries water along the edge of the narrow continental shelf. The southern part of this current goes poleward; the northern component equatorward. The location of the bifurcation is not known with any degree of accuracy. Since the shelf is so narrow it may be assumed that the water on the shelf moves in sympathy with the juxtapositioned current, but to date there are no observations to adequately support this hypothesis. Both along the shelf of Mozambique and the shelf of Madagascar, upwelling cells with enhanced nutrient contents have been observed. These include a cell off the Delagoa Bight along the Mozambican coast and at near Angoche on the southeast coast of Madagascar. The intensity and the persistence of these cells, as well as their biological impact, needs to be determined.

The northern Agulhas Current seems to start somewhere between the cities of Durban and Maputo, according to evidence gleaned from the movement of sediments on the adjacent shelf. The current is strong, intense and increases in depth and in volume flux downstream. Maximum velocities exceed 2m/s on its inshore side; the current being 60 to 100 km wide with an intense inshore edge but a more dispersed offshore border. The current follows the shelf edge quite closely, diverging from the coastline where the continental shelf is wide, such as at the Natal Bight. In general, the trajectory of the current is unusually stable for a current of this nature, the exception occurring during the passage along the current of a Natal pulse. This singular meander commences at the Natal Bight and from there moves down the current at about 20 km/day, growing in amplitude. The coastal effect of the passage of a Natal Pulse is a sudden, but short-lived reversal in current directions. On the whole, the water movement on the adjacent shelf is parallel and in the same direction as the northern Agulhas Current. There are two exceptions: directly south of Durban and over the Natal Bight. South of Durban there is a persistent lee eddy that may be carried off in an incipient Natal Pulse; on the Natal Bight the water movement may be largely dependent on the reigning wind.

At the northern tip of the Natal Bight there is a concentrated and persistent upwelling cell that probably supplies a major part of the bottom water for this part of the shelf. It enhances the local nutrient supply and leads to a marked increase in local phytoplankton density. The precise role of this point source of nutrients for the ecosystem of the region has not been studied.

The path of the southern Agulhas Current – downstream of the city of Port Elizabeth – is much less stable, exhibits meanders as well as attendant shear edge eddies and plumes. It flows past the wide Agulhas Bank and has a decisive influence on this shelf’s water masses. At the eastern corner of the Agulhas Bank there is a persistent upwelling cell. It has been inferred that this cell supplies all the nutrient rich bottom water for the bank. It also enhances the seasonal thermocline. This process may be crucial for the successful spawning of the major pelagic fish species that have been shown to preferentially produce their eggs on this bank. From here the larvae and sprat move to the Benguela upwelling system of the south Atlantic Ocean to support the largest fisheries of the region – therefore a true transboundary ecosystem.

 

Somali Current system

Directly north of the Mozambique Channel, the northward setting current along the coast of the African continent is called the Zanzibar Current. It has as its main source the northern part of the westward flowing South Equatorial Current. During the north east monsoon season the Zanzibar Current is opposed by the southward flowing Somali Current and this meeting point usually shifts southward as the season progresses. It is only the surface expression of the Zanzibar Current that is prevented from moving northward; at depth the current continues as an undercurrent below the Somali Current. The north east monsoon occurs during the months of December to April, and is strongest in February. During the opposite wind phase, the south west monsoon season (June to October, maximum in August), the Zanzibar Current is strengthened considerably and forms the main tributary to the Somali Current which during this period carries water northward in an intense coastal jet. Speeds in this jet may reach very high values of up to 3.5 m/s. The southern part of the current is shallow. Further to the north it deepens.

The northward flow of the Somali Current during the south west monsoon season is not simply alongshore. The flow turns offshore at about 3°N. North of this point a strong upwelling cell has been observed to develop. The biological effect of this transitory upwelling cell has not been adequately studied. Two coastal gyres are then created at the sides of this upwelling cell. These gyres seem characteristic of the flow during the beginning of this season. As the season advances, these upwelling gyres shift northward, join together and by the time of the most intense part of the south west monsoon in August, the Somali Current is established as a continuous western boundary current from the Zanzibar Current in the south to the East Arabian Current in the north.

