188 ATLANTIC OCEAN EQUATORIAL CURRENTS ATLANTIC OCEAN EQUATORIAL CURRENTS S. G. Philander, Princeton University, Princeton, Centered on the equator, and below the westward NJ, USA surface Sow, is an intense eastward jet known as the Equatorial Undercurrent which amounts to a Copyright ^ 2001 Academic Press narrow ribbon that precisely marks the location of doi:10.1006/rwos.2001.0361 the equator. The undercurrent attains speeds on the order of 1 m s\1 has a half-width of approximately Introduction 100 km; its core, in the thermocline, is at a depth of approximately 100 m in the west, and shoals to- The circulations of the tropical Atlantic and PaciRc wards the east. The current exists because the west- Oceans have much in common because similar trade ward trade winds, in addition to driving divergent winds, with similar seasonal Suctuations, prevail westward surface Sow (upwelling is most intense at over both oceans. The salient features of these circu- the equator), also maintain an eastward pressure lations are alternating bands of eastward- and west- force by piling up the warm surface waters in the ward-Sowing currents in the surface layers (see western side of the ocean basin. That pressure force Figure 1). Fluctuations of the currents in the two is associated with equatorward Sow in the thermo- oceans have similarities not only on seasonal but cline because of the Coriolis force. At the equator, even on interannual timescales; the Atlantic has where the Coriolis force vanishes, the pressure force a phenomenon that is the counterpart of El Ninoin is the source of momentum for the eastward Equa- the PaciRc. The two oceans also have signiRcant torial Undercurrent which, in a downstream direc- differences. The Atlantic, but not the PaciRc, has tion, continually loses water because of intense a net transport of heat from the southern into the equatorial upwelling which sustains the divergent, northern hemisphere, mainly because of an intense, poleward Ekman Sow in the surface layers. cross-equatorial coastal current in the Atlantic, the Along the African coast, cold equatorward coastal North Brazil Current. The similarities and differ- currents, the Canary Current off north-west Africa, ences between the tropical Atlantic and PaciRc (and and the Benguela Current off south-west Africa, are also the Indian Ocean) are of enormous interest to driven by the components of the winds parallel to modelers because they provide invaluable checks on the coast. These currents, which are associated with the theories and models that explain and simulate intense coastal upwelling and low sea surface tem- oceanic currents. Those currents play a central role peratures, feed the westward North and South in the Earth’s climate, by inSuencing sea surface Equatorial Currents respectively. temperature patterns for example. Along the coast of South America, the most prominent current is the North Brazil Current, 3 Time-averaged Currents which carries very warm water from about 5 N across the equator. Some of that water feeds the Although the trade winds that prevail over the Equatorial Undercurrent, but much of it continues tropical Atlantic Ocean have a westward compon- into the northern hemisphere. Further south along ent, the currents driven by those winds include the the coast of Brazil, the Sow is southward. eastward North Equatorial Countercurrent, between The net north}south circulation associated with the latitudes 33 and 103N approximately. Sverdrup, the various currents is a northward Sow of warm in one of the early triumphs of dynamical oceanog- surface waters, and a southward return Sow of cold raphy, Rrst pointed out that this current is attribu- water at depth, resulting in a transport of heat table to the curl of the wind. Flanking this eastward from the southern into the northern Atlantic. current are westward currents to its north, the The southward Sow below the thermocline is part North Equatorial Current, and to its south, the of the global thermohaline circulation, which South Equatorial Current. The latter current is par- involves the sinking of cold, saline waters in the ticularly intense at the equator, where it can attain northern Atlantic. The absence of such formation of speeds in excess of 1 m s\1. Figure 1, a schematic deep water in the northern PaciRc } that ocean map of the various currents, actually depicts condi- is less saline than the northern Atlantic } is part of tions between July and September when the south- the reason why there is a northward transport of east trades are particularly intense and penetrate heat across the equator in the Atlantic but not the into the northern hemisphere. PaciRc. ATLANTIC OCEAN EQUATORIAL CURRENTS 189 Longitude 70˚W 60˚ 50˚ 40˚ 30˚ 20˚ 10˚0˚ 10˚E 20˚N Caribbean Current North Equatorial Guyana Current Current 10˚ North Equatorial Countercurrent South Guinea Current North Equatorial Brazil 0˚ Current