Socrates, who lived from about 470-399 BC, was the first to incorrectly suggest that the ocean currents, varying sea water temperatures, and tidal cycles were a result of deep subterranean channels in the Earth that would transport vast quantities of seawater. He proclaimed "in the earth itself, all over its surface, there are many hollow regions, some deeper and more widely spread than in which we lived. All these are joined together underground by many connecting channels...through which...there flows a great volume of water -- monstrous unceasing subterranean rives of waters both hot and cold - and of fire, too..." Aristotle carried Socrates's theories further, and from these early attempts of explaining the sea's currents it was first thought that the ocean was more like a river that encircled the land, flowing in a circular direction. This idea held firm in the European geographic imagination until late in the medieval period
In the mid-1600's, one of the earliest charts of global ocean circulation was produced by the Jesuit priest Athanasius Kircher in his Mundus Subteranneus. Given information from Portuguese explorers and other early natural theorists, he concluded that one of the entrances to the subterreanean channels must be located in the Gulf of Guinea in order to explain the tumultuous currents in this region ( The Guinea Current flows east at approximately 3°N along the western coast of Africa (Henin et al. 1986). When it reaches the Gulf of Guinea, it can obtain velocities close to 100 cm s-1 near 5°W (Richardson and Reverdin 1987). It has at least two sources: the North Equatorial Countercurrent (NECC) and the Canary Current. The seasonal instability of these two currents can affect the seasonal variability of the Guinea Current. For example, during the 1983-1984 Pacific El Nino, the Intertropical Convergence Zone was far south. This led to a relaxation of the trade winds at the equator, which strengthened the NECC and, subsequently, the Guinea Current.
Areas of upwelling and increased biological productivity characterize the Guinea Current, like other eastern ocean boundary currents. Cooler waters at the surface during the boreal summer are associated with the intensification of the current. The Guinea is a geotropically balanced current with isotherms that slope upwards towards the northern coast. As the current intensifies, the slope steepens, which brings the thermocline closer to the surface near the coast. The coastal upwelling and the summer intensification of the Guinea Current are thus related. However, the Guinea Current is unusual among upwelling regions in that there seems to be no correlation between sea surface temperature and wind patterns on a seasonal time scale. According to Binet (1997), the seasonal shallowings are not induced by local wind stress, but by geostrophic adjustment of isotherms, Kelvin waves and cyclonic turbulent eddies. Several experts agree that the Guinea Current experiences a minimum during the winter (November through February) and a maximum during the summer (May through September). A reversal in current direction has been detected during the minima. This change is attributed to variations in the flow of the North Equatorial Countercurrent, the Canary Current, and the Benguela Current, and to the weakening of the easterly winds. However, Lemasson and Rebert (1968) offered an alternative explanation. As they were studying the currents on the continental shelf of the Ivory Coast during December, they observed not only the Guinea Current, but also a current below it. This current, named the Ivoirien Undercurrent, flowed opposite to the Guinea, to the west.
They also noticed that at times the speed of the Guinea Current was higher offshore than onshore. Thus, suggested that the presence of a westward current at the surface near the coast could be due to the surfacing of the Ivoirien Undercurrent and the seaward displacement of the Guinea Current, not due to the reversal of the Guinea Current.
Another explanation is that the apparent reversals are actually caused by cyclonic eddies between the current and the coast. Ingham points out that "the data on which the concept of current reversals is based are either point observations or isolated sections perpendicular to the coastline," which do not clearly depict current reversals in the alongshore direction.
References Bakun, A., 1978: Guinea Current Upwelling. Nature, 271, 147-150. Binet, D., 1997: Climate and pelagic fisheries in the Canary and Guinea currents 1964-1993: The role of Trade Winds and the Southern Oscillation. Oceanologia Acta, 20, 177-190. Boisvert, W.E., 1967: Major currents in the North and South Atlantic Ocean between 64°N and 60°S, Tech. Rep. Hydrogr. Off. Wash., TR-193, 92 p. Colin, C., 1988: Coastal upwelling events in front of Ivory Coast during the FOCAL program. Oceanologia Acta, 11, 125-138.
Hisard, P. and J. Merle, 1979: Onset of the summer cooling in the Gulf of Guinea. Submitted to Deep Sea Research. Hisard, P., C. Henin, R. Houghton, B. Piton, and P. Rual, 1986: Oceanic conditions in the tropical Atlantic during 1983 and 1984. Nature, 322, 243-245. Henin, C., P. Hisard, and B. Piton, 1986: Observations hydrologiques dans l'ocean Atlantique Equatorial, Ed. ORSTOM, FOCAL, 1, 1-191. Ingham, M.C., 1970: Coastal upwelling in the northwestern gulf of Guinea. Bulletin of Marine Science, 20, 1-34. Lemasson, L. and J.-P. Rebert, 1973a: Les courants marins dans le Golfe Ivoirien. Cah. ORSTOM, Ser. Oceanogr., 11, 67-95. Longhurst, A.R., 1962: A review of the Oceanography of the Gulf of Guinea. Bull. Inst. Afr. Noire, 24, 633-663. Marchal, E. and J. Picaut, 1977: Repartition et abondance evaluees par echo-integration des poissons du plateau ivoiro-ghaneen en relation avec les upwellings locaux. J. Rech. Oceanogr., 2, 39-57. Peterson, R.G., L. Stramma, and G. Kortum, 1996: Early concepts in charts and circulation. Progress in Oceanography, 37, 1-115. Philander, S.G.H., 1979: Upwelling in the Gulf of Guinea. Journal of Marine Research, 37, 23-33. Picaut, J., 1983: Propagation of the seasonal upwelling in the eastern equatorial Atlantic. Journal of Physical Oceanography, 13, 18-37. Richardson, P.L. and S.G.H. Philander, 1987: The seasonal variations of surface currents in the tropical Atlantic Ocean: A comparison of ship drift data with results from a general circulation model. Journal of Geophysical Research, 92, 715-724. Richardson, P.L. and G.Reverdin, 1987: Seasonal cycle of velocity in the Atlantic North Equatorial Countercurrent as measured by surface drifters, current meters, and ship drifts. Journal of Geophysical Research, 92, 3691-3708. Verstraete, J.M., 1992: The seasonal upwellings in the Gulf of Guinea. Progress in Oceanography, 29, 1-60. External links http://oceancurrents.rsmas.miami.edu/atlantic/guinea.html http://www7320.nrlssc.navy.mil/global_nlom/globalnlom/eqa.html