The department of Chocó, on the Colombian Pacific coast experiences 8,000 to 13,000 mm of average annual precipitation. Lloró (5°30′N, 76°32′W, 120m) has received above 12,700 mm (1952–1960). Using the NCEP/NCAR Reanalysis data, we show that the ocean-land-atmosphere interaction over the easternmost fringe of the tropical Pacific, enhanced by the dynamics of a low-level westerly jet (“CHOCO”), contributes to explain the existence of such record-breaking hydrological region. Deep convection develops from low-level moisture convergence by the CHOCO jet, combined with high-level easterly trade winds, orographic lifting on the western Andes, low surface pressures and warm air. Precipitation is organized in mesoscale convective complexes, in turn dynamically linked to the jet. The strength of the CHOCO jet (centered at 5°N) is associated with the gradient of surface air temperatures between western Colombia and the Niño 1+2 region, thereby exhibiting strong annual and interannual variability, which contributes to explaining Colombia's hydro-climatology and its anomalies during ENSO.