Mixing in the ocean plays a key role in the ocean's dynamics and thermodynamics and how the ocean interacts with the atmosphere. Here we focus on lateral mixing in the equatorial ocean, and in particular the part played by inertial instability. We demonstrate the effectiveness of the instability in promoting mixing in the equatorial thermocline, that is sufficient to affect the large scale structure of the equatorial ocean, and which rectifies problems in existing ocean models. The spatial and temporal variability of the process is likely to impinge on interannual variations of the ocean and El Ni¿o.