Seasonal cycle of velocity in the Atlantic North Equatorial Countercurrent as measured by surface drifters, current meters, and ship drifts


  • P. L. Richardson,

  • G. Reverdin


This report describes the general circulation and seasonal variation of currents in the equatorial Atlantic, concentrating on the North Equatorial Countercurrent (NECC), using data collected as part of the Seasonal Response of the Equatorial Atlantic and Programme Français Océan et Climat dans l'Atlantique Equatorial experiments plus historical ship drifts. During 1983–1985 the Lagrangian circulation was studied by launching and tracking 30 freely drifting drogued buoys in the North Equatorial Countercurrent and 23 in the South Equatorial Current (SEC). In addition, continuously recording current meters were moored for 20 months at depths from 20 to 300 m near the center of the NECC, at 6°N, 28°W. These measurements are the longest series ever obtained in this current and provide information about its seasonal and interannual variations and zonal, meridional, and vertical structure. The seasonal cycle of the NECC is very regular from year to year. Each year the NECC starts up in May–June. It flows eastward across the Atlantic with surface speeds of up to 143 cm/s in the west, extending down to 350 m at 28°W, and flowing into both the Guinea Current and North Equatorial Current. It disappears or reverses from about January–June west of 18°W. Some near-surface water of the NECC is inferred to downwell and flow equatorward toward the Equatorial Undercurrent. The SEC flows into the North Brazil Current, which during spring continues up the coast into the Caribbean. During fall, however, the whole North Brazil Current retroflects, or turns back on itself, between 45° and 50°W, forming the western NECC. The retroflection establishes a meander pattern in the NECC that slowly propagates westward during fall at a speed of 4 cm/s. The meanders have a displacement of about 300 km in latitude, a wavelength of 900 km, and meridional velocity fluctuations of up to 100 cm/s. The swift currents and time-dependent meanders in the western NECC cause a high eddy kinetic energy, ∼2400 cm2/s2, equivalent to that of the energetic part of the Gulf Stream.