Current Latitude South Equatorial Current 10˚ South Equatorial Brazil Current Current 20˚S Figure 1 Schematic map showing the major surface currents of the tropical Atlantic Ocean between July and September when the North Equatorial Countercurrent flows eastward into the Guinea Current in the Gulf of Guinea. From January to May the North Equatorial Countercurrent disappears and the surface flow is westward everywhere in the western tropical Atlantic. Seasonal Variations of the Currents The upwelling along the west African coast, and the coastal currents too, are subject to large sea- The seasonal variations of the winds are associated sonal Suctuations in response to the variations in with the north}south movements of the Intertropical the local winds. Thus upwelling is most intense off Convergence Zone (ITCZ), the band of cloudiness south-western Africa, and surface temperatures and heavy rains where the south-east and north-east there are at a minimum, during the late northern trades meet. The south-east trades are most intense summer when the local alongshore winds are most and penetrate into the northern hemisphere during intense. Off north-western Africa the season for the northern summer when the ITCZ is between 103 such conditions is the late northern winter. The and 153N. During those months the surface currents northern coast of the Gulf of Guinea (along 53N are particularly strong. The North Brazil Current, approximately) also has seasonal upwelling, with after crossing the equator, veers sharply eastward lowest temperatures during the northern summer, to feed the North Equatorial Countercurrent. The even though the local winds along that coast have Equatorial Undercurrent is also strongest during this almost no seasonal cycle. In that region, changes in season when the east}west slope of the equatorial the depth of the thermocline (which separates warm thermocline is at a maximum. surface waters from the cold water at depth) depend During the summer of the southern hemisphere, on winds everywhere in the equatorial Atlantic, the zone where the north-east and south-east trades even the winds off Brazil which are most intense meet (the ITCZ) shifts equatorward so that the during the northern summer when they cause winds are relaxed along the equator. The North a shoaling of the thermocline throughout the Gulf Brazil Current no longer veers offshore after cross- of Guinea. ing the equator, but continues to Sow along the If the winds over the ocean were suddenly to stop coast into the Gulf of Mexico. It is fed by surface blowing, how long would it be before the currents Sow that is westward at practically all latitudes in in Figure 1 disappear? The answer to this question the tropics because, during this season, the eastward (which is the same as asking how long it would take North Equatorial Countercurrent disappears from for the currents to be generated from a state of rest) the surface layers, as is evident in Figure 2. At this is of central importance in climate studies because, time, the northward heat transport across 103Nis associated with the currents, are sea surface temper- huge } on the order of a peta-watt; during the ature patterns that profoundly affect climate. (From northern summer it is practically zero. a strictly atmospheric perspective, the cause of El 190 ATLANTIC OCEAN EQUATORIAL CURRENTS 15˚N If the winds change gradually rather than ab- NECC ruptly, then the timescale of the gradual changes _ 10 relative to the time it takes the ocean to adjust determines the nature of the oceanic response. Thus 10˚ _ 10 winds that Suctuate on a timescale much longer 0 than the adjustment time of the ocean will force an 20 equilibrium response in which the ocean, at any 34 given time, is in equilibrium with the winds at that 5˚ _ 15 time. (The currents and winds Suctuate essentially _ 10 in phase.) From results such as these it can be _ inferred that the seasonally varying trade winds 60 _ _ Latitude 30 over the tropical Atlantic and PaciRc Oceans should _ 30 _ _ 40 20 0˚ 5 force an equilibrium response near the equator in _ 10_ 20 the case of the small ocean basin, the Atlantic, but not in the case of the much larger PaciRc. The _ 39 _ 30 measurements conRrm this theoretical result: the 5˚ seasonal variations of the currents and of the ther- _ 20 mocline slope are in phase with the variations of the _ winds in the equatorial Atlantic, but not in the 10 R 10˚S equatorial Paci c. J F MAM J J ASON D J Month Interannual Variations Figure 2 The seasonal disappearance of the North Equatorial Countercurrent (NECC) from the western tropical Atlantic. The Given the similarities between the climates of the eastward velocity in cm s\1 (negative values correspond to tropical Atlantic and PaciRc } arid, cool conditions westward flow) is shown as a function of latitude and month, on the eastern sides, along the shores of Peru and starting in January.