The monsoonal wind patterns vary somewhat from year to year and so will, in consequence, the seasonal development of the Somali Current. This inter-annual variability may have a decisive influence on the shelf circulation, on the marine ecosystem of the region and also on the success of the local fisheries. This variability has not been studied in a multi-disciplinary way. In general the coastal currents and the effect of offshore currents on the shelf area inshore of the Somali system have also been investigated in a very patchy and inadequate way. Long-term monitoring of currents, water masses and biota has been deficient.

 

Islands

The islands that lie east of Madagascar (i.e. excluding those in the Mozambique Channel and the Comoros) all lie in the path of the South Equatorial Current. This wind-driven current is shallow and is considered not to change much in strength or direction, either seasonally or inter-annually. However, this conclusion may be the consequence of insufficient observations in the region. The effect of the passing water masses on the narrow shelf regions of most of the islands is not well known, but can be assumed to be a function of their offshore bathymetry and the absence or presence of coral reefs. The flow of the South Equatorial Current over the Mascarene Ridge that lies between Mauritius and the Seychelles has been established only recently. This shallow obstruction to the westward flow causes the current to be concentrated into a number of narrow jets through the deeper parts of the ridge. A seasonal phytoplankton bloom commences along the eastern coast of Madagascar with the onset of winter and progresses as a productivity wave eastwards as the seasonal thermocline deepens and nutrients from below are made available in the euphotic zone.

There is another important perturbation to the envisaged steady and invariant nature of this component of the ASCLME. It has been shown that meridional meanders in the South Equatorial Current travel westward as Rossby waves. Embedded in them are eddies. These eddies intensify as they move westward and may have a decisive influence on the ocean circulation of the islands they pass. It has been demonstrated that they, and eddies that come from the Mozambique Channel, may eventually reach the Agulhas Current and trigger Natal Pulses with all the attendant effects of current changes.

 

Climate impacts and climate change

One of the reasons why international interest in the ASCLME region has grown over the past decade is the general recognition that the ocean region plays a key role in global climate. The leakage of warm water from the Agulhas system into the south Atlantic Ocean is the mechanism that has attracted most attention. This leakage seems to be controlled by mesoscale processes in the source regions of the Agulhas Current about which insufficient information is currently available. Coastal currents also affect the rainfall over the adjacent continental land masses and changes in these currents will therefore have an impact on terrestrial precipitation. This process is currently under investigation using numerical models.

Climate change may also affect the frequency and intensity of extreme events such as hurricanes. This will be of crucial importance for countries like Madagascar and Mozambique. Sea level rise will cause increased erosion of soft coastal plains and may therefore have substantial effects on low lying areas such as a major part of central Mozambique and low-lying small island states. Last, but not least, increased acidification of sea water, which occurs when more carbon dioxide is absorbed, may cause increased bleaching of corals and have major effects on plankton that could have lasting effects on coastal regions of a number of ASCLME countries.

 

Conclusions

The transboundary ecosystems of the west Indian Ocean, their dependence on their physical, chemical and geological environment and the possible effect of climate change on these interrelated systems all remain largely unknown. This holds true for the Agulhas Current system, the Somali Current system and the Mascarene Plateau and the islands of the southwest Indian Ocean. In order to establish some criteria for the proper management of these ecosystems, exploratory investigations are required to establish essential baseline information. This is one of the primary objectives of the ASCLME Project.

For more information:
Lutjeharms, JRE. 2007. Three decades of research on the greater Agulhas Current. Ocean Sci., 3, 129-147.
Lutjeharms, JRE. 2006. The coastal oceans of south-eastern Africa. In The Sea, Vol 14B, editors Robinson, AR and Brink, KH. Harvard University Press, Cambridge MA. Pp. 783-834.

Heileman et al.,  2008. Agulhas and Somali Current LMEs chapters from The UNEP Large Marine Ecosystem Report:  A Perspective on Changing Conditions in LMEs of the World's Regional Seas.