Interhemispheric exchanges of mass and heat in the Atlantic Ocean in January–March 1993

Abstract

Hydrographic, geochemical, and direct velocity measurements along two zonal (7.5°N and 4.5°S) and two meridional (35°W and 4°W) lines occupied in January–March, 1993 in the Atlantic are combined in an inverse model to estimate the circulation. At 4.5°S, the Warm Water (potential temperature θ>4.5°C) originating from the South Atlantic enters the equatorial Atlantic, principally at the western boundary, in the thermocline-intensified North Brazil Undercurrent (33±2.7×10 6 m 3 s −1 northward) and in the surface-intensified South Equatorial Current (8×10 6 m 3 s −1 northward) located to the east of the North Brazil Undercurrent. The Ekman transport at 4.5°S is southward (10.7±1.5×10 6 m 3 s −1). At 7.5°N, the Western Boundary Current (WBC) (17.9±2×10 6 m 3 s −1) is weaker than at 4.5°S, and the northward flow of Warm Water in the WBC is complemented by the basin-wide Ekman flow (12.3±1.0×10 6 m 3 s −1), the net contribution of the geostrophic interior flow of Warm Water being southward. The equatorial Ekman divergence drives a conversion of Thermocline Water (24.58⩽ σ 0<26.75) into Surface Water ( σ 0<24.58) of 7.5±0.5×10 6 m 3 s −1, mostly occurring west of 35°W. The Deep Water of northern origin flows southward at 7.5°N in an energetic (48±3×10 6 m 3 s −1) Deep Western Boundary Current (DWBC), whose transport is in part compensated by a northward recirculation (21±4.5×10 6 m 3 s −1) in the Guiana Basin. At 4.5°S, the DWBC is much less energetic (27±7×10 6 m 3 s −1 southward) than at 7.5°N. It is in part balanced by a deep northward recirculation east of which alternate circulation patterns suggest the existence of an anticyclonic gyre in the central Brazil Basin and a cyclonic gyre further east. The deep equatorial Atlantic is characterized by a convergence of Lower Deep Water (45.90⩽ σ 4<45.83), which creates an upward diapycnal transport of 11.0×10 6 m 3 s −1 across σ 4=45.83. The amplitude of this diapycnal transport is quite sensitive to the a priori hypotheses made in the inverse model. The amplitude of the meridional overturning cell is estimated to be 22×10 6 m 3 s −1 at 7.5°N and 24×10 6 m 3 s −1 at 4.5°S. Northward heat transports are in the range 1.26–1.50 PW at 7.5°N and 0.97–1.29 PW at 4.5°S with best estimates of 1.35 and 1.09 PW.