Intercomparison of the upper layer circulation of the western equatorial Atlantic Ocean:: In situ and satellite data

Thanks to the agreement found between in situ measurements and TOPEX/POSEIDON data in the western tropical Atlantic Ocean, a realistic picture of the spatial and temporal variability over the 1992-1997 period is obtained. The sea level variability clearly emphasizes three ranges of variability. The intraseasonal variability is associated with propagating features north of the equator, consistent with a first baroclinic Rossby wave characteristic. The annual variability, which represents the largest part of the variability, describes the seasonal cycle of the North Equatorial Countercurrent; but, more interesting, is a clear year-to-year variability in the northernmost part of the area. The surface currents also reveal an intraseasonal tendency with a peak of energy at 62 days at 5 degrees N. Upper layer volume transport across 38 degrees W and between 3 degrees N and 9 degrees N shows a regular seasonal contrast mostly due to the 3 degrees N-6 degrees N region. Year-to-year variations are also clearly evidenced. The eastward transport loses about 35% of its strength in the second half of 1995 compared with 1994. This event is followed by a stronger than usual westward transport in early 1996, and in early 1997, the transport seems abnormally eastward. However, contrary to the seasonal cycle, this interannual variability occurs mainly in the 6 degrees N-9 degrees N band. Interhemispheric transport computed across 7 degrees 30'N, between 50 degrees W and 35 degrees W, is northward during the whole period 1992-1995. A seasonal cycle can be detected with maximum transport during boreal winter and minimum in spring. An interesting result is the increasing tendency of the northward transport from April 1993 until boreal fall 1995, when the maximum value for the period is reached.