Summary
Historical data from about 500 surface moorings are used to describe the mean and half-yearly averaged circulation of the Equatorial Atlantic. The emphasis is on the kinematic structure and transports of equatorial countercurrents and undercurrents. West of 30° W they form a single eastward flow between 5° S and 10° N penetrating to the depth of 1500 m. The total transport of this flow is more than 90 Sv. There are three subsurface cores in this eastward flow located approximately at 5° S, at the Equator and at 5° N. The typical velocities are 20, 50 and 30 cm/s respectively. The typical velocity in the Equatorial Undercurrent (EU) deep core is about 20 cm/s. East of 30° W the single eastward flow is separated by two branches of the South Equatorial Current (SEC) flowing westward along 3 to 4° S and 2 to 4° N. This westward current, with maximum velocities of about 30 to 40 cm/s in the subsurface layers, penetrates to a depth of 1000 m. Proceeding to the east, the cores of the North Equatorial Countercurrent (NECC), EU and the South Equatorial Countercurrent (SECC) rise toward the surface and become wider and weaker. Their total transport decreases to about 75 Sv at 22.5° to 25° W. The EU is the strongest of these currents. The volume transport of the NECC is reduced from 30 Sv in the western part to 20 Sv in the eastern part of the Equatorial Atlantic (±10 to 15 per cent).
The SECC and EU are intensified throughout the Equatorial Atlantic during the second half of the year. Their cores deepen and widen at this time and their transports increase. The EU transport doubles while the NECC one rises by about 50 per cent in the Central Equatorial Atlantic. Current shear in upper layers at least doubles.
The circulation patterns of the N-W and N-E parts of the Equatorial Atlantic are characterized by two principal common features: 1) an absence of significant meridional mean gradient transport which can play an essential role in the total mass and heat exchange between equatorial and extraequatorial regions; 2) the occurence of quasisteady intensive eddies. The main reasons for these features are discussed. The principal features of the current field and transport pattern are analyzed. The mean northward mass and heat transports in the upper 300 m layer at 10° N are estimated with at least 50 per cent error as 27 Sv and 2 PW, respectively.
Zusammenfassung
Es werden historische Daten von etwa 500 Oberflächenverankerungen benutzt um die mittlere und halbjährliche gemittelte Zirkulation des äquatorialen Atlantiks zu beschreiben. Schwerpunkt ist die Untersuchung der kinematischen Struktur und des Transports der äquatorialen Gegenströme und Unterströme. Westlich von 30° W bilden sie eine einzelne ostwärts gerichtete Strömung zwischen 5° S und 10° N, die bis in eine Tiefe von 1500 m hinabreicht. Der Gesamttransport dieser Strömung ist größer als 90 Sv. Unter der Oberfläche gibt es in dieser ostwärts gerichteten Strömung drei Kerne die bei 5° S, am Äquator und bei 5° N liegen. Die typischen Geschwindigkeiten betragen 20, 50 und 30 cm/s. Die typische Geschwindigkeit des Tiefenkerns des äquatorialen Unterstroms (EU) liegt bei 20 cm/s. Östlich von 30° W ist der einfache ostwärts gerichtete Strom in zwei Zweige des Südäquatorialstroms (SEC) geteilt, die zwischen 3 und 4° S und 2 und 4° N westwärts strömen. Dieser Weststrom, mit Maximalgeschwindigkeiten von 30 bis 40 cm/s unterhalb der Oberflächenschicht, reicht bis in eine Tiefe von 1000 m. Die ostwärts strömenden Kerne von nordäquatorialem Gegenstrom (NECC), EU und südäquatorialem Gegenstrom (SECC) steigen zur Oberläche auf, dehnen sich aus und werden schwächer. Der Gesamttransport nimmt auf etwa 75 Sv zwischen 22,5 und 25° W ab. Der EU ist der stärkste dieser Ströme. Der Volumentransport des NECC nimmt von 30 SV im westlichen auf 20 Sv im östlichen Teil des äquatorialen Atlantiks ab (±10–15%).
SECC und EU werden im äquatorialen Atlantik während der zweiten Jahreshälfte verstärkt. In dieser Zeit sinken ihre Kerne ab und weiten sich aus. Auch ihr Transport nimmt zu. Im zentralen äquatorialen Atlantik verdoppelt sich der Transport des EU und der des NECC nimmt um 50% zu. Die Stromscherung in den oberen Schichten verstärkt sich mindestens auf das Doppelte.
Die Zirkulationsmuster der N-W und N-E Teile des äquatorialen Atlantiks werden durch zwei generelle Merkmale charakterisiert: 1) das Fehlen eines deutlichen mittleren meridionalen Gradienttransports, der eine wesentliche Rolle im gesamten Massen- und Wärmeaustausch zwischen äquatorialen und außeräquatorialen Gebieten spielt; 2) das Auftreten von quasi-stetigen ausgeprägten Eddies. Die Hauptgründe für diese Merkmale werden diskutiert. Die Hauptmerkmale des Strömungsfeldes und des Transportmusters werden analysiert. Die mittleren nordwärts gerichteten Massen- und Wärmetransporte in den oberen 300 m bei 10° N werden mit einer Fehlerwahrscheinlichkeit von 50% mit 27 Sv bzw. 2 PW bestimmt.
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Polonsky, A.B., Nikolaenko, E.G., Konate, S. et al. A study of the Equatorial Atlantic currents using historical moored data. Deutsche Hydrographische Zeitschrift 45, 219–254 (1993). https://doi.org/10.1007/BF02226382
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DOI: https://doi.org/10.1007/BF02226382