This thesis presents a study of intrusive features in the Western Equatorial Pacific and the role mixing plays in determining the large-scale structure of the Equatorial Pacific. In the thermocline of the Western Equatorial Pacific, intrusions appear as interleaving layers of warm/salty water and cold/fresh water. Using SeaSoar and CTD data, we describe the characteristics of these features; their vertical, meridional and zonal scales, their slopes across isopycnals and their temperature-salinity ratio. Approximately half of the features are shown to be consistent with a mechanism of double-diffusive interleaving, with the diffusive interface dominating the vertical fluxes. Assuming the small-scale variance drives the medium scale intrusion motion, we estimate the fluxes of heat and salt due to double-diffusive interleaving. We show that the estimated fluxes produced by interleaving are comparable with fluxes produced by mesoscale eddies as calculated from mooring measurements in the Western Equatorial Pacific. The effective lateral diffusion coefficient due to interleaving is estimated to be 1 x 10³ m² s⁻¹, a value which is believed to be significant in large-scale models of the Equatorial Ocean. Using the linear stability theory for double-diffusive interleaving on an Equatorial beta plane, we derive the functional form of the vertical and cross- frontal interleaving fluxes. The transfer of tracers produced by interleaving is proportional to the buoyancy frequency and the lateral salinity gradient. Experiments are performed using a two-dimensional model to assess the impact of mixing parameterizations on the large-scale circulation. We find that both the lateral mixing of momentum and tracers influences the characteristics of the resulting tracer and dynamic fields; the Sea Surface Temperature can be changed by O(1°C) and the speed of the Equatorial Undercurrent by O(50%). The heat-salt flux ratio is not important in the Equatorial region. The parameterization scheme is effective at confining the circulation to the Equator when implemented geodesically, isopycnally and assuming that diffusive fluxes dominate above the salinity maximum and finger fluxes below. Allowing diffusive or finger fluxes to dominate everywhere leads to asymmetry in the Equatorial circulation. Lateral mixing also plays a role in the transients of the system when, for example, the zonal wind field reverses direction, via the interplay with vertical mixing.