IN the equatorial Pacific Ocean, easterly trade winds and the Earth’s rotation combine to drive surface currents away from the Equator, thereby causing cold nutrient-rich subsurface water to upwell. The front(1) that forms between this upwelled water and warmer waters north of the Equator is sometimes, visible as a spectacular "line in the sea"(2) between 2 degrees and 6 degrees N. Westward-propagating cusp-shaped disturbances observed along this front(3) have been attributed to the effect of dynamical instabilities in the system of zonal equatorial currents(4-11) but the connection between these phenomena remains unclear. Here we report extensive measurements from shipboard sensors, satellite and drifting buoys which reveal the three-dimensional structure of an anticyclonic eddy (or vortex) 500 km in diameter and centred at 4 degrees N. We suggest that cusp-shaped disturbances of the front are caused by trains of large-amplitude vortices, which are driven by instability, of the mean zonal sheer. We show that these vortices not only play an important role in the meridional transport of heat, salt and momentum, but are also associated with regions of intense horizontal convergence along the front, where dramatic concentrations of marine life are observed.