The mean flow field of the tropical Atlantic Ocean

The mean horizontal flow field of the tropical Atlantic Ocean is described between 20 degrees N and 20 degrees S from observations and literature resu lts for three layers of the upper ocean, Tropical Surface Water, Central Wa ter, and Antarctic Intermediate Water. Compared to the subtropical gyres th e tropical circulation shows several zonal current and countercurrent bands of smaller meridional and vertical extent. The wind-driven Ekman layer in the upper tens of meters of the ocean masks at some places the flow structu re of the Tropical Surface Water layer as is the case for the Angola Gyre i n the eastern tropical South Atlantic. Although there are regions with a st rong seasonal cycle of the Tropical Surface Water circulation, such as the North Equatorial Countercurrent, large regions of the tropics do not show a significant seasonal cycle. In the Central Water layer below, the eastward North and South Equatorial undercurrents appear imbedded in the westward-f lowing South Equatorial Current. The Antarcic Intermediate Water layer cont ains several zonal current bands south of 3 degrees N, but only weak flow e xists north of 3 degrees N. The sparse available data suggest that the Equa torial Intermediate Current as well as the Southern and Northern Intermedia te Countercurrents extend zonally across the entire equatorial basin. Due t o the convergence of northern and southern water masses, the western tropic al Atlantic north of the equator is an important site for the mixture of wa ter masses, but more work is needed to better understand the role of the va rious zonal under- and countercurrents in cross-equatorial water mass trans fer. (C) 1999 Elsevier Science Ltd. All rights reserved.